Effects of Prolonged Food Deprivation on the Ultradian Growth Hormone Rhythm and Immunoreactive Somatostatin Tissue Levels in the Rat*

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Abstract
The effects of prolonged food deprivation on plasma GH, pituitary GH, and tissue somatostatin levels were investigated in the male rat. Six-hour GH secretory profiles obtained from normal animals bearing chronic intracardiac venous cannulae showed the typical ultradian GH rhythm, with most peak GH values being >600 ng/ml. Exposure to 24-h food deprivation resulted in a significant depression in amplitude of GH pulses, with peak values not exceeding 300 ng/ml. The amplitude and duration of the GH secretory episodes declined progressively after 48 and 72-h food deprivation, and normal periodicity was not evident. The mean 6-h GH level in each of the food-deprived groups was significantly less than that of normal animals. A rebound response was observed in rats allowed to refeed for 3 days after 72-h food deprivation, as evidenced by an increased number of GH secretory episodes and a shorter period of the GH rhythm. In a second series of experiments, pituitary GH and tissue levels of somatostatin-like immunoreactivity (SLI) were measured in two groups of rats which were subjected to either 72-h food deprivation or ad libitum feeding. Starvation resulted in a significant increase in pituitary GH concentration when compared to fed controls (88.7 ± 3.2 vs. 66.0 ± 2.4 μg/mg wet wt; P < 0.01), but did not significantly alter medial basal hypothalamic SLI concentration. A significant elevation of the SLI concentration of starved rats was observed in the pancreas, jejunum, and colon. These results indicate that the GH secretory episodes are markedly suppressed in response to prolonged food deprivation via decreased release from the pituitary gland. The finding of an alteration in pancreatic and gastrointestinal SLI concentration in response to deprivation of exogenous nutrients is compatible with the view that somatostatin may play a role in nutrient homeostasis.