Effects of thyroxine on Na+,K+-ATPase and norepinephrine turnover in brown adipose tissue of obese (ob/ob) mice.

Thyroxine treatment improves some of the defective thermogenic properties of obese (ob/ob) mice. Because brown adipose tissue (BAT) is an important thermogenic organ in mice, effects of thyroxine treatment on Na+, K+-ATPase, a thyroid-hormone responsive enzyme, and on rates of norepinephrine (NE) turnover, an indicator of sympathetic nervous system activity, in BAT of lean and obese mice were evaluated. Female mice, six weeks old, were injected with approximately 4 micrograms thyroxine daily for 2 weeks. Numbers of Na+, K+-ATPase enzyme units in BAT were similar in control lean and obese mice. Thyroxine treatment increased numbers of Na+, K+-ATPase enzyme units by 60% and 100% in lean and obese mice, respectively, indicating that the BAT of obese mice was responsive to thyroxine treatment. Fractional rates of NE turnover were 75% faster in BAT of control lean mice than in obese mice. Thyroxine treatment decreased functional rates of NE turnover in BAT of lean mice by 35%, but had no effect on NE turnover in BAT of obese mice. Rates of NE turnover in heart and pancreas of control lean and obese mice were unaffected by phenotype. Although the decreases in fractional rates of NE turnover in heart (-23%) and pancreas (-11%) of lean mice in response to thyroxine injections were not statistically significant, the calculated rates of NE turnover (fractional rate of NE turnover times the NE content of the organ) in these organs of lean mice were decreased 25% to 30% (P less than 0.05) in response to thyroxine. Thyroxine injections did not affect NE turnover in either heart or pancreas of obese mice.(ABSTRACT TRUNCATED AT 250 WORDS)