The regulation by endothelin-1 (ET-1) of cytosolic free calcium ion concentrations ([Ca2+]i) was investigated in single immature rat (testicular) Sertoli cells. [Ca2+]i was estimated in individual gonadal cells by digital imaging videomicroscopy using the calcium indicator dye fura-2/AM. Two concentration-dependent types of ET-1-induced [Ca2+]i signals were observed. Responses to high ET-1 concentrations (1.0-1000 nM) were characterized by a biphasic, rapid, and transient [Ca2+]i rise (spike) within 10 sec, followed by an exponential decrease toward a new steady state level (plateau phase) in 98% of responsive cells. At low concentrations of ET-1 (0.001 or 0.1 nM), the [Ca2+]i increase was slower, reaching peak values 40-100 sec after stimulation and remaining elevated for 2-3 min of observation. There was cell-cell heterogeneity in the amplitude and kinetics of the [Ca2+]i response to the same concentration of ET-1. However, there was a significant ET-1 concentration-dependent increase in the total percentage of cells responding to ET-1. Removal of extracellular Ca2+ or use of Ca2+ channel blockers (verapamil or cobalt) did not affect the ET-1-induced [Ca2+]i spike phase, but abolished the plateau phase, suggesting that ET-1 induces the mobilization of Ca2+ from internal stores, followed by calcium influx from extracellular sources. In cell population experiments, ET-1 attenuated FSH-stimulated cAMP and estradiol accumulation by Sertoli cells. These inhibitory effects were mimicked by phorbol 12-myristate 13-acetate, an activator of protein kinase-C, suggesting that ET-1 action on Sertoli cells might be linked to the protein kinase-C pathway. In conclusion, the present investigations demonstrate that ET-1 activates an intracellular signaling pathway involving [Ca2+]i in single rat Sertoli cells. The sources of the biphasic [Ca2+]i response include mobilization of Ca2+ from internal stores, followed by Ca2+ influx via verapamil- and cobalt-sensitive Ca2+ channels. Increasing ET-1 concentrations recruit an increasing number of individual Sertoli cells responding with a spike-plateau [Ca2+]i signal, thus offering a mechanism at the single cell level for the ET dose-response curve.
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