POTASSIUM AND THALLIUM UPTAKE IN DOG MYOCARDIUM

We sought to ascertain the rates and mechanisms of uptake of markers f or regional myocardial blood flows. Methods: The rates of exchange of potassium and thallium across capillary wails and cell membranes in is olated blood-perfused dog hearts were estimated from multiple indicato r dilution curves recorded for I-131-albumin, K-42 and Tl-201 from the coronary sinus outflow following injection into arterial inflow. Anal ysis involved ming the observed dilution curves with a model composed of a capillary-interstitial fluid-cell exchange region and nonexchangi ng larger vessels. Results: Capillary permeability surface products (P Sc) for potassium and thallium were similar, 0.82 +/- 0.33 (mean +/- s .d., n = 19) and 0.87 +/- 0.32 ml min(-1) g(-1) (n = 24) with a ratio for simultaneous pairs of 1.02 +/- 0.27 (n = 19). For the myocardial c ells, PSpc averaged 3.7 +/- 3.1 ml min(-1) g(-1) (n = 19) for K+ and 9 .5 +/- 3.9 (n = 24) for Tl+; the ratio of potassium to thallium averag ed 0.40 +/- 0.19 (n = 18), thereby omitting a single high value for po tassium. This high cellular influx for thallium is interpreted as due to its passage through ionic channels for both Na+ and K+. Conclusion: The high permeabilities and large volumes of distribution make thalli um and potassium among the best ionic deposition markers for regional flow. Their utility for this purpose is compromised by significant cap illary barrier limitation retarding uptake; so regional flow is undere stimated modestly in high-flow regions particularly.