Growth hormone and thyrotropin secretory profiles and provocative testing in aged rats.

The impact of aging on both basal and induced GH and TSH secretion in male and female rats was investigated. Analysis of the individual GH secretory profiles in young (3-4 month) and old (19-20 month) rats indicated that sex-dependent patterns of GH secretion was preserved in old animals. However, we observed a reduction of individual GH peak amplitudes of 66% in old males and 53% in old females when compared to their respective young animals. Further, the GH response to an intravenous bolus of GH-releasing factor (GRF), morphine and clonidine was dramatically blunted or absent in old male and female animals. In contrast to GH, basal TSH secretion was elevated, while the TSH response to thyrotropin-releasing hormone (TRH) was not significantly affected in old animals of either sex. The present data provide evidence that reduced pituitary sensitivity to GRF may be a possible cause for reduced GH secretion in old animals. Further, the elevation in plasma TSH observed in old animals is not the result of an increased pituitary sensitivity to TRH.