Alteration in hepatic mitochondrial compartment of cold-acclimated rats. Association with enhanced triiodothyronine serum levels. A morphometric/stereologic study by electron microscopy.

In cold-exposed rats, the circulating levels of free and total T3 rapidly increase and reach higher values after 2 h, 70% and 80%, respectively. These values remain constant during cold exposure and return to normal values after few hours of re-exposure to room temperature. A remarkable modification of the mitochondrial compartment in rat hepatic cells is associated with these hormonal variations. The morphometric stereologic analysis shows that after 10 days of cold exposure the number of mitochondria per nucleus is nearly doubled (from 1,912 to 3,414), the total volume of the mitochondrial compartment per nucleus increases by about 40% (from 21% to 29%), and the mitochondrial membrane surfaces per nucleus increase too (from 7,233 square microns to 11,597 square microns for the outer membranes; from 31,572 square microns to 42,603 square microns for the inner membranes + cristae). The mean mitochondrial volume decreases by about 25%. This probably indicates, the formation of new organelles by division of pre-existing mitochondria. On re-exposure to room temperature, there is a return to normal values. The present data suggest the existence of a physiological long-term mechanism for the regulation of cellular thermogenesis during cold exposure (increase in number of mitochondria per cell), associated with enhanced levels of T3.