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Effect of Prolonged Thermal Exposure on Growth Hormone (GH) Secretion in Cattle¹

University of Missouri, Columbia 65201

Abstract

Experiments were conducted to measure the plasma levels of growth hormone [GH] and its clearance rates (k) in cattle following prolonged exposure to thermoneutral (18 C, 50% RH) and high ambient temperatures (35 C, 50% RH). Plasma levels of GH were measured by coated charcoal radioimmunoassay and the clearance from circulation was determined by the labeled GH disappearance method, while the GH distribution space (GHDS) was obtained by extrapolation technique. The GH secretion rate was estimated as the product of these three parameters.

As compared to the respective values at thermoneutrality, all the components of GHSR changed in heat exposed animals. The GHDS increased from 23.2 to 25.9 liters which partially was responsible for a decline in [GH] from 18.2 to 13.5 ng/ml plasma. The drastic decline in GHSR from 16.04 mg to 9.09 mg/cow/day was due to a major decline in k from 38.4 to 26.3 pools/day. The observed decrease in the GHSR have been viewed as a compensatory hormonal response of the homeotherm to prolonged environmental heat stress which may be a factor in depressed milk production in the tropics.

¹ Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 7165. Approved by the Director.

² Present address: University of Saskatchewan, Saskatoon, Canada.

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