Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

Abstract

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+ concentration ([Ca2+]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 [Ca2+]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 Ca2+ entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+ (CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+ that modulates catecholamine release. Targeted aequorins with different Ca2+ affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+ transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]c microdomains in which the local subplasmalemmal [Ca2+]c rises abruptly from 0.1 to approximately 50 microM, triggering CICR, mitochondrial Ca2+ uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+ uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+ redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+ release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]c that regulate the early and late steps of exocytosis.

Acute Enhancement of Synaptic Transmission and Chronic Inhibition of Synaptogenesis Induced by Perfluorooctane Sulfonate through Mediation of Voltage-Dependent Calcium Channel

“…100 µM PFOS not only markedly enhanced ICa but also caused a clear left-shift of the curve ( Figure 3D, P < 0.001, two-way ANOVA). VDCCs are involved in various neuronal functions, among which the L-type has been widely studied (19). To further characterize the PFOS effect on calcium channel subtypes, nifedipine was added to the bath solutions during current recordings.…”

Fast exocytosis mediated by T- and L-type channels in chromaffin cells: distinct voltage-dependence but similar Ca2+-dependence

“…Of particular relevance is whether T-type channels are capable of coupling to secretion and preserve their characteristic low-threshold of activation. In addition, it is interesting to assay how close the secretory events follow the voltage-dependence of Ttype channels and how T-type channels may coexist with the other voltage-gated Ca 2+ channels normally expressed in rodent chromaYn cells, in particular with the L-type channels which are the dominant Ca 2+ channel subtypes controlling secretion in these cells (see Garcia et al 2006). Besides being the most densely expressed Ca 2+ channels in rodent chromaYn cells, L-type channels possess other important features: (1) control 30% of catecholamine release in in-vivo perfused rat adrenal glands while P/Q and N-type channels contribute marginally (Akiyama et al 2004), (2) can be up-and down-modulated by -adrenergic autoreceptors (Hernandez-Guijo et al 1999;Cesetti et al 2003) and aVected by phosphodiesterase type-4 activity , (3) contribute to a lowthreshold "pace-maker" current that sets the action potentials autorythmicity at rest (Carbone et al 2006c).…”

Mitochondria and chromaffin cell function

“…According to the initial formulation, stimulus promotes Ca 2+ entry, which increases the cytosolic Ca 2+ concentration ([Ca 2+ ] C ) and triggers exocytosis [20,21]. Later studies showed that the genesis of the [Ca 2+ ] C signal includes subtle nuances contributed by the intracellular Ca 2+ -handling organelles [5,8,24,25]. To begin with, Ca 2+ entering through the plasma membrane Ca 2+ channels is buffered by cytosolic Ca 2+ buffers, so that the increase of [Ca 2+ ] C is much less than expected.…”

“…In this way high [Ca 2+ ] C microdomains (HCMD) are built at the secretion site whereas [Ca 2+ ] C near the resting level is preserved at the cell core. Mitochondria play a prominent role in shaping the [Ca 2+ ] C signal in chromaffin cells [24]. On the other hand, changes of Ca 2+ concentration in the mitochondrial matrix ([Ca 2+ ] M ) modify the rate of respiration [27,32,47,49,51] and, when excessive, can trigger cell death [22].…”