Effects of hypothyroidism and thyrotoxicosis on metaiodobenzylguanidine kinetics in the rat heart.

Metaiodobenzylguanidine (MIBG), a norepinephrine (NE) analogue, has been used to study cardiac sympathetic innervation. Although MIBG uptake and washout generally parallel the kinetics of cardiac NE (an accepted marker of sympathetic nerve function), quantitative differences exist between NE and MIBG. To determine the value of MIBG as a marker of cardiac sympathetic nerve function, cardiac MIBG uptake and washout were measured in rats with hypothyroidism and thyrotoxicosis which cause increased and decreased sympathetic nerve function, respectively. Rats were made hypothyroid by the ingestion of water containing 0.03% methimazole and thyrotoxic by the intraperitoneal injection of L-thyroxine. After 3 weeks of treatment, rats were injected with 25 mu Ci of 125I-MIBG. Rats were sacrificed at 1.5, 4, 8 and 24 h after injection and heart MIBG activity was determined. Initial uptake and cardiac washout (T1/2) were calculated in both treated groups and compared to controls. MIBG washout was similar in all three groups although it tended to be faster in the hypothyroid group. Uptake was highest in the hypothyroid group (130% of controls) and lowest in the hyperthyroid group (79% of controls). These results suggest that MIBG may be a marker of cardiac sympathetic nerve function although its kinetics may differ from NE.