Alpha 1-adrenergic receptor activation mobilizes cellular Ca2+ in a muscle cell line

Regulation of cellular Ca2+ movements by alpha 1-adrenergic receptors has been studied using 45Ca2+ flux techniques in monolayer cultures of intact BC3H-1 cells. Unidirectional 45Ca2+ efflux from BC3H-1 cells reveals multiphasic kinetics, with a major fraction of cellular Ca2+ residing in a slowly exchanging intracellular compartment. Stimulation of alpha 1-adrenergic receptors by the agonist phenylephrine substantially increases 45Ca2+ unidirectional efflux, accompanied by a far smaller increase in 45Ca2+ influx. The selective enhancement of 45Ca2+ unidirectional efflux upon alpha 1-adrenergic receptor activation results in a net 30-40% decline in total cell Ca2+ content, measured either by radioisotopic equilibrium techniques or by atomic absorption spectroscopy. The relatively large pool of Ca2+ responsive to alpha-adrenergic stimulation is not displaced by La3+ but can be depleted with the Ca2+ ionophore A-23187. These results indicate that alpha 1-adrenergic receptor activation predominantly mobilizes Ca2+ from intracellular stores, together with a much smaller increase in transmembrane Ca2+ permeability. This interpretation is supported by comparative 45Ca2+ flux studies using a sister clone of BC3H-1 cells possessing surface nicotinic acetylcholine receptors but no alpha 1-adrenergic receptors. Agonist stimulation of the cholinergic receptor opens a well characterized transmembrane ion permeability gate. Cholinergic receptor activation greatly enhances the observed 45Ca2+ unidirectional influx relative to efflux, leading to net elevation of cellular Ca2+ content as Ca2+ moves down its inwardly directed concentration gradient.