Date of Award

2001

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

School of Animal Science

First Advisor

Robert A. Godke

Second Advisor

Dale L. Paccamonti

Abstract

Current scientific literature has shown that: (1) NO is a pro-ovulatory agent and inhibition of its production inhibits ovulation, and (2) NO has strong antisteroidogenic activity, especially on estradiol (E2) production by granulosa cells. The experiments described in this dissertation were designed to answer the following questions involving effects of NO on ovarian physiology: (1) is the inhibitory effect of nitric oxide synthase (NOS) inhibitors on ovulation irreversible (follicular atresia) or temporary (delayed ovulation)? and (2) what is the relationship between NO and hCG-induced preovulatory luteinization? Accordingly, cycling mares were subjected to treatment with NOS inhibitors (L-NAME or aminoguanidine) during estrus. Results showed that administration of NOS inhibitors temporarily inhibited hCG-induced ovulation for up to 5 d. Using a similar experimental approach, Holstein cows treated with L-NAME had delayed ovulation and an attenuated post-ovulation rise in plasma progesterone (P4) concentration. In a subsequent experiment, analyses of follicular fluid from mares in estrus showed that NO concentrations were increased after hCG administration. Furthermore, equine and bovine granulosa cell cultures treated with NOS inhibitors showed that inhibition of NOS activity in vitro luteinization as evidenced by decreased P4:E2 ratios. Pharmacokinetic and hemodynamic parameters associated with L-NAME administration in horses demonstrated that L-NAME has a short half life (3.6 min) as opposed to its major metabolite L-NNA (19.5 h). The results of these experiments demonstrated that: (1) administration of NOS inhibitors delayed ovulation in mares and cows; (2) intrafollicular production of NO was up-regulated following hCG administration; (3) inhibition of NO prevented preovulatory follicular luteinization and in vitro luteinization of granulosa cells; (4) both nonspecific and iNOS-specific inhibitors caused significant effects both in vivo and in vitro and (5) L-NAME has a short half life in horses and L-NNA is a major metabolite with a relatively long half life. Finally, these results will contribute to designing pharmacological therapies for manipulating ovarian function for experimental or therapeutic purposes. Future studies investigating the involvement of NO with preovulatory luteinization will likely contribute to the understanding of ovarian dysfunction, such as cystic ovarian syndrome or unruptured luteinized follicles. These conditions affect both animals and humans.

ISBN

9780493563275

Pages

206

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