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Spontaneous Patterns of LH Release in Castrate Male Sheep and the Effects of Exogenous Estradiol¹

Purdue University, West Lafayette, Indiana 47907

Abstract

Blood plasma samples were collected at 10-min. intervals from indwelling cannulae in 12 wethers and LH concentrations were measured by specific radioimmunoassay (RIA). The rhythmicity of the plasma LH fluctuations during each sampling sequence was determined both subjectively and objectively. The average interval between successive rhythmic peaks was approximately 30 min. with a range of 27 to 65 minutes. The increases in plasma LH from nadir to peak associated with almost every rhythmic peak occurred in the 10-min. interval between samples suggesting a brief pulsatile discharge of LH into the circulation. The pulsatile nature of LH release was confirmed by arterio-venous measurements of LH concentrations across the head. Cranial venous LH concentration minus the arterial LH concentration in paired blood samples was defined as cranial output (CO). Large positive CO values were noted just prior to many of the abrupt increases in circulating LH levels further demonstrating that the rhythmic fluctuations in plasma concentrations reflected pituitary release and not merely altered peripheral metabolism.

In three wethers estradiol administration in polydimethylsiloxane implants significantly decreased mean plasma LH levels confirming previous results in ovariectomized ewes. However, mean levels were not as readily suppressed by estradiol to nondetectable concentrations as they had been in castrate females. Estradiol tended to abolish rhythmicity of the LH fluctuations but the inhibition was much less complete than that reported in ovariectomized ewes.

¹ Journal Paper No. 5275, Purdue University Agricultural Experiment Station. A contribution from the Department of Animal Sciences. Supported in part by PHS grant HD 5114.

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