Synthetic alpha-adrenergic agonists are potent alpha-adrenergic blockers in human platelets

Potentiation by adrenaline of agonist-induced responses in normal human platelets in vitro

Adrenaline is not a true platelet agonist, but enhances aggregation, dense granule secretion, and phospholipase C induced by other agonists. In the present work we investigated the effect of adrenaline on other platelet responses. It strongly potentiated ADP-induced shape change in platelet-rich plasma, particularly when aggregation was prevented by EDTA. The degree of potentiation increased with increasing concentrations of ADP. Thrombin-induced α-granule secretion, measured by the release of fibrinogen in gel-filtered platelets, was also potentiated by adrenaline at thrombin concentrations above 0.05 U/ml. In contrast, adrenaline had little effect on thrombin-induced secretion of β-acetyl-hexosaminidase and potentiated very little liberation of arachidonate at high thrombin concentrations. When autocrine stimulation was inhibited by the removal of secreted ADP by creatine phosphate/creatine phosphate kinase and specific blocking of the thromboxane A(2) and fibrinogen receptors, the potentiation of thrombin-induced ADP + ATP secretion by adrenaline was reduced and this reduction was mostly due to the blocking of the thromboxane A(2) receptor. Protein tyrosine phosphorylation by both thrombin and collagen was reduced by adrenaline, and inhibitors of autocrine stimulation counteracted this reduction.

Epinephrine-induced platelet aggregation. A twin study**

A study in healthy adult male twin pairs (17 MZ, 15 DZ) was devised to examine the genetic influence on epinephrine-induced platelet aggregation. Intraclass correlations of for MZ, 0.43 for DZ, and 0.61 for repetitive experiments in the same subjects point to the influence of genetic factors. The antagonizing effect of phentolamine on platelet aggregation did not prove to be under genetic control. There was no significant correlation between epinephrine concentrations which induce half-maximal aggregation and number of alpha-adrenergic receptors as measured by 3H-yohimbine binding. In one MZ twin pair concordantly, and also in two unrelated persons, epinephrine was unable to induce complete platelet aggregation. This phenomenon is not related to the number of alpha-adrenergic binding sites on platelets.

Epinephrine induced platelet aggregation in rat platelet-rich plasma

Epinephrine at even high concentrations neither caused shape change nor aggregation of rat platelets. However, epinephrine induced aggregation in the presence of low (near-threshold) concentrations of collagen at which concentration only platelet shape change was induced without aggregation. The platelet aggregation was also induced by some other catecholamines and clonidine but not by beta-agonist isoproterenol. The aggregatory potencies of R-(-)-isomers, (-)-epinephrine and (-)-norepinephrine were higher than the corresponding desoxy derivatives, epinine and dopamine. In addition, the epinephrine-induced rat platelet aggregation was inhibited by alpha2-antagonists, yohimbine and phentolamine, but not by alpha1-antagonist prazosin or beta-antagonist propranolol. These results suggested that the epinephrine-induced rat platelet aggregation is occurring through alpha2-adrenergic receptors as is the case with human platelets.

Opposing effects of plasma epinephrine and norepinephrine on coronary thrombosis in vivo

BACKGROUND

It is well known that plasma catecholamines and myocardial infarction have a close relation and that coronary artery thrombosis is a major cause of myocardial infarction. In addition, epinephrine is known to be a prothrombogenic agent in vivo. However, the role of the other major circulating catecholamine, norepinephrine, in the development of coronary thrombosis is somewhat uncertain, although the role of norepinephrine is often considered analogous to the role of epinephrine. Therefore, the present study was designed to investigate the effect of norepinephrine and its interaction with epinephrine on coronary thrombosis.

METHODS AND RESULTS

To compare the effects of epinephrine and norepinephrine on coronary thrombosis, we analyzed the frequency of cyclic blood flow reductions (CFRs) in an anesthetized canine model of coronary thrombosis (n = 25). Three experiments were used in the present study. In the first experiment with epinephrine infusion, plasma epinephrine was elevated from 0.46 +/- 0.25 to 27.7 +/- 1.85 nmol/L. The frequency of CFRs increased by more than 60%, from 7.1 +/- 0.5 to 11.5 +/- 0.7 in 40 minutes (P < .01). The second experiment included three experimental periods: control, norepinephrine infusion, and norepinephrine infusion plus epinephrine infusion. Norepinephrine was infused to raise plasma norepinephrine from 1.3 +/- 0.2 to 32.4 +/- 4.3 nmol/L. The frequency of CFRs in the dogs was markedly reduced, from 7.89 +/- 0.42 to 2.41 +/- 1.08 in 40 minutes (P < .01), whereas arterial pressure was elevated from 88 +/- 3 to 118 +/- 5 mm Hg (P < .01). However, when epinephrine infusion was added to the norepinephrine infusion, the frequency of CFRs increased from 2.41 +/- 1.08 to 7.74 +/- 1.12 in 40 minutes (P < .01). In the third experiment, a servocontrol device was used during the norepinephrine infusion to prevent rises in coronary arterial pressure. As a result of the norepinephrine infusion, the frequency of CFRs was reduced from 7.47 +/- 0.71 to 0.83 +/- 0.65 in 40 minutes (P < .01), even though the coronary arterial pressure was not altered.

CONCLUSIONS

The present study demonstrated that infusion of epinephrine stimulated coronary artery thrombosis, whereas infusion of norepinephrine inhibited coronary artery thrombosis. In addition, the inhibitory effect of norepinephrine on coronary thrombosis is independent of increases in coronary arterial pressure. Therefore, the present findings suggest that epinephrine and norepinephrine have opposing effects on coronary thrombosis in dogs.

Pharmacological characterization of epinephrine-stimulated GTPase activity in human platelet membranes

The alpha 2-adrenergic receptor-mediated stimulation of GTPase activity was investigated in human platelet membranes. The stimulatory effect of (-)-epinephrine was strictly dependent on Mg2+ and derived from a high-affinity GTPase activation. (-)-Epinephrine and (-)-norepinephrine stimulated GTPase activity in a concentration-dependent manner with EC50 values of 200 and 600 nM, respectively. These effects were stereospecific, since (+/-)-epinephrine, (+/-)-norepinephrine, and (+)-epinephrine were less potent in stimulating the enzyme activity with EC50 values of 4, 1 and 3 microM, respectively. Thrombin also stimulated GTPase activity concentration dependently with an EC50 value of 0.02 U/mL. The maximal effects of (-)-epinephrine, (-)-norepinephrine, and thrombin were not additive in any combination. Clonidine did not stimulate GTPase activity, whereas another synthetic alpha 2-adrenergic agonist, p-aminoclonidine, had the characteristics of a partial agonist. The rank order of potency for antagonists to inhibit the activation of GTPase by 1 microM (-)-epinephrine was yohimbine = rauwolscine > idazoxan = oxymetazoline = phentolamine = WB4101 = (+)-mianserin > (-)-mianserin > prazosin > (-)-propranolol. Negative logarithms of the IC50 values of these antagonists corresponded well with the negative logarithmic values of Ki(pKi) for the alpha 2A-adrenergic receptors determined by a receptor-binding technique in human platelets. These results indicate that epinephrine stimulates high-affinity GTPase activity of G proteins (putatively Gi2), which are also coupled with thrombin receptors, in a Mg(2+)-dependent and stereospecific manner, via alpha 2A-adrenergic receptor activation in human platelet membrane preparations.

Competitive vs. noncompetitive blockade of epinephrine-sensitive GTPase activity in human platelets

The manner of inhibition of the epinephrine-stimulated GTPase activity by phentolamine and yohimbine was studied in human platelet membranes. (-)-Epinephrine(1-microM)-stimulated GTPase activity was inhibited concentration-dependently by phentolamine and yohimbine with IC50 values of 150 nM and 25 nM, respectively. Addition of increasing concentrations of phentolamine shifted the concentration-response curve to (-)-epinephrine to the right in a parallel manner. Schild plot analysis gave a KB value of 13 nM and a slope factor of 0.99. Yohimbine inhibited the activity, on the contrary, in a noncompetitive fashion. This inhibition by yohimbine was reversible after thorough washing of the membranes.

Human blood platelet alpha adrenoceptor in view of the effects of various imidazol(in)e drugs on aggregation

1. Sixteen imidazol(in)e derivative drugs were tested with regards to their aggregatory and antiaggregatory effects on human blood platelets. 2. Platelet aggregation in response to fixed concentrations of the agents was quantified by the turbidimetric method of Born. 3. Inhibitory effects of the imidazol(in)es against the epinephrine (10(-5) M) induced aggregation was related to concentration of the agents. Of the compounds studied, UK 14,304 elicited an aggregatory effect of a magnitude similar to epinephrine. 4. A strong aggregatory agent appeared also moxonidine. 5. Small aggregation was observed in response to clonidine, antazoline and tetryzoline. 6. For all the group of imidazol(in)e drugs, excepting UK 14,304 and moxonidine, a significant inhibition of the epinephrine induced aggregation was noted. 7. Analysing inhibitory activity of imidazol(in)es one notes the drugs commonly assumed to interact with alpha 2 adrenoceptor among the most potent inhibitors, and those classified as alpha 1 adrenoceptor agonists among the less active agents. 8. The results here obtained suggest some similarity between the platelet adrenoceptor and the alpha 2 subtype of adrenoceptor identified in human circulatory system. 9. Differences appear between the two types of receptors and the existence of a separate alpha 3 class of adrenoceptors in human blood platelets should be considered.

The potentiation of phorbol ester-induced aggregation of human platelets by the prostaglandin endoperoxide analogue, U46619

It was not possible to desensitize human blood platelets to 12-deoxyphorbol-phenylacetate (DOPP) stimulation in a manner analogous with that to platelet aggregating factor (PAF), prostaglandin-endoperoxide analogue (U46619) or adenosine diphosphate (ADP). Platelets previously desensitized to U46619, when challenged with DOPP and ADP, showed an increased aggregation and release of 5-HT. Sub-threshold aggregating doses of U46619 also caused a potentiation of the platelet response and release reaction to DOPP. The concentration of U46619 used to pretreat platelets affected the extent of potentiation of platelet stimulation induced by DOPP. The degree of potentiation was also affected by the time interval between addition of U46619 and DOPP. U46619 did not potentiate the aggregating effects of PAF, or ionophore A23187. The stimulus potentiation of DOPP by U46619 was abolished by prostacyclin (PGI2) and an antibody to U46619, but was unaffected by indomethacin and CP/CPK.

Dissociation of the aggregating effect and the inhibitory effect upon cyclic adenosine monophosphate accumulation by adrenaline and adenosine diphosphate in human platelets

Recent evidence indicates that adrenaline and adenosine diphosphate each have separate stimulus-response pathways for induction of aggregation and inhibition of cAMP accumulation. We have used a natural model to test the validity of this evidence, i.e. patients from two kindreds with an inherited bleeding disorder due to absent aggregation to adrenaline and no secondary wave response to large concentrations of adenosine diphosphate. Our studies showed that the inhibitory effect of adrenaline and adenosine diphosphate on PGE1-induced increase of cAMP in the patients was not different from that of the controls both for adrenaline and adenosine diphosphate. These results therefore support the present evidence that both adrenaline and adenosine diphosphate apply separate pathways in these two platelet functions.

Alpha-adrenoceptor-mediated actions of optical isomers and desoxy analogs of catecholimidazoline and norepinephrine in human platelets: in vitro

Adrenoceptor-mediated effects of the enantiomers of optically active imidazoline, 2-(3,4,alpha-trihydroxybenzyl imidazoline (catecholimidazoline; CI), and norepinephrine (NE), and the corresponding desoxy derivatives, 2-(3,4-dihydroxybenzyl)imidazoline (desoxy-CI) and dopamine, have been investigated in human platelets. Differences between responsiveness of platelets from donor to donor were observed in the presence of the isomers and the desoxy analogs of NE and CI. In certain platelet preparations, all compounds gave concentration-dependent stimulatory responses, whereas in other preparations, only R(-)-NE and R(-)-CI were inducers of platelet aggregation and serotonin release. The rank order of stimulatory potencies (EC50; microM) for CI and NE was R(-)-NE (1.3) greater than R(-)-CI (7.5) greater than S(+)-NE (19) = S(+)-CI (20) = dopamine (22) greater than desoxy-CI (greater than 35). Unlike R(-)-CI, both S(+)-CI and desoxy-CI were either agonists or antagonists of human platelet function. In preparations unresponsive to the S(+)-isomers or desoxy analogs, the potencies (EC50) for R(-)-NE and R(-)-CI were 1.7 and 7.7 microM respectively. The corresponding inactive CIs [S(+)-CI and desoxy-CI] were inhibitors of both primary and secondary phases of aggregation and serotonin release responses to R(-)-CI and R(-)-NE, respectively. In contrast, the aggregation responses to ADP, arachidonic acid or U46619 were not blocked by S(+)-CI or desoxy CI. The rank order of inhibitory potencies for selected alpha-adrenoceptor agents against R(-)-NE was phentolamine greater than clonidine greater than desoxy-CI greater than S(+)-CI. Moreover, the relative inhibitory potencies of phentolamine and desoxy-CI against aggregation responses to R(-)-NE and R(-)-CI, respectively, were the same. These results suggest that the enantiomers and desoxy derivatives of CI and NE mediate their effects in human platelets by an interaction with alpha-adrenoceptors; catecholamines and imidazolines interact with the same alpha-adrenoceptors in human platelets; the stereochemical requirements of both chemical classes for stimulatory activity in human platelets adhere to the Easson-Stedman hypothesis in this alpha 2-adrenoceptor system; and desoxy-CI possessed the highest potency as an antagonist of alpha-adrenoceptors which suggests that the hydroxy group at the benzylic carbon atom of these imidazolines may not be required for maximal binding to adrenoceptors in platelets.

Effects of ADP and epinephrine on macaque and human platelets. Implications for studies on human atherosclerosis

The ranges for near-threshold ADP concentrations for the aggregation of macaque and human citrated platelets overlapped. The minimum concentrations of epinephrine, 0.05 microM to 1.0 microM, that at least doubled the aggregation response at threshold ADP concentrations were comparable for macaque and human citrated platelets. Epinephrine (1.0 microM to 10 mM) alone never aggregated macaque citrated platelets. Biphasic aggregation occurred with both macaque and human citrated platelets. The addition of heparin to a final concentration of 2.2 units/ml had no effect on the threshold ADP concentrations or the sensitivity of macaque or human citrated platelets to epinephrine. One microM phentolamine eliminated the potentiating effect of 1 microM epinephrine on ADP-induced aggregation of macaque and human citrated platelets. The threshold concentrations of ADP for macaque platelets were sharply reduced when heparin was used as an anticoagulant rather than citrate. However, epinephrine induced a similar increase in aggregability with both citrated and heparinized platelets, 0.55 +/- 0.09 SEM% and 0.44 +/- 0.09 SEM%, respectively. These data indicate that macaque and human platelets behave in a similar manner in response to ADP and that epinephrine potentiates the ADP-induced aggregation of macaque and human platelets equally well.

Influence of alpha- and beta-adrenoceptors on thrombin-induced serotonin release in rat platelets

This work was designed to investigate the influence of rat platelet adrenoceptors on the early thrombin-induced serotonin release. In washed platelets prelabeled with [3H]-serotonin, adrenaline and isoproterenol both inhibited, in a dose-dependent manner, the early thrombin-induced secretion of serotonin. Inhibitory responses of both adrenaline and isoproterenol were blocked in the presence of beta-adrenoceptor antagonists, suggesting that the catecholamine acted solely through beta-adrenoceptors. However, isoproterenol inhibited the thrombin-induced serotonin release to a much greater extent than the catecholamine, suggesting that the alpha 2-component of adrenaline might account for the difference observed between the two compounds. Our observation that selective alpha 2-adrenoceptor antagonists as yohimbine and rauwolscine potentiated the inhibitory effect of adrenaline to a level close to that observed with isoproterenol, lends support to the above hypothesis. This latter result suggested that, conversely, alpha 2-adrenergic compounds might exert a counteracting effect on a full beta-adrenoceptor mediated inhibition. Although synthetic alpha 2-adrenergic agents failed to influence isoproterenol inhibitory effect, our study shows that prestimulation of beta-adrenoceptors by isoproterenol, followed by addition of adrenaline or noradrenaline markedly diminished the inhibitory effect of isoproterenol to a level close to that which characterized the inhibition observed with catecholamines, when tested alone. Our work favours the hypothesis that, in rat platelets, early after platelet stimulation, catecholamines might counteract a beta-adrenoceptor- mediated inhibition, through alpha 2-adrenoceptor sites.

Effects of alpha 2-adrenoceptor agonists and of related compounds on aggregation of, and on adenylate cyclase activity in, human platelets

A range of 2-(,5-dihydroimidazolyl)-benzene, -quinoline, and -quinoxaline derivatives and 2-morpholino-4-catechol have been characterized as agonists or partial agonists for human platelet aggregation; and for inhibition of adenylate cyclase by measurement of their effect on platelet [cyclic-3',5'-AMP]. Antagonist activity for these compounds versus adrenaline as agonist has also been assessed for these two responses. The compounds can be divided into 4 groups. Group I contains compounds that are agonists for both responses; group II, compounds that are agonists for inhibition of adenylate cyclase but antagonists for the aggregatory response; group III, compounds that are agonists for the aggregatory response but are antagonists for inhibition of adenylate cyclase by adrenaline; and group IV, compounds that are antagonists for both responses. In group I the EC50 values for induction of aggregation are not significantly different from the EC50 values for inhibition of adenylate cyclase except for 2-morpholino-4-catechol which is significantly more potent as an inhibitor of adenylate cyclase. In group IV a linear correlation is observed between the K1 values for the two responses for 8 compounds but 2 other compounds do not conform to this correlation. The data are not consistent with a model in which a single chi 2-adrenoceptor mediates both the aggregatory response and inhibition of adenylate cyclase and hence support a model in which unique chi 2-adrenoceptors mediate these two responses.

Genetic control of adrenergic receptors on human platelets. A twin study

It was examined whether genetic factors are involved in the expression of alpha-2 adrenergic receptors on human platelets, as measured with 3H-yohimbine as ligand. The twin series comprised 17 monozygotic and 15 dizygotic adult, healthy, male, drug-free twin pairs. For control of intraindividual and interassay variation, 13 unrelated pairs of subjects were examined. Bmax values for 3H-yohimbine binding (range 91-305 fmol/mg protein) proved to be under considerable genetic control; this was not the case for KD values. Different possible genetic mechanisms are discussed.

3H-Rauwolscine binding in platelets from depressed patients and healthy volunteers

3H-Rauwolscine binds specifically and with high affinity to alpha 2-adrenoceptors in human platelets. In a study comparing the binding of 3H-rauwolscine in platelets obtained from 26 control volunteers with 19 hospitalised, untreated, severely depressed patients, the mean maximal binding (Bmax) and mean dissociation constant (Kd) of 3H-rauwolscine binding were found to be identical in both groups. After 7-12 days, treatment with different tricyclic antidepressant drugs there was a significant improvement in the depressive symptoms but no change in the 3H-rauwolscine binding. After an average of 23 days treatment with tricyclic antidepressants, and when the Hamilton Depression Rating Scores had returned to normal, the Kd and Bmax of 3H-rauwolscine binding were still unchanged.

[3H]Rauwolscine and [3H]yohimbine binding to rat cerebral and human platelet membranes: possible heterogeneity of alpha 2-adrenoceptors

The specific binding of the alpha 2-adrenoceptor antagonists [3H]yohimbine and [3H]rauwolscine to membranes prepared from rat cerebral cortex and human platelets was rapid, reversible, saturable and of high affinity. Both ligands labelled an identical population of sites in cerebral or platelet membranes though the affinities of both agents were significantly lower in brain. The specific binding of [3H]yohimbine and [3H]rauwolscine in both tissues was displaced by various adrenergic agents in a manner that suggests that the labelled sites probably represent the alpha 2-adrenoceptor. There were, however, significant differences in the affinities of certain alpha-antagonists between cerebral and platelet preparations, even though these agents generated displacement curves with slopes close to unity in both tissues. Thus, whereas yohimbine, rauwolscine, WB4101 and corynanthine were all weaker in cerebral cortex, prazosin was significantly more potent at cortical than in platelet membranes. A possible heterogeneity of alpha 2-adrenoceptors is discussed.

[3H]Dihydroergocryptine binding to alpha-adrenergic receptors of human platelets. A reassessment using the selective radioligands [3H]prazosin, [3H]yohimbine, and [3H]rauwolscine

Which subtype(s) of the alpha-adrenergic receptor occurs on human platelets? Studies of platelet responsiveness to adrenergic compounds and indirect radioligand binding studies addressing this question have yielded contradictory conclusions. These bindings studies employed the ligand [3H]dihydroergocryptine ( [3H]DHE), an alpha-adrenergic antagonist that does not select between alpha 1- and alpha 2-adrenergic receptors and that also binds to other receptor types in some tissues. To determine the subtype of the platelet alpha-adrenergic receptor, we have examined the binding to intact human platelets of [3H]prazosin (alpha 1-selective), [3H]yohimbine (alpha 2-selective), and [3H]rauwolscine (alpha 2-selective), and we have compared the binding of these selective radioligands with that of [3H]DHE. [3H]Yohimbine and [3H]rauwolscine both bound with high affinity (Kd = 2.7 and 4.6 nM, respectively) to an equal number and a single class (Hill coefficient approximately 1.0) of sites ( approximately 300 per platelet), but [3H]yohimbine yielded lower nonspecific binding than did [3H]rauwolscine. In paired experiments, [3H]DHE bound to 1.5 times as many (phentolamine-displaceable) sites as did [3H]yohimbine or [3H]rauwolscine. Unlabeled yohimbine and epinephrine competed for fewer [3H]DHE binding sites than did phentolamine. Thus, in addition to binding to the alpha 2-adrenergic receptors identified by [3H]yohimbine and [3H]rauwolscine, [3H]DHE seems to bind to other sites on human platelets. The nature of these sites is not clear. We found that [3H]prazosin did not identify alpha 1-adrenergic receptors on platelets, and that phenoxybenzamine only inhibited [3H]yohimbine and [3H]DHE binding at higher concentrations than usually observed for alpha 1-adrenergic receptors. We conclude that (1) all alpha-adrenergic sites on human platelets are of the alpha 2 subtype, (2) [3H]DHE may bind to additional, as yet ill-defined, sites in addition to those sites identified by [3H]yohimbine and [3H]rauwolscine, and (3) [3H]yohimbine is the preferred antagonist radioligand for studying the alpha 2-adrenergic receptors on human platelets.

Plasma catecholamines, platelet aggregation and associated thromboxane formation after physical exercise, smoking or norepinephrine infusion

To study the possible role of catecholamines in platelet activation, platelet aggregation stimulated by ADP, collagen, arachidonic acid and L-epinephrine, thromboxane B2 (TXB2) formation and plasma levels of catecholamines and renin were studied in healthy men both before and after 6 days of propranolol treatment (40 mg three times daily) under control conditions and during sympathoadrenergic stimulation by physical exercise (200 W) or smoking. Exercise markedly increased plasma norepinephrine from 128 +/- 28 to 998 +/- 418 pg/ml (+/- SD), and plasma renin activity from 1.0 +/- 0.5 to 4.2 +/- 1.8 ng AI/ml . hour. Smoking predominantly increased plasma epinephrine, from 47 +/- 25 to 154 +/- 76 pg/ml. Propranolol did not consistently influence these variables, but blunted the circulatory response to exercise and smoking. Despite the marked increases of plasma catecholamines after both stimuli with and without beta blockade, platelet aggregation stimulated by ADP, 1-epinephrine, collagen and arachidonic acid and associated TXB2 formation were not enhanced. Moreover, as already suggested by a trend toward reduced aggregability in these settings, plasma norepinephrine levels in the same range (745 +/- 368 pg/ml) due to infusion (5 micrograms/min) significantly reduced platelet aggregation with low-dose collagen (0.25-0.75 micrograms/ml), I-epinephrine (0.2-1.0 microM) and ADP (0.5-1.5 microM). These data do not support a role of endogenous catecholamines in initiating platelet activation and TXB2 formation.

Alpha 2-adrenoceptors inhibit the cholera-toxin-induced intestinal fluid accumulation

The effects of adrenoceptor agonists and antagonists on the cholera-toxin-induced intestinal fluid accumulation and the mucosal levels of cAMP were investigated in vivo. Cholera toxin produced a marked fluid accumulation. Adrenaline inhibited the effect of the toxin in a dose-dependent manner. An alpha 2-adrenoceptor blocking agent yohimbine antagonized the effect of adrenaline. The alpha 1-adrenoceptor blocking agents prazosin and phenoxybenzamine failed to antagonize the effect of adrenaline. A high dose of a beta-adrenoceptor blocking agent pindolol did not antagonize the effect of adrenaline. Yohimbine or pindolol alone did not produce any effects on the toxin-induced fluid accumulation. However, prazosin and phenoxybenzamine per se inhibited the toxin-induced fluid accumulation. An alpha 2-selective agonist clonidine was slightly more potent than adrenaline, and was about 100-fold more potent than the alpha 1-selective agonist methoxamine in inhibiting the cholera-toxin-induced intestinal secretion. Clonidine, adrenaline and methoxamine failed to reduce the mucosal levels of cAMP, while these alpha-adrenoceptor agonists inhibited the toxin-induced fluid accumulation in the same preparations. These results suggest that the stimulation of alpha 2-adrenoceptors inhibit the cholera-toxin-induced intestinal secretion without reducing the whole mucosal levels of cAMP.

Identification of alpha adrenoceptor subtypes in dog arteries by (3H) yohimbine and (3H) prazosin

Binding of the alpha adrenergic antagonists (3H)prazosin and (3H) yohimbine to membranes of dog arteries exhibit the characteristics expected of alpha adrenoceptors. Binding of both ligands is saturable with dissociation constants of 0.19nM and 1.15nM for (3H)prazosin and (3H)yohimbine respectively. A series of catecholamines inhibit binding of both ligands with a potency in the order epinephrine greater than norepinephrine much greater than isoproterenol, corresponding with the activity of these agents at alpha adrenoceptors in blood vessels. Competition for binding in both instances is stereoselective. l-Phenylephrine has similar potencies in inhibiting (3H)prazosin and (3H)yohimbine specific binding whilst the imidazoline related partial alpha adrenergic agonists clonidine and guanfacine are more potent in inhibiting (3H) yohimbine specific binding. The affinity of prazosin for the (3H)yohimbine binding site is approximately 2500 times less than for the (3H)prazosin site whilst yohimbine is approximately 150 times more potent in inhibiting (3H)yohimbine than (3H)prazosin specific binding. Non-selective alpha adrenergic antagonists have similar affinities for both binding sites. The concentration of (3H)yohimbine binding sites in different arteries vary about two fold whilst for (3H)prazosin in the variation was about three fold. These results indicate that there are two discrete noradrenergic binding sites in the major arteries of dog which have binding properties expected of alpha 1 and alpha 2 adrenoceptors.

Adrenergic receptors in human and experimental pheochromocytoma

Radioligand binding to adrenergic receptors has become an important technique for identification and quantitation of these receptors. In this article we review our recent findings using subtype-selective radioligands for studying alpha 1 (e.g., [3H]prazosin) and alpha 2 (e.g. [3H]yohimbine) receptors in human platelets and rat renal cortical membranes, and we report new results on changes in adrenergic receptors of patients and animals with pheochromocytoma. Five patients with pheochromocytoma were tested prior to surgery. The number of alpha 2-adrenergic receptors on platelets from these patients and the affinity of these receptors for [3H]yohimbine were similar to values for control subjects. In rats with a transplantable pheochromocytoma we found a 50-fold increase in plasma norepinephrine levels. The number of renal cortical beta-adrenergic receptors (identified using [125I]iodohydroxybenzylpindolol) and alpha 1-adrenergic receptors were decreased while the number of alpha 2-adrenergic receptors did not change. Receptor affinities for radioligands were unaltered in animals with pheochromocytoma. These findings indicate that pheochromocytoma is associated with selective decreases in the number of renal beta- and alpha 1-adrenergic receptors without changing alpha 2-adrenergic receptor number or affinity. Furthermore, the number and affinity of alpha 2-adrenergic receptors are not altered in platelets of patients with pheochromocytoma. We conclude that down regulation (an agonist-mediated decrease in receptor number) occurs in vivo for some, but not all, types of adrenergic receptors.

Neuropharmacology of adrenergic neurons in teleost fish

Although this brief review is based on relatively few types of experiments in few species of teleosts, it is possible to summarize some points of interest regarding the similarities and differences in the mechanisms of adrenergic neurotransmission in fish compared to the higher vertebrates. 1. There is a substantial mixing of cranial autonomic ("parasympathetic") and spinal autonomic ("sympathetic") pathways in the cranial nerves. This close relationship between the two systems and the differences in the nature of the neurons of cranial origin (cholinergic, and non-adrenergic, non-cholinergic) and spinal origin (adrenergic, cholinergic and mixed "polynergic") gives a basis in fish also for a complex pattern of innervation of the various organs. 2. Adrenaline is the major transmitter substance in the adrenergic neurons of most teleosts studied, but there are exceptions within the same species. For instance, in the swimbladder mucosa of the cod, noradrenaline dominates, while adrenaline is the major catecholamine in most other organs innervated by adrenergic neurons. The reasons for the regional differences are not known and further studies of the rate of catecholamine turn-over in the adrenergic neurons of fish are clearly indicated. 3. Adrenoceptors of both the alpha- and the beta-type show great similarities with those of mammals. Some differences in the potencies of certain compounds (e.g., clonidine and methoxamine) exist and receptor binding studies should add valuable information about the adrenoceptors of teleosts. The existence of a subtype of beta-adrenoceptor (beta 2) has been proposed and further work is needed to confirm or deny the applicability of the beta 1/beta 2 adrenoceptor terminology in fish. 4. There appears to be some differences in the mode of action of the so called "indirectly acting amines", such as tyramine, between teleosts and mammals. While the uptake of tyramine into the nerve terminals in mammals appears to take place via the cocaine-sensitive neuronal uptake system which is also responsible for catecholamine uptake (uptake 1), tyramine uptake in cod neurons appears to be via a separate pathway. 5. Presynaptic supersensitivity of the type seen in mammals has also been demonstrated in teleost adrenergic neurons. Both denervation (chemical or surgical) and blockade of the neuronal uptake mechanism by cocaine or desipramine produce this type of supersensitivity, while post-synaptic supersensitivity has so far not been described in teleosts. The effects of removal of the uptake system shows that the uptake process may be as important in teleosts as in mammals in the removal of adrenergic transmitter from the synaptic cleft. 6. In the total picture of adrenergic functions in fish, the circulating catecholamines take a special role...

Solubilization of human platelet alpha-adrenergic receptors: evidence that agonist occupancy of the receptor stabilizes receptor--effector interactions

The alpha-adrenergic receptors of human platelet membranes can be directly identified by both a radiolabeled agonist, [3H]epinephrine, and a radiolabeled antagonist, [3H]yohimbine. Digitonin solubilizes a binding component from the membrane that is indistinguishable from the alpha-receptor identified in the native platelet membrane as assessed by (i) order of potency of agonists and antagonists and (ii) affinity of the receptor for [3H]-yohimbine and competing antagonists. However, the solubilized receptor demonstrates a reduced affinity for agonists and a loss of the ability of guanine nucleotides to modulate receptor affinity for agonists. Prelabeling of human platelet membranes with [3H]-epinephrine results in a guanine nucleotide-sensitive agonist-receptor complex that sediments more rapidly in sucrose gradients than do unoccupied or antagonist-occupied receptors. Thus, agonist occupancy of the alpha-receptor prior to membrane solubilization may promote or stabilize receptor interaction with effector components in the membrane, one of which may be the GTP regulatory protein responsible for modulation of receptor affinity.

Stimulation of prostaglandin E2-synthesis by noradrenaline in primary cell cultures from rabbit splenic pulpa is mediated by atypical alpha-adrenoceptors

In primary cell cultures originating from rabbit splenic pulpa the effects of various adrenoceptor agonists on prostaglandin (PG)-synthesis were studied. The cells - microscopically identified as fibroblasts - released PGs into the medium: especially PGE2 besides small amounts of PGF2alpha and PGD2. Noradrenaline increased dose-dependently the amount of PGs released into the medium. Besides noradrenaline, only the catecholamines adrenaline and alpha-methylnoradrenaline strongly activated PG-synthesis. Other alpha-adrenoceptor agonists like the phenylethylamine and imidazoline derivatives were only weak agonists or completely ineffective. All adrenoceptor agonists without intrinsic activity in these cells antagonized the noradrenaline effect on PG-synthesis, the imidazolines being more potent antagonists than the phenylethylamines. The beta-adrenoceptor agonist isoprenaline stimulated PG-synthesis at high concentration only. The effects of both noradrenaline and isoprenaline were inhibited by low concentrations of phentolamine phenoxybenzamine, but not by propranolol. The preferential alpha2-adrenoceptor antagonists yohimbine and rauwolscine were about 50 times more potent in blocking the noradrenaline effect on PG-synthesis than the more alpha1-specific antagonist corynanthine. However, prazosin, another alpha1-adrenoceptor antagonist, was about equipotent with yohimbine. It is concluded that noradrenaline elicits PG-synthesis in rabbit splenic fibroblasts via alpha-adrenoceptor stimulation. The alpha-adrenoceptor involved has properties which are different from those reported so far for alpha1- or alpha2-adrenoceptors.

Interaction of selective alpha-adrenoceptor agonists and antagonists with human and rabbit blood platelets

1 The selectivity of alpha-adrenoceptors mediating the pro-aggregatory response of human and rabbit platelets to adrenaline and the conditions required to permit expression of an aggregatory response to partial agonists at these alpha-adrenoceptors have been studied.2 Yohimbine causes effective blockade of the pro-aggregatory responses whereas indoramin and prazosin are ineffective.3 The clonidine analogue, UK-14304, is nearly as effective as adrenaline in inducing an aggregatory response in human platelets and a pro-aggregatory response in rabbit platelets. Cross-tachyphylaxis between adrenaline and UK-14304 has been demonstrated.4 Clonidine is a weak agonist for the pro-aggregatory response of rabbit platelets and in some donors for the aggregatory response of human platelets.5 Methoxamine induces a pro-aggregatory response in human platelets which is blocked by indoramin or prazosin but not by yohimbine. No such response to methoxamine is observed in rabbit platelets.6 The divalent cation ionophore, A-23187, induces an aggregatory response to clonidine (in platelets from a non-responsive donor), phenylephrine and methoxamine in human platelets and to adrenaline, UK-14304 and clonidine in rabbit platelets. A secretory response to clonidine is also induced by A-23187 in human platelets.7 The adenylate cyclase inhibitor, SQ-22536, is ineffective in either inducing a response to the alpha-agonists or potentiating the effect of A-23187.8 The aggregatory responses to adrenaline and UK-14304 in rabbit platelets and to clonidine in human and rabbit platelets, which can be induced by A-23187, are blocked by yohimbine but not by prazosin or indoramin.9 From these studies we conclude that the pro-aggregatory responses of human and rabbit platelets to adrenaline are mediated primarily by alpha(2)-adrenoceptors. The presence of alpha(1)-adrenoceptors on human platelets is confirmed but these receptors do not appear to be present on rabbit platelets. The conditions required for expression of an aggregatory response to partial agonists at the human and rabbit platelet alpha-adrenoceptors implicate an increase in cytosolic Ca(2+) concentration as a key event in stimulus-response coupling but do not indicate such a role for depression of cyclic adenosine-3',5'-monophosphate concentration.

Mechanisms involved in alpha-adrenergic phenomena: role of calcium ions in actions of catecholamines in liver and other tissues

Epinephrine and norepinephrine binding sites with the physiological characteristics of alpha-adrenergic receptors have been identified in the plasma membranes of liver and other cells. Interaction of catecholamines with these receptors causes a mobilization of calcium ions from mitochondria and perhaps other intracellular stores in liver cells. In other cells, there may also be influx of extracellular calcium ions. Evidence is presented in support of the hypothesis that the rise in cytosolic calcium ions resulting from these changes is responsible for many of the alpha-adrenergic actions of catecholamines. Possible mechanisms by which activation of alpha-adrenergic receptors causes changes in calcium and other aspects of cellular metabolism are discussed.

Alpha adrenoceptors in rat brain: direct identification with prazosin

Tritiated prazosin was used to characterize high affinity binding sites with characteristics similar to alpha 1 adrenoceptors in rat brain membranes. These sites were compared with alpha 2 adrenoceptors labeled with tritiated clonidine. The prazosin sites had an association constant of 2 nM-1 and bound to ligand optimal around pH 7.0. The density of the sites was 300 fmoles per mg of protein; the half time of dissociation of prazosin was 7 min at 30 degrees C. The order or potencies of agonists, determined from binding-inhibition experiments with labeled prazosin, was: naphazoline greater than clonidine greater than adrenaline greater than noradrenaline greater than phenylephrine greater than alpha-methylnoradrenaline greater than dopamine. The order of potencies of antagonists was: prazosin greater than phenoxybenzamine greater than phentolamine greater than clozapine greater than yohimbine. Sodium ions and divalent cations as well as guanyl nucleotides have little or no effect on the binding of the labeled antagonist. This is in contrast to the binding of the labeled agonist clonidine (Glossmann and Presek, 1979a, 1979b). Labeled prazosin may be a useful tool to characterize alpha 1 adrenoceptors.