Biochemical approaches to the study of cytosolic calcium regulation in nerve endings.

The nerve ending cytosol is bounded by the plasma membrane, the mitochondrial inner membrane and the endoplasmic reticulum membrane, transport across each of which is capable, in theory, of regulating the cytosolic free Ca2+ concentration. By parallel monitoring of mitochondrial and plasma membrane potentials, ATP levels, Na+ gradients and intrasynaptosomal Ca2+ distribution in preparations of isolated synaptosomes, we conclude the following: (a) mitochondria in situ represent a major Ca2+ pool, regulating the upper steady-state limit of the cytosolic free Ca2+ concentration by sequestering Ca2+ reversibly; (b) this limit is responsive to the cytosolic Na+ concentration, but is below the concentration required for significant exocytosis; (c) plasma membrane Ca2+ transport can be resolved into a constant slow influx, a voltage-dependent and verapamil-sensitive influx and an ATP-dependent efflux, while Ca2+ efflux driven by the sodium electrochemical potential cannot be detected; (d) Ca2+ regulation by intrasynaptosomal endoplasmic reticulum appears to be of minor significance in the present preparation.