Effect of chronic cold exposure and underfeeding on hind-limb tissue metabolism in pregnant sheep

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Abstract
SUMMARY Muscle metabolism was studied in pregnant sheep over the final 4 weeks of pregnancy, between January and March 1985 and between December and March 1986, to investigate the effect of (i) chronic cold exposure, induced by shearing pregnant ewes 8 weeks before lambing and (ii) undernutrition in shorn and unshorn ewes. This was achieved by measuring hind-limb tissue metabolism using a combination of isotopic and arterio–venous difference techniques. The rates of blood flow, oxygen uptake, carbon dioxide production and calculated heat production were all significantly higher across the hind-limb tissues in shorn than in unshorn ewes. The increase in hind-limb energy metabolism in the shorn group was such that 65% of the difference in wholebody heat production between shorn and unshorn groups could be attributed to muscle tissue. Following underfeeding, all of the difference in whole-body heat production between the two groups was completely attributable to muscle tissue. Nonesterified fatty acid (NEFA) oxidation by muscle accounted for 46% of the difference in whole-body NEFA oxidation rate between fed shorn and unshorn groups. Underfeeding significantly increased the mean arterial plasma concentration of NEFA in shorn and unshorn ewes and the rate of NEFA oxidation across the hind-limb tissues in shorn ewes. The difference in whole-body NEFA oxidation between shorn and unshorn groups could, therefore, be completely accounted for by muscle. No significant differences in the arterial plasma concentrations of glucose, lactate or 3-hydroxybutyrate were recorded between shorn and unshorn ewes, although the rate of glucose uptake across the hind-limb tissued was significantly higher in the shorn group. Calculation of the maximum potential contribution of these substrates to hind-limb metabolism demonstrated that NEFA were the predominant energy source in all experimental conditions studied in all ewes. When data from fed and underfed shorn ewes were pooled, regression analysis revealed a significant negative correlation between the arterial plasma concentrations of glucose and 3-hydroxybutyrate, and significant positive relationships between the arterial concentrations of both glucose and 3-hydroxybutyrate and their rate of uptake across the hind-limb tissues. These relationsips were not observed in unshorn ewes. It is concluded that the muscle mass plays a major role in the nonshivering thermogenic adaptation to chronic cold exposure in shorn pregnant sheep and that NEFA are the major substrate utilized for oxidative metabolism in this tissue.

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