Growth Hormone Release in the Rat: Effects of Somatostatin and ThyrotropinReleasing Factor MARVIN BROWN AND WYLIE VALE Salk Institute for Biological Studies, Lajolla, California 92037 of TRF (30 /xg/100g BW). Structural analogs of TRF with low TSH-releasing activity did not inhibit GH release nor induce tail vibration in MS-treated rats. Pyroglutamyl-3-methyl-histidyl-prolinamide, with 8 times the hypophysiotropic potency of TRF, is similarly more potent than TRF in inhibiting GH release and inducing tail vibration in MS-treated rats. These results suggest the following: 1) MS and PB act at a central nervous system (CNS) site to release GH; 2) TRF may act at a CNS site to inhibit MS- and PB-induced GH release; 3) somatostatin has direct pituitary effects on inhibition of GH, but a CNS site of action cannot be excluded; and 4) TRF stimulates tail motor activity in MS-treated rats. (Endocrinology 97: 1151, 1975)

ABSTRACT. Morphine sulfate (MS) and pentobarbital (PB) stimulate growth hormone (GH) release in the rat hi vivo, but not from enzymatically dissociated anterior pituitary cells in vitro. Somatostatin and thyrotropin-releasing factor (TRF) inhibit the in vivo release of GH induced by MS, with 50% inhibition at ca. 2.3 and 4.6 pig/100 g BW, respectively. Somatostatin and TRF similarly inhibit PBinduced GH release. Prostaglandin E2 stimulates GH release both in vivo and in vitro. Both of these responses are inhibited by somatostatin (50% inhibition at ca. 10 /xg/100g BW), but neither is altered by TRF (100 /xg/100g BW). Both normal and hypophysectomized rats receiving MS exhibited a rapid vibration of the tail immediately after administration

S

OMATOSTATIN wasfirstcharacterized by its ability to inhibit the spontaneous secretion of growth hormone (GH) from monolayered anterior pituitary cells in vitro (1). Subsequently, the in vivo activity of somatostatin was established by its ability to inhibit GH secretion induced by pentobarbital (PB) and morphine sulfate (MS) in the rat (2,3). The mechanism by which PB and MS elicit pituitary release of GH is probably mediated via the central nervous system (CNS) (4,5,6). Thyrotropin-releasing factor (TRF) has been demonstrated to reduce the duration of anesthesia and toxicity induced by PB (7,8). Therefore, we have assessed the effects of this peptide on GH release induced by PB and MS. The effects of somatostatin and TRF on CNS-stimulated GH release by PB and MS are compared to the effects of these peptides on direct pituitary stimulation of GH release by prostaglandin E2 (PGE2). During the course of these experiments, TRF was noted to produce striking motor activity in rats treated with MS and these results are reported. Received May 8, 1975. Supported by NIH Grant AM 1670701.

Materials and Methods Rat Bioassay. Male Sprague-Dawley rats weighing 100 g were used in all experiments. All animals were fed tap water and Purina Rat Chow ad libitum and housed in temperature and humidity-controlled quarters with 14 hours of light and 10 hours of dark (light 0700-2100 h). All experiments were performed between 1400 and 1600 h on animals received from the supplier at least five days previously. Acute administration of MS, PB, and PGE2 immediately followed by hypothalamic peptides was made via the external jugular vein in ether-anesthetized rats. Ten minutes after these iv injections, the animals were decapitated and trunk blood collected for radioimmunoassay of GH. In Vitro Assays. The effects of various substances were tested in vitro on the secretion of GH by primary cultures of enzymatically dissociated rat anterior pituitary cells as previously described (9). Growth Hormone Radioimmunoassay. GH determinations were performed by a double antibody method on plasma samples utilizing the following reagents: NIAMDD rat GH standard (GH-RP-1) NIAMDD monkey anti-rat GH (GHSerum-3); and highly purified rat GH for iodination obtained from Dr. Stanley Ellis, Ames Research Center, Moffett Field, California.

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Endo • 1975 Vol 97 • No 5

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Drugs. MS was obtained from Merck-Sharpe and Dohme. PGE2 was kindly provided by Dr. John Pike, Upjohn Co., Kalamazoo, Michigan. All doses are expressed on a per 100 g BW basis. Hypothalamic Hormones. TRF, most TRF analogs, and somatostatin were synthesized in this laboratory by J. Rivier. Pyroglutamyl-histidyl-azetidine amide was prepared by Drs. E. Nicolaides and C. Rebstock, Parke-Davis, Ann Arbor, Michigan. Doses of peptides are expressed as per lOOg BW. Results Somatostatin,

TRF,

and

3methyl-histidyl-prolinamide

TABLE 1. Effect of TRF and somatostatin (SRIF) on pentobarbital (PB)-induced GH release in vioo Plasma GH ng/ml

Treatment* Control TRF (100 fig) PB (2 mg) PB (2 mg) + TRF (100 fig) PB (2 mg) + SRIF (100 fig)

51.2 50.7 380.0 27.1 10.0

20.7 14.9 >0.05 65.0 0.05

Growth hormone release in the rat: effects of somatostatin and thyrotropin-releasing factor.

Growth Hormone Release in the Rat: Effects of Somatostatin and ThyrotropinReleasing Factor MARVIN BROWN AND WYLIE VALE Salk Institute for Biological S...
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