Potassium and thallium uptake in dog myocardium

We sought to ascertain the rates and mechanisms of uptake of markers for regional myocardial blood flows.

The rates of exchange of potassium and thallium across capillary walls and cell membranes in isolated blood-perfused dog hearts were estimated from multiple indicator dilution curves recorded for 131I-albumin, 42K and 201Tl from the coronary sinus outflow following injection into arterial inflow. Analysis involved fitting the observed dilution curves with a model composed of a capillary-interstitial fluid-cell exchange region and nonexchanging larger vessels.

Capillary permeability surface products (PSc) 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 cells, 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 averaged 0.40 +/- 0.19 (n = 18), thereby omitting a single high value for potassium. This high cellular influx for thallium is interpreted as due to its passage through ionic channels for both Na+ and K+.

The high permeabilities and large volumes of distribution make thallium and potassium among the best ionic deposition markers for regional flow. Their utility for this purpose is compromised by significant capillary barrier limitation retarding uptake; so regional flow is underestimated modestly in high-flow regions particularly.