Hormonal regulation of free intracellular calcium concentrations in small and large ovine luteal cells

The second messengers mediating hormonal regulation of the corpus luteum are incompletely defined, particularly for the primary luteolytic hormone prostaglandin F(2α) (PGF(2α)). In this study, hormonally induced changes in free intracellular calcium concentrations were measured in individual small and large ovine luteal cells by using computer-assisted microscopic imaging of fura-2 fluorescence. This technique could readily detect transient increases in free calcium concentrations within both small and large luteal cells after treatment with 1 μM of the calcium ionophore, A23187. Treatment with PGF(2α) (1 μM) caused a dramatic increase in free calcium concentrations in large (before = 73 ± 2 nM; 2 min after PGF(2α) = 370 ± 21 nM; n = 33 cells) but not in small (before = 66 ± 4 nM; 2 min after PGF(2α) = 69 ± 8 nM; n = 12 cells) luteal cells. The magnitude and timing of the calcium response was dose- and time-dependent. The PGF(2α)-induced increase in free intracellular calcium is probably due to influx of extracellular calcium, since inclusion of inorganic calcium channel blockers (100 μM manganese or cobalt) attenuated the response to PGF(2α) and removal of extracellular calcium eliminated the response. In contrast to PGF(2α), luteinizing hormone (LH) (100 ng/ml) caused no change in intracellular levels of free calcium in small or large luteal cells, even though this dose of LH stimulated (p