Background: To evaluate cardiac sympathetic activity, a simple method should be developed to replace such complex methods as the spillover rate of tritiated norepinephrine (3H-norepinephrine) or microneurography of sympathetic nerve activity. The goal of this study is to evaluate cardiac sympathetic activities by analyzing the washout of I-123 meta-iodobenzylguanidine (123I-MIBG), a radiolabeled norepinephrine analogue, in Dahl salt-sensitive (DS) rats as it relates to the progression of hypertension.
Methods and results: Dahl salt-resistant (DR) rats and DS rats were fed an 8 % salt diet starting at age 5 weeks. Marked hypertension and cardiac hypertrophy developed in the DS rats, whereas DR rats remained normotensive. Then the time-activity curves of 123I-MIBG from 15 to 200 minutes were obtained from both DS and DR strains at ages 8, 11, and 13 weeks using dynamic scintigraphic analysis. We also examined the nonneuronal washout of 123I-MIBG using dynamic scintigraphic studies in desipramine pretreated normal rats. In the preliminary study with desipramine pretreatment, the majority of the nonneuronal 123I-MIBG washout occurred by 90 minutes after injection. Therefore the late-phase washout in the control rats was found to reflect the neuronal washout. We then applied exponential curve fitting to the time activity curves acquired in the 90- to 200-minute period after 123I-MIBG injection in both DR and DS rats. When we compared the coefficients of these washout curves in the DS and DR rats as an index of cardiac sympathetic activities, the coefficient values remained high during all stages in DS rats, whereas they decreased with age in DR rats.
Conclusion: Measurement of late-phase 123I-MIBG washout may be a useful tool for assessing the change in sympathetic activity in the progression of hypertension without the influence of extraneuronal washout of 123I-MIBG and left ventricular hypertrophy.