Influence of nitric oxide and noradrenaline on prostaglandin F(2α)-induced oxytocin secretion and intracellular calcium mobilization in cultured bovine luteal cells

Although prostaglandin (PG) F(2α) released from the uterus has been shown to cause regression of the bovine corpus luteum (CL), the neuroendocrine, paracrine, and autocrine mechanisms regulating luteolysis and PGF(2α) action in the CL are not fully understood. A number of substances produced locally in the CL may be involved in maintaning the equilibrium between luteal development and its regression. The present study was carried out to determine whether noradrenaline (NA) and nitric oxide (NO) regulate the sensitivity of the bovine CL to PGF(2α) in vitro and modulate a positive feedback cascade between PGF(2α) and luteal oxytocin (OT) in cows. Bovine luteal cells (Days 8-12 of the estrous cycle) cultured in glass tubes were pre-exposed to NA (10^-5 M) or an NO donor (S-nitroso-N-acetylpenicillamine [S-NAP]; 10^-4 M) before stimulation with PGF(2α) (10^-6 M). Noradrenaline significantly stimulated the release of progesterone (P₄), OT, PGF(2α), and PGE₂ (P < 0.01); however, S-NAP inhibited P₄ and OT secretion (P < 0.05). Oxytocin secretion and the intracellular level of free Ca²⁺ ([Ca²⁺](i)) were measured as indicators of CL sensitivity to PGF(2α). Prostaglandin F(2α) increased both the amount of OT secretion and [Ca²⁺](i) by approximately two times the amount before (both P < 0.05). The S-NAP amplified the effect of PGF(2α) on [Ca²⁺](i) and OT secretion (both P < 0.001), whereas NA diminished the stimulatory effects of PGF(2α) on [Ca²⁺](i) (P < 0.05). Moreover, PGF(2α) did not exert any additionally effects on OT secretion in NA-pretreated cells. The overall results suggest that adrenergic and nitrergic agents play opposite roles in the regulation of bovine CL function. While NA stimulates P₄ and OT secretion, NO may inhibit it in bovine CL. Both NA and NO are likely to stimulate the synthesis of luteal PGs and to modulate the action of PGF(2α). Noradrenaline may be the factor that is responsible for the limited action of PGF(2α) on CL and may be involved in the protection of the CL against premature luteolysis. In contrast, NO augments PGF(2α) action on CL and it may be involved in the course of luteolysis.