Progressive vasomotor changes in ischaemic myocardium

Under certain conditions, a progressive increase in vascular resistance occurs within ischaemic myocardium during the first three hours after coronary artery stenosis. Measurements of vasodilator reserve in the ischaemic region demonstrated that this is at least partly due to an increase in vascular smooth muscle tone. Two hypotheses were suggested as an explanation: release of vasoconstrictors within the ischaemic area, or a decreasing release of vasodilators. Potential coronary constrictors considered included norepinephrine, PGF2 alpha, thromboxane A2, and high K+. Each of these substances was eliminated as the source of the vasoconstriction by pharmacological studies on ischaemic canine hearts. Measurement of adenosine release from isolated guinea pig hearts provided support for the possibility that the vasoconstriction results from a decreasing release of metabolic vasodilators. Throughout the period of ischaemia, both blood flow and myocardial function were far below the levels that could have been achieved in the presence of the stenosis. We conclude that during moderate ischaemia, myocardial function and blood flow are linked in a positive feedback cycle which promotes reduced ventricular function and coronary blood flow.

Cellular and molecular actions of adrenomedullin in glomerular mesangial cells

Adrenomedullin (AM), a potent vasodilatory and hypotensive peptide produces several biological outcomes in glomerular mesangial cells. Mesangial cells are important in the pathogenesis of glomerulonephritis, and therefore the actions of AM on mesangial cells have important clinical and therapeutic implications. This minireview describes the various actions of AM on mesangial cell function and the signal transduction mechanisms involved. As in other systems, most actions of AM can be explained by increase in cAMP levels in the cell, although a few exceptions remain. The fact that most data obtained to date has been in culture, the physiological significance of the actions of AM in mesangial cells is discussed.

FK506 promotes adenosine release from endothelial cells via inhibition of adenosine kinase

The immunosuppressants, cyclosporin A and tacrolimus (FK506) induce an increase in plasma levels of adenosine and mimic ischemic preconditioning. However, the mechanism of action of the two drugs on adenosine metabolism is not clear. Since inhibition of adenosine kinase promotes an increase in endogenous adenosine release, we tested a hypothesis that FK506 induces adenosine release via inhibition of adenosine kinase activity. In cultured endothelial cells, FK506 enhanced release of tracer adenosine and inhibited uptake of tracer adenosine. It also reduced adenosine kinase activity of the cell membrane fraction. In addition, FK506 does not inhibit membrane transport of tracer adenosine. These observations indicate that FK506 inhibits in situ adenosine kinase activity in endothelial cells. Other cell signaling inhibitors were found to inhibit adenosine uptake via inhibition of adenosine transport. In conclusion, FK506 promotes adenosine release from endothelial cells by a novel mechanism involving inhibition of adenosine kinase activity associated with the membrane.

Microalbuminuria in urban Zimbabwean women

The prevalence of microalbuminuria (MAU) in African populations has not been reported, nor has the relationship between MAU and hypertension been reported for these populations. We collected spot urine samples from 370 women, 25 years and older as a part of a population-based, cross-sectional blood pressure survey in an urban community in Zimbabwe and analysed the samples for albumin and beta2-microglobulin. The age-adjusted prevalence of hypertension was 30% for women 25 years and older in this community. After excluding the samples with hematuria (11%), the prevalence of MAU (3.0 < or = albumin-to-creatinine ratio (ACR, mg/mmol) <25.0) in the study population was 9%. When age-adjusted to the population in the community, the prevalence was 8% among women 25 years and older. The prevalence of MAU was substantially higher in hypertensive (HT) than in normotensive (NT) women (16% vs 4%, P<0.001). A significantly higher level of log ACR in HT was found in each age group except the youngest age group (age 25-34). In age-adjusted multiple regression, percent fat mass was negatively associated with log ACR (beta = -1. 18, 95% CI (-0.23, -2.21), P = 0.02). In a similar regression analysis, higher log beta8-microglobulin-to-creatinine ratio was very strongly associated with higher log ACR (beta = 0.34, 95% CI (0.25, 0.43), P<0.0001) and significantly associated with lower percent fat mass (beta = -1.02, 95% CI (-0.25, -1.8), P = 0.01). These results suggest that MAU is frequently caused by hypertension, but that other diseases may contribute to its presence.

Economic development and women's blood pressure: field evidence from rural Mashonaland, Zimbabwe

A survey of 515 non-pregnant women at 12 geographically chosen research sites in rural Mashonaland shows significant differences in mean blood pressure, controlled by age cohorts. Three levels of economic development are identified: (1) the traditional economy on communal lands, with lowest blood pressure, (2) the wage economy in areas of large-scale commercial agriculture, with elevated blood pressure and (3) the wage economy in mining areas, with the highest elevation of blood pressure. The area is dominated by the primate city, Harare, up to distances of 300 km and beyond, from which forces of change and modernization emanate. It is seen that potassium, sodium and the sodium potassium ratio, are distance-related to Harare and that women's blood pressures tend to follow suit. The rise of body sodium in young persons at risk, often accompanied by declining potassium intake and other changes of modernization, suggest that more attention should be focused on rural areas in Africa, now in the throes of economic change.

Learning the regulation of peripheral blood flow

Students can learn a great deal about the peripheral circulation when teaching is based on five building blocks: hemodynamic principles, neurohumoral control, and three elements of local control of blood flow (metabolic, myogenic, and paracrine). Study of a particular special circulation starts with the application of these building blocks in the context of the function of that tissue. For example, control of skin blood flow is largely concerned with regulation of body temperature (neurohumoral control) and the response to injury (paracrine control). Regulation of coronary blood flow is almost entirely a matter of meeting the metabolic needs of the myocardium (metabolic control). By mixing and matching the five building blocks and keeping in mind the special functions of a particular tissue, students can master the peripheral circulation efficiently.

Effect of variation in environmental temperature on blood pressure: is it important?

OBJECTIVES

To study the effect of variation in environmental temperature on blood pressure and pulse.

DESIGN

Repeated blood pressure and pulse measurements in volunteers on two days when environmental temperatures differed by 10 degrees C.

SETTING

Part of an ongoing community based study.

SUBJECTS

25 Black subjects (23 males and two females) volunteered from a population cohort participating in an ongoing longitudinal study examining cardiovascular risk factors in an urban African Black environment.

MAIN OUTCOME MEASURE

Mean systolic and diastolic blood pressures and pulse rates.

RESULTS

The systolic and diastolic blood pressures were significantly higher when recorded at 15 degrees C than at 25 degrees C, mean difference 32.2 +/- 4.2, p < 0.001 and 19.5 +/- 3.0 p < 0.001) for systolic and diastolic blood pressures respectively. The pulse rate per minute at 15 degrees C was significantly lower than at 25 degrees C (mean difference 11.1 +/- 3.2 p = 0.002).

CONCLUSIONS

A decrease in environmental temperature by 10 degrees C appears to increase blood pressure. Awareness of this phenomenon is important, especially when surprisingly high blood pressures are observed during low ambient temperatures, to avoid over diagnosis of hypertension. This phenomenon, together with that already established of the white coat hypertension, may lead to the erroneous diagnosis of hypertension.

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

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

Comparison of the Omron HEM-713C automated blood pressure monitor with a standard ausculatory method using a mercury manometer

OBJECTIVES

To compare the Omron HEM-713C automated blood pressure machine with the standard ausculatory method using a mercury manometer.

DESIGN

Blood pressures of randomly selected subjects were measured using both the Omron HEM-713C and the mercury manometer.

SETTING

Dombotombo surburb in Marondera, Zimbabwe.

SUBJECTS

One hundred and sixteen subjects 25 years and above (47 males and 69 females) randomly selected in Marondera.

MAIN OUTCOME MEASURE

Systolic blood pressure and diastolic blood pressure.

RESULTS

The Omron HEM-713C passed with a grade B for both systolic and diastolic blood pressures when using the British Hypertension Society protocol. It also passed both systolic and diastolic criteria for Association of the Advancement of Medical Instrumentation.

CONCLUSION

The Omron HEM-713C compares well with the standard mercury manometer, we therefore recommend its use in both research and clinical applications which require blood pressure measurements.

Prevalence of hypertension among women in rural Zimbabwe: a comparison of pregnant and non-pregnant women

OBJECTIVES

To determine the prevalence of pregnancy induced hypertension (PIH) in rural Zimbabwe.

DESIGNS

A blood pressure survey conducted in three socio-economic strata of rural Zimbabwe, communal lands, commercial farms and mining areas.

SETTING

Rural Zimbabwe, Mashonaland West Province.

SUBJECTS

627 pregnant (> 20 weeks gestation) and 483 non pregnant (ages 16 to 45) women.

MAIN OUTCOME MEASURE

Prevalence of PIH.

RESULTS

PIH was defined in two ways: 1. A combination of > or = 2+ proteinuria and systolic and/or diastolic pressure > or = SD above the mean for all pregnant women (> or = 123/75 mmHg). 2. > or = 1+ proteinuria and blood pressure of 140 and/or 90 mmHg. Eight women met definition 1 and seven met definition 2, giving a prevalence of 0.8pc; 1.3pc and 3.2pc in communal lands, commercial farms and mines respectively. In non-pregnant women the prevalence of hypertension (> or = 160 and/or 95 mmHg) was 3.5pc; 5.4pc and 15.1pc in communal lands, commercial farms and mines respectively.

CONCLUSION

PIH is rare in rural Zimbabwe. Hypertension among non-pregnant women of child bearing age is most common in mining communities which have many similarities to urban environments.

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

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

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

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

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

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

Blood pressure response to acute changes in dietary sodium in young Zimbabwean men

OBJECTIVE

The primary objective of this study was to determine the effect of acute alterations in sodium intake upon the blood pressure and hormone levels of young Zimbabwean men.

DESIGN

Blood pressure, 24-h urinary electrolyte excretion and plasma concentrations of angiotensin II, aldosterone, and atrial natriuretic peptide were measured in normotensive black medical students. Three sets of measurements were taken: (1) during free access to sodium (baseline); (2) after 4 days on a low-sodium diet (10 mmol/day); and (3) after 4 days on a high-sodium diet (800 mmol/day).

METHODS

Blood pressure was measured by random zero sphygmomanometry, hormone levels by radioimmunoassay, and urinary electrolytes by flame photometry.

RESULTS

The low-sodium diet caused the range of pulse pressure to narrow, with a decrease in systolic blood pressure (SBP) and an increase in diastolic blood pressure (DBP). With the introduction of the high-sodium diet, SBP increased and DBP decreased. Mean arterial pressure did not change. At the same time, angiotensin II and aldosterone decreased. Plasma atrial natriuretic peptide did not change. A subgroup of the men on the high-sodium diet also received 100 mmol potassium/day. The increase in SBP associated with high sodium was significantly attenuated by the presence of added potassium.

CONCLUSIONS

SBP of young black Zimbabwean men is lowered by dietary sodium restriction and rises with a large increase in dietary sodium for a short duration, but mean arterial pressure does not change due to the opposing decreases in DBP.

Adenosine formation and myocardial energy status during graded hypoxia

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

Interstitial adenosine concentration during norepinephrine infusion in isolated guinea pig hearts

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

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

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

Dirofilaria immitis: alteration of endothelium-dependent relaxation in the in vivo canine femoral artery

Pathogenic mechanisms in filarial diseases are complex and poorly understood. While examining endothelium-dependent vasodilatory responses in the in vivo canine femoral artery, we noticed that dogs with Dirofilaria immitis infection had altered vascular responsiveness. The results reported here extend our original observations on vascular reactivity in dogs with D. immitis infection (L. Kaiser, J. F. Williams, E. A. Meade, and H. V. Sparks, 1987, American Journal of Physiology 253, H1325-H1329). In noninfected dogs, acetylcholine binds to the luminal endothelial cell muscarinic receptor. This results in release of a nonprostaglandin endothelium-derived relaxing factor. The relaxing factor causes an increase in vascular smooth muscle guanylate cyclase and relaxation. However, in dogs with D. immitis infection the mechanism of relaxation to acetylcholine is different. At least two endothelium derived relaxing factors are involved: the major factor is a prostaglandin; the second factor works through vascular smooth muscle cGMP. These data suggest that adult D. immitis release pharmacologically active factors that can alter distal endothelial cell function. The notion that filarial products may alter the physiological function of endothelial cells should be considered in the pursuit of improved understanding of pathogenic mechanisms of filariasis.

Transcapillary adenosine transport and interstitial adenosine concentration in guinea pig hearts

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

d-Propranolol prevents adenosine formation associated with myocardial hypoperfusion

d-Propranolol eliminates the increased adenine nucleoside release from hypoperfused hearts [R. D. Wangler, D. F. DeWitt, and H. V. Sparks, Am. J. Physiol. 247 (Heart Circ. Physiol. 16): H330-H336, 1984]. To determine whether d-propranolol reduces adenosine formation or adenosine release into the vascular compartment, we measured myocardial tissue adenosine (TADO). Decreased formation would lower TADO, whereas decreased release would elevate TADO. Reduction of perfusion pressure by 50% reduced coronary flow (CF), venous oxygen tension (PVO2), and myocardial oxygen consumption (MVO2) by approximately 40, 25, and 35%, respectively. Total adenosine and inosine released during 30 min of hypoperfusion increased 10- and 5-fold, respectively. Also, TADO increased from 2.68 +/- 0.37 to 5.17 +/- 0.67 nmol/g (P less than 0.05). In the presence of d-propranolol, the same reduction in perfusion pressure caused a similar decrease in CF and MVO2. d-Propranolol eliminated the release of adenosine and inosine associated with hypoperfusion. TADO after 30 min of hypoperfusion plus d-propranolol was not significantly increased (3.27 +/- 0.40 nmol/g) and was significantly less than hypoperfused hearts. When severe hypoperfusion was created by reducing perfusion pressure 75%, adenosine release still did not increase if d-propranolol was present. When adenosine release was plotted as a function of oxygen supply-consumption, they were related in a hyperbolic fashion. Despite the severity of hypoperfusion, in the presence of d-propranolol the supply-to-consumption ratio was similar to that of the control perfusion group (no drug). We conclude that d-propranolol blocks nucleoside formation during hypoperfusion by reducing oxygen demand such that a reduction of oxygen supply no longer stimulates adenosine formation.

Distribution of perfusate flow during vasodilation in isolated guinea pig heart

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

Metabolic hyperemia is reduced in adult vs. immature guinea pig hearts

We have previously shown that maturation is associated with decreased maximum coronary flow (CF) elicited pharmacologically or by transient coronary occlusion (Toma, B.S., R.D. Wangler, D.F. DeWitt, and H.V. Sparks. Circ Res. 57: 538-544, 1985). The present study examines the influence of maturation on CF and adenosine release (RADO) during increased myocardial metabolism. Langendorff-perfused hearts from 1- (immature) and 18- (adult) mo-old guinea pigs were given norepinephrine to increase myocardial oxygen consumption (MVO2). CF, oxygen delivery, and venous oxygen tension were significantly lower in the 18-mo-old group for a given MVO2, before and during norepinephrine infusion. Administration of the selective beta-adrenergic agonist isoproterenol, or infusion of norepinephrine during alpha-adrenergic blockade with phentolamine did not significantly increase CF more than norepinephrine at an equivalent MVO2. RADO, relative to MVO2, was greater in 18-mo-old hearts (vs. 1-mo) during norepinephrine infusion. We conclude that in isolated perfused nonworking hearts 1) myocardial metabolic hyperemia decreases during maturation; 2) the observed flow decrement is not the result of increasing alpha-adrenergic activity during maturation; and 3) the flow decrement results in a reduced oxygen supply-to-demand ratio and an increased RADO.

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

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

Alpha-human atrial natriuretic peptide is a coronary vasodilator in the Langendorff-perfused guinea pig heart

The circulating form of atrial natriuretic peptide is now believed to be composed of 28 amino acids (1). Therefore, we studied the coronary vasoactivity of the 28 amino acid, alpha-human atrial natriuretic peptide (alpha-hANP) in five isolated guinea pig hearts Langendorff-perfused at constant pressure (46 mmHg) with Krebs-Henseleit solution. The reactivity of the coronary bed was assured in each heart with bolus injections of norepinephrine, adenosine, and the vasoconstrictor atrial natriuretic peptide, atriopeptin II (APII). APII was a coronary constrictor in each of these five hearts. Nineteen boluses of alpha-hANP were administered to the five hearts, spanning the range 1.6 to 64 nmol/g wet heart weight. alpha-hANP was vasodilator in all five hearts. The equation for the regression of y = flow, % increase, on x = dose, nmol/g, is y = 17.98 logx - 4.11. The correlation coefficient, r, is 0.83, and the coefficient of determination, r2, is 0.69. Analysis of variance of the regression of y on x yields an F statistic of 36.9, P less than 0.00001. These results indicate that coronary vasodilation is correlated with dose of alpha-hANP over much of the range 1.6-64 nmol/g.

Altered endothelial cell-mediated arterial dilation in dogs with D. immitis infection

Vascular endothelial cells regulate arterial diameter in vivo and in vitro. Stimulation of endothelial cell muscarinic receptors by acetylcholine results in the production and release of a nonprostaglandin metabolite of arachidonic acid that causes vascular smooth muscle relaxation. We examined femoral artery endothelium-dependent vasodilator responses in normal dogs and dogs with heartworm (Dirofilaria immitis) infection. Endothelium-dependent vascular reactivity was attenuated in dogs with D. immitis infection studied in the spring but not in the fall. The dilator response was inversely related to the number of female worms but not related to the presence of circulating microfilariae. Indomethacin markedly depressed responses to acetylcholine in dogs with D. immitis but did not alter acetylcholine-induced dilation in normal dogs. These data suggest that D. immitis releases substances that alter distal arterial endothelial cell arachidonic acid metabolism. The seasonal pattern may reflect the onset of maximal reproductive activity in the spring and its decline as the vector season ends in the fall.

Phosphorylation potential and adenosine release during norepinephrine infusion in guinea pig heart

This study tested the hypothesis that adenosine released from isolated guinea pig hearts (n = 5) in response to norepinephrine is related to the cellular phosphorylation potential (PP; [ATP]/[ADP][Pi]), where Pi is inorganic phosphate. 31P-nuclear magnetic resonance (NMR) was used to measure the relative concentrations of Pi, phosphocreatine (PCr), and ATP. Hearts were Langendorff perfused with a physiological salt solution containing 0.1 mM Pi. The venous effluent was collected for measurement of adenosine and partial pressure of oxygen (PO2). After a control period, norepinephrine (6 X 10(-8) M) was infused for 20 min during which 31P-NMR spectra and samples of venous effluent were collected every minute. With norepinephrine infusion, PCr decreased rapidly to 72% of control (P less than 0.05) by 8 min and then recovered to 80% of control for the remaining 12 min. ATP fell slowly to 70% of control (P less than 0.01) over 20 min. Pi increased to a peak at 2 min (P less than 0.01), then declined slowly to a steady state (60% of the peak and 3.5 X control) from 8 to 20 min. Adenosine release increased from 11 +/- 6 to a peak of 250 +/- 68 pmol.min-1.g-1 (P less than 0.01) at 7 min and then slowly fell (P less than 0.05) to a steady state of approximately 110 pmol.min-1.g-1 (P less than 0.01 vs. control) from 10 to 20 min.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Transient increase in release of adenosine during rapid cardiac pacing; transient effects on overdrive suppression of ventricular automaticity

A study was designed to test the hypothesis that endogenous adenosine concentration may increase during episodes of rapid ventricular pacing and, by virtue of its negative chronotropic effects, contribute to the transient suppression of automaticity that follows the period of overdrive. Isolated, perfused, rat ventricular preparations were subjected to periods of 6.0 Hz overdrive stimulation while adenosine release, oxygen consumption, and subsequent suppression of automaticity were measured. At the end of a 1 min episode of overdrive oxygen consumption and adenosine release were significantly increased, and the initial beating rate after 1 min overdrive was suppressed. At the end of 10 min overdrive oxygen consumption was still increased but adenosine release had returned to control values. Suppression of automaticity after 10 min overdrive was similar to that after 1 min overdrive. The relative magnitude of suppression after 1 min overdrive was decreased by theophylline (10(-4) mol.litre-1), and increased by the adenosine deaminase inhibitor, EHNA (10(-5) mol.litre-1). Neither theophylline nor EHNA had any discernible influence on suppression after 10 min overdrive. It its therefore concluded that endogenous adenosine may contribute to the suppression of ventricular automaticity that follows a 1 min episode of overdrive, but because of the transient nature of the increase in adenosine during overdrive endogenous adenosine does not contribute to the suppression that follows prolonged overdrive.

Endothelial cells. Not just a cellophane wrapper

This summary provides an overview of some of the new information about the metabolic and regulatory functions of vascular endothelial cells and discusses possible important consequences of endothelial abnormalities in disease.

Aminophylline and interstitial adenosine during sustained exercise hyperemia

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

Mechanism of adenosine production by isolated rat heart mitochondria

Adenosine production by isolated rat heart mitochondria was examined and was observed to be dependent on an active adenine nucleotide transporter and a functional 5'-nucleotidase. It was found that mitochondria do not transport adenosine. These results suggest that mitochondria provide AMP for an extramitochondrial 5'-nucleotidase and this was verified by direct measurement of extramitochondrial levels of AMP and adenosine. A possible role for mitochondria in myocardial adenosine production is discussed.

Methylene blue and ETYA block flow-dependent dilation in canine femoral artery

Flow-dependent dilation of the canine femoral artery is endothelial cell dependent and is not mediated by prostaglandins, adrenergic or cholinergic receptors, an ascending message from the microcirculation, or by myogenic mechanisms. We investigated the mechanism of flow dilation in 38 pentobarbital anesthetized dogs. A femoral artery-jugular vein shunt was constructed, and femoral artery diameter was continuously measured (sonomicrometer crystals) during control and maximum flow (1 l/min). Inhibition of prostaglandin formation by indomethacin did not alter the dilation response to increased flow, but the lipoxygenase-cyclooxygenase inhibitor 5, 8, 11, 14 eicosatetraynoic acid (ETYA) irreversibly inhibited the dilation response to increased flow. The guanylate cyclase inhibitor, methylene blue, caused a dose-dependent decrease in the dilation response to increased flow. Pretreatment with the H1 receptor antagonist tripelennamine sensitized the vessel to the inhibitory effects of methylene blue. Both methylene blue and ETYA shifted the ED50 for acetylcholine relaxation two orders of magnitude to the right, but did not alter the ability of the vessel to dilate or constrict to other stimuli. These data suggest that both cyclic GMP and a non-prostaglandin metabolite of arachidonic acid are involved in flow dilation. We propose that endothelial cells release a metabolite of arachidonic acid that stimulates vascular smooth muscle guanylate cyclase leading to relaxation. The role of histamine in this system is unknown.

Adenosine production and release by adult rat cardiocytes

We have examined adenosine (ADO) production and transport in a preparation of isolated adult cardiocytes which attach to and form a monolayer on culture dishes. This preparation contains 85% viable cells which are greater than 50% rod shaped and maintain an ATP/ADP ratio of nine. Incubation under control conditions for 15 mins results in a net release of 240 +/- 47 pmol ADO/mg protein (final adenosine concentration in the medium = 47 +/- 9 nM). Both 0.1 mM dinitrophenol (DNP) and 10 mM iodoacetate (IAA) cause a significant increase in ADO (DNP = 1763 +/- 147 and IAA = 612 +/- 90 pmol/mg). Both 20 microM nitrobenzylthioinosine (NBMPR), an inhibitor of the purine nucleoside carrier, and 0.1 mM alpha,beta-methylene adenosine diphosphate (AOPCP), an inhibitor of 5'-nucleotidase activity, attenuate DNP-stimulated ADO release (NBMPR by 62% and ADOCP by 76%). The results are consistent with the hypothesis that under the conditions of our experiments, adenosine is formed by a 5'-nucleotidase in association with transport across the cell membrane, perhaps by an enzyme-carrier complex. In addition, we have examined the effect of 0.1 mM dipyridamole on the extracellular appearance of adenosine in this preparation and found that it causes a significant increase in the amount of adenosine released. These results are consistent with the hypothesis that dipyridamole inhibits adenosine's uptake more than its release in cardiac myocytes.

Adenosine release into venous plasma during free flow exercise

We measured adenosine release into venous plasma as an index of interstitial adenosine concentration during free flow exercise hyperemia. Isolated, blood-perfused dog calf muscles were stimulated at 6 Hz for 10 min at free flow. Plasma samples were collected before, during, and after the exercise period for analysis of plasma adenosine concentration [( ADO]) by HPLC. Adenosine release (Rado) was calculated as plasma flow times venous-arterial [ADO] difference. Rado (nmole/min/100 g) went from -0.1 +/- 0.1 at rest to 6.6 +/- 4.6 during 6-Hz exercise. Isoproterenol infusion, which caused an increase in blood flow equivalent to 6-Hz exercise, did not result in increased Rado. Infusion of the 5'-nucleotidase inhibitor, alpha, beta, methylene adenosine 5'-diphosphate (AOPCP) did not prevent the increase in Rado during exercise. These results support the hypothesis that interstitial adenosine concentration increases during sustained free flow twitch exercise and that this results in increased release of adenosine into venous plasma.

Endothelial cell uptake of adenosine in canine skeletal muscle

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

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.

A role for mitochondria in myocardial adenosine production

The results from the previous work (Bukoski et al, 1983) and those presented here indicate that mitochondria from rat heart are capable of producing adenosine in concert with a sarcolemmal 5'-nucleotidase. In addition, and perhaps more importantly, these data also indicate that mitochondria can produce adenosine in the presence of inhibition of sarcolemmal 5'-nucleotidase. Given the current understanding of the regulation of mitochondrial respiration and thus oxygen consumption, mitochondria may be uniquely situated to sense increases in myocardial oxygen demand and respond to this with a feedback signal (adenosine) which can return the system to a state of balance.

Indicator dilution estimation of capillary endothelial transport

Mechanisms of transport of substrates and small solutes across the endothelial lining of the capillaries include passive diffusion (through clefts between cells or across the plasmalemma) and transporter-mediated flux across the plasmalemma. Because the transport rates are typically high, the multiple indicator dilution technique is usually the method of choice, as it provides the high temporal resolution required. In the simplest version of this technique, a test solute is injected into the inflow simultaneously with reference solutes that are restricted to intravascular and extracellular space. Interpretation of the resulting data requires models; the most precise approach is to fit the model solutions to the data. When appropriate combinations of indicators and sufficiently complex models (those that account for flow heterogeneity, arteriovenous gradients, passive and saturable transport, reaction, and diffusion in multicomponent systems) are used the transporters can be characterized. Features such as the rapidity of intracellular reaction can also be revealed by this technique.

Endothelium-dependent flow-induced dilation of canine femoral and saphenous arteries

We have characterized the dilation response to increased blood flow in the canine femoral and saphenous arteries. An arterio-venous shunt was created and changes in arterial diameter measured by sonomicrometer crystals. Increasing shunt flow approximately 10-fold caused a 9% increase in femoral and 15% increase in saphenous artery diameter. The dilation response consisted of a transient decrease in diameter, followed by a rapid dilation and a slow return to control when flow was decreased. The increased diameter was not a result of decreased transmural pressure or alterations in pulse pressure. After removing the endothelial cells, the vessels did not dilate to increased flow or topical acetylcholine (10(-5) M), but responses to norepinephrine (10(-5) M) and sodium nitroprusside (10(-4) M) were unaltered. Indomethacin, theophylline or propranolol did not affect the flow-induced dilation. Quinacrine, an inhibitor of phospholipase A2, attenuated the dilation response in a dose-dependent manner. We conclude that increased blood flow affects endothelial cells, causing an active dilation of arterial smooth muscle.

Coronary vasoconstrictor effects of atriopeptin II

Atrial natriuretic peptides lower arterial pressure, cardiac filling pressure, and cardiac output. In isolated, Langendorff-perfused guinea pig hearts, atriopeptin II, the 23-amino acid atrial natriuretic peptide, is also a potent coronary vasoconstrictor. The median effective dose for atriopeptin II in guinea pig hearts is 26 nanomoles, the threshold constrictor dose is 5 nanomoles, and flow nearly ceases at a dose of 100 nanomoles in perfused hearts at constant pressure. Similar concentrations of atriopeptin II also cause coronary vasoconstriction in rat and dog heart preparations. The disulfide bridge is necessary for vasoconstrictor activity; reduction of this bridge abolishes the activity, as it does the other biological activities of atrial natriuretic peptides.

Effect of development on coronary vasodilator reserve in the isolated guinea pig heart

Morphological studies have demonstrated an age-related decrease in capillary density and capillary surface area in the developing heart. However, the consequences of these changes on myocardial perfusion are not known. We tested the hypothesis that the decreased capillary density is associated with a reduction in coronary blood flow reserve. To test this hypothesis, we studied coronary responses to adenosine and sodium nitroprusside administration, reactive hyperemia, and autoregulatory capacity. We used a Langendorff-perfused heart preparation from guinea pigs of five different age groups (1 week and 1, 2, 12, and 18 months). Data are expressed as mean +/- SEM. Maximal coronary flows (ml/min per g) in response to adenosine (10(-6) to 10(-5) M) infusion are: 27 +/- 1.3, 18.5 +/- 1.4, 12.2 +/- 0.4, 10.3 +/- 0.3, and 10.6 +/- 0.8 at 1 week, 1, 2, 12, and 18 months, respectively, with the flows at 1 week and 1 month significantly higher than those at 2, 12, and 18 months. There is a similar trend for a decreased maximum coronary perfusion in response to sodium nitroprusside (10(-6) to 10(-5) M) and following a 45-second occlusion of the coronary inlet flow. Despite the decreased maximal pharmacological and reactive hyperemic flow reserve, autoregulation of flow is not altered with growth. The pressure-flow relationship exhibits autoregulation between 25 and 55 mm Hg perfusion pressure for all but the 1-week age group, which autoregulates within a narrower range of pressures (20-45 mm Hg). Total maximal coronary flow (ml/min) increases during development; this indicates that the growth of vessels continues with development.(ABSTRACT TRUNCATED AT 250 WORDS)

Capillary transport of adenosine

We tested the hypothesis that capillary exchange of adenosine is influenced by the ability of endothelial cells (ECs) to take up adenosine. Triple-indicator diffusion experiments were performed by injecting [14C]adenosine, [3H]9-beta-D-arabinofuranosylhypoxanthine ( [3H]araH), and radioiodinated serum albumin (RISA) into the arterial perfusate of isolated nonworking guinea pig hearts. Tracer appearance in venous effluent was observed over time. The early extraction of [14C]adenosine was much higher than that of [3H]araH. Extracted [3H]araH returned to the vascular space, but [14C]adenosine did not. Quantitative analysis of the curves by using a mathematical model indicates that approximately half of the extracted adenosine enters ECs and is metabolized. The remainder enters the interstitium and is taken up by myocytes, ECs, or other cells and is metabolized. We conclude that uptake of adenosine by ECs represents a significant influence on the capillary exchange of adenosine.