BETA-ADRENOCEPTORS IN THE TROUT (ONCORHYNCHUS-MYKISS) HEART - CHARACTERIZATION, QUANTIFICATION, AND EFFECTS OF REPEATED CATECHOLAMINE EXPOSURE

Specific binding of the hydrophilic radioligand [H-3]CGP-12177 to cell surface (functional) beta-adrenoreceptors was quantified in ventricul ar micropunches (2 mm diameter, 350 mu m thickness) from seawater-accl imated rainbow trout held at 7-9 degrees. Binding was stereospecific, saturable, of high affinity, and displaceable by appropriate agonists and antagonists. Phentolamine failed to displace [H-3]CGP at concentra tions up to 10(-4) M, indicating an absence of [H-3]CGP binding to alp ha-adrenergic receptors. Trout ventricular beta-adrenoreceptors are ex clusively of the beta(2) type. This conclusion is based on (1) the IC5 0 value for the beta(2)-antagonist ICI 118551 (2.9 x 10(-6) M); (2) th e inability of the beta(1)-antagonist atenolol to displace [3H]CGP fro m beta-adrenoreceptors; and (3) the order of agonist-binding affinity (isoproterenol > epinephrine much greater than norepinephrine). The B- max and K-d values for [H-3]CGP binding to myocardial tissue were appr oximately 0.04 fmol mu g protein(-1) and 0.25 nM, respectively. The B- max value indicates that the density of cell surface (functional) beta -adrenoreceptors in the ventricle was 12,000 sites per cell or 3.38 si tes per mu m(2) of sarcolemma. The K-d and B-max values for [H-3]CGP b inding to ventricular beta-adrenoreceptors were unaffected by the in v ivo administration of five bolus catecholamine injections (4.0 mu g kg (-1) epinephrine, 2.0 mu g kg(-1) norepinephrine). This suggests that stress-induced increases in plasma catecholamines are unlikely to caus e the down-regulation of heart beta-adrenoreceptors in fish. The metho d described here represents a simple but powerful technique for the qu antification and characterization of adrenergic receptors in the fish heart. (C) 1994 Academic Press, Inc.