Abstract

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+ concentration, [Ca2+](c), depend on at least three efficient regulatory mechanisms: (1) the plasmalemmal Ca2+ channels; (2) the endoplasmic reticulum (ER); and (3) the mitochondria. High-voltage activated Ca2+ channels of the L, N, P/Q, and R subtypes are expressed with different densities in various mammalian species; they are regulated by G proteins coupled to purnergic and opiate receptors, as well as by voltage and the local changes of [Ca2+](c). Targeted aequorin and confocal microscopy show that Ca entry through Ca2+ channels can refill the ER to near millimolar concentrations and causes the release of ER Ca2+ (CICR). We have also seen that, depending on its degree of filling, the ER may act as a sink or source of Ca2+ that modulates the release of catecholamine. Targeted aequorins with different Ca2+ affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+ transients ([Ca2+]M) upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+](c) microdomains at these functional complexes in which the local subplasmalemmal [Ca2+](c) rises abruptly from 0.1 muM to about 50 muM. This triggers CICR, mitochondrial Ca2+ uptake, and exocytosis in nearby secretory active sites. That this is true is shown by the observation that protonophores abolish mitochondrial Ca2+ uptake and drastically increase catecholamine release by 3- to 5-fold. This increase is likely due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; such transport might be controlled by Ca2+ redistribution to the cytoskeleton, through CICR and/or mitochondrial Ca2+ release.