Potassium and Thallium Uptake in Dog

We soughtto ascertainthe ratesand mechanisms of uptakeof markers for regional myocardial blood flows. Methods The rates of exchange of potassium and thallium across capillary walls and cell membranes in isolated blood-perfused dog hearts were estimated from multipleindicator dilutioncurves recorded for 131l-albumin,42K and 201fl fr@, the coronary sinus outflow following injection into arterial inflow.Analysis involvedfittingthe observed dilution curves with a model composed of a capillary-interstitialfluid-cellexchange region and nonexchanging larger vessels. Results Capillaryperme ability surface products (P%) for potassium and thallium were similar,0.82 ± 0.33(mean± s.d., n = 19)and 0.87 ± 0.32mlmin1 g@1(n = 24) with a ratio for simultaneous pairs of 1.02 ± 0.27 (n = 19).Forthemyocardial cells,PS@, averaged 3.7 ± 3.1mlmin1 g1 (n = 19)for K@ and 9.5 ± 3.9(n = 24)forTi@;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@.Conclusion: The high permeabilities and large vol umes 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-flowregions particularly.