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Negative Feedback Control of Luteinizing Hormone Secretion in Prepubertal Beef Heifers at 60 and 200 Days of Age¹

University of Wyoming, Laramie 82071 and and US Department of Agriculture, Montana Agricultural Experiment Station, Miles City 59031

Abstract

Prepubertal beef heifers at 60 and 200 d of age, born in the fall or spring, were assigned randomly to one of three treatment groups: (1) intact = I; (2) bilateral ovariectomy (OVX); or (3) OVX plus estradiol-17β(E₂) administered in silastic implants (OVX + E₂). Luteinizing hormone (LH) was measured in serum samples collected at 20-min intervals for 4 h from heifers on —1, +7, +21, +35 and +49 d after OVX. Luteinizing hormone concentrations increased in the serum by 7 d after OVX in heifers at both 60 and 200 d of age (P<.001; time x treatment). Prior to OVX, the LH patterns were characterized by low levels and infrequent episodic pulses. By 49 d after OVX, the mean LH concentrations increased and the pattern changed to one of rhythmic LH pulses with a periodicity of 1 h (P<.001; time X treatment). Estradiol-treated OVX heifers did not exhibit a postovariectomy rise in serum LH concentrations. Serum E₂ concentration 49 d after OVX in OVX heifers was threefold greater than in I or OVX heifers, thus demonstrating that E₂ exerted negative feedback on pituitary LH secretion in prepubertal heifers. There was no measurable difference in serum E₂ concentrations between I and OVX heifers; however, the contrast in the concentration and pattern of serum LH between the two groups was dramatic and suggested gonadal factors in addition to E₂ are involved in controlling LH secretion. A negative feedback system of ovarian origin plays a role in controlling LH secretion by 60 d of age, but there was no age-related change in the sensitivity of the - hypothalamic-pituitary axis to negative feedback exerted by E₂ in these prepubertal heifers.

¹ This study was a contribution to Western Regional Project W-112, ‘Reproductive Performance in Beef Cattle.’ Publication has been approved by the Director of the Montana Agr. Exp. Sta., Journal Series No. 1183.

² Present Address: The Upjohn Co., Experimental Agr. Sci., Kalamazoo, MI 49001.

³ Division of Anim. Sci., Univ. of Wyoming, Laramie 82071.

⁴ Livestock and Range Res. Station USDA-ARS, Miles City, MT 59301.

⁵ The authors wish to express appreciation to: Dr. G. D. Niswender, Colorado State Univ., Fort Collins, for anti-bovine LH serum; Ms. Ann Darling and laboratory crew for technical assistance; Ms. W. L. Buckner for manuscript preparation.

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