Diurnal variations of plasma growth hormone, thyrotropin, thyroxine, and triiodothyronine in streptozotocin-diabetic and food-restricted rats.

The pattern of spontaneous GH, TSH, T4, and T3 secretion has been studied in male rats in response to a 15-day period of streptozotocin diabetes or food restriction. Beginning at 0900 h, groups of control (C), food-restricted (FR), diabetic (D), and insulin-treated D rats were killed every 60-90 min for a 8-h period. Food restriction resulted in a significant depression of the GH, TSH, T4, and T3 peaks, whereas diabetes caused complete suppression of episodic secretion of each hormone. Insulin (6 U/100 g BW X day for 12 days) administration to D rats restored the normal pattern of secretion. In D and FR rats, pituitary GH concentrations were lower than in C rats, whereas pituitary TSH concentrations were similar to those in controls. Thus, as compared to C rats, FR and D rats showed an inhibition in GH, TSH, T4, and T3 secretion, most marked in D animals. Since diabetes is associated with a deficiency of circulating thyroid hormones, the potential roles of T4 and T3 on pituitary GH concentration and secretion in D rats were evaluated. Treatment of D rats with insulin (3 U/100 g BW X day), T4 (1.8 micrograms/100 g BW X day), or T3 (0.30 microgram/100 g BW X day) for 12 days resulted in a significant but limited increase in pituitary GH content. When administered together with insulin, the net effects of T4 or T3 with insulin appeared additive. T4 administration to D rats produced a significant though limited increase in plasma GH concentrations and weight gain, whereas both values were unaffected by T3. Simultaneous administration of T4 and insulin resulted in significant increased plasma GH concentration to levels greater than those in C rats. However, plasma GH levels in rats treated with T3 plus insulin were greater than those in D rats, but lower than in C animals. The results indicate that the decreased pituitary GH content of D rats can be corrected, at least in part, by T4 and T3.