HEAT INFLUENCES ON PLASMA INSULIN AND GLUCAGON IN RESPONSE TO SECRETOGOGUES IN NON-LACTATING DAIRY COWS F. Itoh, 1 Y. Obara, M.T. Rose, and H. Fuse Department of Animal Physiology, National Institute of Animal Industry, Tsukuba-Norinkenkyu-Danchi PO Box 5, Ibaraki 305-0901, Japan Received November 6, 1997 Accepted May 22, 1998 Four non-lactating cows were offered a maintenance diet of hay wafer and a commercial concentrate. They were housed in a thermoneutral (TN; 20°C) and then a hot (30°C) environment in an artificial climate chamber. Glucose, arginine, butyrate, and insulin were administered through one jugular catheter, and from a catheter on the other side venous blood was collected. The peak increments in plasma insulin after the glucose and butyrate administrations were lower during heat exposure. The response of insulin after arginine injection was smaller in the hot compared with the thermoneutral environment; however, arginine injection resulted in a significantly higher secretion of glucagon in the hot environment. The response area of insulin after the insulin injection was smaller in the hot environment; however, insulin clearance rate was not changed. It is concluded that in non-lactating cows, insulin release is probably lower during heat exposure. With respect to plasma glucose during heat exposure, the lower basal values, lower concentrations after the end of the glucose infusion, and delayed recovery to basal values after the butyrate and insulin administrations observed, may indicate lower gluconeogenesis and glycogenolysis in the hot environment. © Elsevier Science Inc. 1998 INTRODUCTION The influence of heat exposure on physiological responses such as rectal temperature and respiration rate have been widely demonstrated in sheep (1–3), goats (4), calves (5), heifers (6,7), and lactating cows (8 –10). Voluntary feed intake decreases in heat-stressed cattle, resulting in a reduction in metabolic heat production (9,7). However, the adaptation of endocrine mechanisms that control the distribution and utilization of nutrients in the body also seem to play a considerable role. Lower plasma thyroxine levels (10), higher triiodothyronine (T 3 ) (10), and lower T 3 levels (7) have been noted in Holstein cattle when heat stressed. It has also been reported that basal insulin concentrations, which have the anabolic effect of directing nutrients toward tissues, are lower in summer than winter and spring (11), though, in another experiment, basal insulin and glucagon concentrations were not consistently affected after chronic heat exposure in heifers, though metabolic substrates were maintained at altered levels for 14 d (6). Therefore, experiments that observe insulin and glucagon responses to stimuli such as glucose, arginine (12), and butyrate (13) would possibly be more helpful in clarifying changes in the hormonal responses to hot environments. Although earlier reports demonstrated that the glucose- stimulated insulin response is lower in Holstein cattle in summer (11), and higher in sheep exposed to a controlled hot environment (1,14), plasma insulin and glucagon responses to glucose injection vary depending on the physiological state (15). The responsiveness of the endocrine system to heat stress may also differ in different species. DOMESTIC ANIMAL ENDOCRINOLOGY Vol. 15(6):499 –510, 1998 © Elsevier Science Inc. 1998 0739-7240/98/$19.00 655 Avenue of the Americas, New York, NY 10010 PII S0739-7240(98)00038-1