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Plasma Hormone Levels in 6-Methyl-17-Acetoxyprogesterone and Estradiol Benzoate Treated Heifers^{1, 2,}

Cornell University, Ithaca, New York 14853

Abstract

Frequently collected plasma samples from 24 of 48 Holstein heifers subjected to an estrous cycle synchronization regimen involving a 24-day 6-methyl-17-acetoxyprogesterone (MAP) treatment period preceded by single injections of flumethasone (7.5 mg) and followed by single injections of estradiol benzoate (E₂B) (400 µg) were analyzed for luteinizing hormone (LH), progesterone, estrone (E₁), estradiol (E₂), prolactin (PRL) and thyroxine (T₄).

Synchronized estrus was observed in 83% of the E2B-treated and 75% of the non-E₂B-treated animals within a 72-hr period. Treatment with E₂B shortened the mean time to estrus after MAP withdrawal from 76 to 59 hr (P < .001). Frequent bleeding did not affect synchronization, but lowered (P < .05) fertility (32 vs 74% for bled and non-bled animals, respectively). Among the non-bled heifers, E₂B treatment did not improve conception rate (75 vs 42%, P > .05). Flumethasone treatment had no effect on synchronization, fertility or any of the parameters studied, except that it altered the circadian pattern in plasma concentrations of prolactin on the day of injection; the usual nocturnal elevation of plasma prolactin was nearly abolished. Plasma LH peaks similar to those that occur in normal animals were measured in 19 of the 24 heifers at the onset of estrus. Eleven of the 19 heifers had "normal" hormone patterns associated with the synchronized estrus, as indicated by the following criteria a) plasma progesterone levels were low on the day after MAP withdrawal, b) the plasma LH peak occurred no earlier than 4 hr before and no later than 2 hr after the onset of estrus, and c) plasma E₁ and E₂ rose to peak levels at, or slightly before, the onset of estrus. Seven of these 11 heifers conceived to insemination at the synchronized estrus. Some major type of asynchrony between plasma hormone levels and estrus occurred in the remaining eight animals, and none of these conceived. Premature or greatly delayed LH peaks were the types of asynchrony noted. MAP treatment did not interfere with normal corpus luteum function, except on the first day of administration, when a sharp transient decline in plasma progesterone levels occurred in all animals; a marked rise in thyroxine levels in all animals rapidly followed the fall in progesterone. Plasma E₁ and E₂ levels rose within 24 hr after MAP withdrawal and reached peak levels in about 50 hours. E₂B treated animals had significantly higher peak E₁ levels than non-E₂ B-treated animals. Plasma prolactin about 50 hours. E₂B treated animals had significantly higher peak E₁ levels than non-E₂ B-treated animals. Plasma prolactin levels were elevated slightly during the 24 hr before the plasma LH peak and declined steadily during the 9 hr after the plasma LH peak. However, injections of E₂B had no effect on plasma prolactin levels. Following the initial transient rise, plasma thyroxine levels were reduced (P < .01) during the MAP treatment period.

¹ Supported in part by The Regional Research Project — NE-72 entitled, "Control of Reproduction in The Bovine Female, and in part by USPHS Training Grant 00171.

² The authors gratefully acknowledge the assistance of T.Y. Tanabe, W.C. Hobson, R.D. Smith, P.J. Ross and D.M. Tobey in organizing and carrying out the experiments described. Supply of the MAP by The Upjohn Company is also gratefully acknowledged.

³ Present addresses of authors are as follows:

William Hansel and R.R. Saatman, Cornell Univ., Ithaca, 18453,

P.V. Malven, Dept. of Animal Science, Purdue Univ., Lafayette, 47907,

K.R. Simmons, Dept. of Animal Sciences, Univ. of Vermont, Burlington, 05401,

D.L. Black, Laboratory of Reproductive Physiology Univ. of Massachusetts, Amherst, 01002,

A.J. Hackett, Dept. of Reproductive Physiology, Animal Research Institute, Ottawa, Ontario.

R.J. Schechter, Animal Science Department, California State Polytechnical Univ., Pomona, 91768.