Calcium, strontium and barium elicit different patterns of exocytosis in chromaffin cells
We have studied three divalent cations (Ca2+, Sr2+, Ba2+) capable of eliciting secretion in adrenal chromaffin and other secretory cells. These cations enter the cells through voltage-sensitive Ca2+ channels, yet Ba2+ and Sr2+ are poorly cleared from the intracellular space. In addition, Ba2+ blocks Ca2+-activated K+ channels, promoting cell depolarization and action potentials.
The amount of catecholamines released by intact depolarized cells depends on the cation applied extracellulary following order: Ba2+> Sr2+≥ Ca2+. Conversely, amperometric recordings on permeabilized cells (Fig. 1) show that Ca2+ promotes the longest lasting secretion, as Ba2+ only provokes secretion while present and Sr2+ induces intermediate-lasting secretion. Intracellular Ba2+ dialysis provokes exocytosis at concentrations 100-fold higher than those of Ca2+, whereas Sr2+ exhibits an intermediate sensitivity.
Using Fura-2 on intact chromaffin cells, the extracellular application of Ba2+ increases cytosolic Ca2+ concentrations also inducing long-lasting catecholamine release. When secretion is triggered by pressure application of 5 mM BaCl2, the substitution of Ca2+ by Sr2+ largely affects the kinetics of exocytosis elicited by Ba2+, whereas the replacement of Ca2+ by Mg2+ does not cause effects on secretory spikes (Fig. 2).
We also carried out an electrophysiological study under current-clamp configuration. This study shows that extracellular Sr2+ and Ba2+ cause membrane depolarization and action potentials, which are not blocked by Cd2+ but that can be mimicked by tetra-ethyl-ammonium. When applied intracellularly, only Ba2+ provokes action potentials. In addition, voltage-clamp monitoring of Ca2+-activated K+ channels (KCa) shows that Ba2+ reduces outward currents, which are enhanced by Sr2+.
Our results are compatible with the following sequence of events: Ba2+ blocks KCa channels from both the outside and inside of the cell, causing membrane depolarization that, in turn, opens voltage-sensitive Ca2+ channels and favours the entry of Ca2+ and Ba2+. Although Ca2+ is less permeable through its own channels, it is more efficient on triggering exocytosis. Strontium possesses both an intermediate permeability and an intermediate ability to induce secretion.
We have provided further details of the mechanisms underlying the sustained, yet slower secretory effects of Ba2+ in chromaffin cells, and described some novel actions of the lesser studied cation Sr2+ on BK channel currents and cell excitability that contribute to increase its secretagogue capability in comparison with Ba2+ and Ca2+.
Andrés M. Baraibar, Ricardo de Pascual, Marcial Camacho, Natalia Domínguez, J. David Machado, Luis Gandía, Ricardo Borges
Unidad de Farmacología, Fac. Medicina, Univ. La Laguna, Tenerife and Instituto Teófilo Hernando, Dept. Farmacología y Terapéutica, Fac. Medicina, Univ. Autónoma de Madrid, Spain
Distinct patterns of exocytosis elicited by Ca2+, Sr2+ and Ba2+ in bovine chromaffin cells.
Baraibar AM, de Pascual R, Camacho M, Domínguez N, Machado JD, Gandía L, Borges R
Pflugers Arch. 2018 Oct