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Effect of Ovariectomy and Estradiol on LH Patterns in Ewes¹

Purdue University, West Lafayette, Indiana 47907

Abstract

The influences of ovariectomy and of replacement therapy on LH secretory patterns were studied in three anestrous ewes (ages 7 to 10 months) and in two anestrous ewes (age 15 months). Radioimmunoassay was used to measure plasma LH in samples collected via cannula every 10 min. for a 4-hr. sampling period each day. The mean level for each sampling period was calculated and the occurrence of rhythmical LH patterns was determined both subjectively and objectively. The average interval between successive rhythmic peaks was also calculated for each sampling period in which rhythmicity occurred.

In all ewes ovariectomy increased greatly the incidence of sampling periods during which LH fluctuated rhythmically. Mean LH also rose significantly (P<.OS) in the two older ewes within 18 hr. after ovariectomy. In two of the three younger ewes mean plasma LH never reached levels significantly above the pre-ovariectomy controls. In the remaining ewe mean levels increased significantly (P< .01) at 72 hr. after ovariectomy. Rhythmical LH patterns occurred following ovariectomy even when mean levels were unchanged, and mean levels sometimes increased in the absence of rhythmical patterns.

Estradiol administration in polydimethylsiloxane implants immediately abolished all rhythmical LH patterns and significantly (P< .01) decreased mean levels in four out of five ewes. Combined administration of estradiol and progesterone suppressed LH rhythmicity and mean levels at least as well as did estradiol alone. Removal of the combined implants from the ewes allowed LH to rebound in a manner very similar to that following ovariectomy.

¹ Journal Paper No. 4956, Purdue University Agricultural Experiment Station. A contribution from the Department of Animal Sciences. Supported in part by PHS grant HD 5114.

² Present address: Department of Physiology and Biophysics, Colorado State University, Ft. Collins 80521.