Prostaglandin F_2α-induced calcium transient in ovine large luteal cells: I. Alterations in cytosolic-free calcium levels and calcium flux

The effect of prostaglandin F_2α (PGF_2α) on cytosolic calcium homeostasis was studied in suspensions of ovine large or small luteal cells from superovulated ewes. In large cells loaded with fura-2 (AM), resting cytosolic-free calcium ([Ca²⁺]_i) was 62 ± 5 nM (Hanks' medium, pH 7.15), and PGF_2α (0.5 μM) induced a rapid transient increase in [Ca²⁺]_i to 152 ± 6 nM, which then decreased to 97 ± 6 nM within 3 min and remained at this level for the remainder of the treatment period (10-20 min). PGF_2α did not alter intracellular pH (pH_i) in cells loaded with snarf-1 (AM) (pH_i indicator). The transient nature of the [Ca²⁺]_i increase was due, at least in part, to the ability of PGF_2α to stimulate (P < 0.05) ⁴⁵Ca²⁺ efflux. In small cells, resting [Ca²⁺]_i was 57 ± 5 nM, and no change in [Ca²⁺]_i levels or PH_i occurred with the addition of PGF_2α. PGF_2α also did not affect ⁴⁵Ca²⁺ efflux in small cells. Calcium uptake was not significantly altered by PGF_2α in large or small cells. Data from kinetic analysis of the calcium transient was best fit to a two-compartment model consisting of a rapidly effluxing compartment and a slowly effluxing compartment. The size and rate constants were 62 ± 10 nM and 3.6 ± 1 min^-1, respectively, for the rapidly effluxing compartment and 140 ± 9 nM and 0.02 ± 0.002 min^-1, respectively, for the slowly effluxing compartment. These results provide evidence for a direct effect of PGF_2α specifically on the ovine large luteal cell that involves alterations in [Ca²⁺]_i and calcium flux. This effect is likely to be involved in intracellular mediation of the signal for luteal regression.