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a Laboratory of Cardiovascular Biochemistry and
b Laboratory of Cellular Biology of Hypertension, Centre Hospitalier de l'Université de Montréal, Campus Hôtel-Dieu, Department of Medicine, University of Montreal, Quebec, Canada H2W 1T8
c Molecular Reproduction Research Laboratory, Clinical Research Institute of Montreal, Montreal, Quebec, Canada H2W 1T8
All components of the natriuretic peptide (NP) system have been found in the ovary. The purpose of this study was to determine the hormonal regulation of the NP system during follicular growth and ovulation induced by gonadotropins eCG and hCG. Ovarian membrane binding, before and after treatment, revealed the presence of guanylyl cyclase-type receptors exclusively. Equine CG treatment increased Bmax from 225 ± 50 fmol/mg protein in control animals to 354 ± 51 fmol/mg protein, and additional hCG treatment increased it further to 492 ± 130 fmol/mg protein (p < 0.05), without changing receptor affinity. The increased binding was consistent with increased ability of atrial natriuretic peptide (ANP) to activate guanylyl cyclase in the ovarian cells obtained from hormone-treated animals. In confirmation, autoradiography of 125I-tyroCNP and 125I-ANP binding to the rat ovary showed that both guanylyl cyclase GC-A and GC-B receptor subtypes are localized to the granulosa cells of antral follicles. Quantitative analysis of GC-A and GC-B receptors by reverse transcription-polymerase chain reaction showed that the expression level of both receptors started to increase at 2 h and reached maximal levels at 6 h following eCG treatment. Increased levels of GC-B mRNA were also observed 12 h after eCG injection. At 24 and 48 h the receptor levels were below basal. Stimulation of NP receptors by eCG was paralleled by activation of both ovarian ANP and C-type natriuretic peptide (CNP) gene expression. ANP mRNA increased as early as 1 h after eCG injection and remained elevated up to 6 h. CNP mRNA increased at 2 h after eCG injection, peaked (5-fold) at 6 h, and remained elevated 48 h later, a stage at which follicular maturation continues. Incubation of ovaries with ANP significantly decreased eCG-induced estradiol level, indicating the functionality of the ovarian NP system. These results implicate the NP system in the induction and maintenance of fluid balance in the rapidly developing ovarian follicle.
2 Correspondence: Jolanta Gutkowska, Laboratory of Cardiovascular Biochemistry, CHUM Research Center, Campus Hotel-Dieu, 3850 St-Urbain Street, Pavillon Masson, Montreal, PQ, Canada H2W 1T8. FAX: 514 843 2715; gutkowsj{at}ere.umontreal.ca
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