Adriaan De Jonge

VASCULAR SMOOTH-MUSCLE CONTRACTION INITIATED BY POSTSYNAPTIC ALPHA-2-ADRENOCEPTOR ACTIVATION IS INDUCED BY AN INFLUX OF EXTRACELLULAR CALCIUM

Vasoconstriction in pithed, normotensive rats elicited via stimulation of postsynaptic alpha 2-adrenoceptors by B-HT 920 was antagonized by EDTA and the calcium antagonists nifedipine, D 600 and verapamil, whereas pressor responses to the alpha 1-agonist methoxamine were unaffected. This indicates that vasoconstriction in vivo initiated via postsynaptic alpha 2-adrenoceptors requires an influx of extracellular calcium. Thus, the antihypertensive effect of calcium antagonists may be based upon a diminution of vascular tone maintained by postsynaptic alpha 2-adrenoceptors.

A lipophilic, selective α1 -adrenoceptor agonist: 2-(2-chloro-5-trifluoromethylphenylimino) imidazolidine (St 587)

Abstract A new compound (St 587) is described, which is a selective α 1 -adrenoceptor stimulating agent with lipophilic properties. This combination of characteristics is novel, since all α 1 -adrenoceptor agonists developed so far are hydrophilic. The α-adrenergic effects of 2-(2-chloro-5-trifluoromethylphenylimino) imidazolidine (St 587), a derivative of clonidine, were examined in several animal models. St 587 (1–10,000 μg/kg, i.v.) induced vasoconstriction in pithed, normotensive rats. This peripheral pressor activity was strongly antagonized by prazosin (0.1 mg/kg), but not affected by yohimbine (1 mg/kg). In intact, pentobarbitone-anaesthetized normotensive rats, St 587 (1–3,000 μg/kg, i.v.) evoked transient pressor responses, but a secondary fall in blood pressure and cardiac frequency was not observed. In pitched rats, St 587 (1–1,000 μg/kg) failed to modify the increase in heart rate produced by electrical stimulation of the cardioaccelerator sympathetic nerve fibres. St 587 (300 and 1,000 μg/kg) did not display central hypotensive activity, when injected into the left vertebral artery of anaesthetized cats. In addition, no hypotensive effect was observed when St 587 was administered i.v. to anaesthetized normotensive rats and cats. In mice, St 587 (10–10,000 μg/kg, i.p.) lacked sedative properties, since it did not prolong the hexobarbitone (75 mg/kg, i.p.)-induced loss of the righting reflex. The overall lipophilicity (log P′) of St 587 in the octanol/buffer (pH=7.4) reference system at 37°C amounted to 1.54. The experimental data suggest that St 587 is a lipophillic compound with selective α 1 - agonistic activity. The inability of St 587 to cause hypotension and sedation provides further evidence for the view that α 1 -adrenoceptors in the brain are not involved in the central hypotensive action and the sedation, caused by clonidine and related drugs. These effects are solely mediated by homogenous populations of α 2 -adrenoceptors.

Differential selectivities of RU 24969 and 8-OH-DPAT for the purported 5-HT1A and 5-HT1B binding sites. Correlation between 5-HT1A affinity and hypotensive activity

RU 24969 and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) inhibited the specific binding of [3H]5-HT (2 nM) to rat brain membranes with shallow displacement curves. The displacement data were best fitted with a model of two independent, high and low affinity binding sites. Following addition of spiperone (1 microM) as a selective ligand for the putative 5-HT1A recognition site of [3H]5-HT, the displacement curve of RU 24969 underwent a leftward shift, whereas spiperone induced a shift to the right for the displacement curve of 8-OH-DPAT. In contrast to spiperone, pindolol (1 microM) shifted the displacement curve of RU 24969 to the right. These results suggest that RU 24969 possesses preference for the purported 5-HT1B subtype of central 5-HT1 recognition site. The reported significant linear correlation between hypotensive activity following intravenous (i.v.) administration to anesthetized rats and affinity for the central 5-HT1 binding site could only be maintained by incorporation of the affinity of RU 24969 for its low and 8-OH-DPAT for its high affinity binding site. Based on the proposal that the 5-HT1A site corresponds to the high affinity site of 8-OH-DPAT and the low affinity site of RU 24969, it is hypothesized that the late depressor phase of 5-HT agonists in rats is mediated by activation of peripheral (vascular) 5-HT receptors which have similarities with the 5-HT1A subtype of central 5-HT1 recognition site.

SGD 101-75 IS DISTINGUISHED FROM OTHER SELECTIVE ALPHA-1-ADRENOCEPTOR AGONISTS BY THE INHIBITION OF ITS PRESSOR-RESPONSES BY CALCIUM ENTRY BLOCKADE AND VASODILATATION IN PITHED RATS AND CATS

The vasopressor effects of the selective alpha 1-adrenoceptor agonist Sgd 101/75 (2-[2-methylindazol-4-imino]-imidazolidine HCl) were analyzed in pithed rats and cats. Vasodilatation by the beta 2-adrenoceptor agonist salbutamol (1 mg/kg i.v.) or by the converting enzyme inhibitor captopril (5 mg/kg i.v.) antagonized the vasoconstriction by Sgd 101/75 in pithed rats. The effect of salbutamol was abolished by restoration of the baseline diastolic pressure by infusion of vasopressin. Calcium entry blockade by nifedipine (0.1-3 mg/kg i.v.) and (-)-verapamil (0.3 and 1 mg/kg i.v.) dose dependently inhibited the rise in the diastolic pressure induced by Sgd 101/75 pithed rats. This inhibition could not be attenuated by an infusion of vasopressin. In pithed cats, nifedipine most effectively antagonized the pressor effects of Sgd 101/75. In this respect, Sgd 101/75 is different from other alpha 1-adrenoceptor agonists, which are known to elicit a vasoconstriction which is virtually insensitive to vasodilatory measures and calcium entry blockade. These findings may be explained on the basis of a further subdivision of vascular postjunctional alpha 1-adrenoceptors.

The contractions induced in rat and guinea-pig aortic strips by the α2-adrenoceptor selective agonists B-HT 920 and UK 14,304 are mediated by α1-adrenoceptors

The alpha-adrenergic action of the selective alpha 2-adrenoceptor agonists B-HT 920 and UK 14,304 was investigated on helically cut preparations of rat and guinea-pig isolated aorta. The alpha 1-adrenoceptor selective stimulant (-)-phenylephrine was included for comparison. All agonists induced concentration-dependent contractions in both preparations. Calcium entry blockade by D 600 almost abolished the contractions evoked by B-HT 920 and UK 14,304 in rat aorta while those evoked in guinea-pig aorta were less strongly affected. Contractions elicited by (-)-phenylephrine were moderately impaired by D 600 in rat aorta whereas there was only a limited effect in guinea-pig aorta. Analysis of the prazosin and yohimbine antagonism of B-HT 920- and UK 14,304-evoked contractions showed the involvement of alpha 1-like adrenoceptors in rat and guinea-pig aorta, prazosin being approximately 1000 times more potent that yohimbine. The results show that B-HT 920 and UK 14,304 contract rat and guinea-pig aorta via alpha 1-like adrenoceptors which are not identical. It is submitted that rat and guinea-pig alpha 1-adrenoceptors activate different contractile processes.

Inhibitory effect of calcium antagonist drugs on vasoconstriction induced by vascular alpha2-adrenoceptor stimulation

A survey is given of the mechanisms of the antihypertensive effect of calcium entry blockers. The main background of the antihypertensive/hypotensive action is dilatation of precapillary arterioles (resistance vessels that cause a reduction in total peripheral resistance and, hence, a decrease in blood pressure). The vascular relaxation is caused by an inhibition of the transmembranous calcium influx and, probably less so, by interference with calmoduline. Calcium entry blockers significantly reduce the vasoconstriction induced by the excitation of vascular postsynaptic alpha 2 adrenoceptors. The inhibitory effect of calcium entry blockers is reversed by the calcium entry promoter Bay k 8644. The vasoconstriction induced by alpha 1-adrenoceptor stimulation is less generally influenced by calcium entry blockers than the alpha 2 effects. The interference with alpha 2-adrenoceptor-induced vasoconstriction may contribute to the vasodilator action of the calcium entry blockers, especially in hypertensive patients who show a hyperreactivity to pressor responses toward catecholamines.

Calcium Dependency of Vasoconstriction Mediated by α1- and α2-Adrenoceptors

A survey is given of the calcium dependency of vasoconstriction mediated by α1- and α2-adrenoceptors. In all species studied and with all drugs investigated, it has been confirmed that (α2-adrenoceptor-mediated vasoconstriction is sensitive to calcium entry blockade with calcium antagonists, both in vivo and, under suitable experimental circumstances, also in vitro. The calcium entry promotor (calcium agonist) Bay k8644 appears to behave as a mirror image of the calcium-entry blockers. α1-Adrenoceptor-agonist-induced vasoconstriction shows a differential picture with respect to the dependency of extracellular calcium. Accordingly, the vasoconstriction induced by the α1-agonists cirazoline, (-)-phenylephrine, (+)-erythro-methoxamine, and (-)-amidephrine is not primarily dependent upon an influx of calcium, whereas other compounds like St 587 and Sgd 101 75 cause vasoconstriction which is very sensitive to drug-induced calcium entry blockade. It may be possible that different recognition sites of the α1-adrenoceptors are involved. In this connection, the calcium-entry promotor Bay k8644 does not simply behave as the mirror image of calcium entry blockers.

calcium Influx-dependent and -independent α1-adrenoceptor-mediated Processes of Vasoconstriction In Vivo Do Not Operate via Different α1-adrenoceptor Subtypes

Summary: In pithed rats, the selective α1-adrenoceptor agonists St 587 and cirazoline show preponderant calcium influx-dependent and -independent vasoconstriction, respectively. By using these agonists, selective (competitive) antagonists for either process of vasoconstriction were sought. For this purpose, antagonism was analyzed for eight structurally different antagonists (prazosin, BE 2254, AR-C239, R 28935, corynanthine, phentolamine, sulpiride, and chlorpromazine) opposing the pressor responses evoked by cirazoline and St 587. Where pA2 values ( -log dose antagonist evoking a twofold shift for the agonist dose-response curve) could be calculated, no significantly different pA2 values against either agonist resulted. However, with respect to the slopes of the Schild plots, deviations from unity were found for prazosin, R 28935, AR-C239, sulpiride, and chlorpromazine, but not uniformly against both agonists. Following treatment with phenoxybenzamine (PB) (30 μg/kg) and nifedipine (1 mg/kg), which produced calcium influx-sensitive and -insensitive vasoconstriction to cirazoline, respectively, Schild plots were constructed for BE 2254, prazosin, and chlorpromazine. Using cirazoline as an agonist, unity slopes were now obtained for prazosin and chlorpromazine. The Schild plots of BE 2254 versus cirazoline after PB or nifedipine administration, however, exhibited a slope deviating from unity. For prazosin and chlorpromazine, identical pA2 values still resulted against both processes of vasoconstriction to cirazoline. The results are compatible with the view that α1-adrenoceptors mediating calcium influx-dependent and -independent vasoconstriction in vivo are not distinctly different entities, but are separate recognition sites of the same receptor.

Discrimination between peripheral and central α-adrenergic effects using meta-substituted imidazolidines

meta-Substituted 2-chlorophenyl(imino)imidazolidines of the clonidine-type (2,3- and 2,5-substituted derivatives) were used in attempts to discriminate between alpha-adrenergic effects (central hypotensive and peripheral hypertensive activities) in rats. The central hypotensive activity of four pairs of substitution isomers was expressed as a pC20, calculated from log dose-response curves after i.v. administration of the compounds to anaesthetized, normotensive rats. Peripheral hypertensive potency was evaluated in pithed normotensive rats following i.v. injections and quantified by means of pC60. The partition coefficients of the drugs were determined in an octanol/aqueous buffer (pH 7.4, 37 degrees C) system as a measure of lipophilic behaviour. The pressor activities in rats were comparable within each pair of substitution isomers whereas the depressor potencies differed greatly, although the representatives within each pair displayed similar lipophilicity. Regression analysis showed that the central hypotensive activity was governed by the steric bulk of the 5-substituent. On the other hand, this feature did not affect the peripheral hypertensive potency. The discrepancy between central and peripheral alpha-adrenergic activities found for 2,5-substituted imidazolidines is discussed in connection with the selectivity of these compounds for alpha 1 and alpha 2-adrenoceptors.

Heterogeneity of the interaction between α1- and α2-adrenoceptor agonists with their respective receptors in the vascular system of the pithed rat

The subdivision of α1- and α2-adrenoceptor-mediated pressor responses to different agonists based upon the influence of β2-adrenoceptor-mediated vasodilatation was further investigated in the pithed normotensive rat. The effect of salbutamol (4.18 × 10−6 mol/kg) on the α1-adrenoceptor-mediated increase in diastolic pressure due to dopamine and amidephrine as well as on the α2-adrenoceptor-mediated pressor response to azepexole, DP-6,7-ADTN, M-7, TL-99 and dopamine was assessed. The α1-pressor responses to amidephrine and dopamine were only slightly attenuated by salbutamol. The α2-adrenoceptor-mediated increase in diastolic pressure due to B-HT 933 was strongly antagonized by salbutamol in contrast to the effect of dopamine, DP-6,7-ADTN and M-7. TL-99 occupied an intermediate position. The data do not support the existence of distinctly different subtypes of α1- and α2-adrenoceptors but favor the hypothesis that both α1- and α2- adrenoceptors are activated in a unique way by each of their respective agonists.