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Dose-Response Influence of Prostaglandin E₁ and Somatostatin on Plasma Levels of Growth Hormone, Prolactin and Thyrotropin in Sheep

University of Idaho, Moscow 83843

Abstract

Ten ewe lambs (approximately 40 kg) were used to determine the influence of prostaglandin E₁ (PGE₁) and synthetic somatotropin-release inhibiting factor (SRIF) on plasma levels of ovine growth hormone (GH), prolactin (PRL) and thyrotropin (TSH). Intravenous injection of PGE₁ (20 and 100µg/kg) stimulated a dose-related increase in plasma levels of GH (20 ± 6 and 50 ± 10 ng/ml), respectively. Intravenous injection of SRIF (500 µg), significantly (P<.05) depressed the GH stimulation by PGE₁ (20 µg/kg), while lower doses (5 and 50 µg) were ineffective. The highest dose of PGE₁ (100 µg/kg) also stimulated a significant (P<.01) increase in plasma levels of PRL. One and 10 µg doses of PGE¹/kg had no influence, while the 20 µg/kg dose appeared to stimulate PRL secretion although the response was quite variable. Treatment with SRIF (500 µg) significantly (P<.05) reduced the PRL stimulation by PGE₁ at 20 µg/kg. However the variable PRL response to the 20 µg/kg dose of PGE₁ makes this observation inconclusive. Neither PGE₁ nor SRIF exerted an influence on plasma TSH.

These data demonstrate the dose-related stimulatory effect of intravenously administered PGE₁ on secretion of GH and PRL. The GH response to PGE₁ at doses less than 100 µg/kg was more consistent than that for PRL. The observation that the stimulatory effect of PGE₁ on GH is partially inhibited by a single injection of 500 µg of SRIF is in support of previous reports that high doses of SRIF are inhibitory to the GH response to a variety of stimuli in other species. SRIF likely does not exert a physiological effect on the secretion of either PRL or TSH in sheep.

¹ The authors would like to thank Dr. R. Guillemin for providing the synthetic somatostatin, Drs. J. Pike and J. Lauderdale for supplying the PGE₁ and Drs. A. E. Wilhelmi, L. E. Reichert, Jr. and J. G. Pierce for the GH, PRL and TSH, respectively, used for radioiodination. The GH, PRL and TSH reference standards were provided by the National Institutes of Health. These studies were supported in part by a grant from the National Institutes of Health (NICHD #07830). The authors are also grateful to Mr. D. Ohlson for assistance in conducting the experiments and analyzing the blood samples, and to Mrs. Becky Budge for typing the manuscript. University of Idaho publication number 7443.