Control of calcium spiking frequency in pituitary gonadotrophs by a single-pool cytoplasmic oscillator

The mechanisms by which the generation and frequency of cytoplasmic Ca2+ oscillations are controlled were investigated in pituitary gonadotrophs. In these cells, two Ca(2+)-mobilizing receptors, the gonadotropin-releasing hormone and endothelin receptors, induce frequency-modulated Ca2+ spiking at the rate of up to 30 min-1. The cytoplasmic oscillator is also activated by discharge of luminal Ca2+ (initiated by ionomycin, thapsigargin, or thimerosal) but not by increased voltage-sensitive Ca2+ influx or treatment with caffeine. The basic difference between these two types of Ca2+ oscillations is related to their requirement for inositol-1,4,5-triphosphate (InsP3). Thapsigargin-, thimerosal-, and ionomycin-induced spiking occurs without the rise in InsP3 production that is essential for the generation of receptor-controlled oscillatory responses. The differential requirement for InsP3 in the two types of Ca2+ spiking is indicated by two lines of evidence. First, agonist-induced Ca2+ spiking of frequency similar to that of non-receptor-mediated oscillations was accompanied by a significant increase in InsP3, whereas none of the non-receptor-mediated oscillations was associated with measurable changes in inositol phosphate production. Second, agonist-induced InsP3 formation and Ca2+ spiking were abolished by treatment with the phospholipase C inhibitors U73122 and neomycin sulfate, whereas non-receptor-mediated Ca2+ spiking was not affected by these agents. When the oscillator was activated by agents that do not increase InsP3 formation, it operated only at the basal rate of approximately 5 min-1 and spiking frequency did not rise with increasing drug concentrations, in contrast to the situation in agonist-stimulated gonadotrophs. However, both types of oscillations were affected by depletion of luminal Ca2+ and by changes in the intracellular Ca2+ concentration ([Ca2+]i) but were not inhibited by ryanodine. These findings are consistent with the operation of a single-pool Ca2+ oscillator that is responsible for generation of both types of Ca2+ oscillations. The oscillator is controlled by the coagonist actions of InsP3 and Ca2+ on the InsP3 receptor channels and by the activation of Ca(2+)-ATPase by rising [Ca2+]i. It can be induced to operate at low frequency without an increase in InsP3 production by agents that reduce intraluminal [Ca2+]i, and it exhibits a dose-dependent increase in spiking frequency during agonist stimulation.