In order to clarify the uptake and retention mechanisms of radioiodinated meta-iodobenzylguanidine (MIBG) in heart, the kinetics of no-carrier-added [123I]MIBG were studied in the isolated working rat heart in interaction with pharmacologic agents. The tracer was administered in the perfusate as a 10-min pulse, followed by a 90-min washout period. Kinetic analysis of the externally monitored time-activity curves of control hearts showed avid uptake (Ki = 4.4 +/- 0.7 mL/min/g), and monoexponential clearance (ko = 0.0056 +/- 0.0017 l/min), indicating a distribution volume (Vd = Ki/ko) of 834 +/- 214 mL/g. Blocking experiments (n = 41) were performed with neuronal uptake (uptake-1) inhibitor desipramine (DMI; 50-100 nM) and the extraneuronal uptake (uptake-2) inhibitor N-(9-fluorenyl)-N-methyl-beta-chloroethylamine (SKF550; 0.4-0.8 microM). Uptake rate was 27% reduced (P < 0.05) by 50 nM DMI but not significantly affected by 0.4 microM SKF550. Distribution volume was 88% reduced (P < 0.0005) by 50 nM DMI and 28% reduced (P < 0.05) by 0.4 microM SKF550. In DMI-blocked hearts, uptake rate was dramatically decreased (-80%, P < 0.0005) by SKF550 (0.4 microM), indicating uptake-2 transport contributed predominantly to the extraneuronal uptake of the tracer. The slow uptake rate seen with concomitant inhibition of uptake-1 and uptake-2 was further decreased by addition of unlabeled MIBG (1-10 microM) in a concentration-dependent manner, yet unaffected by addition of the vesicular uptake inhibitor Ro 4-1284 (1 microM). Thus, the uptake rate of [123I]MIBG is primarily dependent on uptake-1 and uptake-2 activity. Other possible mechanisms of uptake such as passive diffusion in association with intracellular binding are significant only in conditions where uptake-1 and uptake-2 mechanisms are largely inhibited.