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.