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Molecular Endocrinology 13 (4): 587-603
Copyright © 1999 by The Endocrine Society

Control of Action Potential-Driven Calcium Influx in GT1 Neurons by the Activation Status of Sodium and Calcium Channels

Fredrick Van Goor, Lazar Z. Krsmanovic, Kevin J. Catt and Stanko S. Stojilkovic

Endocrinology and Reproduction Research Branch National Institute of Child Health and Human Development National Institutes of Health Bethesda Maryland 20892

An analysis of the relationship between electrical membrane activity and Ca2+ influx in differentiated GnRH-secreting (GT1) neurons revealed that most cells exhibited spontaneous, extracellular Ca2+-dependent action potentials (APs). Spiking was initiated by a slow pacemaker depolarization from a baseline potential between -75 and -50 mV, and AP frequency increased with membrane depolarization. More hyperpolarized cells fired sharp APs with limited capacity to promote Ca2+ influx, whereas more depolarized cells fired broad APs with enhanced capacity for Ca2+ influx. Characterization of the inward currents in GT1 cells revealed the presence of tetrodotoxin-sensitive Na+, Ni2+-sensitive T-type Ca2+, and dihydropyridine-sensitive L-type Ca2+ components. The availability of Na+ and T-type Ca2+ channels was dependent on the baseline potential, which determined the activation/inactivation status of these channels. Whereas all three channels were involved in the generation of sharp APs, L-type channels were solely responsible for the spike depolarization in cells exhibiting broad APs. Activation of GnRH receptors led to biphasic changes in cytosolic Ca2+ concentration ([Ca2+]i), with an early, extracellular Ca2+-independent peak and a sustained, extracellular Ca2+-dependent phase. During the peak [Ca2+]i response, electrical activity was abolished due to transient hyperpolarization. This was followed by sustained depolarization of cells and resumption of firing of increased frequency with a shift from sharp to broad APs. The GnRH-induced change in firing pattern accounted for about 50% of the elevated Ca2+ influx, the remainder being independent of spiking. Basal [Ca2+]i was also dependent on Ca2+ influx through AP-driven and voltage-insensitive pathways. Thus, in both resting and agonist-stimulated GT1 cells, membrane depolarization limits the participation of Na+ and T-type channels in firing, but facilitates AP-driven Ca2+ influx.




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