Rise in cytoplasmic Ca induced by monensin in bovine medullary chromaffin cells

Monensin, a 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 μM) 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 μM) or tetrodotoxin (3 μM) 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 μM), 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 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.