Selective Alpha 2-adrenoceptor Agonist Clonidine Research Articles

Pharmacological characterization of the postsynaptic alpha-adrenoceptors in human uterine artery.

Prazosin and yohimbine were used to differentiate postjunctional alpha-adrenoceptors in the human uterine artery in-vitro. Two postjunctional alpha-adrenoceptor subtypes were distinguished by the affinities of the receptor for yohimbine and prazosin. The pA2 for prazosin was 8.91 against phenylephrine with a slope not significantly different from unity (0.91), and the pA2 for yohimbine was 7.25 against naphazoline and 8.70 against clonidine, with slopes not significantly different from unity (1.11 and 1.18, respectively). Yohimbine was not very active against phenylephrine, while prazosin was very active against the mixed and selective alpha 2-adrenoceptor agonist noradrenaline and clonidine; the intercepts of the Schild plot were 8.80 and 8.82 but with slopes significantly less than unity (0.77 and 0.67, respectively). Prazosin competitively antagonized phenylephrine at the alpha 1-adrenoceptor, whereas yohimbine competitively antagonized naphazoline and clonidine at the alpha 2-adrenoceptor. It is concluded that both alpha 1- and alpha 2-adrenoceptors are present in the human uterine artery.

Hemodynamic effects of clonidine in two contrasting models of autonomic failure: Multiple system atrophy and pure autonomic failure

We assessed the effects of clonidine on blood pressure (BP) and heart rate (HR) in multiple system atrophy (MSA), where the autonomic nervous system lesion site is preganglionic, and in pure autonomic failure (PAF), where it is postganglionic. In normal subjects, intravenous infusion of the selective alpha2-adrenoceptor agonist clonidine reduces BP and plasma noradrenaline (NA) levels by means of central alpha2-adrenoceptor action, as well as inducing growth hormone (GH) release. Clonidine-induced GH release is impaired in MSA but spared in PAF. However, the hemodynamic effects of clonidine have not been studied extensively in these disorders. We examined intravenous clonidine test results (performed in our autonomic laboratories using the London Autonomic Units protocol) in 58 patients: 39 with probable MSA and 19 with PAF. Systolic BP (SBP), diastolic BP (DBP), HR, and NA levels were measured supine at baseline and for up to 60 minutes after clonidine. Clonidine resulted in a significant BP fall in MSA patients, which occurred earlier (within 15 minutes of clonidine) and to a greater extent than seen in PAF patients. MSA and PAF patients showed reduction in HR after clonidine administration, although this finding was significantly greater in MSA than in PAF patients. NA levels decreased significantly after clonidine administration in both groups. Although basal NA levels were lower in PAF than in MSA patients, there was no difference in NA reduction relative to baseline between groups. MSA patients showed significant negative correlation between basal NA levels and BP response to clonidine. Clonidine infusion reduces BP and HR in both MSA and PAF groups but to a greater extent in MSA patients. The greater vasodepressor action of clonidine in MSA patients suggests that there is partial preservation of brainstem sympathetic outflow pathways in MSA and may reflect its action at sites in the brainstem and spinal cord that were in part functionally preserved in MSA. Despite similar degrees of NA reduction after clonidine administration, the vasodepressor effect of clonidine was attenuated in PAF compared with MSA patients. This attenuation in PAF patients may reflect greater peripheral alpha2-adrenoceptor denervation supersensitivity due to the postganglionic lesion site. These BP differences, thus, may reflect the underlying lesion site in MSA and PAF, and the hemodynamic data after clonidine infusion may help differentiate these conditions.

Interaction of the antidepressant mirtazapine with alpha2-adrenoceptors modulating the release of 5-HT in different rat brain regions in vivo.

Mirtazapine (MIR) is a novel antidepressant, reported to raise extracellular noradrenaline (NA) through blockade of alpha2-autoreceptors and serotonin (5-HT) output via (1) indirect activation of facilitatory alpha1-adrenoceptors on the cell bodies of ascending 5-HT neurones and (2) blockade of presynaptic release-modulating alpha2-heteroreceptors on 5-HT terminals in the forebrain. To further assess the effect of MIR on NA/5-HT system interplay, including putative regional differences in the effects of the drug on 5-HT release in rat forebrain, we used in vivo microdialysis in anaesthetised rats. Probes were implanted in the dorsal hippocampus (DH) and frontal cortex (FCx), representing median and dorsal raphe 5-HT projection areas, respectively. In the DH, MIR (10 mg/kg s.c.) completely blocked the 5-HT release-suppressing action of the selective alpha2-adrenoceptor agonist clonidine (0.1 mg/kg s.c.), but had no effect per se on the 5-HT output. Neither drug significantly changed the 5-HT levels in the FCx. MIR perfused locally (10 microM via reverse-dialysis) also failed to significantly elevate 5-HT output, and did not affect the clonidine response in either brain area. Thus, the data confirm the basic alpha2-adrenoceptor-blocking properties of MIR, but are only partly concordant with previous studies reporting an increase of 5-HT output after MIR alone. Moreover, we find no elevation in 5-HT by the reference alpha2-adrenoceptor antagonist idazoxan (0.3-1.0 mg/kg s.c.). The discrepancies encountered, and the potential ability of alpha2-adrenoceptor antagonists in general to raise the output of 5-HT, are discussed with particular reference to methodological and other factors that may influence the experimental outcome (e.g., brain regional aspects, different alpha2-adrenoceptor subtypes, potential differences in adrenoceptor tone under varying experimental conditions).

Involvement of alpha-adrenoceptors in the excitatory effect of the A2 adenosine receptors agonist 5'-N-ethylcarboxamidoadenosine (NECA) on cardiac automaticity in the isolated right ventricle of the rat.

The effects of the non-selective A2 adenosine receptor agonist 5'-N-ethyl-carboxamidoadenosine (NECA) were studied on ventricular automaticity induced by a local injury in the isolated right ventricle of the rat. In concentrations ranging from 0.1 to 100 nM, NECA significantly increased ventricular automaticity. This effect was not apparent when the nonselective alpha-adrenoceptor blocker phenoxybenzamine was present at a concentration of 10 microM, which antagonizes both alpha 1- and alpha 2-adrenoceptors, as well as when rats were pretreated with reserpine. In non-reserpinized rats, the excitatory effect of NECA was also abolished in the presence of the selective alpha 1-adrenoceptor antagonist prazosin, but not in the presence of the alpha 2-adrenoceptor antagonist idazoxan. In reserpinized rats, the excitatory effect of NECA was restored in the presence of the non specific alpha-adrenoceptor agonist phenylephrine as well as in the presence of the selective alpha 1-adrenoceptor agonist amidephrine but not in the presence of the selective alpha 2-adrenoceptor agonist clonidine. These results suggest that the excitatory effect of NECA on ectopic ventricular automaticity is dependent on endogenous catecholamines and that alpha-adrenoceptors of type 1 are, in some way, involved in this effect.

Neurotransmitter feedback is not important in modulating the noradrenergic component of responses of rat vas deferens to twin pulse electrical field stimulation.

1. Drug effects on the full time-course of tension responses of the rat vas deferens to challenges of twin pulse field stimulation (TPFS) were examined. A microprocessor-controlled system was used to regulate stimulus delivery, on-line data collection and subsequent data analysis. 2. The second, noradrenergic phase of the response to the second stimulus of TPFS was missing when the interpulse interval was set at 3 s but was progressively restored as the interpulse interval was extended to 120 s. 3. With a 3 s interpulse interval, the missing second phase of the response to the second stimulus was not restored by the selective alpha 2-adrenoceptor antagonists yohimbine, imiloxan or idazoxan, indicative that alpha 2-adrenoceptor-mediated feedback inhibition of noradrenaline release is not the predominant mechanism modulating this response component. 4. Incubation with the P1-purinoceptor antagonist 8-phenyl-theophylline also failed to restore the missing noradrenergic component in the response to the second stimulus of TPFS. 5. Nevertheless, both responses to TPFS were impaired by the selective alpha 2-adrenoceptor agonist clonidine and by the P1-purinoceptor agonist 2-chloroadenosine, indicating the presence of functional presynaptic receptors of both types. These agonist-induced inhibitory effects were readily reversed by those antagonists which had failed to restore the missing noradrenergic component in the second response to TPFS.

Interactions of alpha 2-adrenoceptor antagonists with medetomidine and with ethanol in a holeboard test

The effects of two selective alpha 2-adrenoceptor agonists (clonidine and medetomidine) and antagonists (atipamezole and idazoxan) were examined in the holeboard test. The interactions of the two antagonists with ethanol were also investigated. Atipamezole (0.1–3.0 mg/kg) and idazoxan (0.01–0.3 mg/kg) were without effect on either directed exploration or locomotor activity in the holeboard test, whereas clonidine (0.003–0.1 mg/kg) and medetomedine (0.003–0.1 mg/kg) were sedative. Atipamezole (1–3 mg/kg) and idazoxan (0.3–1.0 mg/kg) reversed the behavioral effects of 0.1 mg/kg of medetomidine. When administered with ethanol (2 g/kg), atipamezole (1–3 mg/kg) showed a significant antagonism of the ethanol-induced reduction in exploratory head-dipping: no change in the locomotor stimulant properties of ethanol was seen. A similar trend was seen for exploration after a combination of idazoxan (1–3 mg/kg) with 2 g/kg ethanol; however, the 3 mg/kg dose attenuated the locomotor stimulant effect. Both antagonists caused a dose-related reduction in the increase in head-dipping seen after administration of 1 g/kg of ethanol, without any effect on the locomotor stimulant effect of this dose. These results suggest mediation of alpha 2-adrenoceptors in some of the behavioral effects of ethanol.

The thermogenic actions of α2‐adrenoceptor agonists in reserpinized mice are mediated via a central postsynaptic α2‐adrenoceptor mechanism

1. The dose-related effects of the selective alpha 2-adrenoceptor agonists clonidine, UK-14,304 and B-HT 933 on the body temperature of untreated and reserpine-treated mice were investigated. 2. In untreated mice all three agonists induced a dose-related hypothermia. The highest doses of UK-14,304 and B-HT 933, 3 and 100 mg kg-1 respectively, elicited a marked (10 degrees C) hypothermia, whereas the maximal hypothermic effect of clonidine (5.5 degrees C) was less pronounced and reached a plateau at a dose of 0.5 mg kg-1 i.p. 3. Reserpine (2.5 mg kg-1, s.c.) induced a marked hypothermia in the mouse; 18 h after injection body temperature had decreased to only slightly (0.5-1.5 degrees C) above ambient (19 degrees C). 4. All three alpha 2-agonists produced a partial dose-related reversal of reserpine-induced hypothermia; maximal thermogenic responses (9-10 degrees C increases in body temperature) were elicited by doses of 0.2, 0.5 and 16 mg kg-1 i.p. of clonidine, UK-14,304 and B-HT 933 respectively, and the log dose-response curves for all 3 agonists were bell-shaped. 5. Following intracerebroventricular administration to reserpine-treated mice, the thermogenic response to clonidine was more rapid in onset, and the agonist was 20 fold more potent than when injected i.p. 6. The selective alpha 2-adrenoceptor antagonists, idazoxan (0.05-0.5 mg kg-1), Wy 26392 (0.3-5.0 mg kg-1) and yohimbine (0.1-1.6 mg kg-1) given orally attenuated the thermogenic responses to all 3 agonists in reserpinized mice in a dose-related manner. Pretreatment with a single dose of idazoxan (0.3 mg kg-1, orally) elicited a 6 fold parallel shift to the right in the dose-response curve to clonidine. 7. The selective alpha 1-adrenoceptor antagonists, prazosin (10 mg kg-1) and indoramin (3-10 mg kg-1), and the beta-adrenoceptor antagonist, propranolol (10 mg kg-1), only partially attenuated the thermogenic responses to the alpha 2-agonists in reserpinized mice. These effects were variable and not clearly dose-related. 8. Pretreatment of reserpinized mice with the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, markedly attenuated (60-95%) the thermogenic response to the noradrenaline uptake inhibitor, desipramine (0.13-12.5 mg kg-1, i.p.), but only slightly reduced (10-35%) that to clonidine (0.032-0.5 mg kg-1, i.p.). 9. These results suggest that alpha2-adrenoceptor agonists reverse reserpine-induced hypothermia via a central mechanism involving activation of postsynaptic alpha 2-adrenoceptors.

Selective α2‐adrenoceptor activation by clonidine: effects on 45Ca2 efflux and insulin secretion from isolated rat islets

A possible role for Ca2+ in the alpha-adrenoceptor-induced inhibition of glucose-stimulated insulin secretion was studied in isolated rat islets by the use of the selective alpha 2-adrenoceptor agonist clonidine. We found that clonidine, in contrast to the alpha 1-adrenoceptor agonist phenylephrine, inhibited glucose-stimulated insulin secretion at dose levels below 10(-6) mol l-1. In islets preloaded with 45Ca2+ and perifused at 2 mmol l-1 Ca2+, clonidine (10(-6) mol l-1) reduced the glucose (13.3 mmol l-1)-stimulated 45Ca2+ efflux during both the first and second phases of insulin secretion. Furthermore, the inhibitory effect of clonidine on glucose (13.3 mmol l-1)-stimulated insulin secretion was partially counteracted by raising the extracellular Ca2+ concentrations. Moreover, the calcium channel agonist Bay K 8644 counteracted the inhibition by clonidine on glucose-stimulated insulin secretion. Our results suggest that selective alpha 2-adrenoceptor-induced inhibition of glucose-stimulated insulin secretion is mediated, at least partially, by restraint of Ca2+-influx. This action might in turn be exerted through interference with the voltage-dependent calcium channels.

Alpha 2-adrenoceptor mediated inhibition of the release of radiolabelled 5-hydroxytryptamine and noradrenaline from slices of the dorsal region of the rat brain.

Possible local interactions between noradrenergic and serotonergic systems in the dorsal raphe region of the rat were investigated by studying the effects of various drugs on depolarization (20 mmol/l K+)-induced release of [3H]5-hydroxytryptamine (5-HT) and [3H]noradrenaline (NA) in vitro using a superfusion method. Exogenous 5-HT did not influence the release of [3H]NA. However, NA (in the presence of 10 mumol/l desipramine) as well as the selective alpha 2-adrenoceptor agonists clonidine and oxymetazoline strongly inhibited [3H]5-HT release. The selective alpha 1-adrenoceptor agonists phenylephrine and methoxamine did not affect the release of either [3H]5-HT or [5H]NA. The inhibition by NA of both [3H]5-HT and [3H]NA release was not affected by the beta-adrenoceptor antagonist sotalol nor by the selective alpha 1-adrenoceptor antagonist prazosin. However, phentolamine and the selective alpha 2-adrenoceptor antagonists yohimbine and rauwolscine competitively antagonized the inhibitory effect of NA on [3H]NA release (respective pA2-values 7.5 and 8.3) and on [3H]5-HT release (respective pA2-values 7.7 and 8.2). Moreover, the release of [3H]NA and also, but to a lesser extent, that of [3H]5-HT were increased by the antagonists. It is concluded that the release of both 5-HT and NA in the dorsal raphe region may be subject to presynaptic inhibition by NA via activation of alpha 2-adrenoceptors.

Neuropeptide y- and alpha-adrenergic receptors in pig spleen: Localization, binding characteristics, cyclic amp effects and functional responses in control and denervated animals

The localization of neuropeptide Y binding sites in the pig spleen, as revealed by [ 125I]Bolton-Hunter-labelled porcine neuropeptide Y and alpha 1-adrenergic receptor binding sites, as revealed by [ 125I](2-beta/4-hydroxy-phenyl/-ethylaminomethyl)-tetralone as radioligand, was compared with the distribution of neuropeptide Y and noradrenaline nerves, the latter revealed by tyrosine hydroxylase and dopamine-beta-hydroxylase, using immunohistochemistry. A large degree of codistribution was obtained between [ 125I]neuropeptide Y and alpha 1-binding sites in the capsule, trabeculae, blood vessels and the red pulp of the spleen. Neuropeptide Y and tyrosine hydroxylase as well as dopamine-beta-hydroxylase-positive nerves were identical in the spleen and had a similar gross distribution pattern as the [ 125I]neuropeptide Y and alpha 1 binding sites. In functional studies using the isolated blood-perfused spleen from pentobarbital-anaesthetized pigs, neuropeptide Y, noradrenaline and the alpha 1-selective agonist phenylephrine contracted the capsule and induced vasoconstriction in the spleen in vivo. However, the selective alpha 2-adrenoceptor agonists clonidine and azepexole had no effects on blood flow or perfusion pressure, suggesting that postjunctional alpha-receptors were of the alpha 1 type. Neuropeptide Y inhibited the forskolin-evoked, cyclic adenosine monophosphate formation in vitro. The [ 125I]neuropeptide Y binding, with an equilibrium-dissociation constant of 503 ± 73pM and a maximal number of specific binding sites of 23 ± 3fmol/mg protein, the neuropeptide Y-induced perfusion-pressure increase in vivo and the inhibition of forskolin-evoked cyclic adenosine monophosphate formation in vitro were dependent on the amidation of the C-terminal portion of the peptide molecule. Furthermore, the effects of neuropeptide Y were not changed by alpha- and beta-adrenoceptor blockade using prazosin and propranolol. Two weeks after postganglionic denervation the neuropeptide Y and the noradrenaline contents of the pig spleen were reduced by 97% and 99%, respectively. These changes were associated with a selective supersensitivity for the noradrenaline-induced perfusion-pressure increase in vivo compared with the effect of neuropeptide Y. However, a similar potentiation of the noradrenaline effect was induced by the monoamine-uptake blocker desipramine in the absence of denervation and there was no change in the functional response to phenylephrine after denervation. This indicated that the denervation supersensitivity for noradrenaline was of prejunctional rather than of postjunctional origin, although a significant increase in the maximal number of specific binding sites of the [ 3H]prazosin binding (75%) was observed in vitro. No significant changes in the equilibrium-dissociation constant or in the maximal number of specific binding sites were observed after denervation for the [ 125I]neuropeptide Y binding sites. Furthermore, the neuropeptide Y-induced inhibition of forskolin-induced cyclic adenosine monophosphate formation was not changed after denervation. It is concluded that alpha 1-adrenoceptor and neuropeptide Y binding sites are co-distributed and can be activated in parallel to induce muscle contraction in support of a co-transmitter role for noradrenaline and neuropeptide Y in the sympathetic nervous control of the pig spleen. Denervation was associated with a selective up-regulation of the alpha 1-adrenoceptor binding sites, but no changes in the binding characteristics were observed for neuropeptide Y, suggesting the absence of homeostatic responses in the regulation of neuropeptide Y receptors of the spleen.

Effect of Chronic Experimental Diabetes on Vascular Smooth Muscle Function in Rabbit Carotid Artery

To understand the effects of diabetes on vascular smooth muscle function and the underlying mechanism(s) involved, we examined the responses to alpha-adrenoceptor agents, serotonin (5-HT), K+, and prostaglandins in the carotid artery of male New Zealand white rabbits with chronic diabetes (16 weeks) induced chemically by alloxan (100 mg/kg, intravenously) treatment. Isolated ring segments of diabetic rabbit carotid artery exhibited an increased (20-60%) maximal response to norepinephrine (NE), methoxamine, phenylephrine, and K+ as compared with controls. Responses to 5-HT were not significantly increased. Nevertheless, there were no significant differences in ED50 values of the agonists in either of the groups. Putatively selective alpha 2-adrenoceptor agonists (clonidine and guanabenz), prostaglandin E1 (PGE1) and prostaglandin I2 (PGI2) did not elicit any response in control vessels. In the diabetic state, however, these drugs contracted the artery in a dose-dependent fashion. Isoproterenol (0.1-10 microM) relaxed arterial rings previously contracted with all the agonists except PGE1 and PGI2, which were potentiated by isoproterenol. Contractions to PGE1 or PGI2 alone or in the presence of isoproterenol were reduced or abolished by 10(-5) M phentolamine. Under these conditions, isoproterenol exhibited its typical relaxatory action. Nifedipine was more potent in inhibiting the K+ response in diabetic carotid artery than in the controls. These results suggest an increased reactivity of diabetic rabbit carotid artery to alpha 2-adrenoceptor agonists, K+, PGE1, and PGI2 which may, at least in part, be due to an increased sensitivity of calcium channels in diabetic vessels. Contractile responses to PGE1 and PGI2 could be attributed to their action on adrenergic neurotransmission, thereby facilitating the release of NE from presynaptic nerve terminals. Furthermore, isoproterenol at a high dose (1 microM or more) may directly stimulate alpha-adrenoceptors. Whether or not this effect of isoproterenol is only prostaglandin-dependent is not clear.

Opiate- and alpha 2-adrenoceptor-induced hyperpolarizations of locus ceruleus neurons in brain slices: reversal by cyclic adenosine 3':5'- monophosphate analogues

The purpose of this study was to investigate the ionic and second messenger mechanisms underlying the hyperpolarizations induced by the selective alpha 2-adrenoceptor agonist clonidine and the opiate agonist morphine in the locus ceruleus. Intracellular recordings were carried out in rat brain slices, and drugs at known concentrations were administered in the perfusate. The cyclic adenosine 3':5'-monophosphate (cAMP) analogues 8-bromo-cAMP and dibutyryl cAMP, while not altering basal activity, reversed the hyperpolarizations induced by clonidine or morphine. In contrast, administration of the parent compound adenosine failed to affect these responses. These results are consistent with previous biochemical studies suggesting that alpha 2-adrenergic and opiate agonists might signal their actions by reducing intracellular cAMP levels. Under manual voltage clamp, both clonidine and morphine elicited outward currents. The algebraic sum of the individual currents elicited by morphine and clonidine significantly exceeded the actual current elicited by their co-administration. This nonadditivity, as well as the observation that cAMP analogues reverse the morphine- and clonidine-induced hyperpolarizations, suggests that these compounds hyperpolarize locus ceruleus neurons through a shared ionic mechanism the activation of which might be signaled by a decrease in intracellular cAMP.

Reflex adrenergic inhibition of colonic motility in anesthetized rat caused by nociceptive stimuli of peritoneum. An alpha 2-adrenoceptor-mediated response.

Nociceptive stimulation of the peritoneum inhibits colonic motility via a sympathoadrenergic reflex. This sympathetic reflex most probably induces the inhibition indirectly via a presynaptic blockade of cholinergic transmission. To further clarify this reflex, colonic motility was continuously recorded by a volume method in rats exposed to nociceptive abdominal stimulation and intravenous drug administration. Intraabdominally applied HCl inhibited colonic motility, which was unaffected by nonselective beta-adrenoceptor blockade (propranolol). However, the nonselective alpha-adrenoceptor antagonist phentolamine and the selective alpha 2-adrenoceptor antagonist yohimbine restored motility. Spontaneous colonic motility was unaffected by the nonselective beta-adrenoceptor agonist (isoproterenol) and the selective alpha 1-adrenoceptor agonist (L-phenylephrine). On the other hand, a nonselective alpha-agonist (noradrenaline) and a selective alpha 2-adrenoceptor agonist (clonidine) inhibited spontaneous colonic motility. It is suggested that in the rat sympathetic reflex inhibition of colonic motility caused by abdominal nociception is mediated via presynaptic alpha 2-adrenoceptors inhibiting the excitatory cholinergic neurons.

Effects of clonidine on neuronal firing evoked by a noxious stimulus

When norepinephrine was applied microiontophoretically to certain neurons in the pontine reticular formation of rats, it produced an increase in neuronal firing like that produced by noxious stimulation. Previous studies have shown that both noxious stimulus- and norepinephrine-evoked increases in neuronal firing are mediated by alpha-adrenoceptors. These neurons were unresponsive to non-noxious stimuli, suggesting that they might play a role in nociception. Microiontophoretic or systemic administration of the selective alpha 2-adrenoceptor agonist clonidine significantly attenuated noxious stimulus-evoked firing, but had little effect on firing evoked by norepinephrine. This effect of clonidine could be prevented by the alpha 2-adrenoceptor antagonists piperoxan and yohimbine. These antagonists, when given alone, increased noxious stimulus-evoked firing, but had no effect on firing evoked by norepinephrine. In contrast, the selective alpha 1-adrenoceptor antagonist ARC-239 (2-(2,4-( o-methoxyphenyl)-piperazin-1-yl)-ethyl-4,4-dimethyl-1,3-(2H,4H)isoquinolindione dihydrochloride) attenuated both noxious stimulus- and norepinephrine-evoked firing. These data are consistent with the hypothesis that presynaptic alpha 2-adrenoceptors modulate the release of norepinephrine. Furthermore, these data suggest that the pontine reticular formation is one site at which clonidine could act to produce analgesia.

Characterization of the alpha-adrenoceptors in the female rabbit urethra.

A radioligand binding technique was used to evaluate the proportions of alpha 1- and alpha 2-adrenoceptors in crude membrane preparations obtained from the female rabbit bladder base and urethra. In addition, urethral rings were studied in vitro in an attempt to determine if alpha 1- and/or alpha 2-adrenoceptors are located postjunctionally in the urethral smooth muscle. Studies of the inhibition of [3H]-dihydroergocryptine binding by the selective alpha 1-adrenoceptor antagonist prazosin or the selective alpha 2-adrenoceptor antagonist rauwolscine revealed the alpha-adrenoceptor population to consist of approximately 25% alpha 1-adrenoceptors and 75% alpha 2-adrenoceptors. These proportions were confirmed in saturation studies with [3H]-prazosin and [3H]-rauwolscine. The sum of alpha 1- and alpha 2-adrenoceptors labelled by these selective alpha 1- and alpha 2-adrenoceptor antagonists was about equal to the number labelled by the non-selective alpha-adrenoceptor antagonist [3H]-dihydroergocryptine. Noradrenaline, as well as the selective alpha 1-adrenoceptor agonist phenylephrine and the selective alpha 2-adrenoceptor agonist clonidine, induced contractions of urethral ring preparations. Prazosin blocked contractions induced by phenylephrine to a greater extent than contractions induced by clonidine. The opposite was true for the inhibitory effect of rauwolscine. In addition to showing that both alpha 1- and alpha 2-adrenoceptor binding sites exist in membrane preparations of the rabbit bladder base and urethra, the results reveal the presence of both adrenoceptor subtypes postjunctionally in the rabbit urethra; and both mediate contraction of the smooth muscle.

Alpha 2-Adrenoceptor agonists induced mydriasis in the rat by an action within the central nervous system.

1 The effects of intravenous administration of the selective alpha 2-adrenoceptor agonists clonidine, UK 14,304 and guanoxabenz on rat pupil diameter were investigated. 2 In rats anaesthetized with pentobarbitone, each agonist produced a marked dose-related increase in pupil diameter; the rank order of potency was: clonidine greater than UK 14,304 greater than guanoxabenz. 3 Pretreatment with the selective alpha 2-adrenoceptor antagonist, RX 781094 (0.5 mg/kg, i.v.), produced a parallel 30-40 fold shift to the right of the dose-pupil dilator response curves for the three agonists. Yohimbine (1.5 mg/kg, i.v.) produced about a 10 fold rightward shift of the dose-response curve for guanoxabenz. In contrast, the alpha 1-selective antagonist, prazosin (0.5 mg/kg, i.v.), failed to affect the dose-response relation for guanoxabenz. 4 Several antagonists of varying selectivities towards alpha 1- and alpha 2-adrenoceptors were tested for their ability to reverse the maximal mydriasis induced by guanoxabenz (0.3 mg/kg, i.v.). The rank order of potency of the antagonists producing a 50% reversal of this effect was: RX 781094 greater than yohimbine greater than piperoxan = rauwolscine greater than mianserin greater than RS 21361. Neither corynanthine nor prazosin reversed the guanoxabenz-induced mydriasis. 5 Topical application of RX 781094 (0.1 to 3% w/v solutions) onto one eye produced a slow reversal of guanoxabenz-induced mydriasis; the time course and degree of reversal were virtually the same in both eyes. 6 Intracerebroventricular administration of RX 781094 (1.25-15 micrograms total dose) caused a rapid dose-related reversal of the maximal mydriasis induced by guanoxabenz (0.3 mg/kg, i.v.). 7 Guanoxabenz (0.3 and 1.0 mg/kg, i.v.) did not produce any dilation of the physostigmine-constricted undamaged pupil of the pithed rat. Intravenous adrenaline was found to produce a small mydriatic effect, while atropine completely antagonized the effects of physostigmine in this preparation. 8 These results indicate that alpha 2-adrenoceptor agonists induce mydriasis in the rat through a central alpha 2-adrenoceptor mechanism. However, the site of action within the central nervous system remains to be determined.

Evidence that catecholamines stimulate renal gluconeogenesis through an alpha 1-type of adrenoceptor.

1. Noradrenaline stimulates gluconeogenesis through an alpha-adrenoceptor in renal cortical tubule fragments from fed rats incubated with 5 mM-lactate. 2. The selective alpha 1-adrenoreceptor agonist methoxamine stimulated gluconeogenesis, but the selective alpha 2-adrenoceptor agonist clonidine was ineffective. 3. The selective alpha 1-adrenoceptor antagonist thymoxamine blocked the stimulatory effects on gluconeogenesis of noradrenaline and of oxymetazoline (a synthetic alpha-agonist). The selective alpha 2-adrenoceptor antagonist yohimbine was ineffective in this respect. 4. It is concluded that noradrenaline and oxymetazoline stimulate gluconeogenesis in rat kidney via an alpha 1-rather than an alpha 2-type of adrenoceptor.