Title

The role of nitric oxide in ovarian function in sheep

Date of Completion

January 2006

Keywords

Biology, Physiology

Degree

Ph.D.

Abstract

The corpus luteum (CL) is a transient endocrine gland which produces progesterone (P), the hormone required for the establishment and maintenance of pregnancy. In most species, in the absence of pregnancy, uterine secretion of prostaglandin F2 alpha (PGF2α) induces a cascade of biochemical events which cause luteal regression (luteolysis). In several species, nitric oxide (NO) has been reported to mediate the actions of PGF2α, and the results of six experiments presented here support the role for NO during luteolysis in sheep. In Experiments 1 and 2, an intra-arterial infusion of high concentrations of a nitric oxide synthase (NOS) inhibitor, L-NAME, in sheep with transplanted ovaries (OVTP) bearing mid-estrous cycle 11-day old CL reduced ovarian venous blood flow (BF) but did not alter the secretion rate (SR) of P. In Experiment 3 dispersed heterogenous luteal cells incubated for 120 minutes in the presence of 10-5 M DPTA-NONOate, an NO donor, stimulated the production of P. In Experiment 4, OVTP sheep bearing CL over 30 days of age were infused for 1 hour with 0, 1, 10, 100, or 1000 μg/min DPTA. Sheep treated with the 1000 μg/min concentration of DPTA exhibited a reduced SR of P, while sheep treated with 1, 10 or 1000 μg/min DPTA also exhibited a decline in ovarian BF. In Experiment 5, OVTP sheep on day 11 of the estrous cycle were infused for 1 hour with 0, 1 or 1000 μg/min DPTA. At the 1000 μg/min infusion rate, the SR of P was again reduced, but neither concentration of DPTA altered ovarian BF. In Experiment 6, histological examination of ovaries collected from intact cycling sheep localized the neuronal nitric oxide synthase (nNOS) isoform to the ovary and CL, whereas the endothelial NOS (eNOS) isoform was confined to ovarian stromal tissue. Collectively, these results suggest that large concentrations of an NO donor can reduce the SR of P; that NO can affect ovarian venous BF in a concentration and age-dependent manner; and suggests that the observed effects on BF and steroidogenesis are mediated via separate mechanisms. ^