Differential Regulation of Ca²⁺ Homeostasis in Ovine Large and Small Luteal Cells

This study was undertaken to characterize differences in Ca²⁺ homeostasis between small and large ovine luteal cells. Although increasing extracellular pH (pH^ex) 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 Ca²⁺-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 F_2α (PGF_2α) or CPA. In contrast, after CPA pretreatment, PGF_2α 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 Ca²⁺-buffering capacity than large cells. Progesterone secretion by large cells was significantly inhibited at alkaline pH^ex. In the presence of PGF_2α, progesterone secretion exhibited a distinct pH optimum of 7.0. In contrast, pH^ex 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 PGF_2α. 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 PGF_2αwith alterations in intracellular Ca²⁺ homeostasis and consequent alterations in progesterone secretion.