Renal effects of selective alpha-1 and alpha-2 adrenoceptor agonists in conscious, normotensive rats

The effects of the selective alpha-1 adrenoceptor agonist, cirazoline, and the selective alpha-2 adrenoceptor agonist, B-HT 933, were assessed on renal hemodynamics and on water and solute excretion in conscious, chronically instrumented rats. Infusion (i.v.) of equipressor doses of cirazoline and B-HT 933, 0.04 and 4 mg/kg/hr, respectively, decreased renal plasma flow without changing glomerular filtration rate. Cirazoline infusion did not affect urinary excretion of water, electrolytes or total solutes. In marked contrast, B-HT 933 increased urine flow and sodium excretion significantly (P less than .01) but did not significantly alter potassium and urea excretion. Urine osmolality decreased to hyposmotic levels (from 613 +/- 86 to 172 +/- 8 mOsmol/kg of H2O) during the infusion of B-HT 933, suggesting a possible interaction between the alpha-2 adrenoceptor agonist and the vasopressin system. This diuretic action of the selective alpha-2 adrenoceptor agonist was also observed after the i.v. infusion of a subpressor dose (0.4 mg/kg/hr) of B-HT 933. In rats treated with the ganglionic blocker, hexamethonium (10 mg/kg i.v.), the B-HT 933-induced diuresis was not affected, confirming an action in the periphery, possibly at the level of the kidney. These results suggest that stimulation of renal alpha-2 adrenoceptors in conscious, euvolumic rats modulates the reabsorption of water and sodium at the site of the renal nephron.     

Systemic and regional hemodynamic characterization of alpha-1 and alpha-2 adrenoceptor agonists in pithed rats

In pithed rats, blood pressure dose-response curves to i.v. cirazoline, methoxamine and phenylephrine (full alpha-1 adrenoceptor agonists) exhibited higher maxima than those to B-HT 920, M-7, UK-14,304 (full alpha-2 adrenoceptor agonists) and indanidine (Sgd 101/75: partial alpha-1 adrenoceptor agonist). For an 80 mm Hg increase in blood pressure, full alpha-1 adrenoceptor agonists enhanced total peripheral, renal and mesenteric vascular resistances significantly more than alpha-2 adrenoceptor stimulants or indanidine. In contrast, all compounds produced a similar degree of hindquarter vasoconstriction, suggesting that both types of alpha adrenoceptors have the same functional importance in this skeletal muscle vascular bed. Application of a multivariate discriminant analysis to the drug-induced changes in the total peripheral and mesenteric vascular resistances associated with a pressor effect of 80 mm Hg allowed their assignment to two distinct groups corresponding to the full alpha-1 and the full alpha-2 adrenoceptor agonists plus indanidine. All investigated compounds in low doses increased cardiac output, which returned to base-line values after high doses of alpha-1 but plateaued after high doses of alpha-2 adrenoceptor agonists or indanidine. alpha-1 adrenoceptor agonists decreased whereas alpha-2 stimulants and indanidine successively increased and then decreased renal blood flow. Finally, all investigated compounds increased hindquarter blood flow at low doses but decreased it at high doses. The ratios of the doses of cirazoline required to produce a 100% rise in systemic and local vascular resistances in the presence or in the absence of prazosin were of similar magnitude. This was also true for M-7 when studied in the presence or in the absence of yohimbine. These findings suggest pharmacological identity within alpha-1 as well as within alpha-2 adrenoceptor populations in all investigated vascular beds. Finally, the calcium entry blocker diltiazem did not affect the increases in systemic and regional resistances evoked by cirazoline but depressed profoundly the effects of M-7 and indanidine. In conclusion, full alpha-1 and alpha-2 adrenoceptor agonists can be discriminated easily on the basis of their systemic and regional hemodynamics in the pithed rat. That the hemodynamic effects of the partial alpha-1 adrenoceptor agonist indanidine are similar to those of alpha-2 adrenoceptor agonists and susceptible to calcium channel blockade suggests that the alpha-1 adrenoceptors stimulated by this drug have the same coupling modality as alpha-2 adrenoceptors and share with the latter the same functional expression when stimulated.     

Undifferentiated effects of calcium antagonists on pressor responses to selective alpha-1 and alpha-2 adrenoceptor agonists in anesthetized, spinal dogs

Whether pressor responses mediated by alpha-2 adrenoceptors are more susceptible to calcium antagonists than those mediated by alpha-1 adrenoceptors was investigated in anesthetized, spinal dogs. All drugs were administered i.v. Methoxamine (3-100 mu/kg) or xylazine (3-300 micrograms/kg) produced a sustained increase in mean arterial pressure but almost no effect on heart rate. Both the initial and the sustained phase of the pressor response to methoxamine were selectively antagonized by prazosin, whereas those to xylazine were selectively antagonized by yohimbine. These results indicate that the peripheral arterial bed of the dog comprises alpha-1 and alpha-2 adrenoceptors and that both the initial and sustained phases of the pressor response to methoxamine are predominantly mediated by alpha-1 adrenoceptors, whereas those to xylazine are mediated by alpha-2 adrenoceptors. The calcium antagonists, i.e., nifedipine (0.3-3 micrograms/kg), diltiazem (10-100 micrograms/kg) and KB-944 (10-100 micrograms/kg), administered during the sustained phase of the pressor responses to equieffective doses of methoxamine (100 micrograms/kg) and xylazine (1000 micrograms/kg), lowered mean arterial pressure. The three calcium antagonists in these doses also lowered the baseline mean arterial pressure but to a lesser extent than the elevated one. These results altogether indicate that the calcium antagonists were more effective in lowering mean arterial pressure elevated by either an alpha-1 or alpha-2 adrenoceptor agonist than the base-line mean arterial pressure and that the sustained phase of the pressor response mediated by alpha-1 adrenoceptors would involve Ca++ influx as much as or more than those mediated by alpha-2 adrenoceptors.     

In vitro effects of beta adrenoceptor agonists and antagonists on the rat ovarian suspensory ligament

The mesovarian suspensory ligament of the rat was used to compare the activities of beta adrenoceptor agonists and antagonists. The following beta adrenoceptor agonists, in descending order of potency, inhibited spontaneous activity in a dose-related manner: zinterol greater than isoproterenol much greater than dobutamine. Several noncardioselective, beta-2 adrenoceptor antagonists with intrinsic sympathomimetic activity (ISA) also inhibited the activity of the ligament: pindolol greater than alprenolol = bucindolol = oxprenolol greater than labetalol. Maximal relaxation induced by the antagonists was equivalent to that caused by the beta receptor agonists. Two cardioselective, beta adrenoceptor antagonists with ISA, acebutolol and practolol, did not inhibit the activity of the suspensory ligament but did increase the rate of the isolated right atrium of the rat. The maximal increases in atrial rate evoked by the antagonists were significantly less than those induced by the beta adrenoceptor agonists. Studies with ICI 118,551 or atenolol as beta-2 or beta-1 selective adrenoceptor blockers, respectively, suggest that the beta adrenoceptors of the suspensory ligament are predominantly of the beta-2 subtype. The possible relevance of these results to the induction of mesovarian leiomyomas in rats by noncardioselective beta adrenoceptor agonists and antagonists with ISA is discussed.     

Ca++ utilization in the constriction of rat aorta to full and partial alpha-1 adrenoceptor agonists

l-Norepinephrine and l-phenylephrine were full agonists and cirazoline, SKF d-89748, Sgd 101/75 and SKF l-89748 were partial agonists in contracting rat isolated aortic rings. Clonidine, l-amidephrine and St 587 were found ineffective. Nifedipine (10(-8)-10(-6) M) abolished the contractions to Sgd 101/75 and to high K+ with similar potency but only partially inhibited the contractions to the other alpha adrenoceptor agonists. Norepinephrine, phenylephrine, cirazoline and Sgd 101/75 were full agonists in stimulating 45Ca++ influx, which amounted to 50% of the maximal influx produced by high K+. SKF d- and l-89748 behaved as partial agonists, whereas St 587, amidephrine and clonidine were virtually inactive. Nifedipine was equally effective in blocking the influx of 45Ca++ produced by K+ and the alpha adrenoceptor agonists. Norepinephrine stimulated 45Ca++ efflux to an extent similar to that for high K+. In the following order of decreasing efficacy, phenylephrine, cirazoline and SKF d- and l-89748 caused significant stimulation of 45Ca++ efflux. Sgd 101/75, amidephrine, clonidine and St 587 were without effect. However, Sgd 101/75 (10(-5) M) antagonized the 45Ca++ efflux of norepinephrine. Nifedipine (3 X 10(-7) M) completely suppressed the K+-induced 45Ca++ efflux but only partly affected the 45Ca++ efflux caused by the alpha adrenoceptor stimulants. A highly significant (r = 0.975) linear relationship was found between the nifedipine-resistant contractile response and the 45Ca++ efflux obtained in the presence of nifedipine. The data suggest that the stimulation of alpha-1 adrenoceptors in rat aorta can activate two distinct processes of Ca++ utilization for contraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Discriminative stimulus properties of xylazine in rat: discriminability and effects of putative alpha-2 adrenoceptor agonists and antagonists

In a first experiment, rats were trained initially to discriminate i.p. injections of 2.5 mg/kg of xylazine from saline in a two-lever, food-reinforced, fixed-ratio 10 drug discrimination procedure. The animals were then retrained on progressively lower doses of the training drug. The stimulus generalization gradient of xylazine was established each time the animals had reached criterion on successive training doses; the criterion required that lever selection be appropriate to the saline or xylazine injection on 10 consecutive sessions. The findings indicate the lowest xylazine training dose at which individual rats can reach criterion to range from 2.5 to less than or equal to 0.31 mg/kg; the median lowest discriminable dose was 0.89 mg/kg. The distribution of lowest discriminable dose accommodated a Gaussian shape. The slope of the xylazine gradient did not in a systematic manner vary as a function of training dose, and the ED50 value of xylazine was an almost constant fraction of training dose (1:2.0) at training doses ranging from 2.5 to 0.63 mg/kg. A second experiment examined the xylazine-like agonist and the xylazine-antagonist effects of clonidine, yohimbine, piperoxan and idazoxan in rats discriminating xylazine from saline at a training dose of 2.5 mg/kg. Clonidine produced generalization in all animals and antagonized the xylazine stimulus in one. Idazoxan produced near complete antagonism, and was generalized in one animal. Yohimbine and piperoxan produced magnitudes of agonist and antagonist effects that were intermediate between those of clonidine and idazoxan. Lisuride and d-lysergic acid diethylamide produced up to 71 and 29% generalization, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of vasopressin in response to intrarenal infusions of alpha-2 adrenoceptor agonists

Previous studies have indicated that the effects of renal alpha-2 adrenoceptor stimulation are mediated through the blockade of the renal effects of vasopressin. If this premise is correct then 1) specific antagonists of the antidiuretic effect of vasopressin (V2 antagonists) should mimic alpha-2 adrenoceptor stimulation and 2) in the presence of V2 antagonists, the diuretic and natriuretic effect of clonidine should be attenuated. The renal effects of [d(CH2)5,D-Ile2,Ile4]AVP, a specific V2 antagonist, were studied. On the day of the experiment, uninephrectomized rats were anesthetized, and the carotid artery and jugular vein were cannulated for recording blood pressure and saline infusion, respectively. The left kidney was exposed and the ureter cannulated. A 31-gauge needle was advanced into the renal artery to permit direct i.r. infusion of study drugs. Bolus doses of the V2 antagonist (0, 1, 3, 10, or 30 nmol/kg i.v.) produced a dose-related increase in urine volume and free water clearance at all doses tested. Sodium excretion increased only at the higher doses (10 and 30 nmol/kg). This dose-related dissociation in water and then sodium excretion is similar to that observed after i.r. clonidine infusions. In the presence of the V2 antagonist, clonidine (3 micrograms/kg/min) had no effect on urine volume or free water clearance but significantly decreased the excretion of sodium from control. These results demonstrate that V2 antagonists mimic the effects of i.r. clonidine. As well, in the absence of vasopressin (V2 antagonism), the effects of clonidine are attenuated. Moreover, they are also consistent with not only an antidiuretic role for endogenous vasopressin but also an antinatriuretic one.     

Dopamine1 receptor agonist and alpha-2 adrenoceptor antagonist effects of fenoldopam in rabbits

Neurochemical and circulatory effects of fenoldopam were studied in pithed rabbits with electrically stimulated sympathetic outflow and in strips of the rabbit pulmonary artery. In pithed rabbits, fenoldopam (1-30 micrograms/kg/min) decreased the arterial blood pressure. Fenoldopam (3-30 microgram/kg/min) also increased the norepinephrine spillover rate (the rate at which endogenous norepinephrine enters into the plasma after having been released from postganglionic sympathetic nerves) and decreased the [3H]norepinephrine plasma clearance. The selective dopamine (DA)1 antagonist SCH 23390 (bolus injection of 10 micrograms/kg followed by infusion of 2 micrograms/kg/hr) antagonized markedly and the DA2-selective antagonist domperidone (bolus injection of 200 micrograms/kg followed by infusion of 50 micrograms/kg/hr) antagonized slightly the hypotensive effect. The increase in the norepinephrine spillover rate was enhanced after treatment with desipramine. Clonidine (0.3 microgram/kg/min) reduced the spillover of norepinephrine, and this effect was abolished by fenoldopam (30 micrograms/kg/min). In pulmonary artery strips preincubated with [3H]norepinephrine, fenoldopam (10(-7) and 10(-6) M) increased the electrically evoked overflow of tritium. The effect of fenoldopam (10(-6) M) was prevented in the presence of a supramaximal concentration of clonidine (10(-5) M). The results suggest that fenoldopam lowers blood pressure mainly by activation of vascular smooth muscle DA1 receptors. In addition, however, it blocks prejunctional alpha-2 autoreceptors at postganglionic sympathetic axons.     

Selective alpha-2 adrenoceptor agonists alter fluid and electrolyte transport in mammalian small intestine

The effects of the selective alpha-2 adrenoceptor agonists B-HT 920 and B-HT 933 on fluid and electrolyte transport in mammalian small intestine were assessed in vitro and in vivo. In Ussing flux chamber preparations of rabbit ileum, B-HT 920 reduces basal short-circuit current (Isc) in a concentration-dependent manner. This in vitro effect is inhibited by rauwolscine (KB = 9.7 nM) but not by prazosin. Isotope flux and ion replacement studies suggest that this decrease in Isc is due primarily to stimulation of a HCO3-dependent transport process. B-HT 920 promptly attenuates the 16,16-dimethyl prostaglandin E2 (dmPGE2)-stimulated increase in Isc and completely reverses dmPGE2-stimulated Cl secretion to absorption. Oral administration of B-HT 933 dose-dependently inhibits dmPGE2-induced enteropooling in conscious rats. This effect of B-HT 933 is likewise blocked significantly by rauwolscine but not by prazosin. Similar effects of B-HT 933 are observed on enteropooling in the pithed rat as are the effects of B-HT 920 in the conscious rat. These results indicate that selective alpha-2 adrenoceptor agonists from the azepine class of compounds have significant proabsorptive and antisecretory activities in the rabbit small intestine in vitro and in the rat intestine in vivo. This in vivo effect does not appear to be central nervous system mediated. These studies suggest that these alpha-2 adrenoceptor agonists may be useful in converting the hypersecreting mammalian small bowel to its normal absorptive state.     

Regional differences of responses to adrenoceptor agonists in isolated and perfused canine coronary arteries

Regional differences of the responses to adrenoceptor agonists were investigated in isolated canine coronary arteries by use of a cannula inserting method. Acetylcholine induced a dose-dependent vasodilation. Norepinephrine and epinephrine produced a vasoconstriction followed by a strong vasodilation in large coronary arteries and only a weak vasodilation in small coronary arteries. Phenylephrine (a selective alpha-1 agonist) induced a strong vasoconstriction in both arteries. The threshold dose and ED50 value for phenylephrine in small coronary arteries were much larger than those in large coronary arteries, although the vasoconstrictions by KCl and prostaglandin F2 alpha were not different between in large and small coronary arteries. Clonidine and xylazine (selective alpha-2 agonists) produced a slight vasoconstriction but not dose-dependently and a vasodilation with extremely large doses. ED50 value of vasodilation for salbutamol (a selective beta-2 agonist) was approximately 80 times greater than that for isoproterenol (a non-selective beta-agonist) in large coronary arteries, but was approximately 20 times in small coronary arteries. The maximal dilator response to salbutamol was about the same as that to isoproterenol in small coronary arteries, whereas it was much smaller than that to isoproterenol in large coronary arteries. These results suggest that adrenoceptors are heterogeneous according to the distance from the coronary orifice in canine epicardial coronary arteries.     

Mechanisms involved in the hyperglycemic response induced by clonidine and other alpha-2 adrenoceptor agonists

Clonidine induced a dose-dependent hyperglycemic response in fed rats, a minimal hyperglycemic response in 48-hr fasted rats and had no effect on blood glucose in 16-hr fasted, streptozotocin-diabetic rats. At a dose of 0.1 mg/kg, there was an equivalent hyperglycemic response in fed rats whether clonidine was administered orally, i.v. or i.p. A hyperglycemic effect was also observed with the central and peripheral alpha-2 adrenoceptor agonist, guanabenz (0.1 mg/kg i.v.). In contrast, 2-(3-4-dihydroxyphenylimino) imidazoline, an alpha-2 agonist which does not penetrate into the central nervous system, caused a lowering of blood glucose at the same dose. The hyperglycemic response induced by clonidine was partially inhibited by the selective alpha-2 antagonists, yohimbine and rauwolscine, and the nonselective alpha antagonist, phentolamine. The hyperglycemic response induced by clonidine was not affected by the selective alpha-1 adrenoceptor antagonists, prazosin or corynanthine. Methoxamine, an alpha-1 agonist, had no effect on clonidine-induced hyperglycemia. The hyperglycemic response to clonidine was partially inhibited by 3-mercaptopicolinic acid, an inhibitor of gluconeogenesis, but was not affected by pretreatment with the H2-histamine receptor antagonist, metiamide, the prostaglandin syntheses inhibitor, idomethacin, or the beta adrenoceptor antagonist, propranolol. These results suggest that 1) the hyperglycemic response induced by clonidine and other alpha-2 adrenergic agonists is mediated by alpha-2 adrenergic receptors located within the central nervous system and 2) clonidine-induced hyperglycemia is effected, in part, by enhanced gluconeogenesis.     

Periadventitial angiopoietin-1 gene transfer induces angiogenesis in rabbit carotid arteries

This study was performed to evaluate angiogenic responses of angiopoietin-1 (Ang1) in vivo after adenovirus-mediated gene transfer in the periadventitial space of the rabbit carotid arteries using a collar technique. Adenoviruses encoding LacZ and vascular endothelial growth factor (VEGF) receptor-1-Ig fusion protein (VEGF-R1-Ig) adenoviruses were used as controls. Increased neovessel formation was seen in adventitia of the Ang1 transduced arteries 7 days after the gene transfer. Neovessels in the Ang1 transduced arteries were large in size and well perfused. Ang1 binds to Tie2 (tyrosine kinase with immunoglobulin and epidermal growth factor homology domain) receptors, which were expressed in the endothelium of the neovessels. When VEGF-R1-Ig was used with Ang1, it resulted in a decrease in the number of neovessels, which implies that VEGF-A or some other VEGF-R1 ligand(s) play a crucial role in angiogenesis occurring in response to Ang1. There were no significant differences in the total number of capillaries in the adventitia of the VEGF-R1-Ig transduced arteries as compared to LacZ controls. Neointima formation was not increased in the Ang1 transduced arteries as compared to the controls. We conclude that in the periadventitial space Ang1 shows angiogenic activity and is a potentially useful factor for the induction of therapeutic vascular growth in vivo.     

Interaction of imidazolines with alkylation-sensitive and -resistant alpha-1 adrenoceptor subtypes.

The interaction of imidazolines with alpha-1 adrenoceptor subtypes sensitive and resistant to inactivation by SZL-49 and chlorethylclonidine (CEC) has been evaluated. Clonidine, oxymetazoline, phentolamine and naphazoline or the phenethylamine, phenylephrine, interacted with high- and low-affinity sites labeled by [3H]prazosin. SZL-49 (1-1000 nM) eliminated the high-affinity sites and caused a significant reduction of the low-affinity sites. CEC (1-100 microM) reduced the number of low-affinity sites, while the effect on high-affinity sites was dependent on the route of administration. In control aortic rings the dose-response curves for either clonidine or naphazoline were biphasic, consisting of high- and low-affinity components. Only the high-affinity component was blocked by prazosin. SZL-49 was more potent than CEC at inhibiting agonist-induced contraction of rat aortic rings. The agonist responses obtained after treatment with either SZL-49 or CEC were only weakly antagonized by prazosin. The combination of SZL-49 and CEC produced no greater inhibition of muscle contraction than did SZL-49 alone. These data show that 1) imidazolines interact with different affinity at sites labeled by [3H]prazosin and these sites correspond to the alpha-1a and alpha-1b adrenoceptor subtype designation; 2) imidazolines induce smooth muscle contraction by interacting at high- and low-affinity sites; 3) these low-affinity sites do not appear to have properties of an alpha-1 adrenoceptor; 4) there may be three sites of interaction for imidazolines on the aorta, the alpha-1a and alpha-1b adrenoceptors and a site that does not have alpha-1 adrenoceptor characteristics.     

Differential effects of the calcium entry blocker D 600 on contractions of rat and guinea-pig aortas, elicited by various alpha-1 adrenoceptor agonists

Contractions of rat and guinea-pig aortas to the alpha-adrenoceptor agonists I-norepinephrine, cirazoline , St 587, clonidine and Sgd 101/75 and the effect of calcium entry blockade by D 600 on the responses were evaluated. In rat aorta, D-600 (10(-8) to 10(-5) M) effectively and concentration-dependently reduced the maximal responses to St 587, clonidine and Sgd 101/75 (maximal percentage inhibition 84 +/- 2.1, 86 +/- 4.1 and 65 +/- 2.8, n = 5-6, respectively) in contrast to those of I-norepinephrine and cirazoline (maximal percentage inhibition 23 +/- 2.1 and 21 +/- 4.1, n = 6, respectively). Reducing the receptor number on rat aorta by approximately 85% by means of the irreversible blocker dibenamine did not result in a greater sensitivity of norepinephrine-induced contractions toward D 600. Prazosin was found 800 to 1000 times more potent than yohimbine in antagonizing the contractile effects of St 587, clonidine and cirazoline on rat and guinea-pig aortas. No difference existed between the pA2 values of prazosin and yohimbine against the different agonists; the pA2 values of prazosin and yohimbine were significantly higher in rat aorta than those in guinea-pig aorta. The results can be explained by assuming the existence of two different agonist recognition sites on the alpha-1 adrenoceptor on rat aorta, whereas the alpha-1 adrenoceptor on guinea-pig aorta contains one agonist recognition site only. The architecture of the alpha-1 adrenoceptor on rat aorta must therefore be different from that in guinea-pig aorta.     

The agonistic and antagonistic effects of short acting estrogens: a review

Based on a review of the literature, this paper clarifies the pharmacologic properties of shortacting estrogens and their role in physiology and medicine. Shortacting estrogens display mixed agonist-antagonistic properties when injected in saline. The mixed estrogenic function results from the rapid clearance of these compounds from target tissue. When administered by pellet implant, however, shortacting estrogens act as full agonists. Both the uterotropic and vaginotropic response patterns of these compounds are detailed. Shortacting estrogens stimulate early uterotropic responses while having little effect on true uterine growth when administered by injection in saline. Thus, they have no antagonistic action when examined by shortterm uterotropic assays, but display partial antagonism when longterm uterine growth assays are used. Previous research has suggested that shortacting estrogens would not be effectual or antagonistic if present in a continuous fashion which would result in constant or longterm occupancy of the estrogen receptor. Estradiol, however, does manifest these properties when injected. Shortacting estrogens do not act as antagonists on vaginotropic responses as they do uterotropic responses. The paper also reviews the functions of these compounds in various physiological states, including blood binding, metabolism, menstruation, and pregnancy. Finally, clinical considerations are discussed. Estriol has an apparent selective effect on vaginotropic events. It has been effective in correcting symptoms of menopause, for example. However, estriol is not believed to have a protective effect against breast cancer. When it is present in a continuous fashion, estriol acts as an estrogen, thereby ruling out such an effect.     

Inhibition of vasopressin-stimulated cyclic AMP accumulation by alpha-2 adrenoceptor agonists in isolated papillary collecting ducts

The effects of selective alpha adrenoceptor agonists and antagonists on vasopressin (VP)-sensitive cyclic AMP (cAMP) formation in microdissected rat papillary collecting ducts were examined. In the presence of 10(-10) M VP, norepinephrine and the selective alpha-2 adrenoceptor agonist, B-HT 933, produced almost total inhibition of VP-stimulated cAMP accumulation. Half-maximal inhibition occurred at 1 x 10(-8) M and 6 x 10(-7) M for norepinephrine and B-HT 933, respectively. Cirazoline, a selective alpha-1 adrenoceptor agonist, had no significant effect on VP-stimulated cAMP accumulation. The inhibitory effects of norepinephrine and B-HT 933 were antagonized by rauwolscine but not by prazosin. The antagonism of B-HT 933-induced inhibition of VP-stimulated cAMP accumulation was competitive with an antagonist dissociation constant (KB) of 10.9 x 10(-9) M. Preincubation of papillary collecting ducts with pertussis toxin (1 microgram/ml for 1 hr at 37 degrees C) attenuated, by 65%, the inhibitory effect of B-HT 933 on VP-stimulated cAMP levels. These results demonstrate that alpha-2 adrenoceptors capable of inhibiting VP action are present on the papillary collecting duct. Furthermore, the alpha-2 adrenoceptor-induced inhibition of VP-stimulated cAMP accumulation is pertussis-toxin sensitive. This suggests that alpha-2 adrenoceptors are coupled negatively to adenylate cyclase, via the guanine nucleotide binding protein, in the collecting tubule.     

Postjunctional alpha-1 and alpha-2 adrenoceptors in human skin arteries. An in vitro study

Experiments were performed to characterize the postjunctional alpha adrenoceptors that mediate adrenergic constriction in human skin arteries. Abdominal s.c. arteries were obtained from patients who died 3 to 12 hr before, and vascular segments 2 mm in length and 600 to 1050 microns in external diameter were prepared for isometric tension recording. On application of norepinephrine, phenylephrine (alpha-1 adrenergic agonist) or clonidine (alpha-2 adrenergic agonist) the arteries contracted in a dose-dependent manner and, in terms of the mean EC50 values, the order of potencies was clonidine greater than norepinephrine greater than phenylephrine. With regard to their ability to induce maximal contraction, the order was norepinephrine = phenylephrine greater than clonidine. In the presence of phentolamine (nonspecific alpha adrenergic antagonist) or yohimbine (selective alpha-2 adrenergic antagonist) the control curve for norepinephrine was displaced to the right in a parallel way. Prazosin (selective alpha-1 adrenergic antagonist) depressed both the slope and maximal response of the control curve for norepinephrine but the shift was not significant. Prazosin and yohimbine produced a parallel rightward shift in the control curve for phenylephrine and clonidine, respectively. These results suggest that skin arteries of humans have a mixed population of postjunctional alpha-1 and alpha-2 adrenoceptors and that alpha-2 adrenoceptors are more prominent. They also suggest that the alpha-2 adrenergic component of the response to norepinephrine is a low-maximum effect compared to the alpha-1 adrenergic component. This could be of significance in regulating skin blood flow and thermoregulatory function.     

Relationship between alpha adrenoceptor occupancy and response for the alpha-1 adrenoceptor agonist, cirazoline, and the alpha-2 adrenoceptor agonist, B-HT 933, in canine saphenous vein

The relationship between alpha adrenoceptor occupancy and response was investigated for the alpha-1 adrenoceptor agonist, cirazoline, and the alpha-2 adrenoceptor agonist, B-HT 933, in canine saphenous vein, a tissue known to contain both alpha adrenoceptor subtypes. Both cirazoline and B-HT 933 produced dose-dependent vasoconstrictor responses in canine saphenous vein, with the maximum vasoconstrictor response elicited by B-HT 933 being only 75% of that produced by cirazoline. Dissociation constants were obtained for cirazoline (0.61 microM) and B-HT 933 (4.99 microM) for interaction with postsynaptic vascular alpha-1 and alpha-2 adrenoceptors, respectively, by the method of fractional irreversible receptor inactivation using phenoxybenzamine. Based on this information, alpha-1 and alpha-2 adrenoceptor occupancy-response relationships were constructed. The alpha-1 adrenoceptor occupancy-response relationship obtained for cirazoline was approximately 4-fold more favorable than the alpha-2 adrenoceptor occupancy-response relationship for B-HT 933, although both agonists were associated with a significant receptor reserve. The alpha-1 adrenoceptor occupancy-response relationship of cirazoline and the alpha-2 adrenoceptor occupancy-response relationship of B-HT 933 were both rectangular hyperbolas, suggesting that both compounds have high intrinsic efficacy at their respective alpha adrenoceptor subtypes. The results indicate that a receptor reserve exists for the alpha-1 and alpha-2 adrenoceptor-mediated effects of cirazoline and B-HT 933, respectively, and that the alpha adrenoceptor reserve may be significantly larger for postsynaptic vascular alpha-1 adrenoceptors than for postsynaptic vascular alpha-2 adrenoceptors in canine saphenous vein.(ABSTRACT TRUNCATED AT 250 WORDS)     

An evaluation of the ability of a series of full alpha-1 adrenoceptor agonists to release internal calcium in venous smooth muscle

Our previous studies have shown that activation of postsynaptic alpha-2 adrenoceptors mainly utilizes extracellular calcium whereas activation of postsynaptic alpha-1 adrenoceptors mobilizes both extracellular and intracellular calcium to produce contractions in the canine saphenous vein. In the present study, the abilities of several full alpha-1 adrenoceptor agonists to release internal calcium for contractions in the canine saphenous vein were evaluated. Contractions to these alpha-1 agonists at two different equieffective concentrations (concentrations that produced 50 and 80% of the maximum phenylephrine response, PE50 and PE80, respectively) were determined in both zero external calcium medium and in normal calcium medium containing 5 mM La . If a correlation exists between the efficacy of an agonist and its ability to release internal Ca++, the contractile response to each agonist should be similar under these conditions. The results indicated marked variations in mechanical responses elicited by these alpha-1 agonists at both PE50 and PE80 concentrations. The data suggest a lack of correlation between efficacy and the ability to release internal calcium to induce contractions for a series of full alpha-1 adrenoceptor agonists in venous smooth muscle.