Changes in serum concentrations of ACTH, corticosterone and in growth following morning or evening injection of female rats with porcine growth hormone

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Abstract
The present study was designed to examine the effects of stress, associated with daily handling and placebo injection, on growth and adrenal activity in female rats. A second objective was to examine the effects of porcine GH (pGH) on growth and on serum concentrations of ACTH and corticosterone. Treatments were administered in the morning (08.00 h) in semi-darkness, or in the evening (20.00 h) towards the end of the light period and were timed to coincide with reported periods of high and low adrenal sensitivity to ACTH. Handling of and s.c. injection of saline into female rats (initial body weight 220 g) each morning for 21 days did not affect growth, food intake or organ weights. In contrast, daily handling and saline injection each evening caused a marked reduction in weight gain (25%), food intake (12%) and liver weight (8%), accompanied by a trend towards an increase in adrenal weight (11%), but no change in serum corticosterone concentrations. In a second experiment, treatment of female rats (initial body weight 180 g) with pGH (5·6 mg/kg) daily at 08.00 h caused a significant improvement in weight gain (17%), but food intake and organ weights were unaltered. Daily injection of rats with pGH at 20.00 h caused a greater relative improvement in weight gain (45%) than did treatment at 08.00 h as the growth impairment caused by the stress of handling was counteracted. The adverse effect of evening injections of saline on food intake was also counteracted by pGH. Changes in liver and adrenal weights, however, were not attenuated by pGH treatment. Serum corticosterone concentrations were barely altered by pGH treatment, but serum ACTH concentrations were increased markedly (65–136%) by pGH in response to both morning and evening injection. The results show that mild stress associated with daily handling and injection can have a profound effect on the growth of rats, but lend no support to the hypothesis that the growth impairment is mediated by corticosterone. Female rats handled and treated in the morning provide a good physiological growth model of a non-stressed animal, but rats treated in the evening are more responsive to the anabolic effects of pGH. It is suggested that pGH causes a reduction in adrenal sensitivity to ACTH and, in the absence of increased corticosterone output, high serum ACTH concentrations may contribute to part of the anabolic activity of pGH. Journal of Endocrinology (1989) 123, 113–119