Increased secretion of growth hormone in rats undergoing catch-up growth after fasting.

The pattern of pulsatile secretion of growth hormone (GH) was determined in catheterized undisturbed male Long-Evans rats which had been fasted for 48 hours beginning at 39 days of age and then fed a stock diet ad lib. Plasma GH was determined at 15 minute intervals for 6 and 24 hour periods at 12 to 26 days into recovery during which time catch-up growth is known to occur in this fasted rat model. Secretion in fasted-refed rats occurred in characteristic periodic bursts of pulses resulting in plasma values that tended to be above 200 ng/ml; the intervening troughs had values under one ng/ml. The mean period of secretory bursts ranged from 2.4 to 3.1 hours in different groups and in light and dark periods; periods did not differ significantly by t test between fasted-refed and control rats. In fasted-refed rats sampled for 24 hours the area under the curve of GH concentration plotted against tie was significantly greater in light than in dark; this difference did not occur in controls. In rats sampled for 6 hours in light the area under the curve of GH concentration was significantly greater in the fasted-refed rats than in the controls. The findings show that the GH secretion in rats is increased in the light phase of the diurnal light/dark cycle during catch-up growth after fasting; the increased secretion occurs within the normal periodic pattern of GH secretion. We conclude that GH secretion is linked to the catch-up phenomenon but that the role of GH in catch-up growth remains unclear.