Potassium, sodium, and iodide transcapillary exchange in the dog heart

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Abstract
Simultaneous first-passage dilution curves of the electrolytes were performed in the blood-perfused isolated dog heart. Quantification of transcapillary exchange was accomplished by means of the parameters "fractional extraction" from the blood, E, and "fractional turnover rate of exchange" of the interstitial compartment with the blood, k. The results show that E for Rb86+ and K42+ is 0.69 ± 0.08, for Na22+ 0.74 ± 0.12, and for I131– 0.54 ± 0.03. The values of k for Na22+ ranged between 0.11 and 3.00 min–1 and for I131 between 0.76 and 3.54. The k's varied with the blood flow but apparently the E's did not. There was a high linear correlation between the plasma clearance of the ions and the flow. From the data the heart interstitial fluid volume was calculated as 0.15 ± 0.04 ml/g. In some experiments various ions were studied simultaneously and the E's used to compute permeability-surface products, PS. The PS ratios for sodium and potassium were not significantly different from the ratio of free diffusion in water constants.