The effect of cold adaptation on kinetics of insulin and growth hormone in heifers

Abstract

Ten Holstein heifers 1 yr of age were housed in groups of five in temperature-controlled environmental chambers maintained at either 20 °C (warm environment) or − 17 °C (cold environment). Each heifer received an injection of insulin or growth hormone followed by frequent blood sampling. Plasma samples were analyzed for insulin, insulin-like growth factor I (IGF-1), growth hormone, glucagon, triiodothyronine, thyroxine and glucose. Basal plasma concentrations of all hormones except growth hormone and IGF-1 were elevated (P < 0.05) in the cold environment. Kinetic parameters for growth hormone and insulin were calculated using a two-pool model. Both the zero-time intercept and the rate constant for the slow component of growth hormone were lower (P < 0.05) in cold-adapted animals. The total turnover rate of growth hormone in both pools also tended to be reduced by cold adaptation (P < 0.07). The reduction in growth-hormone turnover may be related to the reduced average daily gains that were observed in the cold (P < 0.05). Plasma glucose concentration following both the insulin and growth-hormone injections was higher in the cold-adapted heifers (P < 0.001 and P < 0.05, respectively). The response of glucagon to insulin injection was greater in the cold (P < 0.05), as was the response of IGF-I to growth-hormone injection (P < 0.10). These cold-induced alterations in the endocrine system appear to be directed toward providing substrates, including glucose, for use in thermogenesis rather than growth. Key words: Cold adaptation, growth hormone, insulin, metabolic clearance rate, hormone kinetics, cattle