Tissue Responses in the Cold-Exposed Rat

¹ From the Department of Physiology, University of Miami School of Medicine and the Howard Hughes Medical Institute, Miami, Florida

Ten to fourteen-week-old rats were exposed to 5 ± 2°C for 5 days. The Qo₂ values of liver, diaphragm, heart and skeletal muscle were elevated; kidney slices gave a variable response; preparations of thymus, testis, brain, spleen, intestinal smooth muscle and lung did not show significant alterations in oxygen consumption. Comparison of these findings with Barker's data concerning the effects of thyroxine injections in vivo reveals that in general the tissues respond in the same way to the cold stimulus and to the hormonal treatment. In hypothyroid animals, the unresponsive tissues listed above do not undergo alterations in oxygen consumption, but diaphragm and heart Qo₂ values are significantly reduced, while liver and kidney oxygen consumption rates are slightly decreased. The survival time of hypothyroid animals is reduced in the cold; 5 days after exposure of the hypothyroid animals to cold, heart and liver Qo₂ values were found to be elevated, but not to the extent demonstrated by tissues taken from euthyroid animals. The hypothesis is advanced that the thyroid gland exerts its effects by way of a few selected tissues only, in which it regulates the level of metabolism so as to provide adequate heat production for suitable adaptation to cold of the entire animal.