Prostaglandin F_2α-induced calcium transient in ovine large luteal cells: II. Modulation of the transient and resting cytosolic free calcium alters progesterone secretion

A previous study demonstrated that prostaglandin F_2α (PGF_2α) stimulates a transient increase in cytosolic free Ca²⁺ levels ([Ca²⁺]_i) in ovine large luteal cells. In the present study, the magnitude of the PGF_2α (0.5 μM)-induced calcium transient in Hanks' medium (87 ± 2 nM increase above resting levels) was reduced (P < 0.05) but not completely eliminated in fura-2 loaded large luteal cells incubated in Ca²⁺-free or phosphate- and carbonate-free medium (10 ± 1 nM, 32 ± 6 nM, above resting levels; respectively). Preincubation for 2 min with 1 mM LaCl₃ (calcium antagonist) eliminated the PGF_2α-induced calcium transient. The inhibitory effect of PGF_2α on secretion of progesterone was reduced in Ca²⁺-free medium or medium plus LaCl₃. Resting [Ca²⁺]_i levels and basal secretion of progesterone were both reduced (P < 0.05) in large cells incubated in Ca²⁺-free medium (27 ± 4 nM; 70 ± 6% control, respectively) or with 5 μM 5,5′-dimethyl bis-(O-aminophenoxy)ethane-N,N,N′N′-tetraacetic acid (40 ± 2 nM; 49 ± 1% control; respectively). In addition, secretion of progesterone was inhibited (P < 0.05) by conditions that increased (P < 0.05) [Ca²⁺]_i; that is LaCl₃ ([Ca²⁺]_i, 120 ± 17 nM; progesterone, 82 ± 8% control) and PGF_2α ([Ca²⁺]_i, 102 ± 10 nM; progesterone, 82 ± 3% control). In small luteal cells, resting [Ca²⁺]_i levels and secretion of progesterone were reduced by incubation in Ca²⁺-free Hanks ([Ca²⁺]_i, 28 ± 2 nM; progesterone, 71 ± 6% control), however, neither LaCl₃ nor PGF_2α increased [Ca²⁺]_i levels or inhibited secretion of progesterone. The findings presented here provide evidence that extracellular as well as intracellular calcium contribute to the PGF_2α-induced [Ca²⁺]_i transient in large cells. Furthermore, whereas an adequate level of [Ca²⁺]_i is required to support progesterone production in both small and large cells, optimal progesterone production in large cells depends upon an appropriate window of [Ca²⁺]_i.