The evidence for α-adrenoceptors in the coronary circulation and their possible relevance to the physiological regulation of myocardial blood flow

Abstract

This contribution reviews the evidence for the existence of a-adrenoceptors mediating vasoconstriction in the coronary vascular bed and discusses their possible physiological, pharmacological and clinical relevance. It attempts to serve as an introduction to the papers in this symposium which deal with the role of the sympathetic nerves in the control of coronary vessel tone. Evidence for the presence of coronary vascular a-adrenoceptors mediating vasoconstrictionThe experimental evidence for the presence of coronary vascular a-adrenoceptors comes both from in-vitro and in-yivo studies: I. a-adrenoceptors in the coronary circulation: evidence from isolated coronary vesselsOne of the problems involved in analysing adrenoceptor types in the coronary circulation' is that most of the commonly used agonists (e.g. adrenaline, noradrenaline) influence both coronary and myocardial a-and [~-adrenoceptors. A drug may have a direct constrictor effect on the coronary vessels (mediated, for example, through coronary vascular smooth muscle a-adrenoceptors) yet increase coronary flow through stimulation of myocardial work and metabolism (mediated, for example, through myocardial [3-adrenoceptors). This separation of direct (vascular) from indirect (myocardial) effects is especially difficult with regard to drugs which influence adrenoceptors. One obvious way of separating metabolically mediated from direct coronary vascular effects is to examine the responses of isolated coronary vessels. It must however, be emphasised that such studies do not yield definitive information about the responses of such vessels in vivo nor about the responses of the coronary arterioles, the main resistance segment of the coronary vascular bed. The classic studies in this field are those of Bohr (1, 2) who examined the responses of canine coronary arteries of different sizes ('small', 250-500 ~ and 'large' 1.5-2.4 mm diameter) to adrenoceptor agonists. A more detailed study of the responses of pig coronary arteries (spiral strips from arteries 1.5-2.4 mm diameter) has been undertaken by Abaitey (3). The predomin-978

Dihydroergotoxine, an ergot alkaloid without marked hypotensive effect, improves blood flow and metabolism in the underperfused canine myocardium

“…The possible therapeutic significance of ct-blockade under conditions of myocardial ischaemia or severe underperfusion has recently drawn enhanced experimental and clinical attention (1,2). A combination of ct sympatholytically mediated coronary dilation and reduction in myocardial oxygen demand is likely to exert a beneficial effect on the underperfused myocardium.…”