Myocardial adenosine formation during hypoxia: effects of ecto-5'-nucleotidase inhibition.

Release of adenosine and AMP into epicardial fluid and coronary venous effluent of isovolumic guinea-pig hearts was examined during normoxic (95% O2) and hypoxic (30% O2) perfusion with and without the ecto-5'-nucleotidase inhibitor alpha,beta-methylene adenosine diphosphate (AOPCP)*. Normoxic epicardial and venous adenosine levels were 221 +/- 27 and 67 +/- 11 nM, respectively, in untreated hearts. During 15 min of hypoxia, epicardial and venous adenosine levels increased in a phasic manner, reaching maximal values of 498 +/- 32 and 441 +/- 43 nM, respectively, during the initial 5 min of hypoxia. Epicardial and venous adenosine levels then declined slightly during the subsequent 10 min to 332 +/- 33 and 224 +/- 34 nM, respectively. Infusion of 50 microM AOPCP significantly reduced venous adenosine levels during normoxia (less than 50% of control), but was without effect on normoxic epicardial adenosine. Epicardial and venous adenosine levels increased during hypoxia with AOPCP but the increases were lower than those for untreated hypoxic hearts. Epicardial and venous adenosine levels recovered to baseline levels following 30 min of reoxygenation in both groups. Epicardial and venous AMP levels were elevated by AOPCP treatment during normoxia and hypoxia. Coronary vascular resistance decreased during hypoxia but the decline in resistance was less in AOPCP treated hearts. It is concluded that whereas basal interstitial adenosine levels appear to be independent of ecto-5'-nucleotidase activity, the hypoxic increase in interstitial adenosine is partially derived from an AOPCP sensitive ecto-5'-nucleotidase. Venous adenosine appears to be significantly dependent on ecto-5'-nucleotidase activity during normoxia and hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationship between coronary flow and adenosine release during severe and mild hypoxia in the isolated perfused rat heart with special reference to time-course change

Summary The contribution of endogenous adenosine to coronary vasodilation induced by global myocardial hypoxia was examined. In isolated rat hearts perfused by means of Langendorff's technique, the relationship between chronological changes in coronary flow and adenosine release during hypoxia was analyzed. The oxygenation level of myoglobin (MbO₂), myocardial oxygen uptake, lactate release, and left ventricular pressure (LVP) was also measured. Adenosine was determined by radio-immunoassay, and the MbO₂ levels by the optical method. Severe hypoxia (20% O₂+75% N₂+5% CO₂) increased coronary flow, adenosine release, and lactate release and decreased both myocardial oxygen uptake and LVP. Mild hypoxia (50% O₂+45%N₂+5%CO₂) also increased coronary flow, adenosine release, and lactate release, while it affected neither myocardial oxygen uptake nor LVP. These results suggest that the oxygen supply is compensated by an increase in coronary flow in mild hypoxia, whereas this does not occur in severe hypoxia. Changes in MbO₂ were the reverse of those in coronary flow during severe hypoxia, confirming that a decrease in intracellular oxygen correlates well with an increase in coronary flow. The pattern of changes in adenosine release, however, was not identical with that in coronary flow in severe and mild hypoxia, indicating that there is no significant relationship between coronary flow and adenosine release in either severe or mild hypoxic hearts. These findings suggest that adenosine is not the only metabolic mediator of regulation of coronary flow in hypoxic hearts.     

Measurement by fluorescence of interstitial adenosine levels in normoxic, hypoxic, and ischemic perfused rat hearts

An improved assay was used to investigate the effects of hypoxia or ischemia on interstitial fluid and coronary venous effluent levels of adenosine in isolated perfused nonworking rat hearts. The adenosine in 5- to 10-microliter samples of left ventricular epicardial surface transudates and coronary effluents was reacted with chloroacetaldehyde, and the fluorescent derivative (1,N6-ethenoadenosine) was quantitated using high pressure liquid chromatography and fluorescence detection. Hearts responding to hypoxia could be separated into two groups. In one group of hearts, the control (normoxic) transudate and effluent adenosine concentrations were 94 +/- 24 and 41 +/- 6 pmol/ml, respectively. These values increased by 118 and 96%, respectively, with 5 minutes of hypoxia (30% O2), and returned to control levels 5 minutes after resumption of normoxia. In a second group of hearts, the normoxic control levels of adenosine in the transudates (42 +/- 7 pmol/ml) and coronary effluents (62 +/- 17 pmol/ml) were increased with hypoxia by 174 and 1,178%, respectively. However, the transudate levels continued to rise for 5 minutes after resumption of normoxic perfusion while effluent levels fell. In another series of hearts, global ischemia for 30 seconds elicited an elevation of transudate adenosine levels by 362 to 641% above control (58 +/- 15 pmol/ml) as determined 30 seconds after resumption of perfusion flow.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heart bioassay of effluent of isolated,perfused guinea pig hearts to examine the role of metabolites regulating coronary flow during hypoxia

Summary The concentrations of adenosine and other metabolic factors are known to be altered in the effluent of hypoxic hearts, but the relative contribution of these factors in elevating coronary flow has not been clarified. Langendorff prepared guinea pig hearts were perfused at constant pressure and were made mildly hypoxic so that flow increased but oxygen consumption remained unchanged. When the effluent of these hearts was reoxygenated and pH conrrected and directed to perfuse similarly prepared recipient (bioassay) hearts, flow remained unchanged in recipient hearts. However, when donor hearts were made severely hypoxic so that oxygen consumption decreased, and recipient hearts were perfused at constant flow, the , pH corrected effluent produced a large vasodilation in recipient hearts. This response was greatly attenuated in the presence of excess adenosine deaminase but completely abolished by theophylline. Thus the apparent loss of adenine compounds into the effluent may account for vasodilation during severe hypoxia, but not during mild hypoxia, if effluent levels of these compounds truly reflect perivascular levels.     

Adenosine receptor blockade enhances isoproterenol-induced increases in cardiac interstitial adenosine

The failure of adenosine receptor antagonists to consistently attenuate metabolic coronary vasodilation suggests that adenosine is not a primary regulator of functional hyperemia. An alternative hypothesis, however, is that metabolic stimulation of the heart in the presence of an adenosine receptor antagonist results in enhanced interstitial levels of adenosine which then might overcome the blockade. To test this hypothesis, interstitial levels of adenosine and inosine were estimated by HPLC analysis of fluid which exudes from the epicardial surface of isolated rat hearts perfused with crystalloid solution at constant flow. Isoproterenol infusion (10 nM) produced increases in heart rate, left ventricular systolic pressure, rate of pressure development, myocardial oxygen consumption and adenosine and inosine concentrations of venous effluent and surface exudate and produced decreases in coronary vascular resistance. The presence of the adenosine receptor antagonist, 8-(4-sulfophenyl) theophylline (spT) (100 microM), in the perfusate had little or no effect upon most of the responses to isoproterenol except that it significantly enhanced the isoproterenol-induced increases in adenosine release and adenosine concentrations in the venous effluent and surface exudate. The isoproterenol-induced change in adenosine concentration per unit change in oxygen consumption was approximately 3-fold greater in the presence of spT than in its absence. This extra adenosine production may tend to overcome the competitive blocking effect of spT and help explain why agents such as spT are not always effective in blocking metabolic vasodilation.     

Ventilation, EELV and diaphragmatic activity in rats during chronic normobaric hypoxia.

We determined the effects of chronic hypoxia on end-expiratory lung volume (EELV), end-expiratory diaphragmatic activity (DE) and ventilation (VE) in 27 intact (awake and anesthetized) and six carotid body-denervated (CBD; anesthetized) rats. Twenty-nine control animals were also studied. Recordings were made during hypoxia and normoxia before and after 2 or 3 weeks of hypoxia (+3 days of recovery from chronic hypoxia). In awake rats, 2 weeks of chronic hypoxia increased only normoxic VE, while 3 weeks of chronic hypoxia did not change VE or DE. In anesthetized intact rats, after both exposures, hypoxic and normoxic VE tended to decrease, DE did not change and hypoxic and normoxic EELV were enlarged. In CBD animals, 2 weeks of chronic hypoxia did not affect hypoxic VE but decreased normoxic ventilation and enlarged EELV similar to the intact animals. After 3 days of recovery in normoxia, all parameters except EELV were restored to prehypoxic values. Also, transition from hypoxia to normoxia induced parallel changes in EELV and DE while chronic hypoxia increased only EELV. Therefore, chronic normobaric hypoxia induced, (1) an increase in normoxic ventilation reflecting a process of acclimatization; (2) an enlargement of EELV that did not depend on changes in DE and carotid chemoreceptors.     

Inhibition of adenosine metabolism increases myocardial interstitial adenosine concentrations and coronary flow.

We employed an isolated guinea-pig heart model perfused at constant pressure (70 cmH2O) to test the hypothesis that inhibition of adenosine metabolism increases interstitial adenosine concentrations (as measured with epicardial discs) and coronary flow. Iodotubercidin (ITU, 1 microM) and EHNA (erythro-9-[2-hydroxy-3-nonyl] adenine, 5 microM) were used to inhibit adenosine kinase and deaminase, respectively during control conditions and during metabolic stimulation with 1 microM isoproterenol. The adenosine receptor blocker 8-phenyltheophylline (8-PT) was used during control conditions to assess whether the response seen was adenosine specific. ITU plus EHNA decreased heart rate (202 +/- 10 to 136 +/- 11 beats/min) and increased coronary flow (8.2 +/- 0.3 to 12.4 +/- 0.9 ml/min/g) without a change in MVO2, developed pressure or dP/dt. ITU plus EHNA increased adenosine concentrations in epicardial fluid (0.24 +/- 0.07 microM to 1.02 +/- 0.09 microM) and venous effluent (40 +/- 3 nM to 262 +/- 32 nM) during control conditions, and adenosine release increased from 389 +/- 96 pmols/min/g to 3480 +/- 365 pmols/min/g. 8-PT infusion reversed the effects on heart rate and coronary flow and resulted in a persistent elevation of epicardial fluid adenosine concentrations. During metabolic stimulation with 1 microM isoproterenol, ITU plus EHNA significantly limited the increase in heart rate and ventricular developed pressure and dP/dt while coronary flow increased to a significantly greater extent. Myocardial oxygen consumption was similar during metabolic stimulation between the two groups (vehicle vs. ITU plus EHNA). Epicardial fluid adenosine concentration in the vehicle-treated group increased from 0.17 +/- 0.3 microM to 0.34 +/- 0.02 microM at 15 min of isoproterenol stimulation whereas it increased from 1.10 +/- 0.02 microM to 2.90 +/- 0.46 microM in the ITU plus EHNA-treated group. Inhibition of adenosine metabolism during metabolic stimulation significantly increased venous adenosine concentrations and adenosine release and reduced inosine and hypoxanthine release proportionately. The release of adenosine+inosine+hypoxanthine was unchanged. Inhibition of adenosine metabolism provides evidence supporting the hypothesis that adenosine plays a role in regulating coronary vascular resistance as well as influencing heart rate and ventricular inotropy.     

Effect of dipyridamole on myocardial adenosine metabolism and coronary flow in hypoxia and reactive hyperemia in the isolated perfused guinea pig heart.

The effect of dipyridamole on adenosine metabolism and coronary flow in the hypoxic intact myocardium was studied in normal and hypertrophied isolated perfused guinea pig hearts. The presence of dipyridamole in the normal guinea pig heart resulted in a significant increase in the level of tissue adenosine (67%) with a concomitant increase in coronary flow (85%) and a decrease in the rate of adenosine release (18%) from the myocardium during perfusion with normoxic and hypoxic solutions. Directional changes in hypertrophied hearts were the same as for normal hearts. Tissue levels of adenosine in normal dipyridamole-treated hearts during normoxic perfusion were significantly increased above those of untreated hearts at the peak of reactive hyperemia (× 1.5), after 30 s of aortic occlusion (× 2.3) and during steady state flow (× 2.0). Furthermore, the presence of dipyridamole in normal hearts during reactive hyperemia, produced by 30 s of aortic occlusion, increased the volume and duration of coronary flow by 44% and 75%, respectively. These findings are in accord with the hypothesis that adenosine is a mediator of coronary flow and that dipyridamole potentiates the vasodilator effect of endogenous as well as exogenous adenosine.     

Acute renovascular hypertension in conscious dogs. Interaction of the renin-angiotensin system and sympathetic nervous system in systemic hemodynamics and regional blood flow responses

The effects of acute renovascular hypertension on the sympathetic nervous system, regional blood flow and cardiac function were studied in conscious dogs submitted to renal artery occlusion by inflation of a cuff implanted previously around one renal artery. We then compared the alterations in plasma renin and catecholamine levels and in the various hemodynamic parameters induced by those maneuvers in intact dogs, to those in dogs pretreated with alpha- and beta-adrenergic receptor blockers. Subsequently, the converting enzyme inhibitor teprotide was administered to inhibit angiotensin formation in both experiments. Our results suggest that both the renin-angiotensin system and the sympathetic system contribute to the rise in blood pressure. The hemodynamic changes and alterations in regional blood flows accompanying this acute hypertension appear to be due mostly to the increase in plasma angiotensin, since prior adrenoceptor blockade only attenuated their magnitude but did not alter their direction. However, angiotensin-induced coronary vasoconstriction was observed only in adrenergically blocked but not intact animals, probably because of the protective effect of baroreceptor-mediated reflex sympathetic coronary vasodilation.     

Coronary vascular responses to hypoxia in the diabetic lamb: independence from adenosine and autonomic mechanisms

We have previously found that the coronary dilator response to infused adenosine is attenuated in diabetic (alloxan) lambs. Adenosine responsiveness is restored by administration of insulin. The present studies tested the hypothesis that coronary flow changes with hypoxia, if mediated by adenosine, would also be modified. Studies were carried out in eight control and six diabetic lambs. The animals were anesthetized and prepared to maintain constant arterial pressure (reservoir), cardiac output (pump), and heart rate (paced). Atropine and practolol were given. Forced inspired oxygen was reduced in steps. Arterial and coronary sinus blood samples were analyzed for Po2, oxygen content, pH, hematocrit, and glucose. Myocardial oxygen delivery and uptake (MVO2) were calculated. Coronary flow increased identically in both control and diabetic animals as PaO2 was reduced below 60 Torr. Oxygen delivery and MVO2 fell equally in both groups. Acidosis potentiated hypoxic coronary flow changes. Alpha blockade (phentolamine) was without effect in control lambs but caused coronary flow to increase in diabetic lambs. Changes in coronary flow with hypoxia were unaffected, however. Insulin caused no change in the coronary dilator response to hypoxia in either control or diabetic lambs. It is concluded that coronary alpha tone is increased in diabetes but does not modify changes in coronary flow during hypoxia. As coronary flow responses to hypoxia were unaltered in diabetic lambs, and unaffected by insulin, adenosine may not be the primary mediator of coronary vascular dilatation. Potentiation of adenosine by tissue acidosis is apparently insufficient to explain these findings. The mechanism for coronary dilatation during hypoxia is unclear but may involve direct effects of reduced oxygenation of coronary vascular smooth muscle.     

Role of adenosine in the maintenance of coronary vasodilation distal to a severe coronary artery stenosis. Observations in conscious domestic swine

The purpose of this study was to test the hypothesis that adenosine is required to maintain arteriolar vasodilation distal to a severe coronary stenosis. Eight closed-chest conscious pigs were prepared by placing a 7.5-mm long stenosis (82% lumenal diameter reduction) in the proximal left anterior descending coronary artery. Regional myocardial blood flow (microsphere technique) was measured at control 1, after 10 minutes of intracoronary infusion of adenosine deaminase (7-10 U/kg per min) distal to the stenosis, and 20-30 minutes after stopping adenosine deaminase infusion. Studies with 125I-labeled adenosine deaminase were conducted in six additional pigs to document the extent to which infused adenosine deaminase penetrated the interstitial space. 125I-labeled adenosine deaminase was infused for 10 minutes (10-11 U/kg per min) into the left anterior descending coronary artery. Calculated interstitial fluid concentrations of adenosine deaminase ranged between 71 and 272 U/ml and were at least one order of magnitude greater than that required to deaminate all the adenosine which would be released into the interstitium in response to 15-30 seconds of coronary occlusion. In the primary group of animals (n = 8), endocardial flow (ml/min per g) distal to stenosis at control 1 (1.15 +/- 0.33) was reduced vs. endocardial flow in the nonobstructed circumflex zone (1.59 +/- 0.38, P less than 0.05). Flows in epicardial layers were comparable at control 1 (distal zone = 1.40 +/- 0.36 vs. circumflex zone = 1.45 +/- 0.41). Distal zone endocardial and epicardial flows did not change vs. control 1 in response to infusion of adenosine deaminase. However, the distal: circumflex epicardial flow ratio declined vs. control 1 (0.98 +/- 0.14) during adenosine deaminase infusion (0.87 +/- 0.17, P less than 0.05). The distal:circumflex endocardial flow ratio during adenosine deaminase (0.72 +/- 0.20) was unchanged vs. control 1 (0.76 +/- 0.22) but was less than control 2 (0.80 +/- 0.18, P less than 0.05). Thus, destruction of all or most interstitial adenosine caused only slight relative reduction in regional myocardial blood flow distal to a severe coronary artery stenosis. Accordingly, adenosine contributes only modestly to maintenance of arteriolar vasodilation in this setting or else its absence is almost fully compensated for by another mechanism(s).     

S-nitrosohemoglobin deficiency: a mechanism for loss of physiological activity in banked blood

RBCs distribute oxygen to tissues, but, paradoxically, blood transfusion does not always improve oxygen delivery and is associated with ischemic events. We hypothesized that storage of blood would result in loss of NO bioactivity, impairing RBC vasodilation and thus compromising blood flow, and that repleting NO bioactivity would restore RBC function. We report that S-nitrosohemoglobin (SNO-Hb) concentrations declined rapidly after storage of fresh venous blood and that hypoxic vasodilation by banked RBCs correlated strongly with the amounts of SNO-Hb (r(2) = 0.90; P < 0.0005). Renitrosylation of banked blood during storage increased the SNO-Hb content and restored its vasodilatory activity. In addition, canine coronary blood flow was greater during infusion of renitrosylated RBCs than during infusion of S-nitrosothiol-depleted RBCs, and this difference in coronary flow was accentuated by hypoxemia (P < 0.001). Our findings indicate that NO bioactivity is depleted in banked blood, impairing the vasodilatory response to hypoxia, and they suggest that SNO-Hb repletion may improve transfusion efficacy.     

Coronary steal: Its role in detrimental effect of isoproterenol after acute coronary occlusion in dogs

Using epicardial electrograms others have established that infusion of isoproterenol increases myocardial injury after acute coronary occlusion. To define the contribution of alterations in collateral blood flow to this increased ischemia, isoproterenol was administered to 10 dogs. After pretreatment with practolol in doses that successfully block inotropic but not vascular effects of beta adrenergic stimulants, intracoronary isoproterenol continued to enhance the magnitude of S-T segment elevation in ischemic areas. Thus, vasodilation induced by isoproterenol appears to divert flow from the ischemic area. To test this hypothesis, intracoronary adenosine was given to cause coronary vasodilation without enhancing inotropy. S-T segment elevation at ischemic and adjacent sites was significantly increased. Neither agent had systemic effects, but each increased coronary blood flow while concomitantly decreasing collateral flow as evidenced by a reduction in retrograde coronary flow and peripheral coronary pressure. In addition, adenosine significantly diminished the rate of xenon-133 clearance from the ischemic myocardium. Thus, isoproterenol, in addition to its positive inotropic effect, increases myocardial injury by its vascular action. Collateral blood flow to acutely ischemic myocardium is diminished by the production of a coronary steal. Intravenously administered isoproterenol additionally diminishes collateral flow by decreasing coronary perfusion pressure. It is postulated that any agent that causes either a primary or secondary coronary vasodilation may cause a coronary steal and subsequently enhance myocardial injury.     

Significance of release of adenosine triphosphate and adenosine induced by hypoxia or adrenaline in perfused rat heart

The status of ATP as a possible coronary vasodilator remains poorly understood. The onset of hypoxia induced a rapid and transient increase of the ATP concentration in the coronary effluent of the isolated perfused rat heart from 0.8 +/- 0.2 nM to the average peak value of 1.3 +/- 0.2 nM (P less than 0.01) at 2 +/- 0.5 min; at the same time the coronary flow increased 2-fold so that the rate of ATP release increased from 10.2 +/- 2.9 to 21.4 +/- 4.2 pmol/g/min (P less than 0.005). Hypoxia also produced a peak rate release of adenosine of 93 +/- 5 nM/g/min occurring only after the peak increase of coronary flow and also after the peak release of ATP; at peak coronary flow, however, the adenosine concentration was sufficient for vasodilation (0.31 +/- 0.19 microM). Peak release of ATP and of adenosine preceded that of lactate dehydrogenase. 10(-6) M adrenaline induced a rapid increase of coronary flow and release of ATP, the concentration of which rose from 0.9 +/- 0.3 nM to an average peak of 1.7 +/- 0.2 nM (P less than 0.01) at 2 +/- 0.3 min. The rate of increase of ATP in the coronary effluent paralleled the rate of early rise of coronary flow, yet adenosine had also risen to vasodilatory values (0.28 +/- 0.5 microM). The absolute changes in the measured concentrations of ATP in the coronary effluent were more variable and 1000 X less in concentration than those of adenosine. Hence coronary dilation could be explained by adenosine without involving ATP, although an additional vasodilatory role for ATP could not be excluded, especially in the early phases of vasodilation. In one condition, hypoxic K-arrested hearts, the increase in coronary flow could not be linked to release of either adenosine or ATP. The changes in concentrations of potential vasodilators measured in the coronary effluent do not necessarily reflect changes in the interstitial fluid.     

Formation of S-adenosylhomocysteine in the heart. I: An index of free intracellular adenosine

To assess the concentration of free intracellular adenosine in the heart the kinetic properties of cytosolic S-adenosylhomocysteine (SAH) hydrolase were utilized at elevated levels of L-homocysteine (adenosine + L-homocysteine in equilibrium with SAH + H2O). Global hypoxia was induced in the isolated perfused guinea pig heart by graded reduction of perfusion medium PO2 in the presence of saturating concentrations of homocysteine (0.2-1.0 mM). Reduction of PO2 from 660 to 165 mm Hg increased the steady-state concentration of total tissue adenosine from 2.0 +/- 0.2 to 2.8 +/- 0.2 nmoles/g, while the rate of SAH formation increased linearly from 0.22 +/- 0.03 to 2.50 +/- 0.13 nmoles/min/g. When adenosine was exogenously applied at a concentration of 100 microM together with homocysteine (1 mM), SAH accumulation rates were much greater: 23.34 +/- 3.31 and 42.11 +/- 1.73 nmoles/min/g with normoxic (95% O2) and hypoxic (30% O2) perfusion, respectively. The apparent Km and Vmax values for SAH-hydrolase in vivo were estimated to be 20 microM and 59 nmoles/min/g wet wt, respectively. Since the relation between SAH formation and adenosine in the physiological concentration range is linear, the measured rate of SAH accumulation during normoxia and hypoxia permitted the calculation of the free intracellular adenosine level, which was 0.061 nmoles/g (0.08 microM) in the normoxic heart. With hypoxia (PO2 165 mm Hg), this value increased to 1.57 nmoles/g (2.0 microM). Free intracellular adenosine closely correlated with the hypoxia-induced changes in coronary flow. The data reveal that measurement of the rate of SAH accumulation during homocysteine infusion can be used for sensitive assessment of free intracellular adenosine levels. Assuming that the intracellular adenosine concentration equals that in the interstitial space, the results furthermore indicate that the degree of intracellular adenosine formation during hypoxic perfusion is quantitatively sufficient to account for most of the observed increases in coronary flow.     

Intermittent hypoxia increases exercise tolerance in elderly men with and without coronary artery disease

BACKGROUND: Intermittent hypoxia has been suggested to increase exercise tolerance by enhancing stress resistance and improving oxygen delivery. Because the improvement of exercise tolerance reduces mortality in the elderly with and without coronary artery disease intermittent hypoxia might be a valuable preventive and therapeutic tool. However, controlled studies are lacking. METHODS AND RESULTS: Sixteen males (50-70 years, 8 with and 8 without prior myocardial infarction) were randomly assigned in a double-blind fashion to receive 15 sessions of passive intermittent hypoxia (hypoxia group) or normoxia (control group) within 3 weeks. For the hypoxia group each session consisted of three to five hypoxic (14-10% oxygen) periods (3-5 min) with 3-min normoxic intervals. Controls inhaled only normoxic air in the same way. Exercise tests were performed before and after the 3-week breathing program. After 3 weeks of intermittent hypoxia peak oxygen consumption had increased compared to normoxic conditions (+ 6.2% vs.-3%, p < 0.001). This improvement was closely related to the enhanced arterial oxygen content after hypoxia (r = 0.9, p < 0.001). Both higher haemoglobin concentration and less arterial oxygen desaturation during exercise contributed to the increase in arterial oxygen content. During sub-maximal exercise (cycling at 1 W/kg) heart rate, systolic blood pressure, blood lactate concentration, and the rating of perceived exertion were diminished after intermittent hypoxia compared to control conditions (all p < 0.05). Changes in responses to exercise after intermittent hypoxia were similar in subjects with and without prior myocardial infarction. CONCLUSIONS: Three weeks of passive short-term intermittent hypoxic exposures increased aerobic capacity and exercise tolerance in elderly men with and without coronary artery disease.     

Effects of regional alpha- and beta-blockade on resting and hyperemic coronary blood flow in conscious, unstressed humans

Our purpose was to determine if there are basal adrenergic influences on the coronary circulation in humans. We studied 56 patients with denervated hearts after cardiac transplantation and 19 normally innervated patients with angiographically normal coronary arteries. Coronary blood flow velocity was measured during cardiac catheterization with a subselective 3F intracoronary Doppler catheter. Heart rate was controlled by atrial pacing. Epicardial coronary artery diameter was measured by automated analysis of digital coronary angiograms. Coronary flow reserve was assessed by intracoronary papaverine hydrochloride (12 mg) injections. Regional sympathetic blockade was produced by intracoronary injections of phentolamine (3 mg, alpha) and propranolol (2 mg, beta) or metoprolol (3 mg, beta 1). After alpha-blockade, mean arterial pressure fell significantly (p less than 0.05) in both the denervated transplant (-5.8 +/- 1.5%) (mean +/- SEM) and normally innervated patients (-12.6 +/- 3.2%). Reductions in coronary flow velocity also were observed in these groups (-8.2 +/- 2.3% and -9.2 +/- 5.8%, respectively). Calculated coronary vascular resistance was unchanged. Similar changes were seen when patients were pretreated with beta-blockade before alpha-blockade. Nonspecific beta-blockade did not affect mean arterial pressure but decreased coronary velocity (innervated, -11.6 +/- 3.9%; denervated, -9.3 +/- 2.4%) and increased coronary vascular resistance (innervated, 15.4 +/- 6.7%; denervated, 10.2 +/- 3.7%). Coronary vascular resistance did not rise in either group after selective beta 1-blockade with metoprolol. Coronary flow reserve did not change in either patient group after either alpha- or beta-blockade. Changes in epicardial coronary artery diameter were small and generally not significant. These data suggest that alpha-receptor-mediated vascular tone is negligible in both denervated transplant patients and normally innervated patients. Additionally, the increase in vascular resistance after nonselective beta-blockade is the result of direct beta 2 vascular effects. Our data further suggest that there is little adrenergically mediated epicardial artery tone (either humoral or neural) at rest and that maximal vasodilator responses are not limited by adrenergically mediated vasomotor tone.     

Effect of adenosine on atrioventricular conduction. II: Modulation of atrioventricular node transmission by adenosine in hypoxic isolated guinea pig hearts

Adenosine as well as hypoxia and ischemia are known to cause atrioventricular conduction block. To test the hypothesis that adenosine is the primary mediator of hypoxia-induced atrioventricular conduction delay in isolated perfused guinea pig hearts, we characterized a) the time courses of hypoxia-induced adenosine release and delay in atrioventricular conduction, b) the relationships between oxygen tension, adenosine concentration in the effluent, and atria-to-His-bundle interval, and c) the adenosine receptor mediating the negative dromotropic effect of hypoxia. Oxygen tension and effluent adenosine levels were linearly related with a correlation coefficient (r) of -0.85 and a slope of -6.3 +/- 0.37 pmol/min/g/torr. Likewise, oxygen tension and atria-to-His-bundle interval prolongation were linearly related with r = -0.85 and a slope of -0.180 +/- 0.013 msec/torr. The EC50 of effluent adenosine in causing atria-to-His-bundle prolongation was 0.26 +/- 0.02 microM. Adenosine deaminase, an enzyme that deaminates adenosine to inosine and is limited to the extracellular space, significantly attenuated (61%) the atria-to-His-bundle interval prolongation caused by hypoxia. This prolongation was further reduced (81%) by a combination of adenosine deaminase and theophylline, an adenosine receptor blocker. Adenosine deaminase also reduced (by 95%) the atria-to-His-bundle interval prolongation in normoxic recipient hearts caused by the effluent of hypoxic donor hearts. Several adenosine antagonists, i.e., theophylline, 8-phenyltheophylline, and 8-(p-sulfophenyl)theophylline antagonized in a dose-dependent manner the negative dromotropic effect of exogenous adenosine and hypoxia. Schild analysis of the antagonism of hypoxia-induced atria-to-His-bundle interval prolongation by 8-(p-sulfophenyl)theophylline yielded the following pA2 values: 5.30 +/- 0.25 and 5.28 +/- 0.31 using oxygen tension and effluent adenosine vs. AH interval prolongation, respectively. 8-(p-Sulfophenyl)theophylline also antagonized to an equal extent atria-to-His-bundle interval prolongations of similar magnitude caused either by adenosine or hypoxia. We conclude that 1) adenosine is the primary mediator of hypoxia-induced atrioventricular conduction delay, and 2) the adenosine receptor that mediates the negative dromotropic effect of hypoxia is similar to that of exogenous adenosine.     

O2-sensitive K+ currents in carotid body chemoreceptor cells from normoxic and chronically hypoxic rats and their roles in hypoxic chemotransduction.

Carotid body-mediated ventilatory increases in response to acute hypoxia are attenuated in animals reared in an hypoxic environment. Normally, O2-sensitive K+ channels in neurosecretory type I carotid body cells are intimately involved in excitation of the intact organ by hypoxia. We have therefore studied K+ channels and their sensitivity to acute hypoxia (PO2 12-20 mmHg) in type I cells isolated from neonatal rats born and reared in normoxic and hypoxic environments. When compared with cells from normoxic rats, K+ current density in cells from hypoxic rats was significantly reduced, whereas Ca2+ current density was unaffected. Charybdotoxin (20 nM) inhibited K+ currents in cells from normoxic rats by approximately 25% but was without significant effect in cells from hypoxic rats. However, hypoxia caused similar, reversible inhibitions of K+ currents in cells from the two groups. Resting membrane potentials (measured at 37 degrees C using the perforated-patch technique) were similar in normoxic and hypoxic rats. However, although acute hypoxia depolarized type I cells of normoxic rats, it was without effect on membrane potential in type I cells from hypoxic animals. Charybdotoxin (20 nM) also depolarized cells from normoxic rats. Our results suggest that type I cells from chronically hypoxic rats, like normoxic rats, possess O2-sensing mechanisms. However, they lack charybdotoxin-sensitive K+ channels that contribute to resting membrane potential in normoxically reared rats, and this appears to prevent them from depolarizing (and hence triggering Ca2+ influx and neurosecretion) during acute hypoxia.     

Role of resistance and exchange vessels in local microvascular control of skeletal muscle oxygenation in the dog.

The effects of reduction in perfusion pressure, arterial hypoxia, muscle contraction, and adrenergic stimulation on the hindlimb muscle circulation were studied. Under normal conditions (venous PO2 greater than or equal to 40 mm Hg), oxygen delivery to the muscle was maintained mainly by large increases in the capillary exchange capacity and the oxygen extraction ratio in accord with tissue demand following the application of the above stresses. The participation of the resistance vessels under these conditions was minimal. The prevailing venous oxygen tension then was reduced by several means and the response of vascular resistance and capillary exchange capacity to the same stresses was reexamined. At the lower prevailing venous PO2, the sensitivity of the resistance vessels to metabolic and hemodynamic disturbances was greatly increased. Consequently, blood flow autoregulation, functional hyperemia, and hypoxic hyperemia were more intense when venous oxygen tension was low. In contrast, the contribution of exchange capacity was diminished, probably owing to the fact that most of the capillaries already are open at low venous PO2. These data suggest that the locus of local microvascular control of muscle oxygenation shifts from the normally more sensitive precapillary sphincters to the proximal flow-controlling arterioles as the prevailing venous oxygen tension falls. Yet, although the relative contribution of the resistance and exchange vessels to intrinsic regulation of tissue oxygenation is related to the prevailing venous oxygen tension, the two compensatory mechanisms operating in concert maintain tissue PO2 above the critical level over a wide range of stresses.