Characterization of a plasma membrane calcium oscillator in rat pituitary somatotrophs

In excitable cells, oscillations in intracellular free calcium concentratio ns ([Ca2+](i)) can arise from action-potential-driven Ca2+ influx, and such signals can have either a localized or global form, depending on the coupl ing of voltage-gated Ca2+ influx to intracellular Ca2+ release pathway. Her e we show that rat pituitary somatotrophs generate spontaneous [Ca2+](i) os cillations, which rise from fluctuations in the influx of external Ca2+ and propagate within the cytoplasm and nucleus. The addition of caffeine and r yanodine, modulators of ryanodine-receptor channels, and the depletion of i ntracellular Ca2+ stores by thapsigargin and ionomycin did not affect the g lobal nature of spontaneous [Ca2+](i) signals. Bay K 8644, an L-type Ca2+ c hannel agonist, initiated [Ca2+](i) signaling in quiescent cells, increased the amplitude of [Ca2+](i) spikes in spontaneously active cells, and stimu lated growth hormone secretion in perifused pituitary cells. Nifedipine, a blocker of L-type Ca2+ channels, decreased the amplitude of spikes and basa l growth hormone secretion, whereas Ni2+, a blocker of T-type Ca2+ channels , abolished spontaneous [Ca2+](i) oscillations, Spiking was also abolished by the removal of extracellular Na+ and by the addition of 10 mM Ca2+, Mg2, or Sr2+, the blockers of cyclic nucleotide-gated channels. Reverse transc riptase-polymerase chain reaction and Southern blot analyses indicated the expression of mRNAs for these channels in mixed pituitary cells and purifie d somatotrophs, Growth hormone-releasing hormone, an agonist that stimulate d cAMP and cGMP productions in a dose-dependent manner, initiated spiking i n quiescent cells and increased the frequency of spiking in spontaneously a ctive cells, These results indicate that in somatotrophs a cyclic nucleotid e-controlled plasma membrane Ca2+ oscillator is capable of generating globa l Ca2+ signals spontaneously and in response to agonist stimulation. The Ca 2+-signaling activity of this oscillator is dependent on voltage-gated Ca2 influx but not on Ca2+ release from intracellular stores.