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Endocrine Response of the Cow to PMSG and Subsequent Multiple Corpora Lutea Regression by Prostaglandin F₂^1,2,

University of Florida⁴, Gainesville 32611

Abstract

The effect of pregnant mare serum gonadotropin (PMSG) on progesterone, 20ß-dihydroprogesterone, luteinizing hormone (LH) and total estrogen concentrations were measured in plasma of 15 parous Angus cows in which estrus was synchronized with prostaglandin F₂-tromethamine salt (PGF₂). Cows were grouped according to number of corpora lutea (CL): 0 to 1 - group 1; 2 to 3 - group 2; and greater than 3 - group 3. Hormone changes with time were similar in all groups following PMSG treatment, although progesterone and estrogen concentrations were higher in cows with a larger number of corpora lutea. Coefficients of correlation between number of CL and plasma progesterone and estrogen were .62 and .54 (P<.01), respectively.

Efficacy of utilizing PGF₂ to regress multiple CL of superovulated cows was tested by giving a single injection of 40 mg PGF₂ tromethamine salt approximately 10 days post-insemination to cows with more than 3 corpora lutea. PGF₂ caused a reduction (P<.01) in plasma progesterone which reached a nadir of 1.8 ± .8 ng/ml within 5 days followed by a low but steady increase to 2.7 ± 1.6 ng/ml 7 days after PGF₂. Unexpectedly, pregnancy continued to parturition in four of five animals in group 3. Progesterone from multiple CL apparently was adequate to maintain pregnancy, even though values reached levels as low as .5 ng/ml in three of five cows.

Maintenance of pregnancy raised the possibility that the conceptus may have protected the CL from complete regression. Alternatively, the dose of PGF₂ may have been inadequate for the increased mass of corpora lutea.

¹ Florida Agricultural Experiment Station Journal Series No. 1137.

² The authors gratefully acknowledge Dr. J. W. Lauderdale, The Upjohn Company for the gift of prostaglandin F₂-tromethamine salt and to Mr. J. A. Wiggins, The University of Florida Foundation for providing the Angus cows. Antisera for the radioimmunoassays were generously supplied by Dr. J. L. Fleeger for progesterone (PR No.24), for estrogens by Dr. V. L. Estergreen and for luteinizing hormone (B225) by Dr. G. Niswender. Purified bovine LH (LER-1072-2) was generously supplied by Dr. L. E. Reichert. We are indebted to Dr. W. W. Thatcher for assistance with the luteinizing hormone assay and to Dr. P. V. Rao for help with the statistical analysis.

³ Facultad de Agronomia, U.C.V., Maracay, Venezuela.

⁴ Department of Animal Science.