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Release of Pituitary Hormones, Cortisol, Testosterone and Insulin in Response to Prostaglandin F₂ Given during Intracarotid Infusion of Somatostatin in Bulls¹

Michigan State University, East Lansing 48824^,5

Abstract

A carotid artery and a jugular vein were cannulated in each of four 1-year-old bulls. In an experiment with latin square design, each bull was given 30-min intracarotid infusions of saline or .5 mg somatostatin, and each was given an intravenous (iv) injection of 0 (saline) or 20 mg PGF₂ (26.6 mg PGF₂ THAM salt) in saline at 5 min after beginning infusions. Blood serum growth hormone declined (P<.05) continuously from 4.8 ± .6 ng/ml at the outset of somatostatin infusion to 1.8 ± .4 ng/ml at the end, then increased (P<.05) to 8.7 ± 3.8 ng/ml within 10 min and remained relatively stable during the 1-hr period after somatostatin. In the absence of somatostatin, PGF₂ caused a threefold increase in blood serum growth hormone. Intracarotid infusion of somatostatin abolished the growth hormone release in response to iv PGF₂ until somatostatin infusion was terminated; then growth hormone increased (P<01) from 3.8 ± .7 to 30.6 ± 17.7 ng/ml within 10 minutes. Possibly, growth hormone mone accumulated in the pituitary during the 30-min somatostatin infusion, resulting in hypersecretion beginning within 10 min after cessation of somatostatin treatment.

In contrast to its effect on growth hormone, intracarotid infusion of somatostatin failed to alter either basal concentrations of prolactin, Cortisol or LH, or release of these hormones in response to iv PGF₂. Blood serum thyroid stimulating hormone was unaltered by either somatostatin or PGF₂. Blood serum insulin decreased (P<.05) continuously from 5.5 ± .4 at the outset of the 30-min intracarotid infusion of somatostatin to 1.6 ± .3 ng/ml at the conclusion, then increased (P<.01) to pre-treatment concentrations within 10 min after cessation of somatostatin treatment. Prostaglandin did not significantly influence blood serum insulin.

We conclude that somatostatin is a potent inhibitor of growth hormone and insulin secretion, but inhibition of growth hormone and insulin disappears completely within 10 min after somatostatin infusion. Somatostatin does not affect blood serum prolactin, Cortisol, LH or TSH in bulls.

¹ Published with the approval of the Director of the Agricultural Experiment Station as journal article no. 7762. This research was supported in part by USPHS Research Grant No. HD 06948. Dr. J. W. Lauderdale (Upjohn Company) supplied the PGF₂ THAM salt, and Dr. R. Deghenghi (Ayerst Research Laboratories) provided the synthetic linear somatostatin (AY-24,910) for this research.

² Present address: Department of Animal and Dairy Science, University of Georgia, Athens.

³ Present address: Department of Physiology and Environmental Studies, University of Nottingham, Sutton Bonington, Loughborough, Leics, LEI 2 5RD England.

⁴ Present address: U. S. Sheep Research Station, Dubois, ID.

⁵ Animal Reproduction Laboratory, Department of Dairy Science.