Effects of prostaglandin F2α and other potential secretagogues on oxytocin secretion and second messenger metabolism in the ovine corpus luteum in vitro

Abstract

Release of oxytocin by sliced or minced sheep luteal tissue in vitro was stimulated up to 1.6- and 2.3-fold by arachidonic acid and the calcium ionophore A23187 respectively. Prostaglandin (PG) F2 alpha and the PGF2 alpha analogue cloprostenol, and other potential agonists known to be active in vivo, including noradrenaline and acetylcholine, were ineffective, as was the phorbol ester tetradecanoylphorbol acetate (TPA). The ineffectiveness of PGF2 alpha was not due to a general unresponsiveness of the tissue in vitro, as PGF2 alpha reduced LH stimulation of tissue concentrations of cyclic AMP and activated inositol lipid hydrolysis. The effect of arachidonic acid was accompanied by release from the tissue of the cytosolic enzyme lactate dehydrogenase (at arachidonic acid concentrations below those required to release oxytocin) and its effect on oxytocin and lactate dehydrogenase release was mimicked by oleic and linolenic acids; arachidonic acid was concluded to act by a non-physiological physicochemical effect without conversion to an eicosanoid. As PGF2 alpha in vitro is known to raise intracellular Ca2+ concentrations in the large luteal cells that secrete oxytocin, and as A23187 stimulates oxytocin release in vitro in the presence and absence of TPA, it is concluded that in-vitro incubation results in an artifactual blockade of the oxytocin-releasing action of PGF2 alpha at an unidentified point distal to the effect on intracellular Ca2+.