[Characteristics of lipolytic activities in the genetically obese Zucker fatty rats--experimental study on (-)-[3H]dihydroalprenolol binding in the intact fat cells].

In this paper, (-)-[3H]dihydroalprenolol ([3H]DHA) binding, cyclic AMP (cAMP) and free fatty acid (FFA) accumulations of fat cells from both the genetically obese Zucker fatty rats and Wistar rats were studied for the purpose of recognizing the characteristics of lipolytic activities in the obese animals. In the experiments of [3H]DHA binding on intact fat cells, it was revealed that some of [3H]DHA bindings had properties of a beta-adrenergic receptor, while the others had those of a non-beta-receptor, the latter showing the lack of stereospecificity and the failure to compete in a potent manner against beta-adrenergic agonist. This concentration of phentolamine displayed no apparent effect on the ability of isoproterenol to stimulate cAMP accumulation in the fat cells. However, even in the presence of 10(-4)M phentolamine, non-beta-receptor binding was observed by increasing the concentration of [3H]DHA to higher than 10(-9)M. Accordingly, the experimental conditions of [3H]DHA binding were as follows: 3 approximately 15 x 10(4) fat cells preincubated with 10(-4)M phentolamine for 5 min were incubated for 12 min in modified Krebs-Ringer phosphate buffer (KRP, pH 7.4, 37 degrees C), at a final volume of 1 ml containing 0.3% bovine serum albumin, 3 x 10(-10)M [3H]DHA and competitors. The content of the tube were then rapidly filtered under vacuum through a single Whatman GF/C filter (pore size 1.2 micrometers). Computer-analysis of the resulting inhibition curve provided total binding, slope factor, half-maximally inhibitory concentration and non-specific binding. In this condition, [3H]DHA binding on intact fat cells showed the stereospecificity and the equilibrium dissociation constant of (-)- and (+)-propranolol, (-)-alprenolol and (-)-isoproterenol approximate to those expected from the biological activities, displaying properties consistent with those of a beta-adrenergic receptor. cAMP and FFA were determined in cell plus medium after incubation for 5 and 30 min, respectively. The data obtained from the [3H]DHA binding studies and cAMP measurements indicate that the equilibrium dissociation constant, maximal binding and isoproterenol-stimulated cAMP accumulations per 10(6) cells did not differ significantly between the Zucker fatty rats and Wistar rats.(ABSTRACT TRUNCATED AT 400 WORDS)