Selective desensitization of cardiac beta adrenoceptors by prolonged in vivo infusion of catecholamines in rats

The effects of prolonged in vivo infusion of isoproterenol (400 micrograms/kg/hr) or norepinephrine (200 micrograms/kg/hr) from a minipump on the physiological reactivity and binding properties of cardiac beta and alpha-1 adrenoceptors were tested in rats. Infusion of either catecholamine significantly reduced the in vitro inotropic and chronotropic potency of isoproterenol in isolated left and right atria, respectively; desensitization was near maximal as early as after 2 hr of infusion. No significant change in the density of [3H]dihydroalprenolol-labeled beta receptors was evident at this time point in either atrial or ventricular tissue, although isoproterenol did decrease binding site density after 7 days of infusion. There was no change in the binding affinity or physiological blocking potency of dihydroalprenolol after isoproterenol infusion. The inotropic potency of phenylephrine in the presence of dihydroalprenolol was unaffected by infusion of either isoproterenol or norepinephrine and methoxamine failed to increase right atrial rate either in control or in isoproterenol-infused rats. There was also no change in the density and affinity of [3H]prazosin binding sites after isoproterenol infusion. These results indicate selective desensitization of cardiac beta receptors without changes in alpha-1 receptors by prolonged in vivo stimulation with catecholamines. This reaction pattern is different from the well documented effects of hypothyroidism, which include decreased sensitivity of cardiac beta and increased sensitivity of cardiac alpha-1 receptor-mediated responses in rats. Thus, the mechanisms responsible for altered receptor function in the two conditions appear to be different.     

Beta-adrenergic regulation of cyclic adenosine 3'',5'' monophosphate accumulation in human gastric epithelial glands. Inhibitory effect of somatostatin

The action of catecholamines and somatostatin on cyclic adenosine 3',5' monophosphate (cyclic AMP) formation in human isolated gastric glands is reported. We show that: (1) there is a beta 2 receptor-mediated stimulation of cyclic AMP production in fundus. Catecholamines act with the order of potencies isoproterenol (ED50 = 50 nmol 1(-1) greater than epinephrine (ED50 = 0.1 mumol 1(-1] greater than norepinephrine (ED50 = 5 mumol 1(-1]. Their action is completely reversed by propranolol at doses 10(3) times lower than practolol, while unaffected by phentolamine; (2) isoproterenol and Vasoactive Intestinal Peptide (VIP) have additive effects on cyclic AMP in fundic glands whereas no additivity is observed between histamine and isoproterenol; this, together with the absence of catecholamine effect in antral glands, suggests that the beta 2 receptor is located on parietal cells; (3) somatostatin (1 mumol 1(-1] non-competitively inhibits the stimulation by catecholamines but does not affect VIP and histamine stimulations. These results suggest a physiological stimulatory effect of catecholamines on gastric acid secretion in man, through a beta 2 receptor coupled to the cyclic AMP system, regulated by somatostatin.     

Direct relationship between mononuclear leukocyte and lung beta-adrenergic receptors and apparent reciprocal regulation of extravascular, but not intravascular, alpha- and beta-adrenergic receptors by the sympathochromaffin system in humans.

To examine putative relationships between adrenergic receptors on accessible circulating cells and relatively inaccessible extravascular catecholamine target tissues, we measured mononuclear leukocyte (MNL) and lung beta-adrenergic receptors and platelet and lung alpha-adrenergic receptors in tissues obtained from 15 patients undergoing pulmonary resection. Plasma catecholamine concentrations were measured concurrently to explore potential regulatory relationships between the activity of the sympathochromaffin system and both intravascular and extravascular adrenergic receptors. MNL and lung membrane beta-adrenergic receptor densities were correlated highly (r = 0.845, P less than 0.001). Platelet alpha 2-adrenergic receptor and lung alpha 1-adrenergic receptor densities were not. Lung alpha 1-adrenergic receptor densities were positively related to plasma norepinephrine (r = 0.840, P less than 0.01) and epinephrine (r = 0.860, P less than 0.01) concentrations; in contrast, lung beta-adrenergic receptor densities were not positively related to plasma catecholamine concentrations (they tended to be inversely related to plasma norepinephrine and epinephrine [r = -0.698, P less than 0.05] levels). This apparent reciprocal regulation of alpha- and beta-adrenergic receptors by the sympathochromaffin system was only demonstrable with adrenergic receptor measurements in the extravascular catecholamine target tissue. Neither MNL beta-adrenergic receptor nor platelet alpha-adrenergic receptor densities were correlated with plasma catecholamine levels. Thus, although measurements of beta-adrenergic receptors on circulating mononuclear leukocytes can be used as indices of extravascular target tissue beta-adrenergic receptor densities (at least in lung and heart), it would appear that extravascular tissues should be used to study adrenergic receptor regulation by endogenous catecholamines in humans. These data provide further support for the concept of up regulation, as well as down regulation, of some adrenergic receptor populations during short-term activation of the sympathochromaffin system in humans.     

Role of circulating catecholamines in the control of pancreatic polypeptide and gastrin release

The influence of circulating catecholamines on the release of pancreatic polypeptide (PP) and gastrin was studied in volunteers. Physical exercise increased plasma epinephrine by 374 +/- 123% and plasma norepinephrine by 167 +/- 30%, but plasma PP concentrations remained unchanged during standardized bicycle ergometry. Immediately after cessation of exercise catecholamine levels decreased rapidly, whereas PP concentrations increased by 55%. In a second series, epinephrine infusion (5, 25, and 75 ng.kg-1.min-1) increased epinephrine levels by 38 +/- 12, 331 +/- 69, and 1229 +/- 131%, respectively, whilst norepinephrine was unaffected. Neither during nor after catecholamine infusion PP secretion was affected. Gastrin release increased by a maximum of 85 +/- 38% (at epinephrine 75 ng.kg-1.min-1). It is concluded, that (1) changes in circulating adrenaline do not significantly influence PP secretion in man; (2) the PP increase immediately following physical exercise cannot be attributed to a rapid fall of catecholamine levels; (3) endogenous catecholamines are of minor importance in the control of gastrin secretion.     

Effects of adrenergic stimulation on ventilation in man

The mechanism by which catecholamines affect ventilation in man is not known. Ventilatory responses to catecholamines were observed in normal subjects before and after adrenergic receptor blockade. Intravenous infusions of norepinephrine and isoproterenol caused significant increases in minute volume and decreases in end-tidal P(Co2) which were blocked by the administration of propranolol, a beta adrenergic receptor blocker. The hyperventilatory response to hypoxia was not altered by propranolol.Intravenous infusion of phenylephrine caused a small but significant decrease in minute volume which was antagonized by phentolamine, an alpha adrenergic receptor blocker. Angiotensin, a nonadrenergic pressor agent, also decreased minute volume significantly.100% oxygen was administered to suppress arterial chemoreceptors. Increases in minute volume and decreases in arterial P(Co2) in response to norepinephrine and isoproterenol were blocked by breathing 100% oxygen. The decrease in minute volume during phenylephrine was not altered by 100% oxygen.THE RESULTS INDICATE THAT: (a) beta adrenergic receptors mediate the hyperventilatory response to norepinephrine and isoproterenol but not to hypoxia. (b) the pressor agents phenylephrine and angiotensin decrease ventilation, and (c) suppression of chemoreceptors blocks the ventilatory response to norepinephrine and isoproterenol but not to phenylephrine. Implications concerning the interaction of adrenergic receptors and chemoreceptors with respect to the hyperventilatory response to catecholamines are discussed.     

Absence of correlations between plasma catecholamine levels and mononuclear leukocyte beta 2-adrenergic receptors in the elderly.

To assess the effects of aging on catecholamine plasma levels and mononuclear leukocyte (NML) beta 2-adrenergic receptors and on the possible relationships between these two parameters, we evaluated two groups of human subjects: 18 elderly volunteers (age 65-70 years) and 13 young volunteers (age 21-35 years). Norepinephrine plasma levels were significantly higher in the elderly subjects compared to the younger ones (P less than 0.05), whereas plasma epinephrine levels were not different. Also MNL beta 2-adrenoceptor density was significantly higher in elderly subjects (P less than 0.05). The binding dissociation constants were not significantly different. In young subjects there was a significant (P less than 0.02), inverse relationship between receptor densities and plasma norepinephrine levels; this relationship was not present in elderly persons. Our data suggest that the increase in beta 2-adrenoceptors may be due to a compensatory phenomenon, owing to the reduced beta-adrenergic sensitivity observed in the elderly subjects; moreover, the regulation of beta-adrenoceptors by plasma catecholamines seems to be altered by aging.     

Perinatal changes in the coupling of beta 1- and beta 3 adrenergic receptors to brown fat adenylyl cyclase.

The stimulation of adenylyl cyclase by catecholamines in neonatal brown adipose tissue (BAT) is markedly biphasic, suggesting the existence of receptors that have both high and low affinities for catecholamines. The identities of these receptors were examined by comparing responses in neonatal BAT membranes to those of Chinese hamster ovary cells which had been transfected to express the cloned rat beta 1 and beta 3 receptors. The results from these experiments indicate that high-affinity stimulation of adenylyl cyclase by catecholamines in BAT is mediated by beta 1 receptors, as evidenced by the potencies of norepinephrine and isoproterenol at this receptor and the potent blockade of the receptor by alprenolol. The low-affinity catecholamine receptor appears to be the beta 3 receptor, as indicated by the low potency of catecholamine agonists and the inability of low concentrations of alprenolol to block this activity. Furthermore, this receptor, like the cloned rat beta 3 receptor, was antagonized by (-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one (CGP 12177) and was stimulated by (R',R')-4-(2-[(2[(3-chlorophenyl)-2- hydroxyethyl]amino)propyl]phenyl)phenoxyacetic acid (BRL 37344). These results indicate that both beta 1 and beta 3 receptors couple to adenylyl cyclase in BAT and that activation of adenylyl cyclase in neonatal BAT is mediated primarily by beta 3 receptors. Beta 3 receptors were also clearly detected in weanling BAT with the beta 3-selective agonist BRL 37344. However, when catecholamines were used to stimulate activity, the activation of adenylyl cyclase by beta 1 receptors, which occurred at low concentrations of catecholamines, obscured the activation of adenylyl cyclase by beta 3 receptors, which occurred only at high concentrations.     

Characterization of beta adrenoceptor subtypes in canine airway smooth muscle by radioligand binding and physiological responses

Beta adrenoceptor subtypes in canine tracheal smooth muscle have been investigated by radioligand binding and by physiological responses to beta agonists and sympathetic nerve stimulation in vitro. Specific binding of [3H]dihydroalprenolol to tracheal smooth muscle membranes was of high affinity (Kd = 1.0 +/- 0.08 nM), as in peripheral lung membranes from the same animals, but the concentration of binding sites (95.6 +/- 4.7 fmol/mg of protein) was much lower than in lung (532 +/- 48 fmol/mg of protein). Binding was stereoselective and agonists competed with the rank order of potency isoproterenol greater than epinephrine greater than norepinephrine, signifying a preponderance of beta-2 receptors. Using selective beta antagonists, we determined the ratio of beta-1/beta-2 receptors in tracheal smooth muscle membranes to be 1:4. The relaxation response of tracheal smooth muscle strips to exogenous beta agonists was mediated by beta-2 receptors, with a very small contribution from beta-1 receptors. However, the relaxation response to electrical field stimulation of sympathetic nerves was mediated predominantly by beta-1 receptors. Our results suggest that most beta receptors in dog tracheal smooth muscle are of the beta-2 subtype and mediate responses to circulating catecholamines, but there is a small concentration of beta-1 receptors which mediate the response to neurally released norepinephrine.     

Modification by Beta-Adrenergic Blockade of the Circulatory Responses to Acute Hypoxia in Man

In 17 healthy men, beta-adrenergic blockade reduced significantly the tachycardia and the elevation of cardiac output associated with inhalation of 7.5% oxygen for 7 to 10 minutes.Hypoxia did not increase plasma concentrations of epinephrine or norepinephrine in six subjects. Furthermore, blockade of alpha and beta receptors in the forearm did not modify the vasodilation in the forearm induced by hypoxia, providing pharmacologic evidence that hypoxia of the degree and duration used was not associated with an increase in the concentrations of circulating catecholamines in man.Part of the increase in cardiac output and heart rate during acute hypoxia in man is produced by stimulation of beta-adrenergic receptors, probably by cardiac sympathetic nerves. The mechanism of the vasodilation in the forearm during hypoxia remains uncertain.     

In situ studies of catecholamine-induced lipolysis in human adipose tissue using microdialysis

The effects of catecholamines on lipolysis in situ were investigated in humans. Subcutaneous adipose tissue was microdialyzed with solvents containing adrenergic agents. Norepinephrine caused a rapid increase in the glycerol level in adipose tissue (lipolysis index) that was further increased by the alpha adrenoreceptor blocker phentolamine. At 10(-11) mol/l of norepinephrine caused a 100% stimulation of lipolysis (P less than .025). In the presence of phentolamine the lipolytic effects of catecholamines at 10(-12) mol/l was isoproterenol greater than epinephrine greater than norepinephrine. All these three lipolytic catecholamines caused a transient increase in the adipose tissue dialysate glycerol level, which peaked after 20 to 30 min of catecholamine exposure and then declined. The apparent tachyphylaxia could not be overcome by a gradual increase of the catecholamine concentration from 10(-12) to 10(-8) mol/l. However, the selective alpha-2 adrenoreceptor agonist clonidine caused a continuous and dose-dependent decrease in the dialysate glycerol level; the minimum effective concentration was 10(-9) mol/l. In conclusion, catecholamines have a lipolytic effect in situ at much lower concentrations than those in the circulation. This effect is transient and is related to beta adrenoreceptors. In additio, catecholamines have alpha adrenoreceptor-mediated effects on lipolysis in situ.     

Comparison of the receptors mediating the catecholamine hyperpolarization and slow inhibitory postsynaptic potential in sympathetic ganglia

We investigated the proposed catecholamine receptor in the superior cervical ganglion of the rabbit with the sucrose-gap technique to characterize the receptor pharmacologically. It has been suggested that this receptor is involved in the slow inhibitory postsynaptic potential in sympathetic ganglia. Epinephrine, norepinephrine and dopamine consistently hyperpolarized the ganglion membrane (N = 60). The order of potency was epinephrine greater than or equal to norepinephrine much greater than dopamine. Clonidine (10(-5) M), phenylephrine (10(-4) M) and isoproterenol (10(-3) M) also hyperpolarized the ganglion. However, apomorphine, even at millimolar concentration, did not hyperpolarize the membrane. The alpha antagonists phentolamine (10(-6) M) and yohimbine (10(-6) M) depressed the response to all catecholamines and shifted the catecholamine concentration-response curve to the right; dopamine and beta antagonists and the alpha-1 antagonist prazosin had no effect on the catecholamine hyperpolarizations. In contrast, the nerve-evoked slow inhibitory postsynaptic potential was selectively depressed only by atropine (10(-7) M). In conclusion, we characterized an alpha-2 adrenergic receptor in the rabbit superior cervical ganglion responsible for the catecholamine hyperpolarization and found that the slow inhibitory postsynaptic potential does not appear to be mediated by the same receptor.     

Effect of dipivalyl derivatives of catecholamines on cardiovascular function in the conscious dog.

Dipivalyl derivatives of epinephrine, norepinephrine and isoproterenol were injected intravenously into conscious dogs while cardiovascular variables were monitored. The dipivalyl compounds produced cardiovascular effects that were comparable to those produced by their respective parent catecholamines except that the responses had a delayed onset and a prolonged duration. The catecholamines were more potent than their derivatives; norepinephrine was 28 times, epinephrine 15 times and isoproterenol 1.7 times as effective as their respective dipivalyl derivatives. Alpha or beta adrenergic responses to the dipivalyl compounds were attenuated or abolished after alpha adrenergic blockade (phenoxybenzamine or phentolamine), or beta adrenergic blockade (propranolol), respectively, or combined alpha and beta blockade. Since other evidence indicates that the dipivalyl derivatives themselves are inactive, our results suggest that these compounds act as prodrugs that exert their cardiovascular effects only after they are biotransformed to catecholamines.     

Effects of dopamine receptor agonists and antagonists on catecholamine release in bovine chromaffin cells.

Dopamine D2 receptors are known to regulate the release of catecholamines from neurons in the central and peripheral nervous systems. In the present study we have evaluated the effects of dopamine D2 agonists and antagonists on the release of endogenous norepinephrine and epinephrine stimulated by 50 microM nicotine in isolated bovine chromaffin cells in order to investigate whether isolated bovine chromaffin cells may contain functional dopamine D2 receptors. Dopamine (10(-4) and 10(-6) M), quinpirole (10(-5) M) and 2-amino-5,6-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (10(-5) M) had no effect on the nicotine-stimulated release of norepinephrine or epinephrine from the bovine chromaffin cells. Pergolide (10(-6) M) and apomorphine (10(-4) M) produced a significant inhibition of nicotine-stimulated release of both norepinephrine and epinephrine from the chromaffin cells. The inhibitory effect of the selective dopamine D2 agonist pergolide on catecholamine release from the chromaffin cells was not reversed by 10(-6) M concentrations of the selective dopamine D2 receptor antagonists haloperidol, domperidone, metaclopramide, fluphenazine, flupentixol, (+)- or (-)-sulpiride, the dopamine D1 receptor antagonist SCH 23390 (10(-7) M) or the alpha receptor antagonist phentolamine (10(-6) M). These data suggest that the inhibition of nicotine-stimulated release of catecholamines from the bovine chromaffin cells is not a receptor-mediated effect. Further studies showed that the inhibitory effect of pergolide on catecholamine release in the bovine chromaffin cells was correlated with an inhibition of nicotine-stimulated 45Ca++ uptake and 22Na+ uptake into these cells. It is concluded that functional dopamine D2 receptors of the classical type do not exist on isolated bovine chromaffin cells.     

Terbutaline-induced desensitization of polymorphonuclear leukocyte beta 2-adrenergic receptors in young and elderly subjects.

Potential age-related differences in cardiovascular responsiveness and receptor regulation induced by short-term administration of a selective beta 2-adrenergic receptor agonist were investigated. Young (age range, 23 to 31 years) and elderly (age range, 64 to 73 years) healthy subjects were treated with terbutaline (5 mg, three times daily) for 5 days. Similar plasma terbutaline concentrations were achieved in the two age groups. The elderly group had higher baseline plasma norepinephrine concentrations and mean arterial pressures, neither of which were altered by terbutaline administration. During terbutaline treatment, heart rate increased in both age groups while subjects were supine but consistently increased only in the young group while subjects were standing. In both age groups, the density of beta 2-adrenergic receptors on polymorphonuclear leukocyte membranes was reduced by 50% during terbutaline administration and returned to baseline values at similar rates after drug administration was stopped. Isoproterenol-stimulated cyclic adenosine monophosphate accumulation in polymorphonuclear leukocytes from elderly subjects was regulated similarly. These findings suggest that the ability of terbutaline to increase standing heart rate is selectively impaired in the elderly, whereas the ability of polymorphonuclear leukocyte beta 2-adrenergic receptors to down-regulate and to return to baseline values is not.     

Effect of beta 2-adrenergic receptor stimulation on interleukin-18-induced intercellular adhesion molecule-1 expression and cytokine production

beta-Adrenergic receptor (AR) agonists have been demonstrated to modulate the production of inflammatory mediators. Recent studies implied that beta 2-AR agonists might be useful for chronic inflammatory diseases caused by interleukin (IL)-18. In the present study, we found that norepinephrine, epinephrine, or isoproterenol down-regulated IL-18 (100 ng/ml)-induced intercellular adhesion molecule (ICAM)-1 expression on monocytes in a dose-dependent manner (10(-8)-10(-4) M), but did not effect B7.1 and B7.2 expression after 24-h incubation. The modulatory effect of these catecholamines on ICAM-1 expression was antagonized by beta 2-AR antagonist, but not by alpha 1-, alpha 2-, or beta 1-AR antagonist. beta 2-AR-selective agonists salbutanol and terbutaline down-regulated IL-18-induced ICAM-1 expression on monocytes, but alpha 1-, alpha 2-, or beta1-AR agonist had no effect. In the same manner, salbutanol and terbutaline as well as norepinephrine, epinephrine, and isoproterenol regulated the IL-18-induced cytokine production, including IL-12, tumor necrosis factor-alpha or interferon-gamma through the stimulation of beta 2-AR. Together with the previous finding that ICAM-1/lymphocyte function-associated antigen-1 interaction plays a crucial role in the IL-18-initiated cytokine network, the present study strongly suggested that the stimulation of beta 2-AR inhibited the IL-18-activated cytokine cascade through the inhibitory effect on ICAM-1 expression, contributing to finding a new method for clinical treatment.     

Free and conjugated catecholamines in patients with cirrhosis

A defect of conjugation may play a role in the elevated plasma free norepinephrine observed in patients with cirrhosis. Plasma free, sulfoconjugated, and glucuronoconjugated catecholamine concentrations were assessed in 15 patients with cirrhosis and in 15 age-matched control subjects. Plasma free norepinephrine and epinephrine levels were significantly higher in patients with cirrhosis (481 +/- 75 and 96 +/- 16 pg/ml, respectively) than in those of the control group (307 +/- 33 and 42 +/- 10 pg/ml, p less than 0.05 and p less than 0.01, respectively). Plasma free dopamine levels were similar in both groups. Sulfoconjugated catecholamines were the predominant form in plasma from both cirrhotic patients and control subjects. The ratio of conjugated to total catecholamines was similar in the two groups. Therefore, it is unlikely that a defect in conjugation of catecholamines is contributing to the excessive plasma free norepinephrine and epinephrine concentrations found in patients with cirrhosis. Moreover, in patients with cirrhosis, no significant relation was found between plasma conjugated catecholamines and the severity of liver disease. This study shows that cirrhosis does not induce alteration in conjugation of catecholamines and that hepatocellular function is not essential for conjugation of circulating catecholamines.     

Quantitative analysis of the contribution of pulmonary and hind limb circulation to the clearance of exogenous catecholamines

The contribution of pulmonary and hind limb circulation to the clearance of exogenous catecholamines was analyzed quantitatively. During infusion of clinical doses of norepinephrine, epinephrine and dopamine in dogs, the plasma level of catecholamine and the plasma flow were measured simultaneously. Percentage of contribution was calculated from the following equation; transorgan difference of plasma catecholamine (nanograms per milliliter) X plasma flow (milliliters per minute) X 100/dose (nanograms per minute). This value means the percentage of the amount of catecholamine cleared by an organ to the amount of catecholamine administered into the body. Small but significant transpulmonary gradients of plasma levels of norepinephrine, epinephrine and dopamine and large translimb gradients of plasma levels of these catecholamines were observed. The plasma flow of pulmonary circulation was increased by infusion of epinephrine and dopamine, whereas it remained unchanged by infusion of norepinephrine. The plasma flow of hind limb circulation showed no significant change by infusion of catecholamines. The calculated contribution values indicate that pulmonary circulation clears 35.7% of norepinephrine (at 0.2 ng X kg-1 X min-1), 27.1% of epinephrine (0.2 ng X kg-1 X min-1) and 21.5% of dopamine (10 micrograms X kg-1 X min-1) administered exogenously, and that the corresponding figures for hind limb circulation are 8.2, 7.8 and 4.5%.     

Characterization of the Human Platelet α-Adrenergic Receptor: CORRELATION OF [3H] DIHYDROERGOCRYPTINE BINDING WITH AGGREGATION AND ADENYLATE CYCLASE INHIBITION

Human platelets aggregate and undergo a release reaction when incubated with catecholamines. Indirect evidence indicates that these events are mediated through alpha-adrenergic receptors. We used [(3)H]dihydroergocryptine, an alpha-adrenergic antagonist, to identify binding sites on platelets that have the characteristics of alpha-adrenergic receptors. Catecholamines compete for the binding sites in a stereo-specific manner with the potency series of (-) epinephrine > (-) norepinephrine > (+/-) phenylephrine > (-) isoproterenol. The dissociation constant (K(d)) of (-) epinephrine is 0.34 muM. Binding is saturable using a platelet particulate fraction but not with intact platelets. There are 0.130 pmol binding sites per milligram protein in fresh platelet membranes. This number represents approximately 100 binding sites per platelet. The K(d) for [(3)H]-dihydroergocryptine was 0.003-0.01 muM. The alpha-adrenergic antagonist phentolamine (K(d) = 0.0069 muM) was much more potent than the beta-adrenergic antagonist (+/-) propranolol (K(d) = 27 muM) in competing for the binding sites. The binding data were correlated with catecholamine-induced platelet aggregation and inhibition of basal and prostaglandin E(1)-stimulated adenylate cyclase. (-) Epinephrine was more potent than (-) norepinephrine in producing aggregation whereas (-) isoproterenol was ineffective. The threshold dose for inducing aggregation by (-) epinephrine was 0.46 muM. Phentolamine and dihydroergocyrptine blocked this response, whereas (+/-) propranolol had no effect. (-) Epinephrine and (-) norepinephrine inhibited basal and prostaglandin E(1)-stimulated adenylate cyclase in a dose-related manner. The concentration of (-) epinephrine inhibiting adenylate cyclase 50% was 0.7 muM. This inhibition was also blocked by phentolamine and dihydroergocryptine but not by (+/-) propranolol. The specificity of binding and the close correlation with alpha-adrenergic receptor-mediated biochemical and physiological responses suggest that the [(3)H]dihydroergocryptine binding site represents, or is closely related to, the human platelet alpha-adrenergic receptor. The ability to assay this receptor directly and to correlate these data with independently measured sequelae of receptor activation should facilitate increased understanding of the physiology and pathophysiology of the human platelet alpha-adrenergic receptor.     

Effects of Adrenergic Receptor Activation and Blockade on the Systolic Preejection Period, Heart Rate, and Arterial Pressure in Man

We have investigated the possibility that alterations in the duration of the systolic preejection period can be used to estimate adrenergic influences on the human left ventricle. The preejection period was determined from high speed, simultaneous recordings of the phonocardiogram, carotid pulse tracing, and electrocardiogram. The preejection period was shortened by isoproterenol, epinephrine, and moderate doses of norepinephrine—all of which activate beta adrenergic receptors—and by cedilanid-D. It was unaltered by changes in heart rate induced by atropine and right atrial electrical pacing. Beta adrenergic receptor blockade by propranolol abolished the shortening effects of the three catecholamines but did not inhibit that due to cedilanid-D. Vasoconstriction, both alpha adrenergic (epinephrine and norepinephrine after propranolol) and nonadrenergic (angiotensin), prolonged the preejection period. Most of the shortening of the preejection period by beta adrenergic receptor activating agents and cedilanid-D and all of the prolongation accompanying pharmacologic vasoconstriction occurred after the onset of the first heart sound, thereby excluding changes in electrical-mechanical delay as a major factor in the observed preejection period responses. Shortening of the preejection period by beta adrenergic activity induced with isoproterenol was dose-related. Increasing doses of propranolol produced parallel shifts to the right in the isoproterenol dose-response curve.     

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.