Effects of catecholamines, histamine, and nitroglycerin on flow, oxygen utilization, and adenosine production in the perfused guinea pig heart

Sex differences and the effects of ovariectomy on the β-adrenergic contractile response

The presence of sex differences in myocardial β-adrenergic responsiveness is controversial, and limited studies have addressed the mechanism underlying these differences. Studies were performed using isolated perfused hearts from male, intact female and ovariectomized female mice to investigate sex differences and the effects of ovarian hormone withdrawal on β-adrenergic receptor function. Female hearts exhibited blunted contractile responses to the β-adrenergic receptor agonist isoproterenol (ISO) compared with males but not ovariectomized females. There were no sex differences in β(1)-adrenergic receptor gene or protein expression. To investigate the role of adenylyl cyclase, phosphodiesterase, and the cAMP-signaling cascade in generating sex differences in the β-adrenergic contractile response, dose-response studies were performed in isolated perfused male and female hearts using forskolin, 3-isobutyl-1-methylxanthine (IBMX), and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP). Males showed a modestly enhanced contractile response to forskolin at 300 nM and 5 μM compared with females, but there were no sex differences in the response to IBMX or CPT-cAMP. The role of the A(1) adenosine receptor (A(1)AR) in antagonizing the β-adrenergic contractile response was investigated using both the A(1)AR agonist 2-chloro-N(6)-cyclopentyl-adenosine and A(1)AR knockout (KO) mice. Intact females showed an enhanced A(1)AR anti-adrenergic effect compared with males and ovariectomized females. The β-adrenergic contractile response was potentiated in both male and female A(1)ARKO hearts, with sex differences no longer present above 1 nM ISO. The β-adrenergic contractile response is greater in male hearts than females, and minor differences in the action of adenylyl cyclase or the A(1)AR may contribute to these sex differences.

Role of cellular acidosis in production of nitric oxide in canine ischemic myocardium

We tested the hypothesis that cellular acidosis modulates the production of nitric oxide (NO) in ischemic hearts. In canine hearts, we decreased coronary blood flow (CBF) to one third of the control by reduction of coronary perfusion pressure (105+/-3 to 41+/-5 mmHg), and thereafter we maintained CBF constant (89.8+/-1.6 to 30.0+/-0.5 ml/100 g/min) with an intracoronary administration of either saline, atropine, rauwolscine, HOE140, 8-sulfophenyltheophylline (8SPT), NaHCO3, or HOE642 (the inhibitor of Na+/H+ exchange). The cardiac NO levels defined as the differences of the nitrate and nitrite levels between coronary venous and arterial blood increased in the saline administration (2.9+/-0.2 to 12.7+/-1.7 micromol/l), and the extents of increases were identical in the condition of either saline, atropine, rauwolscine, HOE140 or 8SPT administration. In the condition with either NaHCO3 or HOE642, the increases in the cardiac NO levels were blunted (4.5+/-0.7 and 4.8+/-0.4 micromol/l, respectively). Cyclic GMP content of epicardial coronary artery in the ischemic area increased, which was also attenuated by either NaHCO3 or HOE642. We confirmed the acidosis-induced NO production in a more severe ischemic myocardium, and also showed that cellular acidosis produced by infusion of HCl increased NO production in non-ischemic myocardium. We conclude that cellular acidosis and subsequent activation of Na+/H+ exchanges modulate production of endogenous NO in canine ischemic myocardium.

Myocardial oxygen consumption modulates adenosine formation by canine right ventricle in absence of hypoxia

Myocardial adenosine formation varies with myocardial oxygen consumption (MVO(2)), but whether concurrent hypoxia is required for adenosine formation is uncertain. Changes in right coronary (RC) perfusion pressure (RCP) produce directionally similar alterations in right ventricular (RV) MVO(2)and in RC venous P O(2)(P(v)O(2)), an index of myocardial P O(2). RCP was varied in 10 anesthetized, open chest dogs to determine if, under these conditions, RV formation of adenosine would increase with MVO(2)in absence of myocardial hypoxia. Dialysis probes were implanted in the mid myocardium of RV free wall for collecting dialysate samples for HPLC analyses to estimate interstitial adenosine and other purines. Coronary venous blood was sampled from a superficial vein draining the RC artery (RCA) perfusion territory. At 115+/-3 mmHg baseline RCP, RC blood flow (RCBF)=0.51+/-0.04 ml/min/g, MVO(2)=4.6+/-0.5 ml/min/100 g, P(v)O(2)=34+/-1.5 mmHg, and dialysate adenosine=0. 27+/-0.03microM. When RCP was lowered to 61+/-1 mmHg by adjusting an occluder on the proximal RCA, RCBF decreased to 0.36+/-0.03 ml/min/g, MVO(2)fell to 3.7+/-0.4 ml/min/100 g, lactate uptake remained positive, P(v)O(2)fell to 30+/-1.7 mmHg, and dialysate adenosine decreased to 0.20+/-0.03microM. Reactive hyperemia of 1.25+/-0.13 ml/min/g was observed when the RCA constriction was released, although dialysate adenosine had fallen. When RCP was elevated to 164+/-2 mmHg by inflating a balloon catheter in the descending aorta, RCBF increased to 0.70+/-0.06 ml/min/g, MVO(2)increased to 5.8+/-1. 0 ml/min/100 g, P(v)O(2)rose to 39+/-2.3 mmHg, and dialysate adenosine increased to 0.33+/-0.04microM. These data indicate that (1) RV oxygen demand varies with RCP; (2) if RV ischemia is absent, myocardial adenosine formation is modulated by MVO(2), with no requirement for hypoxia; (3) pressure-flow autoregulation is relatively ineffective in the RC circulation, where adenosine does not mediate and may even blunt autoregulation.

Comparison of coronary hemodynamics during infusions of dobutamine and adenosine in patients with angina pectoris

Pharmacologic stress tests are used to evaluate the significance of coronary arterial stenosis, and adenosine and dobutamine are often used for this purpose. The aim of this study was to examine coronary hemodynamics in 16 patients with stable exertional angina who had organic stenosis (> 90%) in the left anterior descending coronary arteries and in 13 control subjects at baseline and during separate intravenous infusions of dobutamine (20 micrograms/kg per min [corrected]) and adenosine (140 micrograms/kg per min [corrected]). Measurements of great cardiac vein blood flow and lactate and oxygen contents in the blood from the aortic root and great cardiac vein were analyzed in patients with ischemic signs of myocardial ischemia (ST-segment depression in the precordial ECG leads and/or myocardial lactate production) during the infusions (in 11 patients during adenosine infusion and in 12 patients during dobutamine infusion) and in control subjects. Myocardial oxygen consumption during adenosine infusion did not change compared with the baseline in both groups, whereas it increased during dobutamine infusion in both groups (p < 0.01 compared with the baseline in each case). However, great cardiac vein blood flow was significantly increased compared with the baseline during both infusions in both groups (p < 0.01 compared with baseline in each case), and the oxygen contents of great cardiac vein blood, which contains effluent from the ischemic region, showed no decrease but increased significantly during both infusions in the patients in comparison with control subjects. Although the global oxygen supply was excessive, myocardial ischemia occurred during dobutamine and adenosine infusions in the patients. As myocardial oxygen consumption remained unchanged during adenosine infusion, regional myocardial flow reduction, possibly as a result of flow maldistribution, may contribute, at least partly, to adenosine-induced myocardial ischemia. An increase in myocardial oxygen consumption may be the principal cause of myocardial ischemia during dobutamine infusion, but an inappropriate flow increase in normal myocardium may occur during dobutamine-induced ischemia.

Adenosine attenuation of isoproterenol-stimulated adenylyl cyclase activity is enhanced with aging in the adult heart

Interstitial levels and release of adenosine have been shown to be greater for aged adult hearts compared to young adult hearts. Furthermore, blockade of A1 adenosine receptors in the aged adult heart prevents the reduced contractile and metabolic response to isoproterenol. The aim of this study was to determine whether there is an enhanced antiadrenergic effect of adenosine in the aged adult heart. Ventricular membranes from young and aged adult hearts were incubated in the presence of isoproterenol (ISO) and phenylisopropyladenosine (PIA) either alone or in combination. Basal and ISO-enhanced adenylyl cyclase activity were significantly reduced in the membranes from aged rats. PIA alone, at 0.1 nM to 100 microM, had no direct effect on basal adenylyl cyclase activity in membranes from either group. In the presence of either 100 nM or 1 microM ISO, 100 microM PIA significantly attenuated ISO-enhanced adenylyl cyclase activity to a greater extent in the aged adult heart membranes (78 or 48% for the aged vs. 37 or 25% for the young). Moreover, in the presence of 100 nM ISO the IC50 for the PIA concentration response curve was shifted to the left for the aged ventricular membranes as compared to the membranes from young adults (1.62 x 10(-7) M vs 1.5 x 10(-6) M, aged vs young, respectively). The enhanced inhibition of adenylyl cyclase is associated with an increase in adenosine A1 receptor density (23.7 +/- 3.5 vs 14.7 +/- 1.7 fmol/mg, aged vs young) and Kd (6.1 +/- 1.7 vs 2.2 +/- 0.5 nM, aged vs young) in the aged adult heart membranes as determined by [3H]DPCPX binding. These results suggest that the reduced response to catecholamines in the aged adult heart may be due, at least in part, to an enhanced expression of the antiadrenergic effect of adenosine on beta-adrenergic receptor mediated activation of adenylyl cyclase.

Adenosine is not the sole vasodilator released on exposure of isolated guinea-pig hearts to histamine and isoprenaline

1. The role of adenosine in the control of coronary vessel tone was studied using two guinea-pig hearts in series. Vasodilatory mediator(s) released by the donor heart were assayed by the recipient heart. 2. Adenosine deaminase reduced the activity of vasodilatory mediator(s) released by histamine and isoprenaline by 58.0 +/- 14.4% and 80.5 +/- 5.9% respectively; responses to exogenous adenosine were abolished. 3. Adenosine deaminase added to the donor perfusate supplying the recipient heart caused a significant increase in basal perfusion pressure. 4. Our results suggest adenosine is not the sole mediator of vasodilation in stressed hearts and additionally unstressed hearts appear to release a basal vasodilatory level of adenosine.

Theophylline inhibits isoproterenol-induced coronary dilatation in the isolated perfused rat heart

To study the role of endogenous adenosine in coronary dilatation induced by beta-adrenergic stimulation, isolated rat hearts were perfused at a constant flow rate. Perfusion of the heart with isoproterenol caused increases in the heart rate, left ventricular developed pressure, coronary dilatation, and release of adenosine and its degradation products. Theophylline inhibited isoproterenol-induced coronary dilatation without any significant effects on other contractile properties such as heart rate and left ventricular developed pressure. Coronary dilatation induced by exogenous adenosine was also inhibited by theophylline. These results suggest that beta-adrenergic stimulation induces coronary vasodilatation not only through direct beta-adrenergic responses, but also through increases in extracellular adenosine.

Increased myocardial adenosine production and reduction of beta-adrenergic contractile response in aged hearts

The contractile response of the aged adult heart to beta-adrenergic stimulation is known to be reduced compared with the young adult heart. Since endogenous adenosine exerts an antiadrenergic action in the heart, this study was undertaken to determine if the basal endogenous level of myocardial adenosine increases with age and whether this increase mediates the reduced responsiveness of aged heart to beta-adrenergic stimulation. Young (3-5 months) and aged (12-22 months) Sprague-Dawley adult rat hearts of CD and SD stock were perfused at constant pressure and paced at 270 contractions/min. The two age groups had a similar level of +dP/dtmax (index of contractility) under control conditions. Adenosine release into the coronary effluent was 30 +/- 3 nmol/min/g dry wt from young and 54 +/- 9 nmol/min/g dry wt from aged hearts. Inosine release was also greater from the aged hearts. Isoproterenol (10(-8) M) stimulation increased contractile state by 113% in young hearts and only 69% in aged hearts. Isoproterenol further increased the adenosine and inosine release from both age groups. Theophylline (5 x 10(-5) M), an adenosine antagonist, prevented the difference in the contractile response to isoproterenol stimulation between the young and aged hearts. Elevation of external calcium from 2 to 4 mM increased contractility equally in both age groups without influencing adenosine release. Myocardial oxygen consumption, coronary effluent PO2, oxygen supply-demand ratio, and lactate release were similar for both age groups, indicating that under the conditions studied the elevated release of adenosine by the aged hearts was not due to hypoxia. Aged (10-14 months) adult guinea pig hearts also displayed a reduced responsiveness to the isoproterenol stimulation and released more adenosine compared with young (3-4 months) adult guinea pig hearts. These findings suggest that enhanced adenosine levels that are present in the aged myocardium are responsible, in part, for the reduced contractile responsiveness of the older adult heart to beta-adrenergic stimulation.

Influence of beta-adrenergic stimulation and contraction frequency on rat heart interstitial adenosine

Adenosine (ADO) has an antiadrenergic action in the heart that causes an attenuation of contractile and metabolic responses elicited by beta-adrenergic stimulation. The effect of an increase in oxygen consumption elicited by either beta-adrenergic stimulation or an increase in contraction frequency on interstitial fluid and coronary effluent ADO levels was investigated in isolated perfused isovolumically contracting rat hearts. ADO in left ventricular surface transudates and coronary effluents was rendered fluorescent with chloroacetaldehyde, and the formed ethenoadenosine derivative was quantitated with high-performance liquid chromatography fluorescence detection. Heart preparation integrity was verified by determining the activities of lactate dehydrogenase and ADO deaminase in the transudates. Isoproterenol (10(-8) M) elicited a 45% increase in oxygen consumption and a 54% increase in developed left ventricular pressure in hearts paced at 240 beats/min. With isoproterenol the control transudate ADO concentration (304 pmol/ml) increased 493%, and the control effluent ADO concentration (48 pmol/ml) increased 259%. Increasing the contraction frequency from 180 to 300 beats/min in the presence of 10(-6) M propranolol increased oxygen consumption by 45% and decreased left ventricular pressure by 29%. With the increase in contraction frequency, the transudate ADO concentration did not increase significantly. However, the ADO concentration in the effluent was an average of 269% greater in hearts contracting at the higher frequency. Increasing the contraction frequency of hearts treated with both 10(-6) M propranolol and 10(-5) M atropine also had no significant effect on the level of transudate ADO. The effluent level of ADO increased only 78%. Levels of ADO in transudates were not significantly affected by mesothelial cell metabolism. These results suggest that the beta-adrenergic stimulation the interstitial level of ADO in the heart increases to levels that are sufficient to manifest its antiadrenergic effects. Furthermore, there is not always a correlation between the levels of ADO found in the interstitial and effluent fluid compartments.

Beneficial effects of alpha 2-adrenoceptor activity on ischemic myocardium during coronary hypoperfusion in dogs

We have previously reported that alpha 2-adrenoceptor stimulation enhances adenosine-induced coronary vasodilation. In the present study, we tested the hypothesis that alpha 2-adrenoceptor activity exerts beneficial effects on myocardial ischemia through augmentation of vasodilatory effects of released adenosine. In open-chest dogs, the left anterior descending coronary artery was perfused through an extracorporeal bypass tube from the carotid artery. Propranolol was infused into the bypass tube to exclude the metabolic effects of norepinephrine. When clonidine (0.24 micrograms/kg/min i.c.) was infused for 10 minutes after reduction of coronary blood flow by partial occlusion of the bypass tube, coronary blood flow was increased by 43% from 27 +/- 1 ml/100 g/min despite no changes in coronary perfusion pressure (38 +/- 5 mm Hg) and a slight decrease in adenosine release. Both fractional shortening and lactate extraction ratio of the perfused area were significantly improved (fractional shortening, 1.8 +/- 1.0 to 10.9 +/- 1.5%, p less than 0.001; lactate extraction ratio, -57.8 +/- 6.5 to -31.9 +/- 2.4%, p less than 0.005). Identical results were observed in the denervated hearts, indicating that the beneficial effect of clonidine is not attributed to the prevention of norepinephrine release from the sympathetic nerve terminals. The beneficial effects of clonidine were prevented by yohimbine, an alpha 2-adrenoceptor blocking agent. An adenosine receptor antagonist, 8-phenyltheophylline, also prevented the beneficial effects of clonidine, indicating that these beneficial effects are mediated by effects of adenosine. Furthermore, the extent of augmentation of coronary flow in the ischemic heart was coincided with that of augmentation of exogenous adenosine-induced hyperemic flow (40%) by clonidine. Production of cyclic AMP in the coronary artery during myocardial ischemia was augmented by clonidine. In 12 other dogs, myocardial ischemia was produced by intracoronary embolization of microspheres (15 microns in diameter). Clonidine enhanced (39%) the hyperemic coronary flow and improved both fractional shortening and lactate extraction ratio. Thus, we conclude that alpha 2-adrenoceptor stimulation can ameliorate myocardial ischemia mainly due to enhancement of vasodilatory effects of adenosine released from the ischemic myocardium.

Adenosine production and energy metabolism in ischaemic and metabolically stimulated rat heart

Adenosine may modulate blood flow and electrical activity in heart in response to changes in myocardial energy metabolism. In the present study, 31P NMR spectroscopy was used to examine the relation between cytosolic phosphate metabolite levels and release of adenosine into the venous effluent of isovolumic heart during graded low-flow ischaemia or metabolic stimulation with isoproterenol. When coronary flow rate was varied in steps between 1.6 and 12 ml/min/g, cytosolic ATP levels did not change significantly but the phosphorylation potential exhibited a linear correlation with flow rate below approximately 7 ml/min/g. Purine release (adenosine and inosine) correlated linearly with the cytosolic phosphorylation potential and free AMP concentration. Metabolic stimulation of hearts with isoproterenol (0.4, 3.0, and 60 nM), produced a significant fall in cytosolic ATP levels and decreased the cytosolic phosphorylation potential. Purine release in these hearts increased exponentially as the cytosolic phosphorylation potential dropped, and as cytosolic free AMP increased. These results support a link between the phosphorylation potential and the mechanism of adenosine production during ischaemia and metabolic stimulation. Presumably, this link is the activity of the enzyme 5'-nucleotidase, which is responsible for converting AMP to adenosine, together with the concentration of its substrate, AMP. In low-flow ischaemia, cytosolic AMP may control adenosine formation. With isoproterenol stimulation, a more complex relationship exists, indicating possible allosteric regulation of the enzyme(s) responsible for adenosine formation, in addition to changes in AMP concentration.

Effect of adenosine on histamine release and atrioventricular conduction during guinea pig cardiac anaphylaxis

Anaphylactic events occurring in cardiac tissue can result in severe metabolic imbalances. The present study addresses the question of whether adenosine, produced in response to this stress, influences either the antigen-antibody-induced alterations in cardiac function or the release of histamine, which is known to be one of the important mediators of the anaphylactic reaction. Isolated hearts of passively sensitized guinea pigs were perfused at constant flow in a Langendorff preparation with physiological salt solution. Under control conditions, antigen challenge evoked a rapid transient release of histamine, an increase in coronary vascular resistance and beating rate, and an increase followed by a decrease in left ventricular systolic pressure. The antigen-induced transient increase in adenosine release from 0.26 +/- 0.07 to 4.66 +/- 0.48 nmol/min/g was associated with a 75 +/- 9% increase in the PR interval in all hearts and atrioventricular blocks in six of 17 hearts. Antigen challenge was also conducted in the presence of theophylline, 8-(4-sulfophenyl) theophylline (SP-T), erythro-9-(2-hydroxy-3-nonyl) adenosine hydrochloride (EHNA), or exogenous adenosine. The major findings were that 1) the antigen-induced prolongation of the PR interval was attenuated by the adenosine receptor blockers theophylline (to 23 +/- 6%) and SP-T (to 15 +/- 4%); 2) the incidence of antigen-induced atrioventricular blocks tended to be decreased by theophylline (to three of 10 hearts) and SP-T (to zero of seven hearts) and to be increased by the adenosine deaminase inhibitor, EHNA (to six of 10 hearts); 3) none of the interventions had major influences upon antigen-induced alterations in vascular resistance, atrial automaticity, or systolic pressure; and 4) EHNA and adenosine both significantly increased adenosine levels before anaphylaxis and also enhanced the total histamine release induced by antigen challenge from a control value of 2,321 +/- 244 ng/g to 3,424 +/- 307 ng/g and 4,298 +/- 616 ng/g, respectively. We conclude from our data that increases in levels of endogenous adenosine during cardiac anaphylaxis may contribute to the development of atrioventricular conduction delays and blocks and that increases in levels of adenosine before antigen challenge may increase the amount of histamine released during cardiac anaphylactic reactions.

Adenosine release by the isolated guinea pig heart in response to isoproterenol, acetylcholine, and acidosis: the minimal role of vascular endothelium

The objective of this study was to determine the contribution of endothelial cells to adenosine appearing in venous effluent of isolated perfused guinea pig hearts. The adenine nucleotide pool of endothelial cells was selectively labelled by infusing 3H-adenosine (5 X 10(-8) M) into the heart for 30 minutes. Selective labelling of the endothelial adenine nucleotides was confirmed by measuring the relative specific activities of the nucleotides of coronary endothelial cells (removed from the heart by enzyme treatment). Endothelial ATP, ADP, and AMP had relative specific activities that were 49, 25, and 7 times higher, respectively, than their nucleotide counterparts in total myocardial tissue. Isoproterenol increased the release of both total adenosine and radioactive adenosine, but the relative specific activity of venous adenosine decreased dramatically. Acetylcholine, at a concentration that caused no change in left ventricular pressure but caused a decrease in coronary vascular resistance, increased the release of total adenosine. However, both radioactive adenosine release and the relative specific activity of venous effluent adenosine were decreased with acetylcholine. Infusion of hydrochloric acid caused a sustained reduction in left ventricular pressure and coronary vascular resistance. Total adenosine release fell within one minute and remained reduced during HCl. Radioactive adenosine release was elevated at 15 seconds but fell below control values at 2 minutes and remained reduced during steady-state acidosis. We conclude that the majority of the adenosine released in response to isoproterenol and acetylcholine originates from an unlabelled compartment, most likely the myocytes. Acidosis results in decreased release of adenosine from both the labelled endothelium and the unlabelled cells in the heart.(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

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 radioimmunoassay, 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)

Regulation of adenosine formation by the heart

We think that the available data on adenosine formation suggest the two signals are responsible for adenosine release from cardiac myocytes: (1) the ratio of oxygen supply to demand and (2) agonist-triggered release of extracellular adenine nucleotides. We do not believe that the available data support the oxygen consumption hypothesis. The few studies which allow us to judge the relative importance of these two signals suggest that both hypoxia and sympathetic nerve stimulation release adenosine primarily by decreasing O2 supply:demand. Agonist triggered nucleotide release may be quantitatively important in situations in which decreased O2 supply/demand cannot explain increased release, i.e., isoproterenol and acetylcholine administration.

Phasic release of adenosine during steady state metabolic stimulation in the isolated guinea pig heart

If adenosine is the major factor responsible for myocardial metabolic vasodilation, its release should be sustained as long as oxygen consumption and coronary flow are augmented. To see if adenosine meets this criterion, we examined the time course of its release during norepinephrine infusion in isolated, non-working guinea pig hearts (n = 8). During an 11-minute infusion period (steady state perfusate concentration = 6 X 10(-8) M), the coronary effluent was collected over 30-second intervals for measurements of coronary flow (ml/min per g), and adenosine and inosine release (pmol/min per g). Myocardial oxygen consumption (MVO2 = microliter O2/min per g) was measured at 1, 4, 6.5, and 11 minutes. Control values of coronary flow, myocardial oxygen consumption, and adenosine and inosine release were 7.5 +/- 0.4, 85 +/- 5, 22 +/- 5, and 431 +/- 39, respectively. During norepinephrine infusion, coronary flow, myocardial oxygen consumption, and adenosine release attained maximal levels within one minute (inosine within 2 minutes). These values were 10.6 +/- 0.4, 125 +/- 9, 849 +/- 110, and 2595 +/- 581, respectively. Thereafter, coronary flow and myocardial oxygen consumption values were sustained. In contrast, adenosine and inosine release significantly declined to nadirs by 9.5 minutes. Thereafter, steady state levels were maintained at 117 +/- 24 and 960 +/- 294, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurements of coronary plasma and pericardial infusate adenosine concentrations during exercise in conscious dog: relationship to myocardial oxygen consumption and coronary blood flow

A conscious dog model was employed to evaluate the relationship among myocardial oxygen consumption (MVO2), myocardial adenosine release and coronary blood flow (CNF) during graded treadmill exercise. Two methods were utilized simultaneously as indexes of changes in interstitial adenosine concentrations, (1) a pericardial infusate technique and (2) to measurement of arterial-coronary sinus plasma adenosine concentration differences. Graded exercise was associated with graded increases in MVO2, CBF, pericardial infusate adenosine concentration (PI Ado) and adenosine washout in coronary plasma. Regression analysis demonstrated significant linear relationships for MVO2 v. CBF (r = 0.78, P less than 0.01), MVO2 v. PI Ado (r = 0.52, P less than 0.01), and PI Ado v. CBF (r = 0.76, P less than 0.01). Coronary plasma adenosine arterio-sinus differences, sinus plasma concentrations, and net washout of adenosine also increased with graded exercise, however, a significant inter-animal variance was noted. These data suggest that the plasma adenosine assay is capable of detecting directional (qualitative) changes associated with changes in cardiac metabolism, however, it may not be reliable as a quantitative indicator of interstitial adenosine concentrations due to multiple factors which may influence the plasma adenosine pool. The pericardial infusate technique, which presumably represents a model of diffusion, is relatively invariant by comparison. The results demonstrate a significant correlation among MVO2, PI Ado and CBF, and thereby provide support to the hypothesis that adenosine is a major factor in the coupling of myocardial oxygen demand to oxygen supply under physiological conditions.

Consistent parallel relationships among myocardial oxygen consumption, coronary blood flow, and pericardial infusate adenosine concentration with various interventions and beta-blockade in the dog

Coronary blood flow responds uniquely to changes in myocardial demand, regardless of the stimulus. If adenosine mediates this response, interstitial fluid adenosine concentration should also change in parallel with myocardial oxygen consumption and coronary blood flow during alterations of cardiac work. We tested this hypothesis by measuring coronary blood flow, myocardial oxygen consumption, and the concentration of adenosine in pericardial infusates, an index of interstitial fluid adenosine concentration, during six experimental conditions and control states in anesthetized, open-chest dogs. Significant alterations of myocardial oxygen consumption and coronary blood flow during aortic constriction, vagal stimulation, atrial pacing, or intravenous infusion of calcium chloride, norepinephrine, or isoproterenol were accompanied by significant alterations in pericardial infusate adenosine concentration. Significant linear relationships were determined among myocardial oxygen consumption, coronary blood flow, and pericardial infusate adenosine concentration for each of the experimental stimuli and their paired control values. There were no significant differences among the six different conditions for any of these relationships. In addition, these relationships were not altered by beta-blockade in five dogs subjected to aortic constriction and calcium infusion. Although beta-blockade may alter the effects of a stimulus, myocardial oxygen consumption, coronary blood flow, and adenosine all are affected proportionately. The results suggest that adenosine production responds to alterations of myocardial oxygen consumption independently of the stimulus which produces the change in oxygen demand, and the resultant change in interstitial fluid adenosine concentration may initiate the change in coronary blood flow to maintain the balance between oxygen supply and demand.

Relationship between myocardial oxygen consumption, coronary flow, and adenosine release in an improved isolated working heart preparation of guinea pigs

We describe herein an isolated working heart preparation of guinea pigs, in which coronary perfusion pressure can be varied independently from afterload by directing left ventricular stroke volume into an artificial circulation. This preparation proved to be functionally stable, exhibited hemodynamic features characteristic of the heart in situ, and shows the phenomena of flow autoregulation, reactive hyperemia, and hypoxic and metabolic vasodilation. Myocardial oxygen consumption and coronary flow were tightly coupled when cardiac work was enhanced by either 1.5-6.0 X 10(-9) M isoproterenol (r = 0.975) or changes of afterload (20-100 mm Hg) (r = 0.890). Isoproterenol-induced changes in adenosine release correlated with changes of coronary flow (r = 0.869) and myocardial oxygen consumption (r = 0.894). The concentrations of endogenously formed adenosine were within the vasodilatory range of exogenously applied adenosine. In contrast, afterload-induced changes in myocardial oxygen consumption were not associated with an enhanced release of adenosine, inosine, and hypoxanthine, and did not correlate with coronary resistance (r = 0.422). The specific activity in the effluent perfusate of intracoronarily infused [8-14C]adenosine was increased with elevated afterload, suggesting that less adenosine was liberated by the heart. Our findings indicate that adenosine formed in response to beta-adrenergic stimulation is a major metabolite adjusting coronary flow to myocardial needs. Adenosine, however, does not appear to be involved in the afterload-induced changes in coronary flow when coronary perfusion pressure and, thus, oxygen supply are increased simultaneously. It is likely that formation of adenosine is not triggered by changes in MVO2 as such, but may critically depend on the oxygen supply: demand ratio.

Mechanism of adenosine inhibition of catecholamine-induced responses in heart

The properties of adenosine inhibition of catecholamine-induced responses were investigated, using an isolated rat heart preparation. Perfusion of hearts with 0.1 microM isoproterenol increased myocardial cAMP content 2.8-fold, activation of cAMP-dependent protein kinase 4.4-fold, phosphorylase a formation 3.4-fold, left ventricular pressure 1.8-fold, rate of ventricular pressure development 2.1-fold, and rate of ventricular relaxation 2.2-fold within 1 minute. When perfused with the isoproterenol, 10 microM adenosine reduced the catecholamine-produced increase in cAMP, cAMP-dependent protein kinase, and phosphorylase by 30-40%, and the elevation in left ventricular pressure and rate of ventricular pressure development by 40-70% within 40 seconds. More than 2 minutes were required for the nucleoside to significantly reduce the isoproterenol-elicited increase in the rate of ventricular relaxation. Perfusion of adenosine alone at concentrations from 0.1 to 10 microM were without effect on the above parameters. Theophylline at 50 microM had no effect alone on the above parameters but blocked the inhibitory actions of adenosine on the isoproterenol-induced responses. In the presence of 15 mM Mg++ adenosine reduced by approximately 56% the 2-fold increase in myocardial membrane adenylate cyclase activity produced by 1 microM isoproterenol without affecting basal or fluoride-stimulated activity. Adenosine also reduced the isoproterenol-induced increase in enzyme activity assayed at 1-2 mM Mg++, a level that more closely approximates the intracellular activity of the ion. The results suggest that physiological concentrations of adenosine attenuate the catecholamine-induced increase in cAMP content, cAMP-dependent protein kinase activation, phosphorylase a formation, and contractile parameters in the working heart, via reducing the beta-adrenergic activation of adenylate cyclase.

Measurements of oxygen tension in perfusates from guinea pig isolated hearts and the demonstration of coronary vasodilator material

The release of coronary vasodilator mediator(s) from guinea pig isolated hearts was demonstrated by a technique employing donor and recipient hearts perfused in series. Vasodilator mediator release followed the positive inotropic, chronotropic, and coronary vasodilator responses of donor hearts to isoprenaline and their exposure to hypoxia. Isoprenaline-induced coronary vasodilatation of donor hearts was accompanied by a fall in perfusate pO2, indicative of a raised myocardial oxygen consumption. Measurement of perfusate pO2 levels showed that the regassing procedure was adequate and the fall in pO2 level no longer occurred after isoprenaline. However, at high resolution, small changes in pO2 could still be detected by these were not sufficient to induce vasodilatation of the recipient. The technique was adapted to enable simultaneous measurement of perfusate pO2 and release of vasodilator mediator following administration of isoprenaline. The value of this in examining the relationship between myocardial oxygen consumption and vasodilator mediator release is discussed.

Catecholamine-induced vasodilator metabolite release from guinea-pig hearts is not due to increased myocardial activity

Guinea-pig isolated hearts were perfused in series, the donor heart perfusate supplying the recipient heart. Isoprenaline increased the rate and force of contraction and oxygen consumption of donor hearts and produced a coronary vasodilatation. This was accompanied by the release of vasodilator metabolite as demonstrated by vasodilatation of the recipient heart, the beta-adrenoceptors of which were antagonized by propranolol. During arrest of donor hearts by either carbachol or application of a fibrillating current, isoprenaline still released vasodilator metabolite and increased oxygen consumption but without changes in rate or tension. This release was prevented by beta-adrenoceptor blockade. It is concluded that the sympathomimetic-induced coronary vasodilatation is mediated via the release of a vasoactive metabolite, the trigger for which is not the concomitant mechanical hyperactivity.

Inhibitory action of adenosine on histamine- and dopamine-stimulated cardiac contractility and adenylate cyclase in guinea pigs

Infusion of adenosine into the coronary arteries of isolated guinea pig hearts produced a dose-dependent inhibition of cardiac contractile force development elicited by bolus injections of histamine (7.5 X 10(-9) mol) or dopamine (1.5 X 10(-8) mol). Threshold concentration of adenosine was 10(-7) M and maximal inhibition (90%) was obtained at 3 X 10(-5) M. Adenosine in the effective concentration range did not alter Ca2+-induced increases in contractile force. The rise in tissue levels of cAMP induced by equieffective doses of histamine (7.5 X 10(-9) mol) and dopamine (1.5 X 10(-8) mol) was inhibited by adenosine (3.5 X 10(-5) M) by about 60%. In a particular membrane preparation of guinea pig ventricles the adenylate cyclase activity stimulated by the histamine (10(-5) M) and dopamine (10(-4) M) was inhibited in a dose-dependent manner by adenosine. This effect could be reversed by theophylline (5 X 10(-5) M) in a competitive manner. The hormone-insensitive adenylate cyclase of a Lubrol-PX solubilized membrane preparation stimulated by NaF or 5'-guanylylimido-diphosphate (GppNHp) was also inhibited by adenosine (40% and 90% inhibition at 10(-5) M and 10(-4) M, respectively). Adenosine did not influence the Km value of the adenylate cyclase for ATP, but markedly lowered Vmax of the enzyme. From additional studies with purine-substituted (N6-methyl-adenosine, N6-phenylisopropyl-adenosine) and ribose-substituted (2'-deoxy-adenosine and arabino-furanosyl-adenine) adenosine analogues, we conclude that adenosine may inhibit the inotropic responses to hormones as well as the adenylate cyclase activity by specifically interacting with at least two different sites associated with the adenylate cyclase.

Relationship of cardiac oxygen usage, adenosine content, and coronary resistance in dogs

Analysis of the relationships between cardiac oxygen usage (MVO2), cardiac muscle adenosine levels([Ado]), and coronary vascular resistance (R) in open-chest, anesthetized dogs tested the hypothesis that adenosine is a physiological regulator of coronary flow. Experiments using each dog as its own control showed that [Ado] varied directly with MVO2 as the latter changed spontaneously or in response to atrial pacing, paired pacing, aortic constriction, of beta-adrenergic blockade. In turn, R varied inversely with changes in [Ado]. Stimulating MVO2 with isoproterenol significantly increased the slope of the regression of [Ado] on MVO2 as well as of R-1 on [Ado]. The effect of beta-adrenergic stimulation on [Ado] is unexplained, but its effect on R-1 seems to reflect the combined effects of adenosine and direct beta-adrenergic coronary relaxation. These results support the hypothesis that adenosine mediates the coronary flow responses to changes in MVO2.

Prostacyclin-induced coronary vasodilation. Interactions with adenosine, cyclic AMP and energy charge in the rat heart in vitro

The action of prostacyclin (PGI2) on several biochemical and physiological parameters of myocardial function and coronary perfusion was studied in the rat heart in vitro, perfused according to Langendorff either at constant pressure (65 mmm Hg) or at constant volume (8 ml/min). PGI2 dose-dependently decreased coronary vascular resistance, the ED50, being 5--10 nM, whereas that of adenosine was 550--650 nM. PGI2 (30 nM) did not alter the myocardial energy-rich phosphate content but diminished myocardial cAMP by 23% and adenosine release by 61%. In contrast to adenosine, the coronary dilating activity of PGI2 was not inhibited by isobutyl-methyl-xanthine (MIX; 0.4 microM). There were no direct effects of PGI2 on myocardial contractile force, oxygen consumption or heart rate. The results provide evidence for (1) a direct powerful coronary dilating activity of PGI2 which appears to be independent of adenosine, (2) the absence of any direct activity of the compound on myocardial contraction and energy charge.