The effect of aging on adrenergic and nonadrenergic receptor expression and responsiveness in canine skeletal muscle.

We tested the hypothesis that adrenergic and nonadrenergic receptor responsiveness and protein expression would be altered with advancing age. Young (n = 6; 22 ± 1 mo; mean ± SE) and old (n = 6; 118 ± 9 mo) beagles were instrumented with flow probes and an indwelling catheter for continuous measurement of external iliac blood flow and arterial blood pressure. Vascular conductance (VC) was calculated as hindlimb blood flow/mean arterial pressure. Selective agonists for α-1, α-2, neuropeptide-Y (NPY), and purinergic (P2X) receptors were infused at rest and during treadmill running at moderate (2.5 mph) and heavy (4 mph with 2.5% grade) exercise intensities. Feed arteries were dissected from gracilis muscles, and α-1D, α-1B, α-2A, P2X-4, P2X-1, and NPY-Y1 receptor protein expression was determined. Phenylephrine produced similar decreases (P > 0.05) in VC in young and old beagles at rest (young: -62 ± 5%; old: -59 ± 5%) and during moderate (young: -67 ± 5%; old: -62 ± 4%) and heavy (young: -54 ± 4%; old: -49 ± 3%) exercise. Clonidine caused similar (P > 0.05) decreases in VC in old compared with young dogs at rest (young: -59 ± 8%; old: -70 ± 6%) and during moderate (young: -52 ± 6%; old: -47 ± 5%)- and heavy (young: -42 ± 5%; old: -43 ± 5%)-intensity exercise. NPY infusion resulted in a similar decline in VC in young and old beagles at rest (young: -40 ± 7%; old: -39 ± 9%) and during moderate (young: -47 ± 6%; old: -40 ± 6%)- and heavy (young: -40 ± 3%; old: -38 ± 4%)-intensity exercise. α-ß-Methylene-ATP also produced similar decreases in VC in young and old beagles at rest (young: -36 ± 6%; old: -40 ± 8%) and during exercise at moderate (young: -42 ± 5%; old: -40 ± 9%) and heavy (young: -47 ± 5%; old: -42 ± 8%) intensities. α-1B receptor protein expression was elevated (P < 0.05) in old compared with young dogs, whereas there were no age-related differences in α-1D or α-2A receptor expression and nonadrenergic P2X-4, P2X-1, and NPY-Y1 receptor expression. The present findings indicate that postsynaptic adrenergic and nonadrenergic receptor responsiveness was not altered by advancing age. Moreover, the expression of adrenergic and nonadrenergic receptors in skeletal-muscle feed arteries was largely unaffected by aging.

Endogenous nitric oxide modulates myocardial oxygen consumption in canine right ventricle.

The role of endogenous nitric oxide (NO) in modulating myocardial oxygen consumption (MVO2) is unclear, in part because of systemic and coronary hemodynamic effects of blocking NO release. This study evaluated the effect of NO on right ventricular MVO2 under controlled hemodynamic conditions. In 12 open-chest dogs, N(omega)-nitro-L-arginine methyl ester (L-NAME, 150 microg/min), a NO synthase (NOS) blocker, was infused into the right coronary artery. Heart rate and mean aortic pressure were constant. Right coronary blood flow and right ventricular MVO2 were measured at normal and elevated right coronary perfusion pressures (RCP) before and after L-NAME. To avoid effects of NO synthesis blockade on right coronary blood flow, which might have altered right ventricular MVO2, experiments, were conducted during adenosine-induced maximal coronary vasodilation. L-NAME did not affect right coronary blood flow (P = 0.51). However, L-NAME significantly increased right ventricular MVO2 (6% at RCP 100 mmHg, and 21% at RCP 180 mmHg). Right coronary blood flow varied with perfusion pressure (P < 0.02), and the elevation of MVO2 produced by L-NAME increased at higher flows (P < 0.04), consistent with the greater shear stress-mediated release of NO. These findings indicate that endogenous NO limits right ventricular MVO2.

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.

Insulin improves cardiac contractile function and oxygen utilization efficiency during moderate ischemia without compromising myocardial energetics.

Insulin improves myocardial contractile function during moderate ischemia, but the mechanism is unknown. To determine effects of insulin on myocardial oxygen utilization efficiency (O2UE) and energetics, regional left coronary perfusion pressure (CPP) was lowered sequentially from 100 to 60, 50, and 40 mmHg in 24 anesthetized, open-chest dogs. Regional power index (PI), myocardial oxygen consumption (MVO2), and O2UE index (PI/MVO2) were determined in untreated and insulin treated (4 U/min, i.v.) hearts. Biopsies were obtained from six untreated and six insulin-treated hearts at CPP=40 mmHg for determining high energy phosphates and the cytosolic phosphorylation potential. Measurements were compared with data from normal, untreated myocardium (n=11). MVO2 fell (P<0.05) in all hearts as CPP was lowered to 40 mmHg, and was unaffected by insulin treatment. PI decreased 32 and 75% in untreated hearts at CPP=50 and 40 mmHg, respectively (P<0.05). In insulin treated hearts, PI was not significantly depressed at CPP>40 mmHg, and fell only 26% at CPP=40 mmHg. O2UE increased (P<0.05) in all hearts at CPP=60 mmHg. In insulin treated hearts, O2UE was greater (P<0.05) at CPP=50 and 40 mmHg than at CPP=100 mmHg, and greater (P<0.05) than in untreated hearts at CPP=40 mmHg. Reducing CPP to 40 mmHg produced similar metabolic changes in all hearts. Compared to normal myocardium, ATP content of untreated and treated hearts was unchanged, creatine phosphate content decreased 21 and 14%, creatine content increased 24 and 30%, inorganic phosphate concentration increased 108 and 140%, and phosphorylation potential decreased 80 and 77%. We conclude that insulin markedly improves PI and O2UE without altering cytosolic energetics during moderate myocardial ischemia.

Insulin improves contractile function during moderate ischemia in canine left ventricle.

This study determined the effects of insulin on myocardial contractile function and glucose metabolism during moderate coronary hypoperfusion. Coronary perfusion pressure (CPP) was lowered from 100 to 60, 50, and 40 mmHg in the left anterior descending coronary artery of anesthetized, open-chest dogs. Regional glucose uptake (GU), lactate uptake, myocardial O2 consumption, and percent segment shortening (%SS) were measured without (n = 12) or with intravenous (4 U/min, n = 12) or intracoronary insulin (4 U/min, n = 6). Glucose metabolites were also measured in freeze-clamped biopsies of control heart (n = 6) and hearts treated with intravenous insulin (n = 6) at the completion of the protocol (40 mmHg CPP). GU increased with intravenous and intracoronary insulin (P < 0.01). In all groups, GU was unaffected by reduced CPP, although lactate uptake decreased significantly (P < 0.01). Myocardial O2 consumption fell (P < 0.05) as CPP was lowered in all groups and was not altered significantly by intravenous or intracoronary insulin treatment. Without insulin, %SS decreased 72% (P < 0.05) at 40 mmHg CPP, but in hearts treated with intravenous and intracoronary insulin, %SS was not reduced (P > 0.05). Myocardial glycogen, alanine, lactate, and pyruvate contents were not significantly different in untreated hearts and hearts treated with intravenous insulin. Thus, in moderately ischemic canine myocardium, insulin markedly improved regional contractile function and did not appreciably increase the products of anaerobic glucose metabolism.

Elevated right atrial pressure does not reduce collateral blood flow to ischemic myocardium.

Right atrial pressure (RAP) may become substantially elevated during heart failure and has been reported to reduce collateral flow to the ischemic myocardium of isolated hearts. The effect of elevated RAP on blood flow to collateral-dependent and normal myocardium of in situ hearts was studied in 20 open-chest anesthetized dogs with acute occlusion of the left anterior descending coronary artery. Regional myocardial blood flow was measured with radioactive microspheres while RAP was elevated by restricting right ventricular (RV) outflow with constant aortic pressure. Increasing RAP from 3.8 +/- 0.5 to 21.5 +/- 0.8 and then to 34.3 +/- 0.9 mmHg did not reduce blood flow to any transmural region of LV normal or collateral-dependent myocardium. Further elevation of RAP to 49.3 +/- 1.1 mmHg reduced subepicardial but not subendocardial collateral flow. Blood flow to normal RV increased. Retrograde flow and peripheral coronary pressure increased as RAP was elevated. Previously injected 11-micron microspheres were present in the retrograde flow when RAP was elevated; thus retrograde capillary flow contributed to the retrograde flow. The results explain discrepancies among previous reports, and they are consistent with a waterfall phenomenon in the coronary collateral circulation.