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

GH₃ 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 μM), whereas at higher concentrations (10 μM), it completely abolished them. Ca²⁺ oscillations persisted following exposure to thapsigargin, indicating that inositol 1,4,5- triphosphate-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(1R)) 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,V)) and increased the amplitude of Ca²⁺ oscillations. Ba²⁺ inhibited both K(1R) and K(Ca,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(1R) channels modulate the frequency of Ca²⁺ oscillations and prolactin release, whereas TEA-sensitive K(Ca,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.