Extraction and retention of technetium-99m Q12, technetium-99m sestamibi, and thallium-201 in isolated rat heart during coronary acidemia

Abstract.

Technetium-99m Q12 and ^99mTc-sestamibi are cationic lipophilic myocardial perfusion imaging tracers. Because myocardium in areas of ischemia becomes acidotic, experiments were designed to differentiate the effects of myocardial perfusate pH on radiotracer extraction and retention independent of substrate availability. We hypothesized that ^99mTc-Q12 and ^99mTc-sestamibi single-pass uptake and retention would be unaffected by a modest reduction in coronary perfusate pH. Isolated rat hearts were perfused at constant flow with Krebs-Henseleit buffer enriched with bovine red blood cells (20%). The indicator dilution method was used to measure the maximum extraction (E _max) and net extraction (E _net) of thallium-201 and ^99mTc-Q12 (n = 8) or ²⁰¹Tl and ^99mTc sestamibi (n = 7) during baseline perfusion (pH = 7.4), during acidemic (pH = 6.7) perfusion, and during a restitution period with normal perfusate (pH = 7.4). ²⁰¹Tl E _max (0.71±0.03) was greater than either ^99mTc-Q12 or ^99mTc-sestamibi E _max (0.27±0.02 and 0.26±0.01 respectively, P<0.0001). Acidemia significantly reduced ²⁰¹Tl E _max (0.65±0.03, P<0.02) but not ^99mTc-Q12 or ^99mTc-sestamibi E _max (0.25±0.02 and 0.24±0.02 respectively). During control perfusion E _net of ²⁰¹Tl was greater than that of ^99mTc-Q12 at 3 and 5 min and greater than that of ^99mTc-sestamibi at 3 min. ^99mTc-Q12 E _net was less than ^99mTc-sestamibi E _net at 3, 5, and 10 min. Acidemia decreased ²⁰¹Tl and ^99mTc-sestamibi E _net at 3, 5, and 10 min but had no effect on ^99mTc-Q12 E _net. It is concluded that E _max of ^99mTc-Q12 is less than that of ²⁰¹Tl but is not different from that of ^99mTc-sestamibi. E _net of ^99mTc-Q12 is less than that of ^99mTc-sestamibi.