K(ATP)(+) channels, nitric oxide, and adenosine are not required for local metabolic coronary vasodilation.

The role of ATP-sensitive K(+) (K(ATP)(+)) channels, nitric oxide, and adenosine in coronary exercise hyperemia was investigated. Dogs (n = 10) were chronically instrumented with catheters in the aorta and coronary sinus and instrumented with a flow transducer on the circumflex coronary artery. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous plasma concentrations using a previously tested mathematical model. Experiments were conducted at rest and during graded treadmill exercise with and without combined inhibition of K(ATP)(+) channels (glibenclamide, 1 mg/kg iv), nitric oxide synthesis (N(omega)-nitro-L-arginine, 35 mg/kg iv), and adenosine receptors (8-phenyltheophylline, 3 mg/kg iv). During control exercise, myocardial oxygen consumption increased ~2.9-fold, coronary blood flow increased ~2.6-fold, and coronary venous oxygen tension decreased from 19.9 +/- 0.4 to 13.7 +/- 0.6 mmHg. Triple blockade did not significantly change the myocardial oxygen consumption or coronary blood flow response during exercise but lowered the resting coronary venous oxygen tension to 10.0 +/- 0.4 mmHg and during exercise to 6.2 +/- 0.5 mmHg. Cardiac adenosine levels did not increase sufficiently to overcome the adenosine receptor blockade. These results indicate that combined inhibition of K(ATP)(+) channels, nitric oxide synthesis, and adenosine receptors lowers the balance between total oxygen supply and consumption at rest but that these factors are not required for local metabolic coronary vasodilation during exercise.

Feedforward sympathetic coronary vasodilation in exercising dogs.

The hypothesis that exercise-induced coronary vasodilation is a result of sympathetic activation of coronary smooth muscle beta-adrenoceptors was tested. Ten dogs were chronically instrumented with a flow transducer on the circumflex coronary artery and catheters in the aorta and coronary sinus. During treadmill exercise, coronary venous oxygen tension decreased with increasing myocardial oxygen consumption, indicating an imperfect match between myocardial blood flow and oxygen consumption. This match was improved after alpha-adrenoceptor blockade with phentolamine but was significantly worse than control after alpha + beta-adrenoceptor blockade with phentolamine plus propranolol. The response after alpha-adrenoceptor blockade included local metabolic vasodilation plus a beta-adrenoceptor vasodilator component, whereas the response after alpha + beta-adrenoceptor blockade contained only the local metabolic vasodilator component. The large difference in coronary venous oxygen tensions during exercise between alpha-adrenoceptor blockade and alpha + beta-adrenoceptor blockade indicates that there is significant feedforward beta-adrenoceptor coronary vasodilation in exercising dogs. Coronary venous and estimated myocardial interstitial adenosine concentrations did not increase during exercise before or after alpha + beta-adrenoceptor blockade, indicating that adenosine levels did not increase to compensate for the loss of feedforward beta-adrenoceptor-mediated coronary vasodilation. These results indicate a meaningful role for feedforward beta-receptor-mediated sympathetic coronary vasodilation during exercise.

Quantitative analysis of feedforward sympathetic coronary vasodilation in exercising dogs.

Recent experiments demonstrate that feedforward sympathetic beta-adrenoceptor coronary vasodilation occurs during exercise. The present study quantitatively examined the contributions of epinephrine and norepinephrine to exercise coronary hyperemia and tested the hypothesis that circulating epinephrine causes feedforward beta-receptor-mediated coronary dilation. Dogs (n = 10) were chronically instrumented with a circumflex coronary artery flow transducer and catheters in the aorta and coronary sinus. During strenuous treadmill exercise, myocardial oxygen consumption increased by approximately 3.9-fold, coronary blood flow increased by approximately 3.6-fold, and arterial plasma epinephrine concentration increased by approximately 2.4-fold over resting levels. At arterial concentrations matching those during strenuous exercise, epinephrine infused at rest (n = 6) produced modest increases (18%) in flow and myocardial oxygen consumption but no evidence of direct beta-adrenoceptor-mediated coronary vasodilation. Arterial norepinephrine concentration increased by approximately 5. 4-fold during exercise, and coronary venous norepinephrine was always higher than arterial, indicating norepinephrine release from cardiac sympathetic nerves. With the use of a mathematical model of cardiac capillary norepinephrine transport, these norepinephrine concentrations predict an average interstitial norepinephrine concentration of approximately 12 nM during strenuous exercise. Published dose-response data indicate that this norepinephrine concentration increases isolated coronary arteriolar conductance by approximately 67%, which can account for approximately 25% of the increase in flow observed during exercise. It is concluded that a significant portion of coronary exercise hyperemia ( approximately 25%) can be accounted for by direct feedforward beta-adrenoceptor coronary vascular effects of norepinephrine, with little effect from circulating epinephrine.

Role of K(ATP)(+) channels and adenosine in the control of coronary blood flow during exercise.

The present study was designed to examine the role of ATP-sensitive potassium (K(ATP)(+)) channels during exercise and to test the hypothesis that adenosine increases to compensate for the loss of K(ATP)(+) channel function and adenosine inhibition produced by glibenclamide. Graded treadmill exercise was used to increase myocardial O(2) consumption in dogs before and during K(ATP)(+) channel blockade with glibenclamide (1 mg/kg iv), which also blocks adenosine mediated coronary vasodilation. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous values by using a previously tested mathematical model (Kroll K and Stepp DW. Am J Physiol Heart Circ Physiol 270: H1469-H1483, 1996). Coronary venous O(2) tension was used as an index of the balance between O(2) delivery and myocardial O(2) consumption. During control exercise, myocardial O(2) consumption increased approximately 4-fold, and coronary venous O(2) tension fell from 19 to 14 Torr. After K(ATP)(+) channel blockade, coronary venous O(2) tension was decreased below control vehicle values at rest and during exercise. However, during exercise with glibenclamide, the slope of the line of coronary venous O(2) tension vs. myocardial O(2) consumption was the same as during control exercise. Estimated interstitial adenosine concentration with glibenclamide was not different from control vehicle and was well below the level necessary to overcome the 10-fold shift in the adenosine dose-response curve due to glibenclamide. In conclusion, K(ATP)(+) channel blockade decreases the balance between resting coronary O(2) delivery and myocardial O(2) consumption, but K(ATP)(+) channels are not required for the increase in coronary blood flow during exercise. Furthermore, interstitial adenosine concentration does not increase to compensate for the loss of K(ATP)(+) channel function.

Role of nitric oxide and adenosine in control of coronary blood flow in exercising dogs.

BACKGROUND: Inhibition of nitric oxide (NO) synthesis results in very little change in coronary blood flow, but this is thought to be because cardiac adenosine concentration increases to compensate for the loss of NO vasodilation. Accordingly, in the present study, adenosine measurements were made before and during NO synthesis inhibition during exercise. METHODS AND RESULTS: Experiments were performed in chronically instrumented dogs at rest and during graded treadmill exercise before and during inhibition of NO synthesis with N(omega)-nitro-L-arginine (L-NNA, 35 mg/kg IV). Before inhibition of NO synthesis, myocardial oxygen consumption increased approximately 3.7-fold, and coronary blood flow increased approximately 3.2-fold from rest to the highest level of exercise, and this was not changed by NO synthesis inhibition. Coronary venous oxygen tension was modestly reduced by L-NNA at all levels of myocardial oxygen consumption. However, the slope of the relationship between myocardial oxygen consumption and coronary venous oxygen tension was not altered by L-NNA. Inhibition of NO synthesis did not increase coronary venous plasma or estimated interstitial adenosine concentration. During exercise, estimated interstitial adenosine remained well below the threshold concentration necessary for coronary vasodilation before or after L-NNA. CONCLUSIONS: NO causes a modest coronary vasodilation at rest and during exercise but does not act as a local metabolic vasodilator. Adenosine does not mediate a compensatory local metabolic coronary vasodilation when NO synthesis is inhibited.

Adenosine is not responsible for local metabolic control of coronary blood flow in dogs during exercise.

The purpose of this investigation was to quantitatively evaluate the role of adenosine in coronary exercise hyperemia. Dogs (n = 10) were chronically instrumented with catheters in the aorta and coronary sinus, and a flow probe on the circumflex coronary artery. Cardiac interstitial adenosine concentration was estimated from arterial and coronary venous plasma concentrations using a previously tested mathematical model. Coronary blood flow, myocardial oxygen consumption, heart rate, and aortic pressure were measured at rest and during graded treadmill exercise with and without adenosine receptor blockade with either 8-phenyltheophylline (8-PT) or 8-p-sulfophenyltheophylline (8-PST). In control vehicle dogs, exercise increased myocardial oxygen consumption 4.2-fold, coronary blood flow 3.8-fold, and heart rate 2.5-fold, whereas mean aortic pressure was unchanged. Coronary venous plasma adenosine concentration was little changed with exercise, and the estimated interstitial adenosine concentration remained well below the threshold for coronary vasodilation. Adenosine receptor blockade did not significantly alter myocardial oxygen consumption or coronary blood flow at rest or during exercise. Coronary venous and estimated interstitial adenosine concentration did not increase to overcome the receptor blockade with either 8-PT or 8-PST as would be predicted if adenosine were part of a high-gain, negative-feedback, local metabolic control mechanism. These results demonstrate that adenosine is not responsible for local metabolic control of coronary blood flow in dogs during exercise.

Role of K+ATP channels in local metabolic coronary vasodilation.

ATP-sensitive potassium (K+ATP) channels have been shown to play a role in the maintenance of basal coronary vascular tone in vivo. K+ATP channels are also involved in the coronary vasodilator response to adenosine. The aim of this study was to determine the role of K+ATP channels in local metabolically mediated increases in coronary blood flow during cardiac electrical paired pacing without catecholamine effects. In 10 anesthetized closed-chest dogs, coronary blood flow was measured in the left circumflex coronary artery, and myocardial O2 consumption was calculated using the arteriovenous O2 difference. Cardiac interstitial adenosine concentration was estimated from coronary venous and arterial plasma adenosine measurements using a previously described, multicompartmental, axially distributed, mathematical model. Paired stimulation increased heart rate from 57 to 120 beats/min, myocardial O2 consumption 88%, and coronary blood flow 76%. During K+ATP channel blockade with glibenclamide, baseline coronary blood flow decreased in relation to myocardial O2 consumption and thus coronary sinus O2 tension fell. Paired-pulse pacing with glibenclamide resulted in increases in myocardial O2 consumption and coronary blood flow similar to those during control pacing. Coronary venous and estimated interstitial adenosine concentration did not increase sufficiently to overcome the glibenclamide blockade. In conclusion, K+ATP channels are not required for locally mediated metabolic increases in coronary blood flow that accompany myocardial O2 consumption during pacing tachycardia without catecholamines, and adenosine levels do not increase sufficiently to overcome the glibenclamide blockade.

Inorganic phosphate as regulator of adenosine formation in isolated guinea pig hearts.

This study evaluated cytosolic P(i) as an independent regulator of cardiac adenosine formation by dissociating changes in P(i) from changes in AMP and ADP. Myocardial high-energy phosphates (HEP), measured by (31)P nuclear magnetic resonance spectroscopy, were depleted acutely by perfusing isolated guinea pig hearts with 2-deoxyglucose (2-DG), and the effects of 2-DG were compared with a norepinephrine infusion producing similar changes in HEP. 2-DG treatment resulted in lower adenosine release (R(ado)) (54 +/- 18 vs. 622 +/- 199 pmol x min(-1) x g(-1)) and P(i) concentration ([P(i)]) (0.5 +/- 0.1 vs. 6.0 +/- 0.9 mM) than norepinephrine despite similar AMP concentration ([AMP]). Chronic phosphocreatine depletion produced by beta-guanidinopropionic acid feeding also reduced R(ado) and P(i) during hypoxia. Replacement of perfusate glucose and pyruvate with acetate increased R(ado) (from 39 +/- 12 to 356 +/- 100 pmol x min(-1) x g(-1)) and [P(i)] (from 2.0 +/- 0.5 to 5.1 +/- 0.6 mM) with no change in cytosolic [AMP]. Adenosine kinase isolated from guinea pig hearts was inhibited by [P(i)] values seen during hypoxia or hypoperfusion. We conclude that cytosolic [P(i)] can be an important regulator of cardiac adenosine formation through inhibition of adenosine kinase.

Adenosine formation during hypoxia in isolated hearts: effect of adrenergic blockade.

Adrenergic receptor blockade has been reported to decrease cardiac adenosine formation and release during hypoxia. We wished to determine whether this occurs by an improvement in the energy supply/demand ratio. Isolated guinea pig hearts were perfused at a constant pressure of 50 mm Hg. Hypoxia (30% O2) was maintained for 20 min while adenosine release and venous PO2 were measured in the coronary venous effluent. beta-adrenergic blockade with 5 microM atenolol did not change hypoxic adenosine release (Control: 15.6 +/- 2.7, Atenolol: 23.6 +/- 5.7 nmol/g/20 min). Addition of 6 microM phentolamine with atenolol significantly reduced hypoxic adenosine release (4.4 +/- 1.4 nmol/g/20 min, P < 0.05). Atenolol was without hemodynamic effects, but addition of phentolamine reduced left ventricular pressure development, heart rate, and oxygen consumption prior to hypoxia. Atenolol plus phentolamine did not change venous PO2 during hypoxia. Treatment with phenoxybenzamine (1 microM) plus atenolol also reduced adenosine release (7.4 +/- 0.8 nmol/g/20 min). Control experiments and atenolol plus phentolamine experiments were repeated using 31P-NMR to measure high energy phosphates. Adrenergic blockade had no effect on phosphate concentrations during normoxia, but resulted in higher [PCr], lower [P(i)] and higher phosphorylation potentials during hypoxia. Adrenergic blockade also prevented the hypoxia-induced rise in intracellular [H+], [AMP] and [ADP] seen in control hearts. The changes in phosphorylation potential are correlated with similar changes in adenosine release in adrenergically intact hearts. We conclude that the primary effect of adrenergic blockade during hypoxia is a reduction in ATP use due to alpha-receptor blockade. This leads to improved high energy phosphate concentrations during hypoxia and a reduction in adenosine formation.

Adenosine release and high energy phosphates in intact dog hearts during norepinephrine infusion.

Cardiac adenosine release is thought to depend on the oxygen supply/demand ratio, and this effect may be mediated by changes in high energy phosphate concentrations. Previous studies supporting this hypothesis have been done primarily in isolated hearts. We tested this hypothesis in intact dog hearts. Anesthetized, open-chest dogs were placed in a 4.7-T magnet where 31P nuclear magnetic resonance spectra were acquired via a surface coil over the heart at 2-minute intervals (60 scans, 2-second interpulse delay). Coronary sinus flow was shunted through a flow probe and returned via a jugular vein. After a control period, intracoronary norepinephrine was infused (12 micrograms/min) for 16 minutes and plasma samples were taken every 5 minutes. The phosphocreatine/ATP peak area ratio was used as an index of high energy phosphate changes. During norepinephrine infusion, arterial pressure, heart rate, coronary sinus flow, oxygen consumption, and adenosine release all increased significantly. Adenosine release peaked at 5 minutes but remained elevated after 15 minutes. There was a transient fall in the phosphocreatine/ATP ratio (9.2 +/- 3.1%, p less than 0.05) during the first 7 minutes, but the ratio returned to control levels by 9 minutes. The oxygen supply/consumption ratio increased after 5 minutes of norepinephrine infusion and then returned to control levels. We conclude that during norepinephrine infusion in vivo, persistent adenosine release can occur with only small transient changes in high energy phosphate concentrations and with no decrease in the oxygen supply/demand ratio.

Adenosine formation and energy status in isolated guinea pig hearts perfused with erythrocytes.

The purpose of this study was to examine adenosine release and high-energy phosphate concentrations during norepinephrine (NE) infusion in isolated guinea pig hearts perfused with a physiological salt solution (PSS) containing erythrocytes (RBC). Phosphate concentrations were monitored using 31P-nuclear magnetic resonance spectroscopy while NE was infused at 6 x 10(-10) mol/min. Compared with perfusion with PSS alone, RBC-perfused hearts consumed more oxygen and developed higher left ventricular pressure and first time derivative of left ventricular pressure at lower coronary flow rates. Adenosine release rates were very similar with both perfusates. NE infusion did not produce a decline in ATP concentration ([ATP]) or an increase in calculated [ADP] and [AMP] in RBC-perfused hearts. However, phosphorylation potential ([ATP]/[ADP][Pi]) declined because of increased [Pi]. We conclude that NE infusion does not change adenine nucleotide concentrations in well-oxygenated guinea pig hearts and that changes in nucleotide concentrations are not necessary for increased adenosine release. Phosphorylation potential is a better predictor of adenosine release than any individual nucleotide or phosphate concentration.

Adenosine formation and myocardial energy status during graded hypoxia.

Previous studies using hypoperfusion and 2-deoxyglucose infusion have revealed a biphasic relationship between myocardial energy status and adenosine release (RADO). As energy charge ([ATP] + 1/2[ADP])/([ATP] + [ADP] + [AMP]) or phosphorylation potential ([ATP]/[ADP][Pi]) is lowered there is an initial increase in RADO, but RADO declines from peak levels during severe energy depletion. This study examined the hypothesis that the same pattern of RADO exists during graded hypoxia. Isolated guinea-pig hearts were perfused at constant flow and exposed to mild (30% O2) and severe (0% O2) hypoxia in the presence of norepinephrine (NE, 6 x 10(-8) M). Phosphorylation potential and energy charge were determined using 31P-NMR spectroscopy and adenosine release into coronary venous effluent was measured. Graded hypoxia lowered energy charge and phosphorylation potential, and raised RADO. Although severe hypoxia plus NE lowered energy charge and phosphorylation potential to levels equivalent to those associated with decreased RADO during hypoperfusion or 2-deoxyglucose treatment, RADO during severe hypoxia was greater than during mild hypoxia. HCl was infused during severe hypoxia in order to reproduce the low intracellular pH seen during hypoperfusion, but HCl increased RADO rather than decreasing it. We conclude that during hypoxia, RADO does not have a biphasic relationship to phosphorylation potential or energy charge, suggesting that the regulation of adenosine formation cannot be explained solely in terms of these variables. Furthermore, intracellular acidosis is not responsible for inhibiting RADO at low phosphorylation potential and energy charge during hypoperfusion because it has no effect on RADO during severe hypoxia.

Interstitial adenosine concentration during norepinephrine infusion in isolated guinea pig hearts.

This study determined the effect of norepinephrine (NE) on cardiac interstitial fluid adenosine concentration [( ADO]isf). Isolated guinea pig hearts were perfused with a Krebs-Henseleit buffer solution. Radiolabeled albumin, sucrose, and adenosine were injected under control conditions and after 3 and 20 min of NE infusion to obtain multiple indicator dilution curves that were used to determine capillary transport parameters for adenosine. These parameters together with venous adenosine concentrations were used in a mathematical model to a calculate [ADO]isf. Capillary transport parameters were not changed significantly by NE infusion. Because of uncertainty regarding two model parameters, two sets of [ADO]isf values were calculated. One set used best-fit values obtained from indicator dilution curves, and a second set used parameters chosen to provide the highest [ADO]isf values consistent with indicator dilution curves. Venous adenosine concentrations were 1.9 +/- 0.4 nM under control conditions and 243 +/- 110 and 45 +/- 25 nM after 3 and 20 min of NE infusion, respectively. Calculated [ADO]isf was 2.6-9.4, 591-1,288, and 166-324 nM, respectively, under these same conditions. We conclude that NE infusion greatly increases [ADO]isf, and adenosine is responsible for most of the vasodilation at 3 min. The subsequent fall in venous concentration is due to a fall in [ADO]isf rather than to decreased capillary permeability. Vascular resistance remained low while [ADO]isf fell, which suggests that additional vasodilators are important during maintained NE infusion.

Adenosine formation and energy status during hypoperfusion and 2-deoxyglucose infusion.

The relationship between adenosine (Ado) formation and cytosolic energy status was studied in isolated guinea pig hearts during hypoperfusion plus norepinephrine infusion (0.6 nmol/min) and in isolated rat hearts during 2-deoxyglucose (2-DG) infusion. 31P nuclear magnetic resonance (31P-NMR) was used to measure phosphate concentrations, and both phosphorylation potential (expressed as [ATP]/[ADP][Pi]) and energy charge [expressed as (([ATP] + 1/2[ADP])/([ATP] + [ADP] + [AMP]))] were calculated as indexes of cytosolic energy status. Both progressive flow reductions and increasing length of exposure to 2-DG led to progressive decreases in energy charge and phosphorylation potential. In both cases, steady-state Ado release first increased then declined despite a continued fall in energy status. Inosine release followed a similar pattern. This biphasic pattern of Ado release vs. energy charge is similar to the pattern seen in in vitro studies of cytosolic 5'-nucleotidase, supporting the hypothesis that Ado formation in vivo is regulated by the influence of energy status on this enzyme. However, Ado release in vivo peaked at an energy charge much higher (0.997) than that observed in vitro (0.60-0.86). It is therefore probable that the inhibition of Ado formation in the perfused heart occurs via factor(s) in addition to energy charge.

Transcapillary adenosine transport and interstitial adenosine concentration in guinea pig hearts.

We used the multiple-indicator-dilution technique to observe the capillary transport of adenosine in isolated Krebs-Henseleit-perfused guinea pig hearts. Tracer concentrations of radiolabeled albumin, sucrose, and adenosine were injected into the coronary inflow; outflow samples were collected for 10-25 s and analyzed by high-performance liquid chromatography (HPLC) and by gamma- and beta-counting. The albumin data define the intravascular transport characteristics; the sucrose data define permeation through interendothelial clefts and dilution in interstitial fluid (ISF). Parameters calculated from adenosine data include permeability-surface area products for endothelial cell uptake at the luminal and abluminal membranes and intraendothelial metabolism. We found that in situ endothelial cells avidly take up and metabolize adenosine. Tracer adenosine in the capillary lumen is twice as likely to enter an endothelial cell as it is to permeate the clefts. There was no adenosine in the arterial perfusate. Under control conditions, the steady-state venous adenosine concentration was 3.6 +/- 0.8 nM, which from the flow and the parameters estimated from the tracer data gave a calculated ISF concentration of 6.8 +/- 1.5 nM. During dipyridamole infusion (10 microM) at constant pressure, the cell permeabilities went essentially to zero, whereas the venous adenosine concentration increased to 44.0 +/- 12.6 nM, giving an estimated ISF concentration of 191 +/- 53 nM. With constant flow perfusion, venous concentration during dipyridamole infusion was 30.9 +/- 6.3 nM, and estimated ISF concentration was 88 +/- 20 mM. We conclude that in this preparation, at rest, the ISF adenosine concentration is about twice the venous concentration and the ISF adenosine concentration increases with dipyridamole administration.

Distribution of perfusate flow during vasodilation in isolated guinea pig heart.

The purpose of this study was to determine the effect of vasodilation on the distribution of perfusate flow in the isolated guinea pig heart. Hearts were perfused retrogradely through the aorta with oxygenated Krebs-Henseleit buffer solution at 37 degrees C. Regional myocardial flows were measured with 15-micron radioactive microspheres. Each heart was subdivided into 45 pieces (average size 44 mg), and heterogeneity of flow was quantified as the relative dispersion (standard deviation/mean). Under control conditions at a perfusion pressure of 46 mmHg (60 cm water), the relative dispersion of left ventricular (LV) flow was 30 +/- 2% (n = 8). Vasodilation was induced via infusion of dipyridamole (10(-5) M). When flow was held constant at the resting value, relative flow dispersion increased to 43 +/- 6% (n = 8). When perfusion pressure was held constant and flow allowed to increase, relative dispersion fell to 20 +/- 5% (n = 5). Heterogeneity for the heart as a whole was higher than for the left ventricle but followed the same pattern with vasodilation. In a separate series of hearts (n = 5) equipped with LV balloons but without microsphere flow measurements, vasodilation at constant flow decreased LV pressure development, dP/dt, and O2 consumption. Vasodilation at constant pressure increased O2 consumption, but did not increase LV pressure or dP/dt. We conclude that vasodilation in this preparation will increase flow heterogeneity during constant-flow perfusion but decrease heterogeneity during constant-pressure perfusion. Furthermore, increased flow heterogeneity can compromise ventricular function.

Metabolic interaction between skeletal muscle and liver during bacteremia.

To study the effects of bacteremia on skeletal muscle leucine (LEU) metabolism, mongrel dogs were infused with normal saline or Escherichia coli (10(9)/kg). After a bolus dose (3.6 microCi), L(1-carbon 14) LEU (0.3 microCi/min) was infused directly into the isolated, constant-flow, in vivo gracilis muscle. Arteriovenous differences for amino acids, labeled and unlabeled LEU and alpha-ketoisocaproic acid (KIC), and labeled carbon dioxide were measured at ten-minute intervals for one hour. Bacteremia increased the net release of amino acids and total N2 from muscle. Moreover, plasma LEU that was deaminated and released as KIC was increased, and there was also an increase in decarboxylated plasma LEU during bacteremia. Despite the marked increase in KIC release from skeletal muscle during bacteremia, arterial concentrations were not significantly different from those of controls. An unchanged arterial plasma KIC concentration associated with a marked increase in KIC released from skeletal muscle indicates an increase in LEU metabolism, most likely in the liver. Thus, the increased skeletal muscle catabolism is not a futile cycle but rather an essential event to meet the increased metabolic needs of the body during bacteremia.

Effects of ischemia on VO2, tension, and vascular resistance in contracting canine skeletal muscle.

This study examined the changes in O2 consumption (VO2), vascular resistance, and tension development during skeletal muscle contractions at reduced flow. We tested the hypothesis that when VO2 is limited by O2 supply, the skeletal muscle vasculature is not maximally dilated because of the fall in contractile force that accompanies the decrease in O2 supply. During 30 min of ischemic contractions, tension fell by 45 +/- 4% and VO2 fell 54 +/- 1% from preischemic levels. The O2 cost per unit tension did not change compared with nonischemic muscles. After the initial flow reduction, flow fell an additional 16 +/- 3% over 30 min. Adenosine infusion after 30 min of ischemic contractions increased flow by 42 +/- 3% but increased VO2 by only 9.8 +/- 2.3% and had no effect on tension development. When perfusion pressure was returned to normal after 30 min of ischemic contractions, twitch tension did not begin to recover within 20 min but tetanic tension showed a small improvement. VO2, although increased, remained well below the preischemic level. These results suggest that because of the reduced tension during ischemic contractions, the O2 supply-to-consumption ratio is nearly normal, which could explain the presence of the vasodilator reserve. The defect in tension development is long lived, producing a "stunned" muscle in which excess O2 supply does not restore function or VO2 to normal.

Aminophylline and interstitial adenosine during sustained exercise hyperemia.

We tested the hypothesis that an increase in interstitial fluid (ISF) adenosine concentration contributes to vasodilation of high oxidative skeletal muscle during sustained free-flow exercise. Canine calf muscles were stimulated at 3 Hz for 10 min before and after the infusion of the adenosine receptor antagonist aminophylline (10 mg/kg). The vasodilation that occurred during aminophylline infusion was allowed to decay before the postaminophylline exercise period was begun. This dose of aminophylline shifted the response to infused adenosine 20-fold during rest and reduced the response to a standard dose by 90% during exercise. Aminophylline had no significant effect on blood flow or on O2 consumption at rest or during exercise. Adenosine release (venous minus arterial plasma concentration times plasma flow) increased during 3-Hz exercise both before and after aminophylline infusion, but venous plasma adenosine concentration did not increase in either case. We developed a mathematical model of adenosine movement between ISF and plasma to help us judge whether to use adenosine release or venous concentration as an index of ISF adenosine and decided that venous concentration should be used. We conclude that aminophylline has no effect on sustained 3-Hz exercise hyperemia because under these conditions ISF adenosine concentration does not increase.

Endothelial cell uptake of adenosine in canine skeletal muscle.

The vascularly isolated muscles in the hindlimbs of five dogs were perfused with an oxygenated physiological salt solution. The extractions of adenosine and of a nontransported analogue of adenosine, 9-beta-D-arabinofuranosyl hypoxanthine (AraH), were determined by the single-pass indicator-dilution technique. A bolus containing [125I]albumin (reference tracer), [14C]adenosine, and [3H]AraH was injected into the artery while samples of venous effluent were collected over the next minute. This injection was repeated with dipyridamole (10(-5) M) in the perfusate. Early extractions of AraH (EAra) and adenosine (EAdo) under control conditions were 48 +/- 4 and 80 +/- 4%, respectively. In the presence of dipyridamole, EAra was unchanged (47 +/- 5) while EAdo decreased to 45 +/- 7%. Since early extraction reflects primarily the barrier posed by endothelial cells, these results demonstrate significant endothelial uptake of adenosine. Analysis of these data using a mathematical model of blood-tissue exchange indicates that, under the conditions of these experiments, at least 78% of the adenosine taken up by skeletal muscle entered endothelial cells.