DIFFERENTIAL REGULATION OF CA2+ HOMEOSTASIS IN OVINE LARGE AND SMALL LUTEAL CELLS

This study was undertaken to characterize differences in Ca2+ homeosta sis between small and large ovine luteal cells. Although increasing ex tracellular 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 sarc oplasmic/ endoplasmic reticulum Ca2+-ATPase inhibitors thapsigargin (T G) and cyclopiazonic acid (CPA) increased [Ca2+](in) in both cell type s. Treatment of small cells with CPA resulted in transient increases i n [Ca2+](in), whereas CPA produced sustained increases in [Ca2+](in) i n large cells. In small cells, pretreatment with CPA prevented further increases in [Ca2+](in) in response to TG and vice versa. In large ce lls, TG pretreatment precluded further increases in [Ca2+](in) with ei ther prostaglandin F-2 alpha (PGF(2 alpha)) or CPA. In contrast, after CPA pretreatment, PGF(2 alpha) or TG induced further increases in [Ca 2+](in) in large cells. This suggests that a TG-sensitive, CPA-insensi tive Ca2+ pool is present in large cells but not in small cells. Neith er Na+ removal nor KCl addition affected [Ca2+](in) in either cell typ e, indicating that neither the Na+/Ca2+ exchanger nor voltage-dependen t Ca2+ channels are involved in Ca2+ homeostasis in these cells. Addit ion of the calcium antagonist, LaCl3 (La3+), produced a gradual increa se in [Ca2+](in) in large cells but no changes in [Ca2+](in) in small cells. Additionally, treatment with increasing concentrations of 4-bro mo-A23187 resulted in titratable increases in [Ca2+](in) that are grea ter in large than small cells, suggesting that small cells possess a h igher Ca2+-buffering capacity than large cells. Progesterone secretion by large cells was significantly inhibited at alkaline pH(ex). In the presence of PGE(2 alpha) progesterone secretion exhibited a distinct pH optimum of 7.0. In contrast, pH(ex) did not affect secretion of pro gesterone 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 c ells differ in regulation of ICa2+](in) homeostasis, and that treatmen ts that increase [Ca2+](in) decrease progesterone secretion in large c ells but have no effect in small cells. A reduced capacity of large ce lls to regulate [Ca2+](in), therefore, may facilitate induction of lut eal regression in these cells by PGF(2 alpha). 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 effe ct is only observed in large cells, which support our contention that in large cells, Ca2+ is a more important factor in regulating progeste rone secretion than in small cells. Our data also support that large c ells in the corpus luteum respond to PGF(2 alpha) with alterations in intracellular Ca2+ homeostasis and consequent alterations in progester one secretion.