Comparison of the myocardial uptake of a technetium-labeled isonitrile analogue and thallium.

The myocardial transmicrovascular transport of thallium-201 (201Tl) and technetium-99m hexakis(2-methoxyisobutylisonitrile) (MIBI) were compared during variable blood flow levels in nine blood-perfused, isolated rabbit hearts. Seventeen injections of radiolabeled albumin and EDTA as well as 201Tl and MIBI were performed by indicator-dilution techniques. When coronary flow was varied from 0.52 to 3.19 ml/g/min, myocardial extraction for MIBI averaged 0.38 +/- 0.09 (SD) whereas 201Tl myocardial extraction averaged 0.73 +/- 0.10 (p less than 0.001). Net extraction, which was calculated using end points of 1.8-4.9 minutes, averaged 0.41 +/- 0.15 for MIBI and was less than the 201Tl net extraction of 0.57 +/- 0.13 (p less than 0.001). The mean capillary permeability-surface area product for MIBI (0.44 +/- 0.13 ml/g/min) was one third of 201Tl (1.30 +/- 0.45 ml/g/min; p less than 0.001). However, parenchymal cell permeability-surface area product for MIBI (47.58 +/- 25.85 ml/g/min) was much higher than 201Tl (6.52 +/- 6.51 ml/g/min; p less than 0.0001), and apparent cellular volume of distribution for MIBI (15.15 +/- 3.31 ml/g) was also higher than 201Tl (10.19 +/- 4.00 ml/g; p less than 0.01). These data suggest that capillary permeability for 201Tl is greater than MIBI, but the reverse is true at the parenchymal cell wall. In addition, a new blood-perfused preparation is used for indicator-dilution techniques, and previously developed modeling analyses are also extended to these experiments.