Changes in the metabolism of the shivering hind leg of the young ox during several days of continuous cold exposure.

The effect of 4 days of continuous exposure to a cold environment on blood flow in, and oxygen and energy substrate uptake by the shivering hind leg has been studied in young steers. The animals shivered throughout the period of cold exposure and total oxygen consumption (total VO2) remained 40-50% greater than VO2 during thermoneutrality. Leg blood flow (leg Q) and oxygen uptake (leg VO2) increased two- and four-fold respectively on the first day of cold. Both had declined significantly by the final day, leg Q to a level 37% greater than, and leg VO2 to about double, pre-cold levels. The change in the relationships of leg Q and leg VO2 to total VO2 was examined by linear regression analysis, which suggested that the changing contribution of leg VO2 to total VO2 was entirely due to changes in leg Q, rather than in the arteriovenous difference in blood oxyhaemoglobin saturation across the leg. The net uptakes by the leg of free fatty acids (FFA), acetate, glucose and lactate all increased on the first day of cold. Both glucose and acetate uptakes were greater on day 4 than on day 1 in the cold, in spite of the lower leg VO2, but net uptakes of FFA and lactate were considerably lower. The decrease in net uptake of FFA is attributed mainly to an increase in the rate of lipolysis and release of FFA from fat depots in the leg, because of the associated progressive increase in the release of glycerol and oleic acid from the leg. The molar ratios of net carbohydrate substrate, acetate and FFA uptakes to leg VO2 are compared. The results suggest that tissues other than the leg muscles become increasingly important as sites of heat production, and that there are changes in the utilization of glucose, acetate and FFA by shivering muscle, during prolonged cold exposure.