Mechanisms involved in ATP-evoked Ca2+ oscillations in isolated human granulosa-luteal cells

Using single-cell microfluorimetry, we have shown that ATP evoked repetitive Ca2+ oscillations in intact Fura-2 loaded human granulosa-luteal cells (hG/LCs) in the absence of extracellular Ca2+. Sustained increases in Ca2+(i) required extracellular Ca2+ and ATP depleted stores were refreshed by brief (2 min) incubation with external Ca2+. Basal Ca2+(i) was unaffected by caffeine (1 mM), but 20 mM caffeine inhibited ATP-evoked Ca2+ release in the absence of external Ca2+. Thimerosal (10 μM) evoked repetitive Ca2+ spikes, under Ca2+-free conditions, which fused to form an elevated plateau when external Ca2+ was replaced. Thimerosal-induced changes in Ca2+(i) were reversibly inhibited by the thiol reducing agent dithiothreitol (1 mM). The periodicity and amplitude of the Ca2+(i) oscillations produced by thimerosal and ATP differ. ATP- or thimerosal-evoked changes in Ca2+(i) were unaffected by dantrolene sodium (10 μM). The Ca2+-ATPase inhibitor thapsigargin (1μM) increased Ca2+(i) and attenuated subsequent ATP-evoked changes in Ca2+(i). We conclude that ATP stimulates an oscillatory release of Ca2+ from InsP3-sensitive stores in hG/LCs