An ANALYSIS OF SOME FACTORS INFLUENCINGα‐ADRENOCEPTOR FEED‐BACK AT THE SYMPATHETIC JUNCTION IN THE RAT HEART

Abstract

The effects of the selective prejunctional α‐adrenoceptor antagonist, yohimbine, on the cardio‐acceleration responses to sympathetic stimulation were examined in the pithed rat. Yohimbine reversed the inhibitory effects of the α‐adrenoceptor agonist, clonidine. on the stimulation‐induced tachycardia. Yohimbine failed to potentiate significantly responses to stimulation in the absence of clonidine when stimulation was applied at the optimal level for cardiac responses (C6‐T1). When the pithing rod electrode was moved to T2‐T6, cardioaccelerator responses were smaller and yohimbine produced potentiation at frequencies of ≥ 1 Hz. This potentiation was prejunctional since responses to exogenous noradrenaline (NA) were not increased by yohimbine. In the presence of cocaine, potentiation by yohimbine could be shown at the lower frequency of 0.1 Hz. The possible physiological significance of a negative feed‐back effect of noradrenaline on cardiac sympathetic nerves is discussed.

Contributions of α1‐adrenoceptors, α2‐adrenoceptors and P2x‐purinoceptors to neurotransmission in several rabbit isolated blood vessels: role of neuronal uptake and autofeedback

“…An alternative explanation is that a2-adrenoceptors are junctional' but, in the absence of other physiological factors which permit their expression e.g. angiotensin (Dunn et al, 1989; (Docherty & McGrath, 1979 In some instances cocaine altered the ability of the blocking drugs to affect the responses. Thus rauwolscine had a greater blocking effect in plantaris vein and responses in saphenous vein could be blocked by prazosin and rauwolscine more readily in the presence of cocaine.…”

Pre‐ and Postsynaptic Α‐adrenoceptor Blocking Activity of Raubasine in the Rat Vas Deferens

“…The terminology (a2 for pre-and a, for postsynaptic x-adrenoceptors was originally suggested by Langer (1974) and this concept was generalized by Berthelsen & Pettinger (1977). The two types of a-adrenoceptors differ in their selectivity for agonists and antagonists (Starke, 1972;Dubocovitch & Langer, 1974;Starke, Endo & Taube, 1975;Drew, 1976;Docherty, McDonald & McGrath, 1979;Timmermans, Kwa & Van Zwieten, 1979;Weitzell, Tanaka & Starke, 1979;Docherty & McGrath, 1980;Kobinger & Pichler, 1980). Raubasine (A-yohimbine, ajmalicine) was isolated originally from Rauwolfia serpentina by Popelak, Kaiser & Spingler (1953) and subsequently from Vinca rosea (Kohlmunzer & Krupinska, 1960).…”

Α‐adrenoceptors Can Mediate Chronotropic Responses in the Rat Heart

“…These observations have significance for the study of adrenergic mechanisms in the heart, particularly the involvement of prejunctional a2-adrenoceptors which, hitherto, have been assumed to be the only a-adrenoceptor associated with cardioaccelerator neurotransmission (Docherty & McGrath, 1979;1980). If such receptors occur in man this also has implications for combined therapy with a-andadrenoceptor blocking drugs.…”

“…This procedure avoids activation of prejunctional a2-adrenoceptors which modulate output of neurotransmitter (Docherty & McGrath, 1979;1980).…”