To evaluate the role of transmembrane ion flux on the acute response of luteal cells to LH, the effects of two drugs, ouabain and monensin, and changes in ion composition of the medium were examined. Ouabain, an inhibitor of the Na+ extrusion pump, and monensin, a selective Na+ ionophore, would be expected to increase the intracellular level of Na+. Both drugs produced a highly significant, dose-related and Na+-dependent inhibition of LH-stimulated cAMP accumulation and progesterone secretion. The IC50 values for ouabain and monensin for both responses to LH were about 50 and 0.1 µm, respectively, and inhibition of cAMP accumulation was competitive [inhibitory constant (Ki) = 20 and 0.06 µm, respectively]. Both drugs showed inhibition only in the intact cell, and no nonspecific cytotoxic effects were evident. No effect on the specific binding of gonadotropin or on LH-stimulated adenylate cyclase activity in membranes was seen, nor was inhibition reduced by coincubation of the cells with isobutyl methylxanthine, a cAMP phosphodiesterase inhibitor. Both drugs inhibited (Bu)2cAMP-stimulated progesterone accumulation. No effect of ouabain was seen on cholera toxin- or forskolin-stimulated cAMP accumulation, whereas monensin significantly inhibited this response to both agonists. Preincubation of cells with LH protected against inhibition of cAMP accumulation by ouabain and monensin. Removal of extracellular Na+ completely prevented inhibition by both drugs and slightly blunted the response to LH alone. However, reduction of extracellular Na+ from 128 to 32 mm, treatment of cells with tetrodotoxin (a sodium channel blocker), or increasing extracellular K+ from 5.4 to 66 mm had no effect on the stimulation of cAMP accumulation by LH. On the other hand, removal of extracellular Ca2+ completely blocked inhibition of LH-stimulated cAMP accumulation by ouabain or monensin. It is concluded that ouabain and monensin inhibit the acute stimulation of cAMP accumulation by LH probably as a result of an influx of Na+ into the luteal cell. The increase in intracellular Na+ does not appear to directly inhibit LH-sensitive adenylate cyclase activity, but induces a secondary influx of extracellular Ca2+ which inhibits activation of adenylate cyclase by LH at a site involved in coupling of the receptor to the enzyme. Luteal regression may be initiated by mechanisms similar to that caused by ouabain and monensin, because the characteristics of inhibition by these drugs is identical to that caused by prostaglandin F2α, a physiological luteolysin. Moreover, maintenance of corpus luteum function by LH may occur in part by processes that maintain optimum ionic gradients and low Ca2+ levels in the luteal cell.
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