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Animal Science Department, Oklahoma Agricultural Experiment Station (J.A.V., R.P.W.), Stillwater, Oklahoma 74078-0425; the Department of Physiology and Pharmacology, Auburn University (T.D.B.), Auburn, Alabama 36849-5520; the Department of Physiology, Colorado State University (A.M.T., T.M.N.), Fort Collins, Colorado 80523
Address all correspondence and requests for reprints to: Dr. Robert P. Wettemann, Animal Science Department, Oklahoma State University, Stillwater, Oklahoma 74078. E-mail rpw{at}okway.okstate.edu
Thirty-two nutritionally anestrous cows were used to determine the
effect of the frequency of exogenous GnRH pulses on ovarian follicular
growth, serum concentrations of LH and FSH, and concentrations of LH,
FSH, GnRH receptors (GnRH-R), messenger RNA (mRNA) for GnRH-R, and mRNA
for gonadotropin subunits in the pituitary. Cows were randomly assigned
to one of four treatments: 2 µg GnRH infused (iv) continuously during
1 h, 2 µg GnRH infused during 5 min once every hour, 2 µg GnRH
infused during 5 min once every fourth hour, or saline (control) for 13
days. Infusion of GnRH every hour increased LH concentrations in serum
(P < 0.05), but FSH concentrations were not
affected by GnRH infusion. Luteal activity (LA) was assessed by the
presence of corpora lutea and/or serum progesterone greater than 1
ng/ml. Six of eight cows infused 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 by 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 
and FSHß
mRNA were not influenced by treatment. However, continuous infusion of
GnRH decreased (P < 0.05) LHß mRNA subunit.
Concentrations of GnRH-R (P < 0.1) and GnRH-R mRNA
(P < 0.05) were reduced when GnRH was infused
continuously. We concluded that pulsatile secretion of LH is necessary
for follicular growth and LA in beef cattle, and GnRH treatment
differentially regulates LH and FSH gene transcription and serum
concentrations of LH and FSH in cattle.
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