EFFECT OF GONADOTROPIN-RELEASING-HORMONE (GNRH) PULSE FREQUENCY ON SERUM AND PITUITARY CONCENTRATIONS OF LUTEINIZING-HORMONE AND FOLLICLE-STIMULATING-HORMONE, GNRH RECEPTORS, AND MESSENGER-RIBONUCLEIC-ACID FORGONADOTROPIN SUBUNITS IN COWS

Thirty-two nutritionally anestrous cows were used to determine the eff ect of the frequency of exogenous GnRH pulses on ovarian follicular gr owth, serum concentrations of LH and FSH, and concentrations of LH, FS H, GnRH receptors (GnRH-R), messenger RNA (mRNA) for GnRH-R, and mRNA for gonadotropin subunits in the pituitary. Cows were randomly assigne d to one of four treatments: 2 mu g GnRH infused (iv) continuously dur ing 1 h, 2 mu g GnRH infused during 5 min once every hour, 2 mu g GnRH infused during 5 min once every fourth hour, or saline (control) for 13 days. Infusion of GnRH every hour increased LH concentrations in se rum (P < 0.05), but FSH concentrations were not affected by GnRH infus ion. Luteal activity (LA) was assessed by the presence of corpora lute a and/or serum progesterone greater than 1 ng/ml. Six of eight cows in fused with GnRH every hour had LA by day 13, whereas only 25% of cows infused either continuously or with a pulse every fourth hour had LA b y day 13. None of the control cows had LA during the experiment (P < 0 .01). Concentrations of LH and FSH in the pituitary were significantly reduced when GnRH was infused hourly or continuously. Concentrations of common alpha and FSH beta mRNA were not influenced by treatment. Ho wever, continuous infusion of GnRH decreased (P < 0.05) LH beta mRNA s ubunit. Concentrations of GnRH-R (P < 0.1) and GnRH-R mRNA (P < 0.05) were reduced when GnRH was infused continuously. We concluded that pul satile secretion of LH is necessary for follicular growth and LA in be ef cattle, and GnRH treatment differentially regulates LH and FSH gene transcription and serum concentrations of LH and FSH in cattle.