G.M. Drew

Evidence for two distinct types of postsynaptic α-adrenoceptor in vascular smooth muscle in vivo

1 The effects of the highly selective α1‐adrenoceptor antagonist, prazosin, and the relatively selective α2‐adrenoceptor antagonist, yohimbine, on the pressor responses to intravenous injections of phenylephrine and noradrenaline have been examined in anaesthetized cats and pithed rats in an attempt to determine whether α1‐ and α2‐adrenoceptors are located postsynaptically on vascular smooth muscle. 2 In anaesthetized cats prazosin caused a much greater reduction in the pressor responses to phenylephrine than to noradrenaline or splanchnic nerve stimulation (after adrenalectomy). Yohimbine was of similar potency in reducing the pressor responses to each stimulus. 3 A differential blocking activity of prazosin against intra‐arterial injections of phenylephrine and noradrenaline was also demonstrated in the blood‐perfused cat hind limb. As in the whole animal, prazosin was more potent against phenylephrine than noradrenaline. A similar, though less marked, effect was seen in the mesenteric circulation, but not in the renal circulation, where prazosin was almost equipotent in reducing responses to phenylephrine and noradrenaline. 4 In pithed rats prazosin was a potent, competitive antagonist of phenylephrine, but had little effect against noradrenaline; only the responses to high doses of noradrenaline were reduced by prazosin. Yohimbine was approximately equipotent as an antagonist of phenylephrine and noradrenaline. In the anococcygeus muscle, prazosin was as potent an antagonist of noradrenaline as it was of phenylephrine on vascular smooth muscle. 5 The results suggest that there are two types of α‐adrenoceptor in the vasculature of cats and rats. Phenylephrine produces pressor responses by stimulating one type of postsynaptic α‐adrenoceptor that is blocked by prazosin and yohimbine; these are α1‐adrenoceptors. Noradrenaline exerts some of its effect via these receptors but most of its effect appears to be exerted through prazosin‐insensitive receptors. The latter receptors appear to differ from α2‐adrenoceptors.

Effects of α-adrenoceptor agonists and antagonists on pre- and postsynaptically located α-adrenoceptors

Abstract The effects of α-adrenoceptor agonists and antagonists have been examined at pre- and post-synaptically located α-adrenoceptos in the pithed rat. The presynaptic receptors were those located at the cardiac sympathetic nerve terminals and the postsynaptic receptors were those present in vascular smooth muscle. Clonidine was approximately equipotent at pre- and post-synaptic α-adrenoceptors, whilst LSD and BAY-1470 were more active at the pre- than at post-synaptic sites. Oxymetazoline, naphazoline, methoxamine and phenylephrine were all much more active at the postsynaptic α-adrenoceptors. Phentolamine was the most potent antagonist at both pre- and post-synaptic α-adrenoreceptors. Piperoxan, yohimbine and tolazoline were about 3–7× less potent than phentolamine at both sites. Thymoxamine was about10× less potent than phentolamine at postsynaptic α-adrenoceptors but about 1000× less active at the presynaptic receptors. The differential actions of both agonists and antagonists at pre- and post-synaptic α-adrenoceptors suggest that the receptors may be of different types.

α2-ADRENOCEPTORS MEDIATE CLONIDINE-INDUCED SEDATION IN THE RAT

1 The central α‐adrenoceptors responsible for mediating clonidine‐induced sedation in rats have been characterized according to their sensitivity to α‐adrenoceptor agonists and antagonists. 2 Clonidine, injected intraperitoneally or intracerebroventricularly, caused dose‐dependent sedation, both in terms of a reduction in the time that rats could remain on an accelerating rotarod and in terms of overt sedation assessed visually. Following intracerebroventricular injection, xylazine, naphazoline and methoxamine, but not phenylephrine, produced similar effects. 3 The sedation caused by intraperitoneal injection of clonidine was antagonized by intracerebroventricularly injected phentolamine, yohimbine, piperoxan and tolazoline but not by labetalol, thymoxamine or prazosin. 4 The relative potencies of the agonists in causing sedation and of the antagonists in inhibiting the sedative effect of clonidine clearly demonstrated that the central α‐adrenoceptors mediating clonidine‐induced sedation are the same as the peripheral presynaptic α2‐adrenoceptors. 5 All the α‐adrenoceptor agonists caused hypothermia after intracerebroventricular injection, but their order of potency was different from that in producing sedation. The hypothermic effect of intraperitoneally injected clonidine was little affected by any of the antagonists administered intracerebroventricularly. No conclusions could be drawn concerning the type of receptor responsible for mediating hypothermia.

Pharmacological characterization of the presynaptic α-adrenoceptors regulating cholinergic activity in the guinea-pig ileum

1 The presynaptic α‐adrenoceptors located on the terminals of the cholinergic nerves of the guinea‐pig myenteric plexus have been characterized according to their sensitivities to α‐adrenoceptor agonists and antagonists. 2 Electrical stimulation of the cholinergic nerves supplying the longitudinal muscle of the guinea‐pig ileum caused a twitch response. Clonidine caused a concentration‐dependent inhibition of the twitch response; the maximum inhibition obtained was 80 to 95% of the twitch response. Oxymetazoline and xylazine were qualitatively similar to clonidine but were about 5 times less potent. Phenylephrine and methoxamine also inhibited the twitch response but were at least 10,000 times less potent than clonidine. 3 The twitch‐inhibitory effects of clonidine, oxymetazoline and xylazine, but not those of phenylephrine or methoxamine, were reversed by piperoxan (0.3 to 1.0 μg/ml). 4 Lysergic acid diethylamide (LSD) inhibited the twitch response, but also increased the basal tone of the ileum. Mepyramine prevented the increase in tone but did not affect the inhibitory action of LSD. Piperoxan or phentolamine only partially antagonized the inhibitory effect of LSD. 5 Phentolamine, yohimbine, piperoxan and tolazoline were potent, competitive antagonists of the inhibitory effect of clonidine with pA2 values of 8.51, 7.78, 7.64 and 6.57 respectively. 6 Thymoxamine was a weak antagonist of clonidine; it also antagonized the twitch‐inhibitory effect of morphine. Thus, its effect against clonidine is probably not mediated specifically at presynaptic α‐adrenoceptors. 7 Labetalol, itself, depressed the twitch response but did not antagonize the inhibitory effect of clonidine on the residual twitch. 8 The results demonstrate that the presynaptic α‐adrenoceptors in the guinea‐pig ileum are of the same type as those located presynaptically in sympathetically innervated tissues. They are a2‐adrenoceptors and are different from those located postsynaptically.

THE α‐ AND β‐ADRENOCEPTOR BLOCKING POTENCIES OF LABETALOL AND ITS INDIVIDUAL STEREOISOMERS IN ANAESTHETIZED DOGS AND IN ISOLATED TISSUES

1 The antagonist potencies of labetalol and each of its four stereoisomers have been compared at α1‐, β1‐ and β2‐adrenoceptors in anaesthetized dogs and in isolated tissues. 2 The RR stereoisomer is a potent, non‐selective antagonist at β‐adrenoceptors but has only weak α1‐adrenoceptor blocking activity. 3 The SR stereoisomer was the most potent antagonist at α1‐adrenoceptors, and it also had similar potency as an antagonist at β‐adrenoceptors. 4 The α‐ and β‐adrenoceptor blocking profile of the RS stereoisomer is intermediate between that of the RR and SR, but the SS stereoisomer is a relatively weak antagonist at both α‐ and β‐adrenoceptors. 5 It is concluded that, although most of the α1‐adrenoceptor blocking activity of labetalol is attributable to the SR stereoisomer and nearly all of its β‐adrenoceptor blocking activity resides in the RR stereoisomer, each of the stereoisomers contributes to the overall pharmacological profile of labetalol.

Characteristics of the dopamine receptors in the rabbit isolated splenic artyery

After blockade of alpha- and beta-adrenoceptors, the tension induced by PGF2alpha in splenic artery strips was relaxed by dopamine, 6, 7-ADTN, N-methyldopamine, apomorphine, N, N-di-n-propyl 5, 6-ADTN, N, N-di-n-propyl 6, 7-ADTN and Sandoz 27-403; their equipotent concentrations (relative to dopamine = 1) were 0.2: 0.3: 0.4: 1.1: 3.9 and 3.9 respectively. 5, 6 ADTN, N-methyl 5,6ADTN, N, N-diethyldopamine, N, N-di-n-propyldopamine and SKF 38393 were weakly active or inactive at relaxing the splenic artery strip. Bulbocapnine and cis-alpha-flupenthixol were specific, competitive, reversible antagonists of dopamine. Fluphenazine, clozapine, trifluoperazine, haloperidol and spiroperidol also antagonised dopamine, but were relatively weak antagonists and a small part of their action was non-specific. Sulpiride was inactive against dopamine. SKF 38393 selectively antagonised the effects of dopamine demonstrating that SKF 38393 has affinity, but little efficacy at the dopamine receptors in the splenic artery. The findings with both agonists and antagonists suggest that the vascular dopamine receptors in the rabbit splenic artery resemble those in the dog renal and mesenteric vascular beds.

Analysis of the depressant effect of the endothelium on contractions of rabbit isolated basilar artery to 5‐hydroxytryptamine

1 The effects of endothelium removal and of a number of pharmacological agents known to modify endothelial cell function on the contractile response of rabbit isolated basilar arteries to 5‐hydroxytryptamine (5‐HT) and other vasoconstrictors were studied. 2 Endothelium removal slightly reduced the contractile response to potassium chloride (40 mm) but markedly augmented and potentiated contractions to 5‐HT (1 nm − 10 μm). 3 l‐NG‐nitro‐arginine (l‐NOARG, 1–30 μm), an inhibitor of nitric oxide formation in vascular endothelial cells, evoked endothelium‐dependent contraction, and augmented and potentiated contractions to 5‐HT in endothelium‐intact but not endothelium‐denuded tissues. Prior incubation with l‐arginine (1 mm), but not d‐arginine (1 mm), abolished these effects of l‐NOARG (1 μm). l‐NOARG (30 μm) also augmented contractions of endothelium‐intact tissues to noradrenaline, prostaglandin F2α, and to a lesser degree endothelin‐1. 4 Neither glibenclamide (3 μm) nor N‐ethylmaleimide (1 μm), putative inhibitors of the effects of endothelium‐derived hyperpolarizing factor (EDHF) and of agonist‐stimulated endothelium‐derived relaxing factor (EDRF) release respectively, had any effect on either resting tension or the contractile response to 5‐HT. In some tissues indomethacin (3 μm), a cyclo‐oxygenase inhibitor, produced a small contraction and augmented the contractile response to 5‐HT, but in most cases indomethacin was without effect. 5 In endothelium‐intact tissues precontracted with uridine 5′‐triphosphate (UTP; 100 μm), 5‐HT did not evoke relaxation but rather caused further contraction. Under the same conditions acetylcholine (0.01–10 μm) evoked endothelium‐dependent relaxation. 6 These data demonstrate that the endothelium profoundly depresses contractions of rabbit isolated basilar artery to 5‐HT, and that this phenomenon can be fully accounted for by the release of an l‐NOARG‐sensitive relaxing factor. Neither glibenclamide‐sensitive EDHF nor cyclo‐oxygenase products plays a major role. As we could find no evidence that 5‐HT stimulates the production of EDRF per se, and l‐NOARG caused endothelium‐dependent contraction and augmented contractions to other vasoconstrictor agents, it seems likely that a basal release of EDRF underlies this phenomenon.

Dopamine receptors in human basilar arteries

After phenoxybenzamine (10(-5) M), pretreatment, and in the presence of propranolol (10(-6) M) and indomethacin (2.8 X 10(-6) M), dopamine caused a marked concentration-dependent relaxation of isolated strips of human basilar artery contracted with PGF2 alpha. This effect was mimicked by apomorphine, 6,7-ADTN and SK&F 38393, but N,N-diethyl dopamine, N,N-di-n-propyl-dopamine and 5,6-ADTN caused only slight relaxation. (+)-Butaclamol, cis-alpha-flupenthixol, fluphenazine and haloperidol competitively antagonised the relaxant effects of dopamine, but sulpiride was ineffective in concentrations as high as 1.3 X 10(-4) M. These findings show that the dopamine receptors in the human basilar artery closely resemble those in the smooth muscle of the rabbit isolated mesenteric and splenic arteries, and the dog renal and mesenteric arteries in vivo, but differ from those located presynaptically on sympathetic nerve terminals.

Cardiovascular effects of GR117289, a novel angiotensin AT1 receptor antagonist

1 The effect of GR117289, an angiotensin AT1 receptor antagonist, on diastolic blood pressure (DBP) was determined in angiotensin‐dependent and angiotensin‐independent models of hypertension in rats. In addition, the antagonist profile of GR117289 at angiotensin AT1 receptors was determined in conscious renal hypertensive rats and conscious normotensive rats, dogs and marmosets. 2 Intra‐arterial and oral administration of GR117289 (0.3–3 mg kg−1, i.a.; 1–10 mg kg−1, p.o.) to 6‐day left renal artery ligated hypertensive (RALH) rats (DBP > 140 mmHg) produced significant, dose‐related reductions in DBP with little apparent effect on heart rate (<15%). The antihypertensive effect of GR 117289 developed progressively over several hours and with some doses persisted for 24–48 h after administration. 3 Administration of GR117289 (1 mg kg−1, i.a.) on 5 consecutive days to RALH rats reduced DBP on each day. The antihypertensive effect of GR 117289 was not cumulative as DBP had almost returned to base‐line values, 24 h after administration of each dose. 4 A dose of GR117289 (3 mg kg−1, i.a.), which produced a substantial reduction in DBP (about 70 mmHg) in RALH rats, was administered to rats in which blood pressure was elevated either by unilateral renal artery clipping, sustained infusion of angiotensin II (AII), DOCA‐salt administration or genetic inbreeding. GR 117289 reduced DBP in rats in which the renin‐angiotensin system was activated by renal artery clipping or AII infusion but had little effect in normotensive rats, DOCA‐salt rats and SHR. 5 Systemic administration of AII to RALH rats and to normotensive rats, dogs and marmosets elicited reproducible pressor responses in all species. Systemic or oral administration of GR 117289 (3 mg kg−1) inhibited the pressor responses produced by AII, resulting in parallel, rightward displacements of AII dose‐response curves. 6 Maximal displacements of AII dose‐response curves occurred 1 h and 1–7 h after systemic and oral administration, respectively. GR 117289 produced a 32–246 fold displacement after systemic administration and a 4–12 fold displacement after oral administration. The effect in dogs was short lasting after systemic administration but the effect of GR 117289 lasted for up to 24 h in rats and marmosets and for up to 24 h after oral administration in all species. The antagonist activity appeared specific for angiotensin receptors as GR 117289 did not inhibit pressor responses to phenylephrine or vasopressin. 7 These experiments demonstrate that GR117289 reduces blood pressure in conscious hypertensive rats after both systemic and oral administration, and is an effective antagonist at angiotensin AT1 receptors in conscious rats, dogs and marmosets.

Role of angiotensin AT1 and AT2 receptors in mediating the renal effects of angiotensin II in the anaesthetized dog

1 Experiments were performed using the selective AT1 receptor antagonist, GR117289, and the selective AT2 receptor antagonist, PD123177, to assess the relative importance of AT1 versus AT2 receptors in mediating the renal effects of angiotensin II (AII) in vivo, in salt‐replete pentobarbitone‐anaesthetized dogs. 2 The AT1 receptor antagonist, GR117289 (0.5 mg kg−1 + 1 μg kg−1 min−1, i.v.), caused renal vasodilatation, characterized by a mean increase of 21 ± 5% in renal blood flow, 45 min post‐dose. GR117289 also caused a fall in mean blood pressure (12 ± 4%), but despite this, sodium and urine excretion were not reduced. Indeed, there was a tendency for urine output and sodium excretion to increase, although the changes were not statistically significant. GR117289 caused a reduction in plasma aldosterone levels (−35 ± 16%) 45 min post‐dose, despite increasing plasma renin activity (+ 173 ± 42%). In contrast to GR117289, the AT2 receptor antagonist, PD123177 (20 μg kg−1 min−1 intra‐renal artery; i.r.a.) caused no significant change in blood pressure, renal blood flow, or sodium and urine excretion, indicating that the renal effects of endogenous AII in these salt‐replete animals are mediated predominantly by AT1 receptors. 3 Intra‐renal artery infusion of AII (1–300 ng kg−1 min−1) caused dose‐related renal vasoconstriction, and decreases in urine output, sodium excretion, fractional excretion of sodium, and glomerular filtration rate (GFR). The AT1 receptor antagonist, GR117289 (0.5 mg kg−1 + 1 μg kg−1 min−1, i.v.) antagonized these renal effects of AII, causing 15–38 fold rightward displacements of mean dose‐response curves for these parameters. In contrast, PD123177 (20 μg kg−1 min−1, i.r.a.) failed to antagonize the renal haemodynamic and excretory effects of lower doses of AII (1–10 ng kg−1 min−1, i.r.a.). However, at higher doses of AII (30–300 ng kg−1 min−1, i.r.a.), while PD123177 still failed to antagonize the effects of the peptide on urine output, sodium excretion and GFR, it did cause a small, but significant, degree of inhibition of AII‐induced renal vasoconstriction. In addition, at a higher dose (50 μg kg−1 min−1, i.r.a.), PD123177 caused a greater degree of antagonism of AII‐induced renal vasoconstriction, while renal excretory responses to AII remained unaffected. 4 This study shows that the renal haemodynamic and excretory effects of AII in salt‐replete anaesthetized dogs are mainly mediated by angiotensin AT1 receptors. However, the inhibitory effect of PD123177 on renal vasoconstrictor responses to high doses of AII, raises the possibility that functionally important AT2 receptors are present in the canine renal vasculature.