Somatostatin Release from the Median Eminence of Unanesthetized Rats: Lack of Correlation with Pharmacologically Suppressed Growth Hormone Secretion

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Abstract
We describe a push-pull perfusion technique to investigate the role of somatostatin in the pharmacological suppression of growth hormone (GH) secretion. Immunoreactive somatostatin (IRS) released from the median eminence (ME) was studied in chronically cannulated, unanesthetized male rats. In control rats which received vehicle injection, plasma GH levels showed a normal ultradian rhythm and relatively stable levels of IRS (around 30 pg/15 min) appeared in the perfusate during the 6-h perfusion. Intracerebroventricular (i.c.v) administration of human growth hormone (40 µg), neurotensin (2 µg), glucagon (25 µg) or intravenous (i.v.) injection of oxymetazoline (50 µg/kg b.w.), an α-adrenergic agonist, or endotoxin (150 µg/kg b.w.) suppressed subsequent GH surges. In these rats, however, IRS levels in the ME perfusate failed to change significantly compared to control rats. These results suggest that changes in somatostatin release may play a minor role in the suppression of plasma GH levels caused by these substances, and that major regulatory effects may be achieved via the suppression of growth hormone-releasing factor release.