Abstract
This study evaluated the effects of heat stress (HS) and dietary nano chromium picolinate (nCrPic) on metabolic responses of sheep to an intravenous glucose tolerance test (IVGTT), an intravenous insulin tolerance test (ITT) and an intramuscular adrenocorticotropin hormone (ACTH) challenge in sheep. Thirty-six sheep housed in metabolic cages were randomly allocated within 3 dietary groups (0, 400 and 800 μg/kg supplemental nCrPic) to either thermoneutral (22 °C) or cyclic HS (22 to 40 °C) conditions for 3 wk. Basal plasma glucose tended to be increased during HS (P = 0.052) and decreased by dietary nCrPic (P = 0.013) while plasma non-esterified fatty acid concentrations were decreased (P = 0.010) by HS. Dietary nCrPic reduced the plasma glucose area under the curve (P = 0.012) while there were no significant effects of HS on plasma glucose area under the curve in response to the IVGTT. The plasma insulin response over the first 60 min after the IVGTT was decreased by HS (P = 0.013) and dietary nCrPic (P = 0.022) with the effects being additive. In response to the ITT plasma glucose reached a nadir sooner (P = 0.005) in sheep exposed to HS, although there was no effect on the depth of the nadir. Dietary nCrPic decreased (P = 0.007) the plasma glucose nadir after ITT. Over the duration of the ITT plasma insulin concentrations were lower in sheep exposed to HS (P = 0.013) whereas there was no significant effect of supplemental nCrPic. There was no effect of either HS or nCrPic on cortisol response to ACTH. Dietary nCrPic supplementation decreased (P = 0.013) mitogen-activated protein kinase-8 (JNK) and increased (P = 0.050) carnitine palmitoyltransferase 1B (CPT1B) mRNA expression in skeletal muscle. Results of this experiment demonstrated that animals under HS and supplemented with nCrPic had greater insulin sensitivity.