Distinct patterns of exocytosis elicited by Ca2+, Sr2+ and Ba2+ in bovine chromaffin cells

• Published in: Pflügers Archiv Vol. 470; no. 10; pp. 1459 - 1471
• Main Authors: Baraibar, Andrés M., de Pascual, Ricardo, Camacho, Marcial, Domínguez, Natalia, David Machado, J., Gandía, Luis, Borges, Ricardo
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.10.2018; Springer Nature B.V
• Subjects: Ammonium; Biomedical and Life Sciences; Biomedicine; Calcium (extracellular); Calcium channels; Calcium channels (voltage-gated); Calcium permeability; Catecholamines; Cell Biology; Chromaffin cells; Depolarization; Dialysis; Exocytosis; Fura-2; Human Physiology; Ion Channels; Membrane potential; Molecular Medicine; Neurosciences; Permeability; Potassium; Potassium channels (calcium-gated); Receptors; Receptors and Transporters; Secretion; Strontium; Amperometry; Catecholamines; Secretion; Adrenal medulla
• ISSN: 0031-6768; 1432-2013
• DOI: 10.1007/s00424-018-2166-4

Three divalent cations can elicit secretory responses in most neuroendocrine cells, including chromaffin cells. The extent to which secretion is elicited by the cations in intact depolarized cells was Ba 2+  > Sr 2+  ≥ Ca 2+ , contrasting with that elicited by these cations in permeabilized cells (Ca 2+  > Sr 2+  > Ba 2+ ). Current-clamp recordings show that extracellular Sr 2+ and Ba 2+ cause membrane depolarization and action potentials, which are not blocked by Cd 2+ but that can be mimicked by tetra-ethyl-ammonium. When applied intracellularly, only Ba 2+ provokes action potentials. Voltage-clamp monitoring of Ca 2+ -activated K + channels (K Ca ) shows that Ba 2+ reduces outward currents, which were enhanced by Sr 2+ . Extracellular Ba 2+ increases cytosolic Ca 2+ concentrations in Fura-2-loaded intact cells, and it induces long-lasting catecholamine release. Conversely, amperometric recordings of permeabilized cells show that Ca 2+ promotes the longest lasting secretion, as Ba 2+ only provokes secretion while it is present and Sr 2+ induces intermediate-lasting secretion. Intracellular Ba 2+ dialysis provokes exocytosis at concentrations 100-fold higher than those of Ca 2+ , whereas Sr 2+ exhibits an intermediate sensitivity. These results are compatible with the following sequence of events: Ba 2+ blocks K Ca channels from both the outside and inside of the cell, causing membrane depolarization that, in turn, opens voltage-sensitive Ca 2+ channels and favors the entry of Ca 2+ and Ba 2+ . Although Ca 2+ is less permeable through its own channels, it is more efficient in triggering exocytosis. Strontium possesses both an intermediate permeability and an intermediate ability to induce secretion.