Thyroid hormone beta receptor activation has additive cholesterol lowering activity in combination with atorvastatin in rabbits, dogs and monkeys.

BACKGROUND AND PURPOSE: Thyroid hormone receptor (TR) agonists are in clinical trials for the treatment of hypercholesterolaemia. As statins are the standard of clinical care, any new therapies must have adjunctive activity, when given in combination with statins. As already known for the statins, the cholesterol lowering effect of TR activation involves increased expression of the low-density lipoprotein receptor. Using animal models, we tested whether TR activation would have additive cholesterol lowering activity in the presence of effective doses of a statin. EXPERIMENTAL APPROACH: We evaluated the activity of a liver-targeted prodrug, MB07811, of a novel TH receptor beta agonist, MB07344, as monotherapy and in combination with atorvastatin in rabbits, dogs and monkeys. KEY RESULTS: In rabbits, MB07344 (i.v.) decreased total plasma cholesterol (TPC) comparable to that achieved with a maximally effective dose of atorvastatin (p.o.). The addition of MB07344 to atorvastatin resulted in a further decrease in TPC. Similarly, the addition of MB07811 (p.o.) to atorvastatin treatment decreased TPC beyond the level achieved with either agent as monotherapy. In dogs and monkeys, atorvastatin and MB07811 were administered as monotherapy or in combination. Consistent with the rabbit studies, the combination treatment caused a greater decrease in TPC than either MB07811 or atorvastatin administered as monotherapy. CONCLUSIONS AND IMPLICATIONS: We conclude that the effects of MB07811 and atorvastatin in lowering cholesterol are additive in animals. These results would encourage and support the demonstration of similarly improved efficacy of combination versus monotherapy with such agents in the clinic.

Inhibition of adenosine kinase induces expression of VEGF mRNA and protein in myocardial myoblasts.

We tested whether increased endogenous adenosine produced by the adenosine kinase inhibitor GP-515 (Metabasis Therapeutics) can induce vascular endothelial growth factor (VEGF) expression in cultured rat myocardial myoblasts (RMMs). RMMs were cultured for 18 h in the absence (control) and presence of GP-515, adenosine (Ado), adenosine deaminase (ADA), or GP-515 + ADA. GP-515 (0.2-200 microM) caused a dose-related increase in VEGF protein expression (1.99-2.84 ng/mg total cell protein); control VEGF was 1.84 +/- 0.05 ng/mg. GP-515 at 2 and 20 microM also increased VEGF mRNA by 1.67- and 1. 82-fold, respectively. ADA (10 U/ml) decreased baseline VEGF protein levels by 60% and completely blocked GP-515 induction of VEGF. Ado (20 microM) and GP-515 (20 microM) caused a 59 and 39% increase in VEGF protein expression and a 98 and 33% increase in human umbilical vein endothelial cell proliferation, respectively, after 24 h of exposure. GP-515 (20 microM) had no effect on VEGF protein expression during severe hypoxia (1% O(2)) but increased VEGF by an additional 27% during mild hypoxia (10% O(2)). These results indicate that raising endogenous levels of Ado through inhibition of adenosine kinase can increase the expression of VEGF and stimulate endothelial cell proliferation during normoxic and hypoxic conditions.

Gradual onset of myocardial ischemia results in reduced myocardial infarction. Association with reduced contractile function and metabolic downregulation.

BACKGROUND: With moderate reductions in coronary blood flow, adjustments in myocardial metabolism can occur with a normalization of the imbalance between the decreased O2 delivery and tissue O2 demand. This state of "downregulated" metabolism is associated with reduced function and minimal irreversible injury and has been linked to myocardial hibernation. We hypothesized that (1) this process would occur when perfusion was reduced to severely ischemic levels, but only when flow declined at a slow rate rather than abruptly, and (2) this would result in blunted ischemia and reduced myocardial injury for a given period of low blood flow. METHODS AND RESULTS: The left anterior descending coronary artery in anesthetized open-chest pigs was cannulated and perfused with arterial blood by an extracorporeal perfusion pump. Regional function (percent segment shortening, %SS) was measured with sonomicrometry and a regional coronary vein cannulated for blood gas analysis and lactate measurements. Coronary blood flow (CBF) was reduced to 10% of control either in a step fashion (Fast Ischemia group) or gradually in a linear manner over 70 minutes (Ramp Ischemia group). In all animals, CBF was held for 60 minutes at this 10% level and then followed by 2 hours of reperfusion. In the Ramp Ischemia group, the linear fall in CBF resulted in an initial maintenance of both %SS and myocardial oxygen consumption (MVO2) followed by linear decreases in both variables (r = .98 to .99) as flow fell to the 10% level. The relation of MVO2 to function was linear (r = .99) over the entire flow range. Although %SS, MVO2, CBF, coronary pressure, and hemodynamics during the 10% flow period were not different between groups, the increases in coronary venous lactate and PCO2 and fall in pH were blunted in the Ramp Ischemia group compared with the Fast Ischemia group. With reperfusion, a significant decrease in end-diastolic length was present only in the Fast Ischemia group. Additionally, although the region at risk was not different, infarction was markedly reduced in the Ramp Ischemia group (6.6 +/- 1.9%) compared with the Fast Ischemia group (31.4 +/- 6.9%). CONCLUSIONS: These data are consistent with the hypothesis that the downregulation of myocardial metabolism with gradually decreased flow to severe levels results in reduced myocardial injury for a given period of low flow. We propose that the rate at which blood flow decreases with myocardial underperfusion is a novel determinant of infarct injury. This may have clinical implication in situations in which there is a time-dependent component to the decrease in coronary blood flow in acute ischemic events, ie, thrombus formation at a site or coronary stenosis.

Effect of platelet depletion and inhibition of platelet cyclooxygenase on C5a-mediated myocardial ischemia.

Activation of the complement cascade is involved in the myocardial injury resulting from transient ischemia and reperfusion. We previously showed that the complement anaphylatoxin C5a causes myocardial ischemia in vivo, mediated in part via thromboxane (Tx) A2. In the present study, we assess the role of platelets in the C5a-induced myocardial ischemia and Tx release. The left anterior descending coronary artery of anesthetized pigs was perfused with arterial blood at constant pressure and measured flow (coronary blood flow). Segment function (percent segment shortening) was measured with sonomicrometry, and regional coronary venous blood was sampled and assayed for TxB2 (by radioimmunoassay). We found that the C5a-induced decrease in coronary blood flow and percent segment shortening and the release of Tx were indistinguishable whether the left anterior descending coronary artery bed was perfused with normal arterial blood, with arterial blood obtained from animals depleted of platelets (cyclophosphamide, n = 6), or with arterial blood from aspirin-treated animals (n = 9) in which the platelets were unable to produce Tx. These data demonstrate that platelet-derived Tx does not contribute to the C5a-induced myocardial ischemia and Tx release in this model and that these cells do not play an integral role in this phenomenon.

Intracoronary versus intravenous effects of cocaine on coronary flow and ventricular function.

BACKGROUND: Cocaine use has been associated with cardiomyopathy and ischemic coronary syndromes. However, the pathophysiological mechanisms responsible for these syndromes are not clear and have been suggested to involve direct effects of cocaine on myocyte contractility and coronary resistance as well as indirect effects via altered autonomic tone, secondary mediators, and myocardial metabolism. We sought to distinguish direct from indirect effects of cocaine on ventricular function and coronary resistance by comparison of the administration of intracoronary cocaine (0.12 to 0.36 mg/min constant infusion) with intravenous cocaine (5 mg/kg bolus infusion) in an in vivo anesthetized pig preparation. METHODS AND RESULTS: To control for changes in coronary resistance secondary to autoregulation and myocardial metabolism, the left anterior descending coronary artery was perfused at constant coronary pressure and the interventricular vein was cannulated for coronary venous oxygen saturation measurement. Coronary blood flow, regional percent segment shortening, myocardial oxygen consumption, and serum cocaine concentrations were measured. Intracoronary cocaine produced a dose-dependent decrease in percent segment shortening in the absence of significant changes in coronary flow or systemic hemodynamics. In contrast, intravenous cocaine had mild biphasic effects on coronary resistance with an initial brief vasodilation (30.0 +/- 5% increase in flow from control) followed by more prolonged vasoconstriction (17.0 +/- 3.3% decrease in flow from control), which were independent of autoregulation or myocardial metabolism. In addition, intravenous cocaine caused an early 48% decrease in percent segment shortening, at which time the measured cocaine concentration was 20.1 micrograms/mL blood. This was comparable to the intracoronary cocaine concentration of 17.1 micrograms/mL blood, which produced a similar 48% decrease in percent segment shortening. CONCLUSIONS: We conclude that the effects of acute cocaine exposure on ventricular function are predominantly direct but of brief duration and therefore probably not clinically relevant. The effects of cocaine on coronary tone are predominantly indirect and biphasic, with early vasodilation followed by mild and more prolonged vasoconstriction. In the absence of coronary stenosis or ventricular hypertrophy, this small amount of vasoconstriction is unlikely to cause ischemia.

Severe neutrophil depletion by leucocyte filters or cytotoxic drug does not improve recovery of contractile function in stunned porcine myocardium.

OBJECTIVE: The contribution of neutrophils to myocardial injury during stunning remains controversial because of conflicting results in neutropenic animals. The goal of this study was to compare the recovery of function in stunned myocardium using two distinct methods for inducing neutropenia in pigs. METHODS: Three groups of pigs were studied: control (n = 6), and made neutropenic by either Leukopak blood filters (n = 7), or cyclophosphamide (n = 7, 50 mg.kg-1 intravenously 4 d prior to study). In anaesthetised open chest pigs, with the heart paced at 110 beats.min-1, the left anterior descending coronary artery was perfused with an extracorporeal circuit at controlled coronary pressure (CP) and the regional coronary blood flow was measured. Systolic wall thickening was determined by sonomicrometry in the left anterior descending and circumflex coronary artery regions. The protocol consisted of 15 min of low flow ischaemia (CP = 40 mm Hg), followed by a staged reperfusion over 10 min back to baseline (CP = 90 mm Hg), and continued for 2 h. Blood filtration was initiated prior to ischaemia and stopped after 90 min reperfusion. RESULTS: In both treated groups during ischaemia and the initial 60 min of reperfusion the neutrophil count was severely depleted to < 5% compared to the control group. Aortic pressure, coronary blood flow during ischaemia, area at risk, and systolic wall thickening in the circumflex region were similar between groups. Recovery of systolic wall thickening in the left anterior descending region after reperfusion was equivalent in all three groups. In the filter group, arrhythmias during ischaemia and reperfusion were significantly less. CONCLUSION: As assessed in the pig model of myocardial stunning and using two different methods, severe neutropenia does not reduce the depression of contractile function.

Role of cardiac mast cells in complement C5a-induced myocardial ischemia.

Activated complement component C5a causes myocardial ischemia mediated by thromboxane (Tx) A2 and leukotrienes C4/D4. Blood cells are not involved in either the mediator release or the myocardial effects of C5a, suggesting that a C5a-sensitive, cardiac resident inflammatory cell is responsible. The goals of this study were to determine whether 1) cardiac mast cell activation accompanies the C5a response, 2) inhibition of mast cell degranulation inhibits the response, and 3) histamine release plays a role in the C5a-induced myocardial ischemia. The left anterior descending coronary artery (LAD) of open-chest pigs (n = 13) was perfused with arterial blood at constant pressure (95 mmHg). Coronary blood flow (CBF) was measured (in-line flowmeter) and regional function [percent segment shortening (%SS)] determined with sonomicrometry. A coronary vein was cannulated for measurement of plasma TxB2 and histamine (a marker of mast cell degranulation). Intracoronary C5a (500 ng) decreased coronary blood flow (45% of preinfusion levels) and LAD %SS (65% of preinfusion) and was accompanied by increases in coronary venous TxB2 (delta 63.3 ng/ml) and histamine (delta 200 nM). Mast cell inhibition with lodoxamide (2 mg/kg iv, n = 8) attenuated the C5a-induced fall in CBF (14 vs. 53% decrease, P < 0.01) and %SS (10 vs. 38% decrease, P < 0.01) and also reduced the C5a-induced increase in both coronary venous histamine (delta 26 vs. 278 nM, P < 0.05) and TxB2 (delta 0.34 vs. 63.3 ng/ml, P < 0.01). However, histamine H1 (pyrilamine) and H2 (ranitidine) receptor blockade had no effect on the C5a-induced fall in CBF or LAD %SS.(ABSTRACT TRUNCATED AT 250 WORDS)

Complement C5a-mediated myocardial ischemia and neutrophil sequestration: two independent phenomena.

The intracoronary infusion of complement C5a causes a decrease in coronary blood flow and contractile dysfunction mediated by thromboxane (TxA2) and leukotrienes. Although these effects are accompanied by polymorphonuclear leukocyte (PMN) sequestration, the role of PMNs and the source of these eicosanoids remain unknown. To assess the contribution of PMNs to the C5a-induced myocardial ischemic response, the left anterior descending (LAD) coronary artery of pigs (n = 13) was cannulated and pump perfused at constant pressure with either normal arterial blood or neutropenic arterial blood (PMN count 0.02 x 10(3) cells/microliters) obtained from animals treated with cyclophosphamide (50 mg/kg iv, given 4 days before). The coronary vein draining the LAD region was cannulated for measurement of leukocyte count and TxB2 levels. Two groups of animals were studied: group 1 (n = 7) neutropenic animals were instrumented and normal animals served as blood donors and group 2 (n = 6) normal animals were instrumented and neutropenic animals served as blood donors. The myocardial response to intracoronary C5a (500 ng) was determined in each animal during coronary perfusion with normal arterial blood and also with neutropenic arterial blood. During perfusion with normal arterial blood, C5a decreased coronary flow to 52.3% and contractile function to 58.8% of preinfusion values. This was accompanied by a transient myocardial accumulation of PMNs (arterial-coronary venous gradient of 5.4 x 10(3) cells/microliters) and increased TxB2 levels in coronary venous blood (from 0.31 to 17.5 ng/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

Intracoronary C5a induces myocardial ischemia by mechanisms independent of the neutrophil: leukocyte filters desensitize the myocardium to C5a.

Activation of the complement cascade with the generation of anaphylatoxins accompanies the inflammatory response elicited by acute myocardial ischemia and reperfusion. Although complement is activated in the interstitium during acute myocardial ischemia, we have studied mechanisms whereby complement might exacerbate ischemia by using a model employing intracoronary injection of C5a in nonischemic hearts. Intracoronary injection of complement component C5a induces transient myocardial ischemia, mediated through the production of the coronary vasoconstrictors thromboxane A2 and peptidoleukotrienes (LTC4, LTD4), and causes sequestration of polymorphonuclear leukocytes (PMN) in the coronary vascular bed. To further investigate the role of the PMN in the C5a-induced vasoconstriction, the left anterior descending coronary artery (LAD) in pigs was perfused at constant pressure and measurements of coronary blood flow, myocardial contractile function (sonomicrometry), arterial/coronary venous blood PMN count, and thromboxane B2 (TxB2) levels were performed. The myocardial response to intracoronary C5a (500 ng) was determined before, during, and after perfusion with blood depleted of PMNs using leukocyte filters (Sepacell R-500, Pall PL-100). In additional animals, the myocardial response to the PMN chemotactic agent, LTB4, and the effects of intracoronary C5a during constant flow perfusion were measured. Control intracoronary injection of C5a decreased flow (41% of baseline) and contractile function (39% of baseline), PMNs were trapped (5.1 x 10(3) cells/microliters), and TxB2 concentration increased in coronary venous blood. The response to C5a during coronary perfusion with arterial blood depleted of PMNs with Sepacell or Pall filters (less than 0.1 x 10(3) cells/microliters) was greatly blunted, with flow and contractile function falling by less than 14 and 8%, respectively, from baseline, and release of TxB2 was greatly attenuated. However, the myocardial ischemia and TxB2 release remained depressed in response to C5a after removal of the filters and perfusion with either arterial blood containing normal levels of PMNs or stored arterial blood never exposed to filters. In contrast, the repeat C5a challenge resulted in equivalent myocardial extraction of PMNs, thus indicating a dissociation of PMN sequestration from the acute ischemic response and release of TxB2. In separate experiments, the intracoronary injection of LTB4 also resulted in a pronounced myocardial extraction of PMNs (8.6 x 10(3) cells/microliters) greater than during C5a, but did not depress coronary flow or function. Perfusion at constant flow greatly diminished the ischemic response to C5a, indicating that vasoconstriction and resultant ischemia is the main cause of the contractile dysfunction. These data indicate that leukocyte filters inhibit the myocardial ischemia and release of TxB2 induced by C5a via mechanisms not related to PMN depletion.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of transient coronary occlusion on coronary blood flow autoregulation, vasodilator reserve and response to adenosine in the dog.

Myocardial ischemia of short duration (15 to 20 min) produces myocardial "stunning" during reperfusion. The vasoregulatory and contractile status of reperfused myocardium during normal and reduced perfusion pressures is of interest in the treatment of patients with unstable angina. In the present study the effects of 15 min of reversible ischemic injury on several aspects of coronary vasoregulation were assessed with use of pressure-flow curves in anesthetized open chest dogs. The left anterior descending coronary artery was cannulated and perfused with arterial blood with use of a servo-controlled roller pump. The autoregulatory gain and an adenosine dose-response curve for coronary flow before and after ischemia and reperfusion were obtained. The maximal autoregulatory gain values in the pressure range of 140 to 60 mm Hg were not significantly different before and after ischemia and reperfusion (0.41 +/- 0.08 vs. 0.5 +/- 0.06, p greater than 0.1). The adenosine dose-response curve was significantly shifted to the right after reperfusion; however, coronary blood flows during maximal adenosine vasodilation over a large range of perfusion pressures (140 to 60 mm Hg) were significantly greater after ischemia and reperfusion. The pressure-dependent decrease in segment shortening (sonomicrometry) over the coronary pressure range of 160 to 30 mm Hg was similar in myocardium before and after stunning. Contractile function in the stunned myocardium at normal (100 mm Hg) and low (40 mm Hg) coronary perfusion pressures was similarly and significantly enhanced by the administration of adenosine. It is concluded that 1) coronary autoregulation is unchanged after brief ischemia and reperfusion; 2) although maximal coronary vascular conductance assessed with adenosine is greater after ischemia, the coronary circulation shows a decreased coronary sensitivity to exogenous adenosine; 3) the relation of contractile function to coronary pressure before and after stunning is unchanged; and 4) enhancement of function in stunned myocardium by vasodilation with adenosine occurs at low and normal perfusion pressures.

Thromboxane A2 and peptidoleukotrienes contribute to the myocardial ischemia and contractile dysfunction in response to intracoronary infusion of complement C5a in pigs.

Intracoronary infusions of activated complement C5a result in myocardial ischemia, contractile dysfunction, and leukocyte accumulation. The hypothesis was tested that the generation of the coronary vasoconstrictors, thromboxane A2 and the 5-lipoxygenase leukotrienes (LTC4 and LTD4), contributes to the C5a-induced decrease in coronary blood flow and contractile function. The left anterior descending coronary artery in anesthetized swine was cannulated and servo pump-perfused with arterial blood at constant pressure and measured flow. Regional subendocardial contractile function was assessed with sonomicrometry. The interventricular vein was cannulated for sampling of coronary venous blood for leukocyte count. The responses in left anterior descending coronary artery blood flow and percent segment shortening to intracoronary infusions of LTC4 (1 microgram), LTD4 (1 microgram), thromboxane agonist U46619 (7.5 micrograms), and C5a (500 ng) were assessed before and after 1) LTD4/LTE4 receptor blockade with leukotriene receptor blocker LY171883 (10 mg/kg i.v.) (n = 5), 2) thromboxane A2/prostaglandin H2 receptor blockade with thromboxane receptor blocker BM13505 (2 mg/kg i.v.) (n = 5), and 3) combined thromboxane and leukotriene receptor blockade (n = 5). In the absence of receptor blockade, intracoronary C5a decreased coronary flow (50-60%) and regional segment function (60-70%) compared with the preinfusion levels. This was accompanied by a fall in coronary venous blood leukocyte levels by 5-6 x 10(6) cells/ml in the absence of alterations in arterial blood leukocyte count. Intracoronary injections of LTD4, LTC4, or U46619 also resulted in prompt decreases in coronary blood flow (50-60%) and segment function (70-80%) from preinfusion levels. Leukotriene receptor blockade with LY171883 abolished these responses to LTD4 and LTC4. Administration of LY171883 also attenuated (p less than 0.05) the myocardial response to C5a; coronary flow and segment function decreased by approximately 28% from preinfusion levels. Thromboxane receptor blockade with BM13505 eliminated the response in coronary flow and segment function to intracoronary U46619. Similar to LY171883, administration of BM13505 blunted (p less than 0.05) the C5a-induced decreases in coronary flow and contractile function, which fell by approximately 20-25% from the preinfusion level. After the combined LTD4/LTE4 receptor and thromboxane A2/prostaglandin H2 receptor blockade, intracoronary C5a resulted in little change in both coronary blood flow and segment shortening. In contrast to the flow and function effects, the C5a-induced myocardial leukocyte extraction was not decreased by leukotriene and/or thromboxane receptor blockade.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of leukocyte activation on myocardial vascular resistance.

The contribution of the leukocyte, particularly the granulocyte, to the tissue injury resulting from the inflammatory response accompanying organ ischemia is a subject of intense, current interest. Leukocytes are large and viscous cells which adhere to vascular endothelium, and are a source of a variety of toxic and vasoactive substances. There are several lines of evidence indicating their involvement in the development of abnormal and heterogeneous tissue perfusion in a wide variety of pathologic states. They have been implicated in the capillary stasis and no-reflow following hemorrhagic shock, and in ischemia and reperfusion of skeletal muscle, brain, and heart. The mechanisms responsible for the detrimental influence of the granulocyte on tissue perfusion include their inherent rheologic properties, their role in the generation of vascular smooth muscle-constricting substances, and their potential for damaging vascular endothelium. One contributing aspect of the inflammatory response is leukocyte activation by products of the complement cascade. In our in vivo model system, stimulation of the granulocyte with activated complement C5a (intracoronary) is associated with myocardial ischemia and a transient myocardial accumulation of granulocytes. The enhanced generation of thromboxane A2 and leukotrienes appears to be primarily responsible for this increase in coronary vascular resistance.

Thromboxane is produced in response to intracoronary infusions of complement C5a in pigs. Cyclooxygenase blockade does not reduce the myocardial ischemia and leukocyte accumulation.

Activated polymorphonuclear leukocytes (PMNs) contribute to myocardial injury during ischemia and reperfusion. There is evidence that activation of the complement pathway may be one of the mechanisms of PMN activation during ischemia. Intracoronary infusion of complement C5a during normal perfusion pressure is associated with decreased coronary flow, contractile dysfunction, and PMN accumulation. The mechanisms responsible for these changes have not been identified. Thromboxane A2 (TXA2) is a potential mediator of this myocardial ischemic response. Activated PMNs produce TXA2, a known coronary vasoconstrictor, and TXA2 was shown to be a mediator of the pulmonary hypertensive response to activated complement. The goal of the present study was to determine if an enhanced TXA2 production is associated with the myocardial response to C5a and whether cyclooxygenase blockade would reduce the myocardial ischemia. In open-chest pigs, intracoronary C5a (500 ng) caused reversible reductions in blood flow (50.0% of control), regional contractile function (25.8% of control), leukocyte trapping (1.0 x 10(6) cells/g myocardium or a peak artery-coronary venous difference of 5.3 x 10(3) cells/microliters blood), and increased coronary venous TXB2 (the TXA2 breakdown product) from 1.6 pmol/ml to a peak of 6.9 pmol/ml. Cyclooxygenase blockade with aspirin or indomethacin, which prevented TXB2 production, did not alter the response in flow, function, or PMN trapping. Ibuprofen, a known direct inhibitor of PMNs in addition to its cyclooxygenase blockade effect, reduced the response slightly. The pig coronary vascular bed was responsive to the TXA2 agonist U46619, which reduced flow and function without PMN trapping. Mechanical reductions in coronary flow to levels equivalent to those during the C5a infusions did not increase coronary venous TXB2 nor cause PMN trapping but did cause equivalent contractile dysfunction. Incubation of whole blood with C5a at concentrations equivalent to those achieved in vivo did not cause TXB2 production. We conclude that 1) TXA2 is produced in response to intracoronary C5a and 2) cyclooxygenase blockade does not prevent the C5a-induced myocardial ischemia, contractile dysfunction, and PMN trapping. The TXA2 production likely involves a vascular site or a blood cell-vascular interaction. This model system indicates the potential for persistently activated PMNs to cause continued ischemia during myocardial reperfusion.

Microvascular and collateral adaptation in swine hearts following progressive coronary artery stenosis.

We chronically implanted hygroscopic occluders around the left circumflex coronary artery in 49 anesthetized young male domestic pigs and we studied the development of a collateral circulation at 4, 8, 12, and 26 weeks after implantation. At these time intervals groups of animals were again anesthetized, the hearts were explanted and perfused in Langendorff-fashion with leucocyte-filtered pig blood. Maximal coronary vasodilation was induced with adenosine and global (electromagnetic), and regional (tracer microspheres) blood flow was measured at 40, 60, 80, and 100 mm Hg of perfusion pressure. At 4 weeks after occluder implantation maximal left circumflex collateral blood flow was about 20% of normal maximal flow. Collateral flow rose to 60% of maximal normal flow between 4 and 8 weeks and did not improve further with longer time intervals. In contrast to the canine heart numerous small vessels develop in response to ischemia in the pig heart. These vessels develop throughout the entire risk region with a slight preference for the subendocardium. They appear on tomographic angiograms as a dense "blush". The study of the relationship between peripheral coronary pressure vs collateral flow showed a relationship much steeper than that of normal maximal flow vs aortic perfusion pressure which indicates that the minimal resistance of the risk region was decreased as part of the mechanism to ensure adequate blood supply in a situation of progressive coronary narrowing.

Reversibly injured, postischemic canine myocardium retains normal contractile reserve.

Transient coronary occlusion (15 minutes) does not result in irreversible myocardial injury but is associated with a depression of contractile function sustained for several hours to days ("stunned myocardium"). The defect in the contractile process responsible for this phenomenon has been suggested to be causally related to a reduced energetic state, altered excitation or excitation-contraction coupling, or damaged contractile filaments. The purpose of this study was to attempt to exclude one or more of these hypotheses by evaluating the contractile reserve of reperfused myocardium. Regional subendocardial segment function was measured (sonomicrometry) in a control region and in an area (treatment region) perfused by a carotid artery to anterior descending coronary artery bypass in 13 chloralose-anesthetized dogs. Dose-response curves were constructed from changes in segment shortening (%SS) in response to intracoronary calcium infusion before ischemia and following 5 or 15 minutes of occlusion and reperfusion (30 minutes). Calcium infusion before ischemia resulted in dose-dependent increases in %SS in the treatment area to a maximum value of 36.6% from a preinfusion value of 25.5% (p less than 0.01), in the absence of changes in control region shortening (23.7%). After 15 minutes of occlusion and reperfusion, treatment area %SS had fallen to a depressed but stable level (46% of preischemic values; p less than 0.01). Subsequent calcium infusion at the same doses as in the preischemic trial produced increases in treatment segment function with return of shortening to control levels at an intermediate dose. At the highest dose, %SS was 35.4%, which was not different from the maximal value found in the preischemic trial. Alterations in heart rate and left ventricular systolic and diastolic pressures during calcium infusion were minor and similar before and after ischemia. Calcium-induced increases in regional segment shortening above control levels (113% of control) in reperfused myocardium were sustained with continuous infusion (30 minutes) without deleterious effects on subsequent function. These results demonstrate that stunned myocardium in this model retains a normal contractile reserve in response to calcium, suggesting that the mechanism responsible for postischemic contractile dysfunction involves calcium.

Influence of mioflazine on canine coronary blood flow and on adenine nucleotide and nucleoside content under normal and ischemic conditions.

Intravenous injection of mioflazine, a nucleoside transport antagonist, caused maximal coronary vasodilation in canine hearts. This was completely reversed by intravenous injection of the enzyme adenosine deaminase. Coronary vasodilation was induced again by the adenosine deaminase inhibitor EHNA [Erythro-9(2-hydroxy-3-nonyl)adenine]; however, without previous injection of mioflazine, EHNA did not produce coronary vasodilation. Mioflazine-induced coronary vasodilation was antagonized by theophylline, but it was not associated with increased plasma levels of adenosine. Under the influence of mioflazine, ischemic myocardium contained adenosine and inosine at a ratio of 65:30, which is the reverse of the control ratio. Total nucleoside content following mioflazine showed reduced nucleoside losses as compared with control. A significant amount of the accumulated adenosine is extracellular since it was accessible to exogenous adenosine deaminase. Reperfusion of ischemic myocardium did not result in increased rates of adenosine phosphorylation, another indicator of its extracellular accumulation. The data are best explained by assuming release of adenosine by mioflazine in addition to its known effect of inhibiting nucleoside transport. The adenosine release occurs most probably into the interstitial space where it occupies smooth muscle adenosine receptors. The existence of nonsymmetric transport (uptake is more inhibited than release) is postulated for the myocyte, as well as for the endothelial cell plasma membrane.

Carotid chemoreceptor reflex parasympathetic coronary vasodilation in the dog.

The hypothesis that carotid body chemoreceptor activation with hypoxic-hypercapnic blood elicits reflex coronary vasodilation was investigated. Circumflex or anterior descending coronary artery blood flow was measured in alpha-chloralose-anesthetized, closed-chest dogs. To minimize changes in cardiac metabolism, the heart was paced at a constant rate after atrioventricular heart block, propranolol (1 mg/kg) was given to prevent beta-receptor-mediated alterations in cardiac contractility, and aortic blood pressure was stabilized by means of a blood reservoir. The carotid body regions were vascularly isolated and perfused at constant pressure with arterial blood or hypoxic-hypercapnic blood. Under these conditions, carotid body chemoreceptor stimulation with hypoxic or hypoxic-hypercapnic blood for 90 s produced atrial bradycardia and a transient increase in coronary blood flow of 36-53% above prestimulation values. The augmented coronary flow was accompanied by a transient increase in coronary sinus O2 tension of 4.6-5.7 mmHg. Aortic blood pressure varied less than 10 mmHg. Intracarotid injections of nicotine (0.1 microgram/kg) or cyanide (150 micrograms) produced similar results. The coronary response to chemoreceptor stimulation with hypoxic blood or drugs was abolished when the reflex arc was interrupted with atropine (0.5 mg/kg). It is concluded that transient reflex parasympathetic coronary vasodilation is elicited by hypoxic or hypoxic-hypercapnic stimulation of carotid body chemoreceptors.

Parasympathetic control of transmural coronary blood flow in dogs.

The transmural distribution of coronary blood flow was studied during vagal stimulation in closed-chest, morphine- and alpha-chloralose-anesthetized dogs. The left main coronary artery was cannulated and perfused at constant pressure. Bradycardia during vagal stimulation was prevented by atrioventricular heart block and ventricular pacing. Beta-adrenergic receptors were blocked with propranolol (1 mg/kg iv), and aortic pressure was stabilized by means of a pressure reservoir. Regional myocardial blood flow was measured with 9-micron radioactive microspheres during vagal stimulation and during intracoronary acetylcholine infusion. Vagal stimulation increased coronary blood flow uniformly across the left ventricular wall. In contrast, intracoronary acetylcholine infusion, at a rate selected to increase total flow to the same degree, vasodilated the subendocardium more than the subepicardium, increasing the inner/outer blood flow ratio. It is concluded that both vagal activation and acetylcholine produce coronary vasodilation that is independent of left ventricular preload, afterload, and heart rate. Acetylcholine vasodilation preferentially vasodilates the subendocardium, increasing the inner/outer flow ratio, but vagal stimulation produces uniform vasodilation across the left ventricular wall.

Carotid baroreceptor reflex coronary vasodilation in the dog.

The hypothesis that neurally mediated coronary vasodilation occurs as part of the carotid baroreceptor reflex was investigated. The left main coronary artery was cannulated and perfused at constant pressure (100 mm Hg) in closed-chest, chloralose-anesthetized dogs. The heart was paced at a constant rate between 60 and 70 beats/min after atrioventricular heart block. Propranolol (1 mg/kg) was given to prevent beta-receptor-mediated alterations in contractility. Aortic blood pressure was stabilized with a blood reservoir. The aortic depressor nerves were cut bilaterally to prevent the buffering influence of aortic arch baroreceptors on the carotid baroreceptor reflex. The carotid sinuses were vascularly isolated and perfused with arterial blood at controlled pressures. Under these conditions, a step change in carotid sinus pressure from 70 to 210 mm Hg produced a 0.29 ml/min per g increase in coronary flow above control and a 10 mm Hg increase in coronary sinus blood oxygen tension. A step in carotid sinus pressure from 70 to 150 mm Hg resulted in a flow increase of 0.13 ml/min per g and a coronary sinus oxygen tension increase of 5.3 mm Hg relative to prestimulation values. Atropine (0.5 mg/kg, iv) blocked most of the reflex coronary vasodilation, indicating a parasympathetic component, and the addition of adrenergic alpha-receptor blockade with phenoxybenzamine (0.25 mg/kg, ic) abolished the remaining response, demonstrating sympathetic participation. The reflex nature of the coronary response was confirmed with carotid sinus denervation and vagotomy. It is concluded that carotid sinus hypertension results in a graded reflex neural coronary vasodilation independent of myocardial metabolic factors. The major component is due to activation of parasympathetic coronary vasodilator fibers, but there is also inhibition of sympathetic vasoconstrictor fibers.

Technique for producing heart block in closed-chest dogs without electrical recording.

An instrument and technique have been devised for the production of complete atrioventricular heart block in anesthetized, closed-chest animals. The instrument is inserted into the right atrium via the right external jugular vein. Localization of the atrioventricular node region by the visualization of intracardiac landmarks with fluoroscopy makes electrical recording unnecessary. Discrete injections of a radiopaque necrosing solution produces the heart block. Procedure time is under 30 min a success rate of 90%.