Plasma catecholamine modulation of alpha 2 adrenoreceptor agonist affinity and sensitivity in normotensive and hypertensive human platelets.

We measured alpha 2-adrenoreceptor density as well as affinity for and sensitivity to agonist on intact platelets of normotensive and hypertensive subjects before and after physiological increases in plasma catecholamines. In normotensives, posture-induced rises in plasma catecholamines correlated with reduced alpha 2-adrenoreceptor agonist affinity and fewer high affinity state receptors. Platelet aggregation and inhibition of adenylate cyclase by L-epinephrine also was reduced. Hypertensive subjects had similar rises in plasma catecholamines with upright posture, but showed no change in receptor affinity or sensitivity. No change in platelet alpha 2-adrenoreceptor number occurred in these studies. In vitro incubation with L-epinephrine revealed that platelets from hypertensives had slower desensitization than those from normotensives. Binding studies at different temperatures and with varying sodium concentrations found no thermodynamic or sodium-dependent differences between normotensive and hypertensive groups. These studies demonstrate that platelets from hypertensive subjects exhibit a defect in the ability of physiological concentrations of agonist to desensitize the alpha 2-adrenoreceptor.

Acute reduction in human platelet alpha 2-adrenoreceptor affinity for agonist by endogenous and exogenous catecholamines.

The binding characteristics of l-epinephrine to intact human platelets were assessed under conditions of physiological and pharmacological variations in plasma catecholamine concentration. In competition with the alpha 2-adrenoreceptor antagonist yohimbine, mean platelet receptor affinity for l-epinephrine was decreased 3.4-fold after 2 h of upright posture and exercise. This change in agonist affinity correlated significantly with the increases in plasma epinephrine and norepinephrine that were stimulated by upright posture and exercise. Supine subjects infused with l-norepinephrine or l-epinephrine for 2 h also averaged a 3.3- and 2.7-fold decrease in platelet alpha 2-adrenoreceptor affinity for agonist with no change in receptor number or antagonist affinity. The alpha 2-adrenoreceptor agonist affinity changes were specific for alpha-agonists since they were blocked by phentolamine, and incubation with 10(-5) M isoproterenol produced no change in alpha 2-adrenoreceptor affinity for l-epinephrine. In vitro exposure of intact human platelets to 10(-6) to 10(-10) M l-epinephrine for 2 h produced a concentration-related decrease in alpha 2-adrenoreceptor affinity for agonist. In all three paradigms, average slope factors approached 1.0 as affinity decreased, which is consistent with a heterogeneous receptor population that becomes more homogeneous after agonist exposure. Incubation of platelet-rich plasma with 10(-6) to 10(-8) M l-epinephrine resulted in a dose- and time-related loss of aggregatory response to l-epinephrine; this demonstrates that agonist affinity changes are correlated with changes in receptor sensitivity. These observations demonstrate that physiological variations in plasma catecholamines acutely modulate the intact human platelet alpha 2-adrenoreceptor's affinity for agonist, and can thereby alter the sensitivity of platelets to alpha 2-adrenergic agonist.

Use of alpha 2 adrenoreceptor agonists and antagonists in the functional assessment of the sympathetic nervous system.

We studied the effects of clonidine, an alpha 2-adrenoreceptor agonist, and yohimbine, an alpha 2-adrenoreceptor antagonist, on blood pressure, heart rate, and plasma catecholamines in 12 patients with autonomic dysfunction. Clonidine (0.1 mg, orally) lowered blood pressure 18 +/- 3 torr in six subjects and raised it 5 +/- 1 torr in six. The change in blood pressure in response to this dose of clonidine was inversely proportional to the supine level of norepinephrine (P less than 0.05). Yohimbine (4-64 micrograms/kg) raised plasma norepinephrine and blood pressure in six patients with degenerative autonomic dysfunction. Yohimbine also attenuated by 50% (P less than 0.05) the hypotensive response to head-up tilt of patients with degenerative autonomic dysfunction. Clonidine depends upon postjunctional hypersensitivity and the degree of autonomic insufficiency to elicit its pressor response. In contrast, the pressor response to yohimbine is related to the capacity of the sympathetic nervous system to be activated and release norepinephrine. If plasma norepinephrine levels following yohimbine administration are monitored, the biochemical and hemodynamic response to the drug may provide a sensitive indication of the capacity of the sympathetic nervous system to be activated in patients with autonomic dysfunction.

Clearance of atrial natriuretic factor by lung, liver, and kidney in human subjects and the dog.

We determined human and canine plasma clearance of atrial natriuretic factor (ANF) by lung, liver, and kidney from arteriovenous differences in plasma ANF and measured organ plasma flow. Human subjects had lower plasma ANF concentrations in the pulmonary vein or the pulmonary capillary wedge position when compared with the pulmonary artery, and both sites yielded pulmonary ANF extraction ratios of 24%. Canine lung ANF extraction was 19 +/- 3% and pulmonary ANF clearance was 328 +/- 78 ml/min per m2 vs. 357 +/- 53 ml/min per m2 in man. Hepatic plasma ANF clearance was 216 +/- 26 ml/min with an extraction ratio of 30 +/- 3% in humans and 199 +/- 89 ml/min and 36 +/- 6% in the dog. Renal plasma ANF clearance in human subjects was 78 +/- 12 ml/min per kidney and correlated well with each kidney's creatinine clearance (r = 0.58, P less than 0.05). The mean renal ANF extraction ratio was 35 +/- 4% in human subjects and 42 +/- 6% in the dog. These data quantitate the specific organ ANF clearances by lung, liver, and kidney in human subjects and in dogs and provide a rationale for elevated plasma ANF levels in cirrhosis, renal failure, and diseases accompanied by reduced perfusion of these organs. These findings support the conclusion that plasma ANF concentrations are dependent upon both the stimuli for ANF secretion as well as the specific organ clearances of ANF.

Micro-CT evaluation of rheumatoid arthritis mouse model disease progression: Manual tracings versus semi-automated routines.

PURPOSE: The primary goal of this study was to demonstrate the value of micro-CT imaging in a rheumatoid arthritis (RA) mouse model. The secondary goal was to assess whether manual correction of the articular surface regions of interest (ROI) identification of the semi-automated methods may result in more effective assessment of bone volume and density loss. MATERIALS AND METHODS: Collagen-induced arthritis (CIA) was induced in six DBA/1J mice at 12 weeks of age and three other DBA/1J identical mice served as controls. Micro-CT images were acquired at baseline and at four, seven, and nine weeks post-induction. Disease was monitored via ROI analysis, and ROIs were first generated using semi-automated techniques. These ROIs were manually manipulated so that a variety of edge irregularities were corrected. Effort was focused on the proximal and distal humerus and the distal femur. ROI volume and density were calculated, and data were compared. A histologic analysis of the study mice was also performed after the last time frame. RESULTS: There was a significant difference between the volume data comparison between the manually manipulated data and the semi-automated routine data across all time frames and across both humeri and femurs. There was no significant difference in densities calculated in Hounsfield units across any of the time frames, humeri or femurs, except for one time frame. CONCLUSION: Our findings suggest that the manual correction technique of semi-automated data can be used to quantify and evaluate bone volume, density, and joint surface architecture changes in a RA mouse model.

Reflexes unique to myocardial ischemia and infarction.

Although arrhythmias caused by myocardial ischemia are a well recognized cause of sudden death, the potential influence of cardiogenic reflexes originating in areas of ischemia has received less attention. In this study, 12 patients with well documented single vessel coronary artery spasm, with a total of 2,240 episodes of transient transmural ischemia, are described. Continuous electrocardiographic and hemodynamic recordings were analyzed to determine possible relations between the anatomic area of ischemia and patterns of change in blood pressure and heart rate. Of seven patients with ischemia of the posterior or inferior left ventricular wall, six had associated bradycardia and hypotension, an apparent Bezold-Jarisch response. Only one of five patients with anterior ischemia had a similar response. A hypertensive, tachycardiac response resembling the James reflex was seen in two of the patients with anterior ischemia, with an increase in blood pressure of 36/22 +/- 12/6 mm Hg and an increase in heart rate of 8 +/- 3 beats/min. This increase began before the onset of chest pain and was seen even in asymptomatic episodes. These reflexly mediated hemodynamic responses may modulate the direct effects of myocardial ischemia and could play a role in sudden cardiac death.

Atrial pressure and secretion of atrial natriuretic factor into the human central circulation.

Atrial natriuretic factor, a peptide found in mammalian cardiac atria, has natriuretic and vasodilatory properties that may be important in the regulation of intravascular volume. To study factors related to its release in human subjects, intracardiac pressures and plasma atrial natriuretic factor concentrations in the central circulation were measured in 34 patients with a variety of cardiovascular disorders. Plasma atrial natriuretic factor concentration increased from the inferior vena cava to the right atrium (76 +/- 24 to 162 +/- 37 pg/ml, p less than 0.001) and from the vena cava to the aorta (76 +/- 24 to 177 +/- 46 pg/ml, p less than 0.001). Mean right atrial pressure was positively correlated with atrial natriuretic factor concentration in the pulmonary artery (r = 0.58, p less than 0.001), and mean pulmonary capillary wedge pressure was positively correlated with concentration in the aorta (r = 0.64, p less than 0.001). In six patients whose atrial natriuretic factor concentrations were measured at two different levels of atrial pressure, increased atrial pressure was accompanied by increased atrial natriuretic factor concentration in the pulmonary artery (p less than 0.01) and aorta (p less than 0.01). Atrial natriuretic factor levels measured in fresh myocardium from a patient undergoing cardiac transplantation showed tissue concentrations in the atria 500-fold higher than tissue concentrations in the ventricles. These data document that atrial natriuretic factor is found in human atrial myocardium and suggest that it may be released in response to increased atrial pressure. Such a secretory release mechanism is consistent with the hypothesis that atrial natriuretic factor plays a role in the regulation of circulatory volume.

Increased vascular beta2-adrenoceptor responsiveness in autonomic dysfunction.

Responsiveness to the vasopressor, vasodepressor and chronotropic effects of several sympathomimetic amines was assessed in 12 patients with severe autonomic dysfunction and in 8 age-matched control subjects. The patients with autonomic dysfunction showed a profound increase in responsiveness to both isoproterenol and phenylephrine as compared with control subjects. The mean bolus dose of isoproterenol required to increase heart rate by 25 beats/min was 0.9 + 0.2 microgram in the patients and 5.4 + 2.1 micrograms in the control subjects. The dose of isoproterenol required to reduce mean blood pressure by 25 mm Hg was 0.3 + 0.2 and 5.2 + 1.8 micrograms, respectively. Thus, although there is a 6-fold increase in responsiveness to the chronotropic effect of isoproterenol in autonomic dysfunction, the responsiveness to the drug's depressor effect is increased 17-fold. This enhanced depressor sensitivity is quite marked, even with oral beta-adrenoceptor agonists. Beta-adrenoceptor agonists must be used with caution in conditions associated with autonomic dysfunction if dangerous hypotension is to be avoided.

Influence of yohimbine on blood pressure, autonomic reflexes, and plasma catecholamines in humans.

We studied the influence of the alpha 2-adrenoreceptor-blocking drug, yohimbine, on blood pressure, plasma norepinephrine, and other measures of autonomic function in normal male volunteers. These studies were designed to evaluate the role of alpha 2-receptors in the tonic regulation of sympathetic outflow in humans. In a dose-ranging study, we found that yohimbine HCl (0.016-0.125 mg/kg) elicited dose-related rises in mean, systolic, and diastolic pressures. At the maximal dose used (0.125 mg/kg), respective increments in mean, systolic, and diastolic pressures were 14 +/- 1 torr; 28 +/- 3 torr; and 8 +/- 1 torr (p less than 0.01) (mean +/- SE). No significant changes in heart rate occurred. Associated with the rise in blood pressure were enhanced pressor and heart rate responses to the cold pressor, isometric handgrip, and Valsalva maneuvers. In a double-blind study, yohimbine (0.125 mg/kg bolus, 0.001 mg/kg/min infusion) induced a two-to-threefold rise in plasma norepinephrine (p less than 0.01), without significantly altering plasma epinephrine or plasma renin activity. Ex vivo platelet aggregation in response to epinephrine was inhibited during yohimbine, showing that non-innervated alpha 2-adrenoreceptors were inhibited. Central effects of yohimbine were evaluated through use of linear analog mood rating scales which showed a shift from calm toward excited ends of these scales. If yohimbine is acting through blockade of alpha 2 receptors, then these receptors tonically suppress sympathetic outflow in humans.

Sodium loading and posture modulate human atrial natriuretic factor plasma levels.

Atrial natriuretic factor is postulated to act through atrial stretch receptors as a volume regulatory hormone that stimulates diuresis and natriuresis in response to increased atrial pressure. To characterize the stimuli associated with the release of atrial natriuretic factor in humans, we studied 14 normal subjects, both in the supine position and after 10 minutes in an upright posture, while they were on a regular diet (Day 0) and during 3 days of supplemental sodium chloride intake (8 g/day). Radioimmunoassay of plasma atrial natriuretic factor was performed with rabbit antibody to the human hormone amino acids (102-126). Urinary sodium excretion increased from 111 +/- 13 mEq/day (mean +/- SEM) on Day 0 to 275 +/- 15 mEq/day by the third day (Day 3) of high sodium intake. The level of atrial natriuretic factor in the supine position rose from 17 +/- 4 pg/ml (Day 0) to 76 +/- 13 pg/ml on Day 3 (p less than 0.001) and after 10 minutes in an upright posture on Day 3, the level fell to 32 +/- 10 (p less than 0.005). Plasma concentrations of atrial natriuretic factor correlated positively with spot and 24-hour urinary sodium excretion and weight gain, and correlated negatively with plasma aldosterone and renin activity. We conclude that the response of atrial natriuretic factor to sodium loading and posture change in humans is appropriate for a volume regulatory hormone.

Antihypertensive metabolites of alpha-methyldopa.

alpha-Methyldopa acts through a metabolite in the central nervous system. Of the three metabolites most prominently considered as potential mediators of alpha-methyldopa hypotension--alpha-methyldopamine, alpha-methylnorepinephrine (MNE), and alpha-methylepinephrine (ME)--ME is the most potent depressor agent following intravenous infusion into rats, following injection into the lateral ventricle, and following injection into the solitary tract nucleus (NTS). The depressor effect of ME in the NTS is attenuated as effectively by timolol as by yohimbine, while the combination of both timolol and yohimbine completely abolishes the pharmacological activity of ME in the NTS. ME is approximately eightfold more potent than MNE in the NTS and also has a greater susceptibility to timolol attenuation.

Reduction of liver plasma flow by caffeine and theophylline.

Caffeine and theophylline block the vasodilating effects of adenosine and may act to enhance sympathoadrenal discharge and activate the renin-angiotensin system. To determine if these methylxanthines might thereby have effects on regional blood flow, we studied the influence of caffeine and theophylline on apparent liver plasma flow (LPF) in normal subjects as assessed by indocyanine green clearance. Oral caffeine, 250 mg, reduced LPF by 19% from 630 +/- 150 to 510 +/- 120 ml/min (P less than 0.001). Intravenous theophylline (4.3 mg/kg) reduced LPF by 15% from 550 +/- 50 to 470 +/- 90 ml/min (P less than 0.05). These methylxanthine-induced falls in LPF may alter the disposition of concomitantly administered drugs. Because of their widespread use in Western society, caffeine and theophylline may be major determinants of liver blood flow in the general population. They may therefore prolong the t1/2 and increase steady-state levels of hepatically eliminated drugs.

Oral yohimbine in human autonomic failure.

Yohimbine is an alpha 2-adrenoreceptor antagonist that acts to enhance sympathetic nervous system discharge and potentiate sympathetically mediated cardiovascular reflex responses. We therefore assessed the ability of yohimbine to increase sympathoadrenal discharge and raise blood pressure (BP) in patients with autonomic failure characterized by profound orthostatic hypotension. Yohimbine 5 mg orally in eight seated patients significantly elevated mean systolic BP by 33 mm Hg from 136 +/- 15 (mean +/- SD) to a maximum of 169 +/- 23 mm Hg (p less than 0.01), mean diastolic BP by 16 mm Hg from 77 +/- 9 to a maximum of 93 +/- 15 mm Hg (p less than 0.01), and mean heart rate (HR) by 10 beats per minute (BPM) from 68 +/- 12 to a maximum of 78 +/- 17 BPM (p less than 0.01). Plasma norepinephrine (NE) increased from 104 +/- 71 to a maximum of 196 +/- 182 pg/ml (p less than 0.05), but plasma epinephrine (E) did not increase significantly (31 +/- 18 versus a maximum of 39 +/- 21 pg/ml). In five patients given yohimbine 2.5 mg orally, BP, HR, NE, and E tended to increase, but the changes were not significant. Plasma yohimbine levels correlated significantly with the changes in mean arterial pressure (r = 0.61, p less than 0.01). Yohimbine raises BP and HR in patients with autonomic failure. These effects are dose- and concentration-dependent and mediated through increased sympathetic discharge. Yohimbine may be useful in the treatment of orthostatic hypotension associated with autonomic failure. It is unique among current modes of therapy for this disorder in that it enhances discharge of the patient's own sympathetic system.

Management of chronic orthostatic hypotension.

Chronic orthostatic hypotension is characterized by recurrent symptoms of cerebral hypoperfusion due to low upright blood pressure levels. The initial approach should be to identify and correct reversible causes. Persistence of orthostatic hypotension suggests autonomic failure. The goal of management is to minimize symptoms and maximize functional capacity; therefore the magnitude of blood pressure fall is not as important as the advent of symptoms. Therapy is based upon the underlying pathophysiology and the risk/benefit ratio of interventions. Patient education and nondrug measures form the cornerstone of management. Drug therapy is often limited by unacceptable supine hypertension. Rational drug use can be governed by individualized trials of therapy. Patients with moderate or severe orthostatic hypotension are difficult to treat, but can be helped toward resumption of a normal life.

Clonidine raises blood pressure in severe idiopathic orthostatic hypotension.

The hemodynamic effects of clonidine were studied in four patients with sever idiopathic orthostatic hypotension and one patient with baroreceptor dysfunction. No depressor response to clonidine was observed in any patient with idiopathic orthostatic hypotension at any dosage. Rather, two patients responded to 0.4 mg of oral clonidine with a 40 mm Hg increment in systolic blood pressure lasting several hours. Each has been receiving clonidine, 0.4 mg twice daily, for one year with greatly increased functional capacity. The other two patients with idiopathic orthostatic hypotension had an even greater pressor response to 0.8 mg of oral clonidine, but adverse effects prevented continued therapeutic use. In marked contrast, the patient with baroreceptor dysfunction had a profound depressor response to 0.2 mg of clonidine. In the treatment of idiopathic orthostatic hypotension, the major advantage of clonidine over other pressor agents is its longer duration of action. The major adverse effects of the drug in these patients are sedation, dry mouth, altered mentation, and excessive hypertension. The drug should not be given to patients with mild idiopathic orthostatic hypotension or selective baroreceptor dysfunction, since severe hypotension may result.

Caffeine and hypertension.

The effect of prolonged caffeine administration on blood pressure in hypertensive subjects was assessed in a double-blind placebo-controlled study. Eighteen hypertensive subjects participated, nine of whom received placebo throughout the study and nine of whom received placebo during the first three days, caffeine during the subsequent seven days, and placebo during the final four days of the two-week study. Those who received caffeine were given 250 mg with meals three times daily. There were no untoward reactions in the course of the study, but one subject with unacceptably high blood pressures while receiving placebo had to be discharged from the study to resume antihypertensive therapy. Systolic blood pressure was immediately increased (9.2 +/- 3.4 mm Hg) within 15 minutes after the first dose of 250 mg of caffeine. On the first day of caffeine, systolic pressure was increased a mean of 7.3 +/- 4.0 mm Hg, but this was no longer significant after the initial day of caffeine administration. Diastolic pressure showed a trend toward increasing, but this never reached significance. The minor increases in plasma catecholamine levels and plasma renin activity were not significant on either a short- or long-term basis. After discontinuation of caffeine, no overshoot phenomena were observed. It is concluded that prolonged administration of caffeine is not associated with significant elevation in blood pressure, plasma catecholamine levels, or plasma renin activity in patients with borderline hypertension.

Prediction of serum vancomycin concentrations following intraperitoneal loading doses in continuous ambulatory peritoneal dialysis patients with peritonitis.

The pharmacokinetics of vancomycin were studied in continuous ambulatory peritoneal dialysis patients with peritonitis. Six patients received an intraperitoneal loading dose of 15 mg/kg and 4 received an intraperitoneal dose of 25 mg/L. The ability of 2 methods to predict serum concentrations during the loading dose exchange was determined. The mean serum concentration after the exchange was 17.8 +/- 2.2 mg/L in patients receiving the loading dose. The mean dialysis clearance in all patients was 0.94 +/- 0.34 L/h. 66.6 +/- 13.4% of a dose was absorbed into the circulation in 4 h. The volume of distribution was 0.61 +/- 0.46 L/kg, and the half-life for equilibration of vancomycin into the circulation from dialysate was 2.76 +/- 0.94 h. Two methods of predicting serum vancomycin concentrations were tested, with 1 method predicting values significantly different from measured concentrations while the other did not. Serum vancomycin concentrations can be accurately predicted during a loading dose exchange.

Atrial natriuretic factor and hypertension. A review and metaanalysis.

Atrial natriuretic factor (ANF) is a recently discovered, volume responsive hormone with multiple potent antihypertensive actions. This article reviews data supporting hypothetical associations between ANF and essential hypertension, examines reports of plasma ANF concentrations in hypertension, discusses the efficacy of ANF and its analogs in the treatment of hypertension, and reviews future issues in ANF research. ANF has been shown to elicit vasodilatation, suppress plasma renin activity, inhibit the synthesis and release of aldosterone, antagonize sympathetically-mediated release of norepinephrine, and promote diuresis and natriuresis. A metaanalysis of plasma ANF concentrations reported in normal and hypertensive subjects reveals a 5 +/- 19 pg/mL (pooled, weighted mean and standard deviation) higher ANF level in age-matched, untreated hypertensives without evidence of end-organ damage. This difference may be inappropriately low given the increase in atrial filling pressures found in hypertension. Low doses of ANF elicit greater reductions in blood pressure in hypertensive subjects than in normals. Recently, inhibitors of the ANF-degrading enzyme, neutral endopeptidase, and of the ANF "clearance" receptor have enhanced the antihypertensive actions of endogenous or exogenously administered ANF. Human studies are currently in progress testing the antihypertensive efficacy of orally administered neutral endopeptidase inhibitors. The discovery of ANF has led to the elucidation of a family of natriuretic peptides from brain, heart, and kidney, and promises to enlarge our understanding of volume regulation in normal and pathophysiological states. The possibility that essential hypertension is associated with inappropriately low plasma ANF levels or altered responsiveness to ANF may offer new insights into the pathogenesis and treatment of hypertension.

Humoral factor, intraplatelet calcium, and hypertension.

Plasma humoral factors which modulate the transmembrane distribution of sodium and calcium have been identified in hypertensive patients and have been hypothesized to be involved in the etiology of essential hypertension. In cross-incubation experiments, we have found that plasma of hypertensive subjects elevated basal and stimulated intraplatelet calcium levels, while plasma of normal subjects has an opposite effect on platelets from hypertensives. Basal intraplatelet calcium in normal platelets was 108 +/- 5 nmol/L and rose to 142 +/- 3 nmol/L (P less than .001) after incubation in plasma from hypertensive patients. Platelets from hypertensive patients had basal calcium levels of 182 +/- 11 nmol/L which fell to 127 +/- 11 nmol/L (P less than .01) after incubation in normal plasma. Hypertensive plasma potentiated the rise in intraplatelet calcium in response to ADP and PAF. Hypertensive patients treated experimentally with plasmapheresis exhibited a disappearance of the plasma factor responsible for elevating intraplatelet calcium. These results indicate the presence of a plasma humoral factor in hypertensives which elevates intraplatelet calcium and sensitizes platelets to agonist stimuli.

Role of cyclic AMP in adenosine inhibition of intracellular calcium rise in human platelets. Comparison of adenosine effects on thrombin- and epinephrine-induced platelet stimulation.

Thrombin-induced platelet aggregation is associated with an increase in intracellular calcium. Epinephrine provokes aggregation in the absence of a rise in intracellular calcium. Adenosine has been postulated as an endogenous inhibitor of platelet aggregation. In this study, the authors examine the effect of adenosine on the rise in intracellular calcium and on platelet aggregation, and the role of cyclic AMP (cAMP) in these actions. Human platelets were obtained from citrated plasma containing 5 micrograms/mL of indomethacin. Intracellular calcium was determined by fura-2 fluorescent dye. Adenosine inhibited thrombin-induced platelet aggregation and the rise in intracellular calcium in a dose-dependent manner. At a concentration of 100 mumol/L, adenosine completely inhibited thrombin-induced aggregation, but only partly inhibited the rise in intracellular calcium (55%). Adenosine also partially inhibited the rise in calcium produced by thrombin in both calcium-containing and calcium-free media, suggesting that adenosine inhibits both calcium influx and calcium mobilization. The effects of adenosine on intracellular calcium, as in the case of platelet aggregation, appear to be linked to adenylate cyclase, since they were prevented by the adenylate cyclase inhibitor 2',5'-dideoxyadenosine (1-mmol/L) and were potentiated by phospho-diesterase inhibition with papaverine (1 mumol/L). Adenosine and dibutyryl-cAMP also inhibited epinephrine-stimulated platelet aggregation in a dose-dependent manner. Thus, it appears that adenosine may inhibit platelet aggregation independently of its ability to decrease cytosolic free calcium.