Adrenomedullin in rat corpus luteum and its role in steroidogenesis during pregnancy

Abstract

Adrenomedullin (ADM), a novel vasorelaxant peptide, was found in human/rat ovaries and uteri. Plasma ADM level increases in pregnant women and pregnant rats. The expression of ADM in rat corpus luteum (CL) and the effects of ADM on steroidogenesis were studied to find out the possible function of ADM in CL during pregnancy. In immature Sprague-Dawley rats, after injection of pregnant mare serum gonadotropin (PMSG) or PMSG and human chorionic gonadotropin (HCG), small antral follicles (diameter of 200-400 µm), large antral follicles (diameter above 900 µm) and newly-formed CL were isolated. Gene expression levels of Adm and its receptor components were analyzed by real-time PCR (qPCR). CL and large antral follicles showed higher gene expression levels of Adm and one of its receptor components, Ramp2, than small antral follicles. To study the expression profile of Adm in CL during pregnancy, CL from early- (7-day), mid- (12-day) and late- (17-day) pregnant rats were isolated. The gene expression levels of Adm and its receptor components were measured by qPCR. Adm expression increased 3-fold in mid- and late- pregnancy while calcrl, Ramp2 and Ramp3 all showed an increase in gene expression in late pregnancy. In ovaries from early-, mid- and late- pregnant rats and non-pregnant rats immunohistochemical (IHC) staining showed heavy ADM staining in the CL but the most intense ADM staining was observed in the center of CL from late-pregnant rats, which might be related to the reported anti-apoptosis function of ADM. To investigate the function of ADM in CL during pregnancy, CL isolated from early- and mid- pregnant rats were incubated with ADM in vitro. The effect of ADM on progesterone secretion was quantified by Enzyme Immunoassay. ADM suppressed progesterone production from the CL in early pregnancy. It enhanced progesterone production in mid-pregnancy when its production was high -- a finding which agrees with the possible luteogenic (anti-luteolytic) action of ADM when the placentae are secreting progesterone and less rCG. In this way, ADM might also provide the fine-tuning of progesterone level from the CL during pregnancy. To conclude, the expression levels of Adm and its receptor component(s) increased during CL formation and were maintained at a high level during pregnancy with an increase at mid- to late pregnancy. One of the roles of ADM in CL during pregnancy may be the maintenance of a stable progesterone production.