AUTOREGULATORY INDEX, ADRENERGIC RESPONSES, AND INTERACTION BETWEEN ADRENOCEPTORS AND PROSTACYCLIN IN THE CORONARY SYSTEM OF RAINBOW-TROUT

Some salient regulatory properties of the coronary system in nonworkin g heart preparation of trout are studied, in particular: (1) the press ure dependence of coronary resistance; (2) the reactivity to adrenergi c compounds; and (3) the interaction between prostacyclin and specific adrenergic agonists. Coronary resistance was not influenced by variat ion in ventricle weight or heart rate among various preparations used throughout this study. A positive autoregulatory index (ArI) was deter mined in the pressure range of 2.5-3.5 kPa, suggesting the existence o f an active vasoconstrictive response probably involving local regulat ory mechanisms. Noradrenaline was a powerful vasoconstrictor and prost acyclin (PGI(2)) was a vasodilator when perfused separately. Both the alpha(1)-agonist phenylephrine and the alpha(2)-agonist clonidine indu ced dose-dependent vasoconstriction, while the beta-agonist isoprotere nol elicited a dose-dependent vasodilation. The beta-antagonists propr anolol (beta-aspecific) and atenolol (beta(1)-specific) induced vasoco nstriction, with atenolol as the more potent (minimal effective concen tration 10(-15) M; maximal effect > 50% vasoconstriction). These resul ts suggest the existence of a beta-adrenergic tone in the preparation. The vasoactivity of PGI(2) was not affected by the alpha-agonists, wh ile a significant vasoconstriction was observed at 10(-8) M in presenc e of 10(-5) M of isoproterenol. This suggests the occurrence of a spec ific interaction between prostacyclin and beta-adrenoreceptors in the coronary system of trout. (C) 1996 Wiley-Liss, Inc.