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Luteinizing Hormone Inhibits Conversion of Pregnenolone to Progesterone in Luteal Cells from Rats on Day 19 of Pregnancy¹

^a Laboratorio de Reproducción y Lactancia, LARLAC-CONICET, 5500 Mendoza, Argentina

We have previously reported that intrabursal ovarian administration of LH at the end of pregnancy in rats induces a decrease in luteal progesterone (P₄) synthesis and an increase in P₄ metabolism. However, whether this local luteolytic effect of LH is exerted directly on luteal cells or on other structures, such as follicular or stromal cells, to modify luteal function is unknown. The aim of the present study was to determine the effect of LH on isolated luteal cells obtained on Day 19 of pregnancy. Incubation of luteal cells with 1, 10, 100, or 1000 ng/ml of ovine LH (oLH) for 6 h did not modify basal P₄ production. The addition to the culture medium of 22(R)-hydroxycholesterol (22R-HC, 10 µg/ml), a membrane-permeable P₄ precursor, or pregnenolone (10^-2 µM) induced a significant increase in P₄ accumulation in the medium in relation to the control value. When luteal cells were preincubated for 2 h with oLH, a significant (p < 0.01) reduction in the 22R-HC- or pregnenolone-stimulated P₄ accumulation was observed. Incubation of luteal cells with dibutyryl cAMP (1 mM, a cAMP analogue) plus isobutylmethylxanthine (1 mM, a phosphodiesterase inhibitor) also inhibited pregnenolone-stimulated P₄ accumulation. Incubation with an inositol triphosphate synthesis inhibitor, neomycin (1 mM), or an inhibitor of intracellular Ca²⁺ mobilization, (8,9-N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (1 mM), did not prevent the decrease in pregnenolone-stimulated P₄ secretion induced by oLH. It was concluded that the luteolytic action of LH in late pregnancy is due, at least in part, to a direct action on the luteal cells and that an increase in intracellular cAMP level might mediate this effect.

¹ This work was supported by grant No. PMT-PICT0098 from Agencia Nacional de Promoción Científica y Tecnológica and by grant PLI 325/8 PRE 023/98 from the PLACIRH (Programa Latinoamericano de Capacitación e Investigación en Reproducción Humana).

² Correspondence and current address: Department of Physiology and Biophysics (M/C 901), University of Illinois at Chicago, 835 South Wolcott Avenue, Chicago, IL 60612–7342. FAX: 312 413 0159; costocco{at}tigger.uic.edu

³ Reprint requests: Ricardo P. Deis, Laboratorio de Reproducción y Lactancia, LARLAC-CONICET Casilla de Correo 855, 5500 Mendoza, Argentina. FAX: 54-61-273976; LarLac{at}Lab.cricyT.edu.ar

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