L-type Ca2+ channels and K+ channels specifically modulate the frequency and amplitude of spontaneous Ca2+ oscillations and have distinct roles in prolactin release in GH3 cells

GH(3) cells showed spontaneous rhythmic oscillations in intracellular calcium concentration ([Ca2+](i)) and spontaneous prolactin release. The L-type Ca2+ channel inhibitor nimodipine reduced the frequency of Ca2+ oscillations at lower concentrations (100nm-1 mu M), whereas at higher concentrations (10 mu M), it completely abolished them. Ca2+ oscillations persisted following exposure to thapsigarsn, indicating that inositol 1,4,5-trisphosphate-sensitive intracellular Ca2+ stores were not required for spontaneous activity. The K+ channel inhibitors Ba2+, Cs+, and tetraethylammonium (TEA) had distinct effects on different K+ currents, as well as on Ca2+ oscillations and prolactin release. Cs+ inhibited the inward rectifier K+ current (K-IR) and increased the frequency of Ca2+ oscillations. TEA inhibited outward K+ currents activated at voltages above -40 mV (grouped within the category of Ca2+ and voltage-activated currents, K-Ca,K-V) and increased the amplitude of Ca2+ oscillations. Ba2+ inhibited both K-IR and K-Ca,K-V and increased both the amplitude and the frequency of Ca2+ oscillations. Prolactin release was increased by Ba2+ and Cs+ but not by TEA. These results indicate that L-type Ca2+ channels and K-IR channels modulate the frequency of Ca2+ oscillations and prolactin release, whereas TEA-sensitive K-Ca,K-V channels modulate the amplitude of Ca2+ oscillations without altering prolactin release. Differential regulation of these channels can produce frequency or amplitude modulation of calcium signaling that stimulates specific pituitary cell functions.