INTERACTIONS OF AGONISTS AND ANTAGONISTS WITH BETA-ADRENERGIC RECEPTORS ON INTACT L6 MUSCLE-CELLS

Abstract

A binding assay was developed to characterize .beta.-adrenergic receptors on intact L6 [fetal rat] muscle cells. The affinity of .beta.-adrenergic receptors for the radioligand iodohydroxybenzylpindolol (IHYP) was the same in membrane preparations and in intact cells when determined by equilibrium binding or kinetic analysis. The number of specific IHYP binding sites per cell was approximately the same on intact cells as on membranes. The pharmacological properties of antagonists indicated that the receptors on intact cells were identical to those on membranes. The .beta.-adrenergic receptors on intact cells had a 100 to 400-fold lower affinity at equilibrium for the agonist isoproterenol than did .beta.-adrenergic receptors on membranes. This low affinity of the receptor for agonists as measured by inhibition of radioligand binding in intact cells was observed in C6 [rat glioma cell line] and S49 [murine lymphoma] cells. .beta.-Receptors on intact cells may initially have a high affinity for agonists as measured by inhibition of IHYP binding. An estimate of the Kd value of isoproterenol for .beta.-receptors on intact cells after a 1 min incubation was similar to the Kd value for isoproterenol measured in membranes at equilibrium in the presence of GTP. After 1-2 min of exposure to a low concentration of agonist, binding of IHYP was no longer inhibited. Agonists rapidly convert the .beta.-receptors on intact cells to a state which has a low affinity for agonists. The affinity of the receptor for antagonists did not change during the incubation.