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Two distinct Na+ currents control cytosolic Ca2+ pulsing in Xenopus laevis pituitary melanotrophs



Two distinct Na+ currents control cytosolic Ca2+ pulsing in Xenopus laevis pituitary melanotrophs



Cell Calcium 21(3): 241-251



Studies with the Na+ channel blocker tetrodotoxin (TTx) on Ca-2+-dependent hormone release by mammalian and amphibian pituitary melanotrophs have suggested that the Na+ spikes these cells generate are not responsible for triggering Ca-2+ influx and consequently secretion. In contrast, we found in Xenopus laevis melanotrophs that the spontaneously occurring elevations in cytosolic free Ca-2+ concentration ('Ca-2+ pulses') were dependent on the presence of extracellular Na+ and sensitive to TTx and the Na+ channel activator, veratridine. However, an inhibitory effect of TTx could only be demonstrated when the extracellular Na+ concentration was lowered to near-threshold levels. In voltage-clamp experiments, two distinct Na+ currents were recorded, one sensitive to TTx and the other insensitive to TTx but blocked by micromolar concentrations of Cd-2+. Together they appeared to control action potential activity and spontaneous Ca-2+ pulsing. These data strongly suggest that Na+ action potentials do regulate cytosolic free Ca-2+ concentration in melanotrophs.

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Accession: 009683506

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PMID: 9105733


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