The Alpha-1 adrenergic receptors
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Renal nerve stimulation (RNS)-induced sodium retention is normally mediated by alpha 1-adrenoceptors. Clinically, chronic prazosin therapy increases rather than decreases sodium retention. In preliminary studies, chronic prazosin (3 days, 3 mg/kg per day) increased renal alpha 2-adrenoceptor density (mean +/- s.e.m., 153 +/- 14 versus 209 +/- 11 fmol/mg protein) in normal rats. We therefore postulated that following chronic prazosin treatment, alpha 2-adrenoceptors may mediate RNS-induced sodium retention. The effect of chronic prazosin pretreatment on the alpha-adrenoceptor subtype mediating RNS-induced antinatriuresis was studied in the perfused rat kidney. In control rats, subpressor levels of RNS (approximately 1 Hz, 10 V, 1 ms) decreased sodium and water excretion. Prazosin 28 nmol/l (alpha 1-blockade) reversed this effect of RNS. Yohimbine 300 nmol/l (alpha 2-blockade) had no effect. In rats treated with prazosin, RNS again decreased sodium and water excretion. In these rats, yohimbine as well as prazosin was necessary to reverse the effects of RNS. Thus, following prazosin pretreatment postjunctional alpha 2- as well as alpha 1-adrenoceptors mediated RNS-induced antinatriuresis. A similar function of the elevated renal alpha 2-adrenoceptors in genetic hypertension in rats could possibly explain the mechanism by which excess sodium retention occurs and thereby the elevation of blood pressure.
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The characteristics of bunazosin binding to alpha-adrenoceptors in the porcine aortic membranes were investigated using [3H]prazosin and [3H]yohimbine binding assays to identify alpha 1- and alpha 2-adrenoceptors, respectively. The extent of the inhibition (Ki values) of [3H]prazosin binding to alpha 1-adrenoceptors induced by bunazosin was 0.29 nmol/l and about the same as that induced by prazosin (Ki = 0.10 nmol/l). The Ki value of bunazosin inhibition of [3H]yohimbine binding to alpha 2-adrenoceptors was 350 nmol/l. There was over a 1000-fold difference in Ki value for bunazosin between alpha 1-and alpha 2-adrenoceptors. Thus, bunazosin is a highly alpha 1 selective agent in vascular smooth muscle.
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Effects of intravenously administered clonidine and DJ-7141 on blood pressure and gastric acid output were studied in anesthetized rats. Both drugs caused a dose-dependent increase in blood pressure. Clonidine-induced increases in the blood pressure were attenuated by yohimbine and prazosin. DJ-7141-induced increases in blood pressure were also attenuated by yohimbine, but were little affected even by a large dose of prazosin. Splanchnic nerve stimulation-induced increases in blood pressure were attenuated by clonidine and DJ-7141. The inhibitory effects of DJ-7141 exceeded those of clonidine. On the other hand, these agonists had no effect on exogenously applied norepinephrine-induced increases in blood pressure. Vagally stimulated gastric acid output was inhibited by small doses of clonidine and large doses of DJ-7141. Clonidine-induced inhibition of acid output was abolished by yohimbine (5 mg/kg), but not by prazosin (5 mg/kg). In contrast, the DJ-7141-induced inhibition of acid output was not affected by yohimbine (5 mg/kg), but was attenuated by a small dose of prazosin (0.1 mg/kg). These results indicate that DJ-7141 had no effect on alpha-2 adrenoceptors in gastric parasympathetic nerves. Furthermore, the alpha-2 adrenoceptors on parasympathetic nerves in the gastric wall are probably pharmacologically different from those on sympathetic nerves and blood vessels in rats.
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The purpose of this study was to determine whether exogenous norepinephrine mediates cerebrovascular constriction via α 1 - or α 2 -adrenoceptors in anesthetized neonatal pigs. Diameters of pial arterioles in anesthetized piglets, 1–6 days old, were investigated using a “closed” cranial window. We examined constrictor effects of norepinephrine on pial arterioles in the absence and presence of relatively selective α 1 ,- (prazosin) and α 2 -(yohimbine) adrenoceptor antagonists (1 mg/kg i.v.). Yohimbine and prazosin inhibited pial arteriolar constriction induced by topical application of clonidine and phenylephrine (10 −6 and 10 −4 M, respectively), and yohimbine did not affect the response to topical phenylephrine. In one group diameter was 188 ± 13 (mean ± SEM) μm during control and 146 ± 12 (μm during 10 −5 M norepinephrine (22 ± 5% constriction). Following yohimbine the same vessels did not constrict significantly. In another group 10 −5 M norepinephrine constricted arterioles by 22 ± 5%, and this response was unaffected by prazosin (24 ± 5% constriction). We conclude that pial arterioles are responsive to both α 1 - and α 2 -adrenoceptor agonists, that intravenous administration of prazosin and yohimbine results in these drugs crossing the blood–brain barrier and inhibiting constrictor effects of agonists, and that norepinephrine constricts pial arterioles predominantly via α 2 -adrenoceptors.
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Postjunctional alpha-1 and alpha-2 adrenoceptors in the coronaries of the perfused guinea-pig heart.
Prazosin and yohimbine were used to differentiate postjunctional alpha adrenoceptors in the coronaries of the perfused guinea-pig heart. Two postjunctional alpha adrenoceptor subtypes were distinguished by the affinities of the receptor for yohimbine and prazosin. The pA2 for yohimbine were 8.74 against alpha-methylnorepinephrine and 8.98 against BHT-920, and the pA2 for prazosin was 9.84 against phenylephrine. Yohimbine was not very active against the alpha-1 selective agonist as was prazosin against the alpha-2 selective agonists. Alpha-1 and alpha-2 postjunctional adrenoceptors mediate vasoconstriction in the whole coronary bed of the perfused guinea-pig heart.
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The nature of alpha adrenergic receptors in human cerebral arteries was characterized and alteration of these receptors after subarachnoid hemorrhage (SAH) was examined using a radioligand binding assay. The specific 3H-prazosin binding to human cerebral arteries was saturable and of high affinity (KD = 4.1 nM) with a Bmax of 92 fmol/mg protein. Specific 3H-yohimbine binding to these tissues was also saturable and of high affinity (KD = 23 nM) with a Bmax 250 fmol/mg protein. IC50 values of adrenergic agents for 3H-prazosin binding were as follows: prazosin, 1.2 X 10(-10) M; phentolamine, 1.3 x 10(-6) M; yohimbine, 1.2 x 10(-5); norepinephrine, 4.9 x 10(-4) M; epinephrine greater than 1 x 10(-3) M. IC50 values of adrenergic agents for 3H-yohimbine binding were as follows: phentolamine, 1.7 x 10(-7) M; yohimbine, 4.2 x 10(-7) M; prazosin, 1.9 x 10(-5) M; epinephrine, 4.4 x 10(-5) M; norepinephrine, 7.9 x 10(-4) M. KD and Bmax of 3H-prazosin and 3H-yohimbine binding after SAH were compared with findings in ...
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The EEG-effects induced by intraperitoneal administration of clonidine, prazosin and yohimbine to 8 and 22 month old rats were compared. Clonidine (0.01 mg/kg) and prazosin (1 mg/kg) increased spectral powers, yohimbine (0.5 mg/kg) decreased them. In the older rats, EEG variations were smaller for prazosin and yohimbine, but larger for clonidine. These findings show that alpha receptor mediated influences on EEG are changed during aging and show that quantified EEG gives a picture of age related changes in the functional state of the neurotransmitter systems.
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