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This study investigates the incidence of high-salt diet in blood pressures, renal alpha(2)-adrenoceptor subtypes distribution, and gene expression in salt-sensitive (SBH) and salt-resistant (SBN) Sabra rats. Comparisons have been made between SBH and SBN rats submitted to a normal or a high-salt diet for 6 weeks. Only alpha(2)B-adrenoceptors are detected in kidneys of SBH rats, whatever the diet. In contrast, mRNA corresponding to alpha(2)A- and alpha(2)B-subtypes are found in this substrain. In these rats, high-salt diet increases blood pressures and up-regulates gene expression and density of only alpha(2)B-adrenoceptors. Inversely, alpha(2)A- and alpha(2)B-adrenoceptors and corresponding mRNA are found in kidneys of SBN rats. In these rats, a high-salt diet does not affect blood pressures but increases gene expression and densities of both alpha(2)A- and alpha(2)B-adrenoceptors. If the up-regulation of renal alpha(2)B-adrenoceptor subtypes is indicative of the hypertensive phenotype, the present study shows that this mechanism is also present in normotensive salt-resistant Sabra rats. In fact, the absence of alpha(2)A-adrenoceptors in SBH could be responsible for the lack of adequate receptor-mediated renal functions predisposing to salt-sensitivity and consequently the development of hypertension. Conversely, the presence of this receptor in SBN rats and its up-regulation could be protective change against the increase of alpha(2)B-adrenoceptors induced by the salt overload and could consequently be responsible for the resistance to salt-induced hypertension.

Obesity is a major cause of human essential hypertension and there are clear evidences that abnormal kidney functions play a key role in obesity hypertension. Feeding rats a cafeteria diet has been extensively used as an experimental model to study obesity and energy balance expenditure. The present study investigated whether rats fed a cafeteria diet develop hypertension with alterations in renal alpha2-adrenoceptor subtype distribution. Weight gain induced by feeding rats a cafeteria diet during 8 weeks was associated with a marked increase in blood pressure. Insulin levels were higher in these hypertensive rats, leading to a decreased plasma glucose/insulin ratio. Based on radioligand-binding studies using [3H]-RX821002 and selective competitors, a raise in alpha2-adrenoceptor density that was solely due to an increased alpha2B-adrenoceptor subtype density was detected in the kidney of the cafeteria-fed rat. Furthermore, reverse transcription-polymerase chain reaction (RT-PCR) experiments showed an overexpression of the gene encoding the alpha2B-adrenoceptor subtype in these rats. On the other hand, despite a similar mRNA level, the alpha2A-adrenoceptor subtype was no more detectable by radioligand-binding studies in the kidney of the cafeteria-fed rat. In conclusion, cafeteria-fed rats are hypertensive, with renal alterations in alpha2-adrenoceptor distribution. These alterations, which are not related to genetic factors, may play a key role in the onset of hypertension.

Purpose: The present study evaluated the expression of alpha1-adrenoceptor subtypes in human corpus cavernosum., Materials and Methods: The mRNA encoding alpha1a, alpha1b and alpha1d subtypes were assessed by RNA-directed complementary cDNA synthesis followed by Taq DNA amplification. The level of alpha1 mRNA was calculated in arbitrary optical density units and normalized with respect to the length of the respective cDNA fragments., Results: We found that alpha1a, alpha1b and alpha1d adrenoceptor subtypes are expressed in human corpus cavernosum, with a predominant expression of the alpha1a subtype., Conclusion: These results suggest that alpha1a-adrenoceptor subtype is important and that understanding the biochemical and functional characteristics of this subtype may lead to the development of specific antagonists in the treatment of impotence.

Studies in humans have suggested that hyperinsulinemia might play an important role in salt sensitivity and in the later development of high blood pressure. This possibility has been tested in this study on Sabra rats, an animal model of salt-induced hypertension. Salt-sensitive (SBH) and salt-resistant (SBN) Sabra rats have been submitted to either a normal (0.2% NaCl) or a high salt (8% NaCl) diet for 6 weeks. Comparisons of blood pressure, basal glucose and insulin levels, and insulin response to glucose overload (1 g/kg) have been made. As pancreatic alpha2-adrenergic receptors are implicated in the control of insulin release, their densities have been determined on plasma membranes by saturation studies with [3H]-RX-821002 as the specific radioligand. Under normal diet, blood pressures were respectively 133 +/- 9 and 108 +/- 10 mm Hg (n = 6) in SBH and SBN. Basal glucose and insulin levels and insulin response to glucose overload were found to be significantly higher in SBH than in SBN. In contrast, alpha2-adrenergic receptor densities were lower (P < .001) in SBH when compared to SBN. High salt diet increased (P < .01) blood pressure, decreased basal glucose (P < .01) and insulin (P < .001) levels only in SBH. However, when compared to SBN the insulin response to glucose overload was maintained higher in SBH. Alpha2-adrenergic receptor densities and difference between SBH and SBN did not differ from those found in normal diet. In conclusion, the salt-induced high blood pressure of salt-sensitive Sabra rats is not associated with hyperinsulinemia and insulin resistance. Indeed, an improvement in the insulin sensitivity appears to be induced by either a high salt diet or high blood pressure.

alpha 2-adrenoceptors are involved in various renal functions regulating blood pressure. They were classified in subtypes whom genes were identified in both humans and rats. In rat renal cortex it was evidenced that the alpha 2B isoform is predominant. This result was confirmed in Sabra rats. However, the renal cortex alpha 2B density is higher in salt-sensitive (SBH) than in salt-resistant (SBN) Sabra rats. alpha 2B-adrenoceptors were recently subclassified in two pharmacologically distinct subtypes exhibiting high and low affinity for guanoxabenz and respectively called alpha 2B1 and alpha 2B2. We studied sodium loading effect on alpha 2B1 and alpha 2B2 distribution in Sabra rat renal cortex using competition experiments between [3H]-yohimbine and guanoxabenz. The rats were submitted to normal (0.2%) or high sodium diet (8%) for six weeks. Under normal diet, proportion alpha 2B1 and alpha 2B2 was similar in SBH and SBN. Nevertheless, their respective densities were significantly higher in SBH as compared to SBN (alpha 2B1: 90.6 +/- 4.1 vs 57.4 +/- 2.5 fmoles/mg prot, p < 0.0001; n = 5; alpha 2B2: 102.7 +/- 4.0 vs 66.4 +/- 4.6 fmoles/mg prot; p < 0.0001; n = 5). Under high sodium diet the distribution of these two isoforms was altered. The densities of alpha 2B1 were decreased by 27.0 +/- 5.9% in SBH (68.0 +/- 4.0 fmoles/mg prot; p < 0.0001, n = 5) and by 47.3 +/- 7.4% for SBN (29.2 +/- 3.1 fmoles/mg prot; p < 0.0001; n = 5). Conversely, the densities of alpha 2B2 were increased by 28.3 +/- 5.4% in SBH (131.1 +/- 9.5 fmoles/mg prot; p < 0.001; n = 5) and by 75.0 +/- 17% in SBN (123.2 +/- 9.1 fmoles/mg prot; p < 0.0001; n = 5). In conclusion, alpha 2B1- and alpha 2B2-adrenoceptor subtypes are found in renal cortex of both SBH and SBN. Our data demonstrated an equal distribution of these two isoforms between SBH and SBN under normal salt diet. This distribution is largely altered, especially in SBN, by the high sodium diet. From these modifications might result differential renal responses to activation of alpha 2B-adrenoceptors between SBH and SBN, and consequently responsible for normal or high blood pressure after high sodium diet.

Objective: To assess whether alterations of alpha 2-adrenoceptor subtypes in distribution and gene expression in the renal cortex could explain the predisposition to salt-sensitivity or salt-resistance in Sabra rats., Design: Studies were performed using plasma membranes and RNA preparations from renal cortex of 8- to 10-week-old Sabra salt-sensitive (SBH) and salt-resistant (SBN) rats on a normal-sodium diet., Methods: The alpha 2-adrenoceptor subtypes were determined by competition experiments with [3H]-yohimbine or [3H]-RX821002. Their gene expression was studied by RNA-directed complementary DNA synthesis followed by Taq DNA polymerase amplification., Results: Binding studies showed that alpha 2B- and alpha 2A-adrenoceptor subtypes represented in SBN 72 and 28% of the maximal binding capacities of the two radioligands, respectively. In contrast, only the alpha 2B subtype was detected in the SBH rat. However, the use of guanoxabenz disclosed alpha 2B-adrenoceptors in alpha 2B1 and alpha 2B2 subtypes. The densities of those alpha 2B subtypes appeared to be higher in the SBH rat than in the SBN rat. Messenger RNA corresponding to alpha 2A and alpha 2B subtypes were found both in SBH rats and in SBN rats. The expression of the alpha 2B subtype was permanently higher in the SBH rats than in the SBN rats. The expression of the alpha 2A gene in the SBH rats suggests a specific SBH post-transcriptional regulation resulting in the absence of alpha 2A-adrenoceptor., Conclusions: Differences exist in the renal cortex concerning expression and distribution of alpha 2-adrenoceptor subtypes between SBH and SBN rats. From these differences there might result different alpha 2-adrenoceptor-mediated renal functions in SBH and in SBN rats, which could lead to a predisposition to sensitivity or resistance to a high sodium intake.

In vivo, testosterone-treatment of female hamsters for 4 days promotes a doubling of alpha 2-adrenoreceptor protein in parametrial adipocytes, with a concomitant accumulation of the alpha 2A-adrenoreceptor subtype mRNA. During in vitro incubation of minced parametrial fat pads for 6 to 48h with testosterone or dihydrotestosterone (100 nM), alpha 2A-adrenoreceptor protein and mRNA levels were also increased and remained to control levels when an antiandrogen or actinomycin D were added in the medium. It is concluded that in hamster adipocytes, androgens upregulate alpha 2A-adrenoreceptor subtype expression at the mRNA level by an androgen receptor-dependent transcriptional activation.

The influence of the androgenic status on the steady-state amounts of Gi alpha 1-2 and Gs alpha subunits was compared in hamster fat cell membranes from the femoral subcutaneous (FSC) and epididymal (EP) adipose tissues, using immunoblotting experiments. In sham-operated hamsters, Gi alpha 1-2 and Gs alpha steady-state amounts found in FSC fat cells were 38% and 40% reduced, respectively, as compared to EP adipocytes. In EP fat cells, castration induced a down-regulation of both Gi alpha 1-2 (-39%) and Gs alpha (-33%), whereas testosterone replacement restored Gs alpha, but not Gi alpha 1-2 levels, to control values. In contrast, these G protein alpha-subunits were insensitive to the androgenic status in FSC fat cells. These data provide the first evidence that the androgenic status can modulate the expression of both the Gi alpha 1-2 and Gs alpha subunits of the fat cell adenylate cyclase regulatory Gi and Gs proteins and that this modulation depends on the anatomical origin of these cells.

Existing evidences indicate that a crossed regulation between alpha 2-adrenergic receptors and Na+/H+ exchanger(s) exists, that Na decreases the affinity of alpha 2-adrenergic receptors for agonists and antagonists, that intracellular Na+ and H+ ion concentrations regulate Na+/H+ exchanger activity, that intracellular pH controls the affinity of the alpha 2-adrenergic receptors for their agonists and antagonists. Alterations of alpha 2-adrenergic receptor densities and allosteric regulation by sodium have been demonstrated in sodium-dependent hypertension in rats. Increased Na+/H+ exchanger activity has been reported in genetic hypertension. Nevertheless, cosegregation experiments and human genetic polymorphism suggest that the exchanger could not be related to hypertension. We propose the following hypothesis: the increased Na+/H+ exchanger characteristic of hypertension could be secondary to the abnormalities of the alpha 2-adrenergic receptors found in hypertension, probably through the alteration of the sodium allosteric effect on these receptors.

In the present study, pharmacological and biochemical binding characteristics of [3H]idazoxan, an originally thought alpha 2-adrenoceptor antagonist, have been determined in smooth muscle of rabbit urethra. It is shown that [3H]idazoxan labels with high affinity non-adrenergic binding sites. Specific binding of [3H]idazoxan is inhibited by compounds possessing an imidazoline or a guanidinium moiety whereas phenylethanolamines and classical alpha 2-antagonists are ineffective competitors which suggests an imidazoline-preferring binding site. However, imidazolidines such as clonidine and paminoclonidine are poorly effective, which differs considerably from pharmacological characteristics of imidazoline binding sites previously reported in the central nervous system. In addition, it is shown that K+ and Mn2+ inhibit [3H]idazoxan binding in a competitive and non-competitive manner, respectively. Other cations such as Na+, Li+ and Mg2+ have no significant effect. It is shown that K+ accelerates the dissociation of [3H]idazoxan binding while Mn2+ does not produce any modification. These results suggest that K+ may bind to an allosteric site, while Mn2+ may bind with a membrane component susceptible to alter [3H]idazoxan binding sites.

Sodium ions markedly decreased in vitro renal alpha 2-adrenoceptor affinity for epinephrine in Sabra hypertensive (SBH) but not in normotensive (SBN) rats. Under these conditions, affinity of alpha 1-adrenoceptor for epinephrine was unchanged in SBH and SBN rats. If these data could be confirmed in vivo, the sodium ion, by acting as an inhibitor, could modify the effect of agonists on renal alpha 2-adrenoceptors in SBH rats. Conversely, the absence of sodium regulation in SBN rats might represent a genetically mediated change responsible for the resistance to the development of salt-induced hypertension.

In the present study, we have provided evidence that [3H] rauwolscine and [3H] idazoxan bind to different sites in rabbit urethra. The [3H] idazoxan capacity and affinity was 215 +/- 14 fmol/mg protein and 1.59 +/- 0.16 nM while [3H] rauwolscine binding parameters were 45.9 +/- 3.4 fmol/mg protein and 2.39 +/- 0.27 nM. [3H] idazoxan specific binding was inhibited only by compounds possessing an imidazoli(di)ne or a guanidinium moiety, while [3H] rauwolscine specific binding was inhibited by phenylethanolamines and classical alpha 2-antagonists. [3H] idazoxan was inhibited by KCl in a competitive and by MnCl2 in a non-competitive way, while other cations such as Na+, Li+ and Mg2+ did not inhibit [3H] idazoxan binding. Moreover, we investigated the regional distribution of [3H] idazoxan and [3H] rauwolscine along the rabbit urethra using quantitative autoradiography. Analysis of the films revealed a different distribution of these two binding sites on the urethral sections.

Imidazoline binding sites have been characterized in organs modulating blood pressure, such as brain and kidney with (3H)-p-aminoclonidine and (3H)-Idazoxan respectively. However, the pharmacological characteristics of the imidazoline-preferring binding sites differ considerably depending on the species investigated and the radioligand used. Little is known about the physiological relevance of the non-adrenergic (3H)-idazoxan binding sites. As some imidazolines and certain alpha-adrenoceptor agonists possess antihypertensive activity, an alteration of these binding sites should be considered as a possible causes in the development of hypertension. In the present study, we performed binding studies with the imidazoline ligand (3H)-idazoxan in renal cortex of hypertensive salt-sensitive (SBH) and normotensive salt-resistant (SBN) Sabra rats. (3H)-idazoxan binding capacities were higher in SBH than in SBN rats. Competition studies have shown for (3H)-idazoxan specific binding non-adrenergic characteristics exclusively. In these both substrains, (3H)-idazoxan binding exhibit pharmacological profile of imidazoline binding sites. However, theses sites have also high affinity for guanidino compounds and amiloride. Surprisingly, amiloride and some analogues were significantly more potent in SBN than in SBH rats. From this study, it is difficult to elucidate the physiological role of imidazoline binding sites in renal cortex. However, differences observed between SBN and SBH suggest that these sites may play a role in the development of hypertension in Sabra rats.

Binding of [3-H]-dihydroalprenolol ([3-H]-DHA) to rat cardiac membranes was rapid and reversible (k1 = 0.633-0.701 x 10(6) M(-1) S(-1) And k(-1) = 0.0017-0.0043 s(-1). 2 [3-H]-DHA bound to a single class of binding sites with an equilibrium dissociation constant (Kd25degreesc) of 5.7+/-1.1 x 10(-9) M. 3 This binding was specific and the order of potency of adrenoceptor agonists in competing for the binding sites was (-)-isoproterenol greater than (+/-)-isoproternol greater than (+)-isoproterenol greater than (-)-adrenaline greater than (-)-noradrenaline. This was in agreement with the beta1 nature of the cardiac beta-receptors. 4 Cardioselective beta-blockers (i.e. metoprolol, acebutolol and practolol) were shown to have lower binding site affinities, when compared to other blockers. This may be related to steric hindrance by the side-chain at the aromatic end of these molecules.

Acidification of a T7 DNA sample was found to be partly irreversible as ultraviolet difference spectra measured at various sub-melting temperatures were different from those observed for a 'normal' DNA sample. This implies some subtle conformational change which is not reversed by return to neutral pH. In the same conditions, only poly(purine)-poly(pyrimidine) polymers behaved in a different manner, during premelting, according to whether they were previously acidified or not. The properties of acidified and reneutralized T7 DNA were also investigated for Escherichia coli RNA polymerase binding and transcription. An inhibition of RNA synthesis and chain initiation was observed. The results suggest that the binding of the enzyme is affected. RNA synthesized is specific but there is a decrease in the number and in the stability of the RNA-polymerase-DNA complexes.

Successively are learned in this paper general characteristics of cental alpha-adrenergic receptors and individualization of alpha 1- and alpha 2-adrenergic sub-groups, biochemical factors which are able to affect fixation of their respective ligands and finally physiological and pathological variations of alpha 1- and alpha 2-adrenergic receptors, which will allow to discuss their participation in different regulations and disorders.

Cerebral and renal alpha 2-adrenoceptors are implicated in the control of sympathetic activity and of sodium reabsorption respectively. In addition, sodium ions play an important role in the regulation of either alpha 2-adrenoceptor densities and affinities for adrenergic agonists. In the present study, alpha-adrenoceptor properties were investigated in genetically predetermined salt-sensitive and salt-resistant Dahl and Sabra rats. Cerebral alpha 2-adrenoceptor densities were higher in salt-resistant than in salt-sensitive Dahl and Sabra rats. In contrast, renal alpha 2-adrenoceptor density was higher in salt-sensitive than in salt-resistant rats. No difference in cerebral and renal alpha 1-adrenoceptor densities was observed between Dahl and Sabra substrains. Noradrenaline content in cerebral and renal cortex were also similar in both these rat substrains. Sodium ions markedly increased cerebral and renal high-affinity alpha 2-adrenoceptor densities in salt-sensitive but not in salt-resistant rats. Cerebral and renal alpha 1-adrenoceptor densities were unchanged in salt-sensitive and salt-resistant substrains of Dahl and Sabra rats. In addition, sodium ions reduced the affinity of adrenaline for renal alpha 2-adrenoceptors in salt-sensitive rats but not in salt-resistant rats. We can conclude that there exist genetically determined differences in the densities and properties of cerebral and renal alpha 2-adrenoceptors between salt-sensitive and salt-resistant rat strains. Abnormal densities of alpha 2-adrenoceptors may play a primary role in the role in the development of hypertension in salt-sensitive animals. These results also suggest an association between absence of sodium regulation of alpha 2-adrenoceptors and resistance to salt-induced hypertension. The absence of sodium regulation in salt-resistant rats may be linked either to a particular receptor conformation or to an abnormal structure of the receptor system. This property may represent a genetically-mediated change responsible for the resistance to the development of salt-induced hypertension.

In rat vas deferens, (+/-)-170 150 proved to be an alpha 2-adrenoceptor blocking agent with a high alpha 2/alpha 1 selectivity ratio. In rat cerebrocortical membranes, (+/-)-170 150 was seven times more potent than yohimbine for inhibiting specific [3H]clonidine binding. However, (+/-)-170 150 appeared to be 3.5 times less selective for alpha 2-adrenoceptors sites than yohimbine. These results indicate that (+/-)-170 150 is a potent preferential alpha 2-adrenoceptor blocking agent, as had been shown by in vivo experiments.

In vitro studies of the residual thymocyte population isolated 48 h after in vivo hydrocortisone injection showed: 1. These cells are partly sensitive to steroid as demonstrated by uridine incorporation inhibition. 2. This residual cell fraction appears heterogeneous after centrifugation on a bovine serum albumine gradient. 3. These cells exhibit low steroid binding and DNA synthesis capacities.

The binding characteristics of [3H]idazoxan and [3H]rauwolscine, two potent alpha 2-adrenoceptor antagonists, were compared in the rabbit urethral smooth muscle. The maximal binding capacity was 6 times higher for [3H]idazoxan than for [3H]rauwolscine in male rabbits. No difference was observed for these radioligands in female rabbits. There were marked differences in the ability of alpha 2-adrenergic compounds to inhibit [3H]idazoxan and [3H]rauwolscine binding. These results were consistent with the existence of non-alpha 2-adrenoceptor sites for [3H]idazoxan in the rabbit urethral smooth muscle.

Stimulation of washed rat platelets with thrombin resulted in an increased turnover of phosphoinositides. Adrenaline and isoproterenol both inhibited thrombin-induced phosphatidic acid formation in a dose-dependent manner. Inhibitory responses of both compounds were blocked by a beta-adrenoceptor antagonist. However, isoproterenol was a more potent inhibitor than adrenaline. Addition of a selective alpha2-adrenoceptor antagonist potentiated the inhibitory effect of adrenaline up to the level observed with isoproterenol. Prestimulation of beta-adrenoceptors with isoproterenol, followed by addition of adrenaline (or noradrenaline) markedly diminished the inhibitory effect induced by the full beta-adrenoceptor agonist. Our results indicate that, in rat platelets, catecholamines are able to counteract, via alpha2-receptors, the beta-adrenoceptor-mediated inhibition of thrombin-induced phosphatidic acid formation. This suggests that catecholamines, by controlling cAMP level, may modulate phospholipase C activity and thereby platelet reactivity.

Prazosin is know to block postsynaptic alpha-adrenoceptors. In this study 3H-prazosin has been used to label biochemically central alpha-adrenoceptors. In rat brain membranes 3H-prazosin bound specifically in a rapid, reversible and saturable manner to a single class of high affinity sites. The relative order of potencies for inhibition of 3H-prazosin binding was WB4101 greater than ARC 239 greater than phentolamine greater than piperoxane greater than yohimbine which is a characteristic of the alpha 1 type of adrenoceptors. In contrast, the relative order of potencies for inhibition of 3H-clonidine binding was yohimbine greater than piperoxane greater than WB4101 greater than ARC239 greater than prazosin which is a characteristic of the alpha 2 type of adrenoceptors. These results indicate that 3H-prazosin binds to central 'alpha 1'-receptors and 3H-clonidine to 'alpha 2'-receptors and confirm the presence of two classes of alpha-adrenoceptors in rat brain membranes.

1. Separation of mouse thymocytes by centrifugation on a discontinuous bovine serum albumin gradient leads to the isolation of four subpopulations of cells. 2. The study of I13H]uridine incorporation in vitro by these subpopulation in the presence of steroid shows that one of them is corticoresistant. 3. However, the binding capacity of these subpopulations measured by incubation with [3H]dexamethasone is very similar. 4. It is therefore concluded that in mouse thymus, in contrast with lymphoma cells, corticoresistance may not be explained by a defect of cytoplasmic glucocorticoid receptors.

The interaction of a new antihypertensive agent, labetalol was demonstrated in subcellular preparations enriched in alpha- and beta-adrenergic receptors. This result give direct confirmation of pharmacological data observed in isolated organs and in vivo. It illustrates the value of biochemical studies of receptors in order to define the mechanism of action of drugs.

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.

1. 3H-prazosin binds to brain alpha 1-adrenoceptors with high affinity (K(D) 25 degrees C approximately or equal to 1 nM). The number of binding sites in different regions of the brain varied from 44-114 fmoles/mg of protein: striatum = medulla oblongata less than hypothalamus less than cortex. 2. 3H-p-aminoclonidine labels brain alpha 1-adrenoceptors. Its binding capacity (66-209 fmoles/mg prot). is higher in the hypothalamus greater than cortex greater than striatum greater than or equal to medulla oblongata. Its affinity varies from one region to another (0.85 mM in cortex, 3.0 nM in hypothalamus). 3. S 3341 in a potent inhibitor of 3H-p-aminoclonidine like clonidine and a weak inhibitor of 3H-prazosin. 4. Comparison of the inhibitory effect of S 3341 and clonidine on the binding of 3H-prazosin and 3H-aminoclonidine indicates that S 3341 is 2-3 times more specific for alpha 2-adrenoceptors than clonidine. 5. Inhibition constants of S 3341 and clonidine (K1) are not identical in various brain regions but clonidine and S 3341 constants vary in parallel.