This study was undertaken to characterize differences in Ca2+ homeostasis between small and large ovine luteal cells. Although increasing extracellular pH (pHex) resulted in increases in intracellular calcium ([Ca2+]in) in both cell types, the large cells exhibited a greater sensitivity, suggesting that distinct [Ca2+]in regulatory mechanisms with distinct pH optima are operating in the two cell types. The sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase inhibitors thapsigargin (TG) and cyclopiazonic acid (CPA) increased [Ca2+]in in both cell types. Treatment of small cells with CPA resulted in transient increases in [Ca2+]in, whereas CPA produced sustained increases in [Ca2+]in in large cells. In small cells, pretreatment with CPA prevented further increases in [Ca2+]in in response to TG and vice versa. In large cells, TG pretreatment precluded further increases in [Ca2+]in with either prostaglandin F2α (PGF2α) or CPA. In contrast, after CPA pretreatment, PGF2α or TG induced further increases in [Ca2+]in in large cells. This suggests that a TG-sensitive, CPA-insensitive Ca2+ pool is present in large cells but not in small cells. Neither Na+ removal nor KCl addition affected [Ca2+]in in either cell type, indicating that neither the Na+/Ca2+ exchanger nor voltage-dependent Ca2+ channels are involved in Ca2+ homeostasis in these cells. Addition of the calcium antagonist, LaCl3 (La3+), produced a gradual increase in [Ca2+]in in large cells but no changes in [Ca2+]in in small cells. Additionally, treatment with increasing concentrations of 4-bromo-A23187 resulted in titratable increases in [Ca2+]in that are greater in large than small cells, suggesting that small cells possess a higher Ca(2+)-buffering capacity than large cells. Progesterone secretion by large cells was significantly inhibited at alkaline pHex. In the presence of PGF2α, progesterone secretion exhibited a distinct pH optimum of 7.0. In contrast, pHex did not affect secretion of progesterone in small cells under any of the conditions tested. TG, CPA, and La3+ all reduced secretion of progesterone in large, but not small, cells. These results demonstrate that ovine large and small luteal cells differ in regulation of [Ca2+]in homeostasis, and that treatments that increase [Ca2+]in decrease progesterone secretion in large cells but have no effect in small cells. A reduced capacity of large cells to regulate [Ca2+]in, therefore, may facilitate induction of luteal regression in these cells by PGF2a Our data also indicate that increases in [Ca2+]in by either increasing pH or by inhibiting Ca2+ ATPases result in decreases in progesterone secretion. This effect is only observed in large cells, which support our contention that in large cells, Ca2+ is a more important factor in regulating progesterone secretion than in small cells. Our data also support that large cells in the corpus luteum respond to PGF2a with alterations in intracellular Ca2+ homeostasis and consequent alterations in progesterone secretion.
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