Ultradian Growth Hormone Rhythm in the Rat: Effects of Feeding, Hyperglycemia, and Insulin-Induced Hypoglycemia

Abstract

Temporal patterns of plasma GH [growth hormone] immunoreactive insulin (IRI) and glucose were defined by obtaining serial blood samples from freely-moving male rats bearing chronic intracardiac venous cannulae. Blood was withdrawn every 15 min for periods of 6 h. Plasma GH and IRI were determined by radioimmunoassay. The typical ultradian rhythm of GH secretion was evident in each undisturbed animal (peaks > 200 ng/ml; troughs < 1 ng/ml; mean period: 3.40 .+-. 0.08 h). Basal plasma IRI and glucose levels fluctuated minimally. There was no significant correlation between plasma GH and IRI, GH and glucose or IRI and glucose levels in unfed rats. The rhythmic GH secretory patterns of feeding animals (mean period: 3.12 .+-. 0.16 h; peaks > 200 ng/ml; troughs < 1 ng/ml) were similar to those of non-feeding animals (mean period: 3.34 .+-. 0.15 h; peaks > 200 ng/ml; troughs < 1 ng/ml) despite large fluctuations in plasma IRI levels and a wide variation in the number and size of the meals taken. No consistent relation was observed between the ingestion of meals and the bursts of GH secretion. The mean period of the GH rhythm was not significantly altered by hyperglycemia (mean period: 3.25 .+-. 0.08 h), although the amplitude of the pulses of half of the hyperglycemic rats was markedly depressed. Insulin-induced hypoglycemia significantly depressed the amplitude of the GH pulses; however, the pattern of this response was not consistent. Despite wide variability in the GH response, the magnitude and time course of recovery of the plasma glucose levels was similar in all animals. GH secretion in the rat is probably regulated primarily by an endogenous ultradian rhythm which is not dependent on changes in plasma glucose or IRI levels, and continues to function independently of feeding behavior. It is unlikely that GH is an important physiologic regulator of glucose homeostasis in this species.