Growth Hormone Secretion in the Stunted Head-Irradiated Rat

ABSTRACT: Pulsatile secretion profiles of pituitary growth hormone (GH) and size and number of cells of brain, heart ventricles, liver, kidney, and gastrocnemius muscle were determined in male Long-Evans rats which received 600 rad x-irradiation to the head only at 2 days of age. Controls consisted of sham-irradiated littermates. The irradiated rats showed significant stunting of body weight and tail length beginning prior to weaning and lasting throughout the period (64 days) of observation. In irradiated rats at 20–21 days of age, just prior to weaning, organ weight was significantly reduced in all organs studied. Brain showed a decrease in organ/body ratio (p < 0.0005) and in total DNA content {p < 0.0005), but these values were not significantly changed in the other organs. DNA/organ ratio was increased significantly in heart (P < 0.025) and gastrocnemius muscle (p < 0.025); brain, liver, and kidney had nonsignificant increases. Protein/DNA ratios were decreased significantly in brain (p < 0.005), heart (p < 0.01), and gastrocnemius muscle (p < 0.05); liver and kidney had nonsignificant decreases. Blood samples were removed for GH determination from cannulated undisturbed irradiated and control rats at 15-min intervals for 18-h periods (9 h light and 9 h dark) at 47–64 days of age. Irradiated rats had normal periodicity of bursts of GH secretion. The area under the curve of GH concentration versus time of the irradiated rat was decreased in light (p < 0.025) and in dark (p < 0.05). Assessments of cell size and cell number suggest that neonatal hypopituitarism and/or undernutrition are unlikely causes of the delayed growth of the head-irradiated rat, and the GH results show that brain controls of rhythmic secretion of GH are intact in this model. The finding of reduced GH secretion, is compatible with the hypothesis that the head irradiation has altered a centrally located control of catch-up growth.