Abstract
We have examined the uncoupling (UCP) protein gene expression in euthyroid and hypothyroid rats. UCP mRNA levels were estimated by northern blot analysis of total RNA from brown adipose tissue (BAT). Stimuli were endogenous (cold) and exogenous norepinephrine (NE), isoproterenol, T3, and T4. While the euthyroid rats UCP mRNA levels increase 2- to 3-fold by 2 h after NE or overnight cold exposure, these stimuli and isoproterenol are ineffective in hypothyroid rats. One single dose of T4, equal to the daily production rate, brings about a normal response in hypothyroid rats exposed to cold overnight. Hypothyroid rats recover their responsiveness to NE approximately 4 h after a receptor saturating dose of T3. On the other hand, such a dose of T3 induces a 3- to 4-fold increase in UCP mRNA levels in hypothyroid rats without the need of exogenous NE, and this response is not reduced by raising ambient temperature to thermoneutrality (28 C). However, the following evidence indicates that T3 requires adrenergic input to stimulate the accumulation of UCP mRNA: first, euthyroid animals maintained at 28 C do not respond to such a treatment. Second, when T3 was injected to hypothyroid rats with unilaterally denervated BAT, only the intact side responded to T3 with an elevation of the UCP mRNA levels, but both sides remained responsive to T3 + NE.(ABSTRACT TRUNCATED AT 250 WORDS)
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