Autoregulatory index, adrenergic responses, and interaction between adrenoreceptors and prostacyclin in the coronary system of rainbow trout

Some salient regulatory properties of the coronary system in nonworking heart preparation of trout are studied, in particular: (1) the pressure dependence of coronary resistance; (2) the reactivity to adrenergic compounds; and (3) the interaction between prostacyclin and specific adrenergic agonists. Coronary resistance was not influenced by variation in ventricle weight or heart rate among various preparations used throughout this study. A positive autoregulatory index (ArI) was determined in the pressure range of 2.5–3.5 kPa, suggesting the existence of an active vasoconstrictive response probably involving local regulatory mechanisms. Noradrenaline was a powerful vasoconstrictor and prostacyclin (PGI2) was a vasodilator when perfused separately. Both the α1-agonist phenylephrine and the α2-agonist clonidine induced dose-dependent vasoconstriction, while the β-agonist isoproterenol elicited a dose-dependent vasodilation. The β-antagonists propranolol (β-specific) and atenolol (β1-specific) induced vasoconstriction, with atenolol as the more potent (minimal effective concentration 10−15 M; maximal effect > 50% vasoconstriction). These results suggest the existence of a β-adrenergic tone in the preparation. The vasoactivity of PGI2 was not affected by the α-agonists, while a significant vasoconstriction was observed at 10−8 M in presence of 10−5 M of isoproterenol. This suggests the occurrence of a specific interaction between prostacyclin and β-adrenoreceptors in the coronary system of trout.