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Estimates of Repeatability in the Temporal Patterns of Secretion of Growth Hormone (GH), Prolactin (PRL) and Thyrotropin (TSH) in Sheep^{1 ,3,}

University of Idaho², Moscow 83843

Abstract

Studies were conducted to determine if variables of GH, PRL and TSH secretion, within an individual animal, were repeatable in sheep over a short-term (2 days) and a long-term (3 month) period. Secretion of GH, PRL and, to a lesser extent, TSH was observed to be episodic. The repeatability of GH was .41 to .79 within month (R_w) and over months (R_o) for overall mean GH concentration and mean amplitude of GH secretory spikes. Similarly, R_o and R_w were .57 to .71 for overall mean and baseline mean TSH concentrations. In contrast, R_w for overall PRL concentration was .46, but was <.20 for other variables of PRL secretion. R_o for all variables of PRL secretion was essentially zero, primarily because of the dramatic seasonal influence on PRL secretion. It is noteworthy that an apparent seasonal effect on amplitude (but not other variables) of GH was detected (higher in December than in September).

These results demonstrate that individual animal secretory patterns of GH or TSH are similar when examined at different times, for short or long intervals. Because of the seasonal effect on PRL secretion, secretory patterns over months are dissimilar. We suggest that the data are consistent with the hypothesis that each individual animal has a characteristic secretory pattern of each hormone that may be inherently or genetically predetermined. The exact nature of each hormone secretory pattern that is ultimately expressed (phenotype) would then be determined, in part, by the inherent ability of the animal to respond to changes in the internal and external environmental stimuli.

¹ Idaho Agr. Exp. Sta. Pub. No. 78412. These studies were supported in part by the Univ. of Idaho Agr. Exp. Sta. and a research grant from the Merck Institute for Therapeutic Research, Rahway, NJ.

³ The authors gratefully acknowledge the National Pituitary agency for supplies of purified ovine GH, PRL and TSH and Ms. D. Blann for expert technical assistance.

² Department of Animal Sciences.