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.
Nassar-Gentina
V
V
Laboratorio de Fisiología Celular, Facultad de Ciencias, Universidad de Chile, Viña del Mar.
Rojas
E
E
Luxoro
M
M
D000313:Adrenal Medulla / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects; D002417:Cattle; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D008985:Monensin / Q000494:pharmacology*; D009543:Nifedipine / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D015222:Sodium Channels / Q000187:drug effects; D013779:Tetrodotoxin / Q000494:pharmacology
D015220:Calcium Channels; D015222:Sodium Channels; D002110:Caffeine; D013779:Tetrodotoxin; D008985:Monensin; D012964:Sodium; D009543:Nifedipine; D002118:Calcium