Postsynaptic alpha 1- and alpha 2-adrenoceptors in the circulatory system of the pithed rat: selective stimulation of the alpha 2-type by B-HT 933

Characterization of alpha-adrenoceptors participating in the central hypotensive and sedative effects of clonidine using yohimbine, rauwolscine and corynanthine

The central alpha-adrenoceptors responsible for mediating the clonidine-induced central hypotension in anaesthetized cats and sedation in mice have been characterized according to their sensitivities to the alpha-adrenoceptor antagonist yohimbine and its two diastereomeric congeners rauwolscine and corynanthine. Yohimbine and rauwolscine (1-10 microgram/kg) dose-dependently antagonized the central hypotensive response to clonidine (1 microgram/kg) applied 15 min later. Greater amounts of corynanthine (30-100 micrograms/kg) had to be administered to diminish the central depressor effect of clonidine. In these studies the drugs were infused via the left vertebral artery. The prolongation of the hexobarbitone-induced loss of the righting reflex in mice by clonidine (0.3 mg/kg, i.p.) was inhibited by previous treatment with yohimbine and rauwolscine (0.04-5 mg/kg, i.p.) in a dose-dependent manner, but not by corynanthine. Binding experiments with rat isolated cerebral membranes demonstrated the higher affinity of yohimbine and rauwolscine for the [3H] clonidine- than for the [3H]prazosin-specific binding sites. The reverse was found for corynanthine. The relative potencies of yohimbine, rauwolscine and corynanthine in inhibiting these central effects of clonidine are comparable to their order of efficacies in blocking peripheral alpha 2-adrenoceptors. Accordingly, clonidine-induced central hypotension and sedation are mediated by alpha 2-adrenoceptors.

Mini-review. The postsynaptic alpha 2-adrenoreceptor

The discovery of postsynaptic alpha 2-adrenoreceptors with the drug specificities of presynaptic alpha 2-adrenoreceptors has contributed to a refinement of the classification of alpha-adrenoreceptors. postsynaptic alpha 2-adrenoreceptors have been identified by pharmacological means and with the aid of direct radioligand-receptor binding studies. The evidence for the existence of this class of alpha 2-adrenoreceptors in the brain and in vascular smooth muscle is particularly strong. Central postsynaptic alpha 2-adrenoreceptors play a major part in the hypotensive action of centrally acting antihypertensive drugs such as clonidine and alpha-methyl-DOPA. Vascular smooth muscle cells contain postsynaptic alpha 2-adrenoreceptors which mediate vasoconstriction, like the more classical alpha 1-adrenoreceptors. The simultaneous occurrence of contractile alpha 1- and alpha 2-adrenoreceptors in vascular smooth muscle offers a simple model for the characterization of alpha-adrenoreceptor agonists and antagonists. At present, highly selective agonists of alpha 2-adrenoreceptors have been found. These new compounds may be useful for the classification of other alpha-adrenoreceptor populations. It has been suggested that the vascular postsynaptic alpha 2-adrenoreceptor might be located at extrasynaptic sites. Accordingly, adrenaline released by the adrenal medulla would be the endogenous stimulant. Finally, observations have been made in vivo indicating that a transmembrane influx of calcium ions is necessary for linking the drug-induced activation of these alpha 2-adrenoreceptors to vasoconstriction.

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.

Evaluation of the selectivity of alpha-adrenoreceptor blocking drugs for postsynaptic alpha 1- and alpha 2-adrenoreceptors in a simple animal model

1 The increase of diastolic pressure of pithed, normotensive rats was determined after i.v. administration of the alpha-adrenoreceptor agonists L-phenylephrine and B-HT 933. 2 alpha-Adrenoreceptor antagonists varied widely in their relative inhibitory effects towards either L-phenylephrine- or B-HT 933-induced vasoconstrictor responses. Prazosin displayed the highest affinity for the postsynaptic alpha-adrenoreceptor triggered by L-phenylephrine. The rank order of potency was further: phenotlamine greater than dihydroergotamine greater than clozapine greater than corynanthine greater than azapetine greater than yohimbine greater than piperoxan greater than tolazoline greater than mainserin greater than rauwolscine. On the other hand, the rank order of potency towards B-HT 933 was: dihydroergotamine greater than rauwolscine greater than yohimbine greater than phentolamine greater than piperoxan greater than prazosin greater than tolazoline greater than mainserin greater than corynanthine greater than azapetine greater than clozapine. These data are in general agreement with the classification of alpha 1-(triggered by L-phenylephrine) and alpha 2-(triggered by B-HT 933) adrenoreceptors. Both populations are present postsynaptically in vascular smooth muscle of the pithed rat and are involved in vasoconstriction. 3 The ratio of KB post alpha 2/KB post alpha 1 was calculated as a measure of selectivity for either alpha-adrenoreceptor site. The alpha-adrenoreceptor antagonists used cover a 20,000-fold range of activity ratios. The antagonists most selective for either type were prazosin (alpha 1) and rauwolscine (alpha 2). The selectivity of the alpha-adrenoreceptor antagonists for postsynaptic alpha 1- and alpha 2-adrenoreceptors in the intact circulatory system of the pithed rat is comparable with the reported selectivity of these blocking agents for alpha 1 (postsynaptic)- and alpha 2(presynaptic)-adrenoreceptors in the rabbit isolated pulmonary artery. 4 It is concluded that two distinct types of postsynaptic alpha-adrenoreceptors participate in vasoconstriction in the pithed rat. Apart from the classical alpha 1-adrenoreceptor, vascular smooth muscle of the pithed rat contains postsynaptic alpha 2-adrenoreceptors resembling those previously found mainly presynaptically. The presence of separate classes of postsynaptic alpha 1- and alpha 2-adrenoreceptors in the intact circulatory system of the pithed rat offers the possibility to use this relatively simple animal model as an in vivo test system for the pharmacological characterization of alpha-adrenoreceptors agonists and antagonists.

The blockade of alpha 2-adrenoceptors by the PNMT inhibitaor SK&F 64139

Three tetrahydroisoquinolines were tested for activity on central alpha 2-adrenoceptors by determining their ability to inhibit [3H]clonidine binding to rat cerebral cortical membrane homogenates. The compounds included the PNMT inhibitors SK&F 64139 (7,8-dichloro-1,2,3,4-tetrahydroisoquinoline) and SK&F 29661 (1,2,3,4-tetrahydroisoquinoline 7-sulfonamide) as well as SK&F 72223 (5,8-dimethoxy-1,2,3,4-tetrahydroisoquinoline) a structural analogue inactive as a PNMT inhibitor. SK&F 64139 and SK&F 72223 but not SK&F 29661 inhibited [3H]clonidine binding. Studies in the isolated guinea pig atrium confirmed that SK&F 64139 and SK&F 72223 are alpha 2-adrenoceptor antagonists.

Vasoconstriction mediated by postsynaptic alpha 2-adrenoceptor stimulation

The postsynaptic alpha-adrenoceptors involved in vasoconstriction brought about by B-HT 933 (2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxazolo-[5,4-d]-azepin) administered i.v. to pithed, normotensive rats were characterized. The rate of onset of the hypertensive response to i.v. B-HT 933 is slower than that induced by (-)-phenylephrine, an agonist of alpha 1-adrenoceptors. The antagonism of the alpha-adrenoceptor blocking drugs rauwolscine, yohimbine and corynanthine was quantified towards B-HT 933-induced increases in diastolic pressure. Rauwolscine (pA2 = 7.06) and yohimbine (pA2 = 6.83) were effective antagonists, whereas corynanthine proved much less potent (pA2 = 5.03). On the basis of the reported selectivity of yohimbine and its two diastereoisomers rauwolscine and corynanthine for alpha 1- and alpha 2-adrenoceptors, it is concluded that the postsynaptic alpha-adrenoceptors triggered by B-HT 933 are of the alpha 2-type. B-HT 933 identifies a subclass of postsynaptic alpha 2-adrenoceptors in vascular smooth muscle distinct from postsynaptic alpha 1-adrenoceptors. Both types of alpha-adrenoceptors are likely to be involved in the mediation of vasoconstriction.