Regional distribution of cardiac output in young lambs: effect of cold exposure and treatment with catecholamines.

1. Lambs less than 3 days old, exposed to thermoneutral or intensely cold conditions in a respiration chamber, were infused with adrenaline or noradrenaline, 1 or 10 mug/kg.min; and the effects on oxygen consumption, cardiac output and its distribution to skin, skeletal and cardiac muscle, liver, spleen, kidney, gut, brown adipose tissue and brain were determined. Cardiac output was estimated by the Fick and dye dilution methods and the distribution of cardiac output by Sapirstein's method of fractional distribution of indicators.2. Under thermoneutral conditions, metabolic rate was stimulated by both doses of noradrenaline and by the low, but not the high dose of adrenaline. Under cold conditions, the low dose of catecholamines had little effect on the already elevated metabolic rate, but the high doses depressed the metabolic response to cold.3. The low dose of adrenaline increased cardiac output under thermoneutral conditions whereas the high dose decreased cardiac output; the effects of noradrenaline were less marked, in contrast to reported effects in new-born rabbits. The low doses of catecholamine given under cold conditions had little effect on the already elevated cardiac output, but the high doses, particularly of adrenaline, decreased cardiac output.4. Blood flow through the skin of the extremities was markedly reduced by cold exposure, while flow through the peri-renal fat was doubled, flow through the skeletal muscle was quadrupled and flow through the cardiac muscle was trebled. These increases, particularly in skeletal muscle, were due to increased cardiac output and to vasodilation, as indicated by the reduced ratio of blood pressure to blood flow. Results are contrasted with published reports that blood flow through brown fat in new-born rabbits was greatly increased by cold, but muscle flow was scarcely altered.5. In almost all organs examined the high doses of adrenaline infused in either environment markedly reduced blood flow, presumably by generalized vasoconstriction. Changes due to noradrenaline were small under thermoneutral conditions and flow through brown fat was increased by only 60% during infusion of 10 mug/kg.min. Much greater increases have been reported in new-born rabbits. Under cold conditions the high doses of noradrenaline tended to decrease flow in most organs including brown fat and muscle.6. The results provide likely explanations for published reports that adrenaline failed to stimulate non-shivering thermogenesis or suppressed the mobilization of metabolites and the metabolic response to cold.