Rise in cytoplasmic Ca2+ induced by monensin in bovine medullary chromaffin cells

Monensin, a Na+/H+ exchanger, induces catecholamine secretion from adrenal chromaffin cells by an unknown mechanism. We found and report here that in bovine chromaffin cells, monensin evokes profound changes in [Ca2+]i which were measured by means of the fluorescent Ca2+ indicator Indo-1. Application of monensin (10 microM) generated a marked [Ca2+]i rise. Removal of external Ca2+ did not prevent the elevation of [Ca2+]i, though it was significantly decreased. In the presence of nifedipine (10 microM) or tetrodotoxin (3 microM) the monensin-induced [Ca2+]i rise remained unchanged. In contrast, in the absence of extracellular Na+ the [Ca2+]i rise was abolished. Addition of caffeine (40 mM) at the peak response generated by monensin produced a further increase in [Ca2+]i, which was independent of external [Ca2+] or [Na+]. After depletion of the IP3-sensitive compartment by thapsigargin (1 microM), caffeine still induced a rise in [Ca2+]i while the monensin response was absent. We concluded that the origin of the Ca2+ for the [Ca2+]i increase elicited by the Na+/H+ exchanger in chromaffin cells is not the extracellular space. Clearly there seems to be at least two intracellular Ca2+ stores, one of which is affected by monensin. This Ca2+ pool, which is different than the pool stimulated by caffeine, is sensitive to the extracellular [Ca2+] and to thapsigargin. Our data are compatible with the idea that the monensin mediated Na+ entry could activate the production of inositol trisphosphate and this in turn could trigger Ca2+ release from the endoplasmic reticulum.