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Age-Associated Changes in Secretory Patterns of Growth Hormone, Prolactin and Thyrotropin and the Hormonal Responses to Thyrotropin-Releasing Hormone in Rams^1,2,

University of Idaho^,4, Moscow 83843

Abstract

The objectives were to determine the relationship between advancing age in rams and (1) secretory patterns of growth hormone (GH), prolactin (PRL) and thyrotropin (TSH); (2) clearance rate of thyrotropin-releasing hormone (TRH) from blood plasma; (3) ability of the pituitary to respond to exogenous TRH stimulation, and (4) weights of kidneys and various endocrine glands. Random episodic secretion of GH, PRL and TSH was observed in rams of all three ages (2, 3 and 5 years; n = 6, 6 and 4 respectively). Significant increases in mean overall and mean baseline GH concentrations were found with increasing age. Frequency and amplitude of GH secretory spikes were not significantly influenced by age. PRL secretion was highly variable among rams. Although not statistically significant, mean overall and baseline concentrations appeared to increase with advancing age. Amplitude and frequency of PRL secretory spikes were significantly lower in the older animals. In contrast to GH, secretory patterns of TSH were unaffected by age. The mean t_1/2 of TRH did not differ among age groups. Furthermore, the metabolic clearance rate (MCR) and secretion rate (SR) of TRH were found to be similar at different ages. Injection of TRH significantly increased PRL and TSH concentration in rams of all ages. The degree of the PRL and TSH responses to TRH did not differ among ages. Body weights of these rams, as well as the absolute and relative weights of the kidneys, adrenal glands, anterior pituitaries, posterior pituitaries, thyroids and testicles, did not change significantly with age. Based on the results of this research, the following conclusions are suggested: (1) advancing age in mature sheep is associated with an increase in the overall and baseline concentrations of GH, while spike amplitude and frequency do not change significantly; (2) the amplitude of PRL secretory spikes is decreased in 5-year-old rams; (3) advancing age has no effect on the secretory pattern of TSH in rams; (4) the clearance and secretion rates of TRH are not influenced by age, and (5) body, kidney and endocrine gland weights do not change appreciably in rams up to 5 years old. These observations disagree with the hypothesis that advancing age is associated with a reduction in secretion of GH, PRL or TSH in rams.

¹ Univ. of Idaho Agr. Exp. Sta. No. 8045.

² This research was conducted in partial fulfillment of the requirements for the M.S. degree of M. Wayne Morrison and was funded principally by the Idaho Agr. Exp. Sta.

³ The authors thank D. L. Ohlson and Ms. D. Blann and Dr. J. Klindt for technical assistance and advice; Dr. J. G. Pierce for purified bovine TSH; Dr. L. E. Reichert, Jr., for purified ovine PRL, and Dr. A. E. Wilhelmi for purified GH used for radioiodination. We are also indebted to the National Pituitary Agency, NIH, for providing ovine hormone preparations used as reference standards.

⁴ Dept. of Anim. Sci.