POSSIBLE ROLE OF ADRENERGIC MECHANISM IN STARVATION-INDUCED REDUCTION IN CIRCULATING THYROXINE AND TRIIODOTHYRONINE IN RATS

To elucidate the possible role of adrenergic mechanism in thyroid hormone metabolism during starvation, serum thyrotropin (TSH), thyroxine (T4), and 3,5,3'-triiodothyronine (T3) levels and conversion of T4 to T3 in perfused liver were investigated in fasting (60 h) and fed rats. Propranolol (0.5 mg/kg), yohimbine (0.3 mg/kg) or phentolamine (5.0 mg/kg) was subcutaneously injected to the rat every 12 h. Serum levels of TSH, T4, and T3 were significantly lower in fasting rats than in fed rats. Although propranolol, yohimbine, and phentolamine administration did not significantly alter circulating TSH, T4 and T3 levels in fed rats, phentolamine partly inhibited the starvation-induced reduction in circulating TSH, T4, and T3. Thyroxine uptake and T3 production in perfused liver were significantly lower in fasting rats than in fed rats. Phentolamine treatment did not alter the T4 uptake and T3 production in perfused liver of fasting rats. These results suggest that alpha-adrenergic mechanism may have some role in starvation-induced reduction in circulating T4 and T3, and that phentolamine partly inhibited this phenomenon probably through the inhibitory effect on reduction in circulating TSH during starvation.