Substrates for cold thermogenesis in thyrotoxic dogs.

The influence of thyrotoxicosis on energy supply during cold exposure was studied in normal and chronically thyroxine (T4)-treated normothermic dogs exposed to neutral (Ta,N = +25 degrees C) or cold (Ta,C = -21 degrees C) ambient temperatures. At Ta,N, T4 treatment significantly increased VO2, glucose turnover, and plasma 3-hydroxybutyrate concentration. The percentage of glucose turnover derived from alanine also increased in spite of lower alanine release. In cold, T4 treatment did not significantly modify O2 consumption, glucose turnover, or plasma alanine concentration, but plasma hydroxybutyrate, alanine clearance, and alanine conversion into glucose were significantly increased compared with control. It is suggested that in cold the main effect of thyrotoxicosis on energy supply, in addition to a trend toward lipid mobilization, is an increase in hepatic alanine extraction and conversion into glucose in the presence of lower muscular alanine delivery.