Monensin can transport calcium across cell membranes in a sodium independent fashion in the crayfish Procambarus clarkii.
Monensin, a Na(+)-selective ionophore, enhances transmitter release when applied to crustacean and frog neuromuscular junctions. Monensin is believed to raise intracellular sodium ([Na+]i) which in turn elevates intracellular calcium ([Ca2+]i). Using the fluorescent indicator fura-2, we measured [Ca2+]i in crayfish Procambarus clarkii presynaptic terminals during monensin application in normal Ringer, zero-calcium Ringer and zero-sodium Ringer to determine if [Ca2+]i increases with monensin application and if so by what mechanism. In normal Ringer, monensin, 10 microM and 100 microM, elevated [Ca2+]i by 440 nM and 7 microM respectively. This rise in [Ca2+]i was dependent on external calcium, as [Ca2+]i did not increase in zero-calcium Ringer. However, in a zero-sodium Ringer, monensin (10 microM) elevated [Ca2+]i by 370 nM. It is important to recognize that monensin, thought to be a sodium-selective ionophore, can transport calcium across the cytoplasmic membrane in a sodium-independent manner.
Mulkey
R M
RM
Department of Molecular and Cell Biology, University of California, Berkeley 94720.
Zucker
R S
RS
D000818:Animals; D016477:Artifacts; D003400:Astacoidea / Q000378:metabolism*; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D007552:Isotonic Solutions; D008564:Membrane Potentials / Q000187:drug effects; D008985:Monensin / Q000494:pharmacology*; D009469:Neuromuscular Junction / Q000187:drug effects* / Q000378:metabolism; D000077331:Ringer's Solution; D012964:Sodium / Q000494:pharmacology
D007552:Isotonic Solutions; D004533:Egtazic Acid; D000077331:Ringer's Solution; D008985:Monensin; D012964:Sodium; D002118:Calcium