Polymeric endoaortic paving: Mechanical, thermoforming, and degradation properties of polycaprolactone/polyurethane blends for cardiovascular applications.

Polymeric endoaortic paving (PEAP) is a process by which a polymer is endovascularly delivered and thermoformed to coat or "pave" the lumen of the aorta. This method may offer an improvement to conventional endoaortic therapy in allowing conformal graft application with reduced risk of endoleak and customization to complex patient geometries. Polycaprolactone (PCL)/polyurethane (PU) blends of various blend ratios were assessed as a potential material for PEAP by characterizing their mechanical, thermoforming and degradation properties. Biaxial tension testing revealed that the blends' stiffness is similar to that of aortic tissue, is higher for blends with more PCL content, and may be affected by thermoforming and degradation. Tubes of blends were able to maintain a higher diameter increase after thermoforming at higher PCL content and higher heating temperatures; 50/50 blend tubes heated to 55 °C were able to maintain 90% of the diameter increase applied. Delamination forces of the blends ranged from 41 to 235 N m⁻². In a Pseudomonas lipase solution, the 50/50 blend had a 94% lower degradation rate than pure PCL, and the 10/90 blend exhibited no degradation. These results indicate that PEAP, consisting of a PCL/PU blend, may be useful in developing the next generation of endoaortic therapy.

Simultaneous administration of thromboxane A2- and serotonin S2-receptor antagonists markedly enhances thrombolysis and prevents or delays reocclusion after tissue-type plasminogen activator in a canine model of coronary thrombosis.

Dynamic changes of the thrombus after its formation due to platelet activation may affect the speed of thrombolysis. In the present study, we wanted to evaluate the role played by thromboxane A2 (TXA2) and serotonin (5HT) in mediating platelet activation during lysis of intracoronary thrombi with human recombinant tissue-type plasminogen activator (t-PA). Coronary thrombi were induced in 26 anesthetized, open-chest dogs by inserting a copper coil into the left anterior descending coronary artery (LAD). LAD blood flow was monitored throughout the experiment by means of a Doppler flow probe placed proximally to the coil. Presence of the thrombus was documented for 30 minutes. The dogs were then assigned to one of four groups as follows: group 1 dogs (n = 8), serving as controls, received a bolus of heparin (200 units/kg) and a bolus of t-PA (80 micrograms/kg) followed by a continuous infusion (8 micrograms/kg/min) for up to 90 minutes or until reperfusion was achieved; group 2 dogs (n = 10) received, immediately before heparin and t-PA, an intravenous bolus of SQ29548 (SQ) (0.4 mg/kg, a selective TXA2-receptor antagonist) and LY53857 (LY) (0.2 mg/kg, a selective serotonin S2-receptor antagonist); group 3 dogs (n = 7) received, before heparin and t-PA, an intravenous bolus of SQ alone (0.4 mg/kg); and group 4 dogs (n = 7) received, before heparin and t-PA, an intravenous bolus of LY alone (0.2 mg/kg). After thrombolysis, all dogs were monitored for 90 minutes or until a persistent reocclusion occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin S2 and thromboxane A2-prostaglandin H2 receptor blockade provide protection against epinephrine-induced cyclic flow variations in severely narrowed canine coronary arteries.

The object of this study was to test the hypothesis that administration of both serotonin S2 and thromboxane A2-prostaglandin H2 (PGH2) receptor antagonists provides significant protection against epinephrine-induced cyclic coronary artery flow variations in open chest, anesthetized dogs with severe proximal coronary artery stenosis and endothelial injury. Three groups of dogs were studied. In Group 1 (n = 7) and Group 2 (n = 6), cyclic coronary flow variations were initiated after placement of a concentric constrictor around the left anterior descending coronary artery and were abolished by administration of either a thromboxane A2-prostaglandin H2 receptor antagonist, SQ29,548 (SQ) (Group 1), or a serotonin S2 receptor antagonist, LY53,857 (LY) (Group 2). Cyclic flow variations were restored with an epinephrine infusion and the second antagonist (LY for Group 1; SQ for Group 2) was administered to abolish epinephrine-induced cyclic flow variations. The rate of epinephrine infusion was increased until cyclic coronary flow variations returned (n = 8) or significant hemodynamic changes occurred. Plasma epinephrine concentrations were determined during a control period of cyclic coronary flow variations, after epinephrine restored cyclic flow variations in the presence of either SQ or LY, and again after epinephrine restored cyclic flow variations in the presence of both SQ and LY. A third group of dogs (Group 3, n = 9) required both SQ and LY to eliminate the initial cyclic coronary flow variations and infused epinephrine restored cyclic flow variations (n = 8). Plasma epinephrine concentrations were determined during a control period and after cyclic coronary flow variation restoration with epinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Local platelet activation causes vasoconstriction of large epicardial canine coronary arteries in vivo. Thromboxane A2 and serotonin are possible mediators.

The goal of the present study was to demonstrate that intracoronary platelet deposition may trigger intense vasoconstriction of large epicardial coronary arteries in vivo and that this is largely mediated by thromboxane A2 and serotonin released by activated platelets. Cyclic flow variations (progressive declines in blood flow followed by sudden restorations of flow) due to recurrent intracoronary platelet activation and thrombus formation were induced by damaging the endothelium and placing a cylindrical constrictor on the left anterior descending coronary artery (LAD) in open-chest, anesthetized dogs. Coronary diameters were measured in vivo by means of ultrasonic crystals sutured on the LAD immediately distal to the constrictor (LAD1) and 1 cm below (LAD2) and on the circumflex coronary artery (Cx). Coronary artery diastolic diameters were measured continuously before and during cyclic flow variations and after they were abolished by administration of LY53857, a serotonin-receptor antagonist (group 1, n = 7), or SQ29548, a thromboxane-receptor antagonist (group 2, n = 7). During cyclic flow variations, at the nadir of coronary flow, LAD1 (a site of maximal platelet accumulation) cross-sectional area decreased by 52 +/- 10% and 38 +/- 6% in group 1 and 2 animals, respectively (p less than 0.001 compared with values recorded during a brief LAD occlusion obtained by a suture snare), whereas LAD2 (a site of minimal or no platelet accumulation) cross-sectional area did not differ from that recorded during the brief LAD occlusion. SQ29548 abolished cyclic flow variations in seven of seven dogs and LY53857 in six of seven, but they affected the increased coronary vasoconstriction differently: LAD1 cross-sectional area increased by 32 +/- 6% of the control value in SQ29548-treated animals, whereas it returned to baseline dimension values in the LY53857-treated group as these interventions also abolished the cyclic flow variations. We conclude that a marked coronary vasoconstriction may be triggered by local platelet deposition and that thromboxane A2 and serotonin are mediators of this vasoconstriction.

Effect of thromboxane and serotonin receptor antagonists on intracoronary platelet deposition in dogs with experimentally stenosed coronary arteries.

We have reported previously that thromboxane A2 (TXA2) and serotonin (5-HT, 5-hydroxytryptamine) are important mediators of cyclic flow variations (CFVs) in a canine model of coronary artery stenosis and endothelial injury. The present study tested the hypothesis that a TXA2 receptor antagonist is more effective in reducing intracoronary platelet deposition at sites of endothelial injury and severe stenosis than a 5-HT2 receptor antagonist. CFVs developed after placing a plastic constrictor around the left anterior descending coronary artery (LAD) in 51 of 56 dogs. Autologous platelets labeled with 111In were injected in 48 animals. Ten control dogs (group 1A) were killed after CFVs were observed for 1 hour at the nadir of coronary blood flow. Five dogs (group 1B) did not develop CFVs after placement of the constrictor. CFVs were abolished with SQ 28668 (2.75 +/- 0.36 mg/kg, group 2) and SQ 29548 (0.45 +/- 0.1 mg/kg, group 3), two different TXA2 and PGH2 receptor antagonists, in eight of 10 and six of seven dogs, respectively. In eight of 10 dogs (group 4), CFVs were abolished with ketanserin (0.66 +/- 0.12 mg/kg), a 5-HT2 receptor antagonist. In group 2, 3, and 4 dogs, the respective drugs were given so that the minimal dose required to abolished CFVs was administered. In six of six dogs (group 5), a higher dose of ketanserin (i.e., 1.5 mg/kg) was used to abolish CFVs. At death, intracoronary platelet deposition was evaluated by calculating the LAD platelet accumulation ratio (111In activity in the LAD/111In activity in the circumflex coronary artery) in 43 dogs and, in 22 dogs, by microscopic examination of the LAD. A marked LAD platelet accumulation ratio was found in group 1A dogs at the stenotic site and in segments immediately distal to it. The LAD platelet accumulation ratio was significantly reduced by both the low and the high doses of ketanserin compared with group 1A dogs (p less than 0.001). However, the two TXA2 receptor antagonists further reduced the LAD platelet accumulation ratio compared with ketanserin-treated animals (p less than 0.01). Microscopic examination confirmed these findings. We conclude that SQ 28668 and SQ 29548, two different TXA2 receptor antagonists, reduce residual intracoronary platelet deposition associated with CFVs in this canine model more effectively than ketanserin, a 5-HT2 receptor antagonist.

Mediation of reocclusion by thromboxane A2 and serotonin after thrombolysis with tissue-type plasminogen activator in a canine preparation of coronary thrombosis.

Human recombinant tissue-type plasminogen activator (rt-PA) has been shown to be an effective and safe agent for coronary thrombolysis in patients with acute myocardial infarction. However, thrombolysis is associated with a high rate of acute reocclusion after discontinuation of rt-PA. The goals of the present study were to assess whether reocclusion after thrombolysis is caused by intracoronary platelet aggregation and to determine the role of thromboxane A2 (TxA2) and serotonin (5HT) in mediating this phenomenon. Accordingly, coronary thrombosis was induced in anesthetized, open-chest dogs by insertion of a copper coil into the left anterior descending coronary artery (LAD). LAD blood flow was monitored throughout the experiment by means of a Doppler flow probe placed proximally to the coil. Thrombolysis was achieved with rt-PA (0.05 mg/kg bolus + micrograms/kg/min infusion) in 23 +/- 3 min. rt-PA was then discontinued and each animal received a bolus of heparin (150 U/kg) every hour. Reperfusion was followed by repeated cycles of gradual occlusions followed by spontaneous restorations of blood flow (cyclic flow variations, CFVs) before a persistent occlusion recurred. In control dogs (n = 6), heparin alone did not prevent CFVs and reocclusion time was 25 +/- 4 min. Administration of an intravenous bolus of 0.2 +/- 0.06 mg/kg SQ29548, a TxA2/prostaglandin H2-receptor antagonist, and an intravenous bolus of 0.2 +/- 0.04 mg/kg ketanserin, a 5HT2-receptor antagonist, completely abolished CFVs in six of six dogs and reocclusion time was greater than 158 +/- 14 min (p less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Cooperative mediation by serotonin S2 and thromboxane A2/prostaglandin H2 receptor activation of cyclic flow variations in dogs with severe coronary artery stenoses.

We have reported previously that thromboxane A2/prostaglandin (PG)H2 and serotonin independently mediate the occurrence of cyclic flow variations (CFVs) in a canine preparation of severe coronary artery narrowing. This may be due to an effect of these substances on platelets and/or the vascular wall. We tested the hypothesis that there is a cooperative effect between thromboxane A2/PGH2 and serotonin receptor stimulation in the development of CFVs in this animal preparation. After placement of a hard plastic cylindrical constrictor around the left anterior descending coronary artery, CFVs develop and are characterized by repetitive cycles of declines in coronary blood flow and abrupt increases in flow. In a control group of dogs, CFV frequency (cycles/hour) and severity (lowest coronary blood flow just before its restoration) did not change significantly over a 3 hr interval. In a second group of dogs, CFVs were established after constrictor placement, abolished with the serotonin (5HT2) receptor antagonist ketanserin, and reestablished by the continuous infusion of serotonin into the left atrium. Serotonin-induced CFVs were then abolished with a thromboxane A2/PGH2 receptor antagonist, SQ29,548, or a thromboxane synthetase inhibitor, dazoxiben (UK37,248). The relative specificity of the respective antagonists, SQ29,548 and ketanserin, was determined in canine platelets and rat aortic vascular strips. No significant cross-reactivity between ketanserin and SQ29,548 was found. Thus, the data obtained in these studies demonstrate a cooperative interaction between thromboxane A2/PGH2 and serotonin S2 receptors that contributes to the development of CFVs in this experimental preparation.

Inhibition of cyclic flow variations in stenosed canine coronary arteries by thromboxane A2/prostaglandin H2 receptor antagonists.

We tested the hypothesis that thromboxane A2 and thromboxane A2/PGH2 receptor occupation are important in mediating cyclical reductions in coronary blood flow (CFVs) in concentrically narrowed canine coronary arteries. Two potent and selective thromboxane A2/PGH2 receptor antagonists, SQ29,548 and SQ28,668 eliminated CFVs and restored a normal pattern of blood flow through the severely narrowed vessels in 77 and 75% of the dogs, respectively. CFVs were eliminated within several minutes of an intravenous bolus injection of SQ29,548 or SQ28,688. A continuous infusion of SQ29,548 (0.2 mg/kg X min) prevented the recurrence of CFVs throughout the duration of its infusion. Left atrial infusion of the thromboxane A2 mimetic, U46619, restored CFVs in 5 of 8 SQ29,548-treated and in 5 of 7 SQ28,668-treated dogs. Circulating concentrations of the stable metabolites of TxA2 and PGI2, TxB2 and 6-keto-PGF1 alpha, respectively, were unaffected by administration of SQ29,548. However, stenosed vascular segments of the left anterior descending coronary artery (LAD) of SQ29,548-treated dogs produced significantly less thromboxane A2 than comparable segments from untreated dogs. Morphologic studies showed that stenosed coronary arteries in which CFVs had been abolished by either SQ29,548 or SQ28,668 had relatively few adherent platelets, whereas comparable coronary segments removed from untreated animals had relatively large, platelet-rich mural thrombi. SQ29,548 did not alter the synthesis of TxB2 by platelets. 6-keto-PGF1 alpha concentrations in the stenosed LAD and nonstenosed circumflex coronary arteries were not altered by SQ29,548 administration. These data suggest that the thromboxane A2/PGH2 receptor antagonists, SQ29,548 and SQ28,688, inhibit cyclic reductions in coronary blood flow in this model by preventing the accumulation of platelets at the site of a critical coronary arterial stenosis. The data also suggest that TxA2 is important in mediating the interaction between platelets and the constricted coronary artery that is responsible for the development and maintenance of CFVs in this experimental model.

Reflex cardiovascular responses caused by stimulation of pulmonary C-fibers with capsaicin in dogs.

The purpose of these studies was to determine quantitatively the reflex cardiovascular responses to stimulation of the pulmonary C-fibers in dogs. We used a preparation in which the airway, pulmonary artery, and the pulmonary veins to the left lung were cannulated in situ. Ventilation and perfusion of the right lung maintained the animal in relatively normal homeostasis. Capsaicin, a decylenic acid amide of vanillylamine that selectively stimulates nerve endings of unmyelinated fibers (C-fibers), was injected into the left pulmonary artery in 5-ml boluses. Maximal reflex responses were obtained with concentrations as low as 0.8-1.6 X micrograms-1 X kg-1. Heart rate, hindlimb resistance, and left ventricular contractility were lowered transiently (the maximal responses showing declines of 40, 13, and 15.2%, respectively). As a result of these changes, combined with vasodilation in other resistance vessels, cardiac output fell 28% and blood pressure fell 35%. Interrupting the afferent neural pathway by severing the ipsilateral cervical vagus nerve eliminated these responses, confirming the distribution of their reflex origin. Although the role of these reflexes in homeostasis has not been decided, the results of this study suggest that the lungs of dogs, if appropriately stimulated, potentially can exert a major inhibitory influence on the neural regulation of cardiovascular function.

Serotonin as a mediator of cyclic flow variations in stenosed canine coronary arteries.

The data obtained in this study demonstrate that the concentration of serotonin is markedly elevated (18- to 27-fold) at the site of a coronary arterial stenosis in open-chest, anesthetized dogs with cyclic flow variations. Cyclic flow variations in this experimental preparation were abolished by ketanserin, a 5-hydroxytryptamine antagonist, but serotonin concentration at the site of the coronary stenosis remained elevated. The intra-atrial administration of serotonin (0.16 to 1 mg/min) restored cyclic flow variations after they had been abolished by ketanserin. Taken together, these data suggest that serotonin may be one of the important mediators of cyclic flow variations in this experimental preparation.

Apparent kinetics of angiotensin converting enzyme: hydrolysis of [3H]benzoyl-phenylalanyl-alanyl-proline in the isolated perfused lung.

Isolated perfused rabbit lungs were used to study the hydrolysis of [3H]benzoyl-phenylalanyl-alanyl-proline [( 3H]BPAP), a synthetic substrate for angiotensin converting enzyme (ACE). Lungs were perfused, at constant flow rates, with physiologic medium containing added BPAP and the concentration of its metabolite, [3H]benzoyl-phenylalaline, was measured in lung effluent. Hydrolysis of BPAP (4.2 microM) was 64.1 +/- 3.3% at 37 degrees C and was significantly decreased (P less than .01) to 10.1 +/- 8.7% by the addition of the ACE inhibitor, MK422 (10(-6) M). Disappearance (i.e., hydrolysis) of immunoreactive angiotensin I was also inhibited by MK422. Hydrolysis of BPAP was saturable and calculated apparent kinetic constants were Km = 13 microM and Vmax = 50 nmol/sec/lung. When the perfusion medium temperature was 10 degrees C, apparent Km was unchanged, whereas Vmax was significantly (P less than .05) decreased. At BPAP concentrations sufficient to depress metabolism to less than 20%, perfusion pressure was unchanged. Hydrolysis of BPAP under first-order conditions was independent of flow over the range 10 to 50 ml/min. However, increase in flow rate to 100 ml/min/lung was associated with decreased BPAP metabolism. These data indicate that BPAP is a substrate for pulmonary ACE in vitro and substantiate its use in intact animals because: 1) it is without physiologic effect in high doses; and 2) calculated apparent kinetics in isolated lungs under these conditions of steady-state concentrations were similar to those obtained from earlier studies that utilized bolus injection techniques in intact animals.

Reflex depression of cardiovascular function during lung inflation.

These experiments were intended 1) to determine the cardiovascular reflex response initiated by lung inflation and 2) to determine the autonomic neural mechanisms that mediate the reflex responses. To test only the reflex effects of lung inflation and eliminate the associated mechanical effects thereof, we isolated the left airway and left pulmonary artery of dogs and performed static inflations of the left lung. Left lung inflation (LLI) to 30 cmH2O caused left ventricular systolic pressure, heart rate, and hindlimb vascular resistance to fall 20, 31, and 15%, respectively. When the hearts were paced to eliminate Bowditch effects, LLI caused left ventricular contractility, which was measured as dP/dt at 30 mmHg developed pressure and as the maximal rate of shortening of the contractile element, to fall 23 and 20%, respectively, and cardiac output (stroke volume) to fall 31%. Transection of the left cervical vagus nerve, interrupting most afferent and some efferent pathways from the left lung, virtually eliminated subsequent cardiovascular responses to LLI. Hindlimb vasodilation was mediated by alpha-adrenergic activity, and the fall in left ventricular contractile state was caused by a reduction in beta-adrenergic activity. The reflex bradycardia was mediated by increased cholinergic and decreased beta-adrenergic activity. These data establish that reflex mechanisms exist whereby expansion of the lung can reflexly modulate heart rate, ventricular contractility, systemic vasomotor tone, cardiac output, and stroke volume independent of the direct mechanical influence of lung expansion on these circulatory phenomena.

Responses to inflation of vagal afferents with endings in the lung of dogs.

In dogs, inflating the lungs to pressures of 9 cm H2O or less reflexly increases heart rate, whereas inflating the lungs to pressures of 10-30 cm H2O reflexly decreases heart rate. The afferent fibers responsible for the cardioacceleration travel in the vagus nerves and are believed to be pulmonary stretch receptors, whereas the afferent responsible for the deceleration also travel in the vagus nerves, but are believed to be lung C-fibers. To identify the afferents responsible for these effects, we recorded the impulse activity of vagal afferents with endings in the left lung, while we slowly inflated that lung to 30-45 cm H2O. We found that 12 slowly adapting receptors fired at significantly lower inflation pressures than did 10 rapidly adapting receptors (5.8 +/- 1.5 vs. 13.5 +/- 2.2 cm H2O, respectively). We also found that 13 pulmonary C-fibers fired at significantly lower inflation pressures than did 10 bronchial C-fibers (16.4 +/- 1.8 vs 26.5 %/- 2.9 cm H2O, respectively). We conclude that slowly adapting receptors are likely to be responsible for the cardioacceleration evoked by low levels of inflation, and that both pulmonary and bronchial C-fibers are likely to be responsible for the cardiodeceleration evoked by high levels of inflation.

Reflex cardiovascular depression induced by capsaicin injection into canine liver.

Capsaicin was injected into the portal circulation of 29 dogs after a blood delay pathway was constructed between the liver and right heart, through which capsaicin-contaminated blood could be replaced while systemic hemodynamics were maintained constant. Capsaicin (500 micrograms) rapidly decreased left ventricular systolic pressure (-10%), mean arterial pressure (-12%), heart rate (-4%), renal vascular resistance (-7%), maximal rate of left ventricular pressure rise (dP/dtmax) (-12%), and dP/dt at 25 mmHg developed left ventricular pressure (-15%) in animals with paced hearts. Left ventricular end-diastolic pressure did not change. Vagus nerve interruption at the level of the diaphragm did not alter hemodynamic changes occurring during capsaicin injections, but anterior hepatic nerve interruption eliminated the changes, suggesting that the cardiovascular responses were reflex in origin and that the principal afferent pathway traverses the hepatic nerve. This study demonstrates that activation of afferent fiber receptors within the liver tissue can contribute to neural regulation of the cardiovascular system, but the natural stimulus for these receptors is not known.