Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

Instituto Teófilo Hernando, Departamento de Farmacología y Terapéutica, and Servicio de Farmacología Clínica e Instituto Universitario de Investigación Gerontológica y Metabólica, Hospital Universitario de la Princesa, Facultad de Medicina, Universidad Autónoma de Madrid; Unidad de Farmacología, Facultad de Medicina, Universidad de la Laguna; and Instituto de Biología y Genética Molecular, Universidad de Valladolid y CSIC, Departamento de Fisiología, Facultad de Medicina, Valladolid, Spain At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca 2+ concentration ([Ca 2+ ] c ) depends on at least four efficient regulatory systems: 1 ) plasmalemmal calcium channels, 2 ) endoplasmic reticulum, 3 ) mitochondria, and 4 ) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca 2+ ] c . Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K + , acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca 2+ entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca 2+ (CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca 2+ that modulates catecholamine release. Targeted aequorins with different Ca 2+ affinities show that mitochondria undergo surprisingly rapid millimolar Ca 2+ transients, upon stimulation of chromaffin cells with ACh, high K + , or caffeine. Physiological stimuli generate [Ca 2+ ] c microdomains in which the local subplasmalemmal [Ca 2+ ] c rises abruptly from 0.1 to 50 µM, triggering CICR, mitochondrial Ca 2+ uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca 2+ uptake, and increase catecholamine release three- to fivefold, support the