CHARACTERIZATION OF THE SUBTYPE OF PRESYNAPTIC ALPHA-2-ADRENOCEPTORS MODULATING NORADRENALINE RELEASE IN CAT AND BOVINE CEREBRAL-ARTERIES

The possible existence of a heterogeneous population of alpha-2-adrenoceptors (alpha-2A and alpha-2B, demonstrated by binding studies) in adrenergic nerve endings of cat and bovine cerebral arteries modulating noradrenaline release was investigated. Electrical field stimulation elicited an increase of tritium secretion from these vessels preincubated with (+/-)-[H-3]noradrenaline, which was reduced by the alpha-2-agonists, clonidine (1-mu-m) and B-HT 920 (0.01 and 0.1-mu-M), in cat cerebral arteries but only by B-HT 920 in bovine cerebral arteries. This reduction was inhibited by the antagonist of the alpha-2B-subtype, prazosin, and the antagonists of alpha-2A- and alpha-2B-subtypes yohimbine and particularly rauwolscine. The effect of B-HT 920 was partially inhibited by clonidine in bovine, but not in cat cerebral arteries. In both types of arteries, prazosin, yohimbine and the alpha-1-agonist methoxamine (all at 1-mu-M) failed to modify the stimulated radioactivity liberation, wheras it was increased by 1-mu-M rauwolscine, and by yohimibine plus prazosin in cat cerebral arteries. The basal tritium release was enhanced by rauwolscine and prazosin in cate cerebral arteries but only by the latter in bovine cerebral arteries. These results suggest: (1) the existence of presynaptic alpha-2-adrenoceptors, mainly of the alpha-2B-subtype, in these vessels negatively modulating noradrenaline release, their activity being greater in cat than in bovine cerebral arteries, and (2) clonidine has no agonistic but a weak antagonistic action in the latter vessels.