CHARACTERIZATION OF POSTSYNAPTIC ALPHA-ADRENOCEPTORS IN THE ARTERIES SUPPLYING THE OVIDUCT

1 In vitro experiments were designed to characterize postjunctional alpha-adrenoceptor subtypes in ring segments (1 mm length; outer diameter 300-500-mu-m) from arteries supplying the oviduct of the heifer. 2 Noradrenaline, adrenaline and phenylephrine evoked concentration-dependent contractile responses. The pD2 values were 5.67, 5.89 and 5.93, respectively. Medetomidine, clonidine and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetra-hydro-4H-(thiazo)-4,5-d-azepine) were ineffective. 3 The alpha-adrenoceptor selective antagonists, prazosin (1 nM-0.1-mu-M) and rauwolscine (0.1-10-mu-M) competitively antagonized the response to noradrenaline. The pA2 values were 9.38 and 6.83, respectively. 4 The dissociation constant (K(D)) for noradrenaline calculated by use of the irreversible antagonist, dibenamine, was 3.95 (2.09-5.81) mu-M. The occupancy-response relationship was non-linear. Half-maximal response to noradrenaline was obtained with 22% receptor occupancy while maximal response required 100% occupancy. 5 B-HT 920 evoked a biphasic contractile concentration-dependent response in preparations incubated in a physiological solution containing 20 mM K+, 0.1-mu-M prazosin and 1-mu-M propranolol. Rauwolscine 0.1-mu-M significantly (P < 0.01) blocked the first component of the B-HT 920 concentration-response curve with an apparent pA2 value of 8.52 (7.86-9.18). 6 These results strongly suggest that alpha-adrenoceptors in oviductal arteries are mainly of the alpha-1 subtype, although a possible role for alpha-2-adrenoceptors cannot be excluded.