Dissociation of Catch-Up Growth Control and Neural Control of Growth Hormone Secretion in the Stunted Head-Irradiated Rat

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Abstract
Male Long-Evans rats were irradiated to the head only at 2 days of age; littermates of the same sex were sham-irradiated. At 40 days of age the irradiated rats were divided into two groups, one of which was fasted 48 h and the other fed a normal diet. The irradiated rats, fasted and nonfasted, were cannulated in the superior vena cava at 48 to 50 days of age. Between 54 and 58 days of age the cannulated undisturbed rats had blood samples withdrawn at 15-min intervals over an 18-h period (9 h light and 9 h dark). Body weight and tail length data showed characteristic stunting following irradiation. The superimposed fast caused transient growth retardation; on refeeding, the fasted rats showed a pattern of catch-up growth limited to the irradiated non-fasted body size. Plasma growth hormone (GH) concentration in the fastedrefed rats as compared with the nonfasted irradiated rats showed no change in the average period of the bursts of GH secretion, the numbers of values in ranges of GH concentration, or the area under the curve of the plasma GH concentration versus time. No difference in these parameters was present in light or dark, considered separately. We conclude 1) that the link between the catch-up growth control and neural mechanisms controlling GH secretion is impaired as a consequence of the neonatal head-irradiation and 2) that catch-up growth acceleration is not dependent on increased GH secretion.

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