COLD-INDUCED ALTERATIONS IN UNCOUPLING PROTEIN AND ITS MESSENGER-RNA ARE SEASONALLY DEPENDENT IN-GROUND SQUIRRELS

We were interested in determining whether season affects the ability o f cold exposure to increase brown adipose tissue (BAT) thermogenic fun ction in 13-lined ground squirrels after acute and chronic cold (4 deg rees C) exposure. Tissues were collected from animals in April and Sep tember after cold exposure for 12, 24, or 48 h. Animals chronically ex posed to the cold (10 days) were killed in early May and mid-August. W e found that mitochondrial uncoupling protein (UCP) concentrations var ied seasonally, with concentrations in control animals (at 23 degrees C) higher in late summer (mid-August and September) than in the spring (April and early May). Cold exposure in late summer did not induce fu rther increases in UCP concentrations. In contrast, when animals were cold exposed in the spring, UCP concentrations and total UCP increased . Surprisingly, 10 days at 4 degrees C did not cause a greater increas e in UCP concentrations than did 24 h at 4 degrees C. Chronic cold exp osure increased the UCP mRNA-to-beta-actin mRNA ratio 48% in May, wher eas a fivefold increase occurred in August. GDP binding was increased after 12 h at 4 degrees C in April; in contrast, animals attempted to hibernate when placed in the cold in September, and no increase in GDP binding was observed. Chronic cold exposure caused GDP binding to inc rease at both times. These results indicate that mitochondrial UCP con centrations are seasonally regulated in the 13-lined ground squirrel.