Abstract

The BC3H-1 cell is a stable cell line of probable smooth muscle origin which expresses nicotinic acetylcholine, alpha1- and beta2-adrenergic receptors on its cell surface. Stimulation of the alpha1 receptor mobilizes 70% of the intracellular Ca2+ within a 2-3 minute interval. To delineate further the linkage between alpha1-receptor occupation and response, we have examined the quantitative relationship between fractional occupation of the receptor, the turnover of inositol-containing phospholipids, and the Ca2+ efflux. Alpha1-receptor activation stimulates the incorporation of [3H]inositol into phosphatidylinositol and the enhanced incorporation is linear over a 60-min interval. In contrast, agonist-elicited increases in hydrolysis of phosphatidylinositol and phosphatidylinositol mono- and bisphosphate develop more slowly, and a 5-min lag in enhanced formation of inositol trisphosphate, inositol bisphosphate, and inositol monophosphate is evident. The increased rate of Ca2+ efflux and enhanced rate of inositol incorporation into phosphatidylinositol elicited by phenylephrine exhibit virtually identical dependencies on agonist concentrations. Moreover, fractional inactivation of receptors with phenoxybenzamine shows equivalent increments in the reduction of the two intracellular responses. Both responses are linearly related to the residual receptor sites remaining after fractional inactivation. These findings indicate an absence of a receptor reserve in activating these intracellular events. Moreover, although alpha receptor occupation stimulates phosphatidylinositol hydrolysis, no evidence is provided that this event would precede Ca2+ release. Should inositol trisphosphate mediate intracellular Ca2+ mobilization in these cells, it would be active in extremely low concentrations or occur as a tightly coupled event in a microscopic environment.