Cardiac output and organ blood flow in warm- and cold-acclimated rats exposed to cold

Cold acclimation increased the cardiac output of unanesthetized rats when measured at 30 °C. After exposure to 9 °C for 70 min cardiac output further increased by 46% in both warm- and cold-acclimated rats. From the changes in the fractional distribution of cardiac output after cold exposure it was shown that the blood flow increased significantly in muscular organs (heart, diaphragm, skeletal muscles) and in the adrenals of warm-acclimated rats. In cold-acclimated rats the blood flow to the brown and white adipose tissues, pancreas, kidney, intestine, liver, and other internal organs was also increased in a cold environment, and accounted for 65% of the increase in blood flow during exposure to cold compared with only 36% in warm-acclimated rats. It is estimated that the extramuscular thermogenesis can account for a greater proportion of the total nonshivering thermogenesis in cold-acclimated rats. The contribution of brown adipose tissue is estimated not to exceed about 6% of the total heat production increase in cold-acclimated rats during exposure to cold.