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Pulsatile or Continuous Infusion of Luteinizing Hormone-Releasing Hormone and Hormonal Concentrations in Prepubertal Beef Heifers¹

Kansas State University^,4, Manhattan 66506

Abstract

An experiment was conducted to determine if exogenous luteinizing hormone-releasing hormone (LHRH) administered iv intermittently as pulses (P) or by continuous sc infusion (I) using osmotic minipumps could sustain pulsatile LH release and induce estrous cyclicity in prepubertal heifers. Prepubertal heifers were assigned randomly to: 1) receive pulses of LHRH (n=6; 2.5 µg LHRH/2 h for 72 h), 2) be infused with LHRH (n=11; 1.25 µg LHRH/h for 72 h), or 3) serve as controls (n=16). Blood was collected at 20-min intervals for 8 h (0900 to 1700 h) from six heifers in each group on d 1, 2, 3 (during treatment), and on d 4 (during 8 h after terminating LHRH treatments). Heifers given LHRH had higher (P<.01) LH concentrations than controls. Preovulatory-like LH surges occurred in three I, two P and no control heifers during treatment. Pulse frequencies of LH (no. LH pulses/8 h) were greater (P<.001) for P heifers than for I and control heifers due to pulsatile LHRH treatment. Serum estradiol was higher (P<.01) during treatment for LHRH-treated heifers than for controls. Serum follicle-stimulating hormone, cortisol, and progesterone were unchanged during treatment. High levels of cortisol on d 1 declined (P<.001) to baseline by d 2. Characteristic progesterone rises or short luteal phases occurred within 10 d of treatment initiation in more (P<.05) LHRH-treated heifers (I=45%, P=33%) than controls (6%), although days to first observed estrus and first ovulation were unaffected by treatments. Although both continuous and pulsatile administration of LHRH successfully induced LH and estradiol release as well as preovulatory-like LH surges in some heifers, earlier initiation of estrous cycles was not achieved. Estrous cycles appeared to be delayed by exposure to continuous LHRH infusions during the peripubertal period.

¹ Contribution No. 85-283-J, Kansas Agr. Exp. Sta., Manhattan 66506. The authors acknowledge Dr. G. D. Niswender, Colorado State Univ., Fort Collins, for providing ovine LH antiserum; Dr. T. M. Nett, Colorado State Univ., Fort Collins, for assaying samples for FSH; Dr. N. R. Mason, Eli Lilly and Co. Indianapolis, IN for providing the estradiol-17ß antiserum; and the Hormone Distribution Office, NIAMDD, Bethesda, MD for donating the LH (NIH-LH-B4). Gratitude also is expressed to Susan Durham for her assistance in laboratory analyses and data collection and to Cindy Barrett for typing this manuscript.

² Present address: Dept. of Anim. Sci., Iowa State Univ., Ames 50011.

³ Address reprint requests to this author.

⁴ Dept. of Anim. Sci. and Ind.

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