The effect of prostaglandin F2 α(PGF2 α) 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 PGF2 α(0.5 μM) induced a rapid transient increase in [Ca2+]; 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). PGF2 αdid not alter intracellular pH (pHi) in cells loaded with snarf-1 (AM) (pHiindicator). The transient nature of the [Ca2+]iincrease was due, at least in part, to the ability of PGF2 αto stimulate (P < 0.05)45Ca2+efflux. In small cells, resting [Ca2+]iwas 57 ± 5 nM, and no change in [Ca2+]ilevels or pHioccurred with the addition of PGF2 α. PGF2 αalso did not affect45Ca2+efflux in small cells. Calcium uptake was not significantly altered by PGF2 αin 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 PGF2 αspecifically on the ovine large luteal cell that involves alterations in [Ca2+]iand calcium flux. This effect is likely to be involved in intracellular mediation of the signal for luteal regression.
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