Capillary permeability in the isolated rabbit heart as measured by local tissue clearance

The relatively simple method of local tissue clearance was used to measure capillary permeability-surface area products (PS) in the isolated, Ringer-perfused rabbit heart. Ten microliters of a mixture of [3H]inulin and [14C]sucrose was injected at a depth of 2 mm into the left ventricular myocardium and clearance rate constants (k in min-1) were determined by analyzing the draining perfusion fluid. PS (ml/min per 100 ml) for each solute was calculated by the following equation: PS = -F ln(1 - lambda k/F), where F is perfusate flow (ml/min per 100 ml) and lambda is the equilibrium tissue/Ringer partition coefficient. At a perfusion pressure of 40 mm Hg, F = 133 +/- 9.7 (mean +/- SEM), PSsucrose = 77 +/- 8.0, and PSinulin = 13.9 +/- 0.7. These PS products are within the range of values previously reported by others using several different techniques. The mean inulin/sucrose permeability ratio was 0.189 +/- 0.018 which is significantly less than the separately measured free diffusion coefficient ratio (= 0.41 +/- 0.005), thus indicating that sucrose and inulin crossed myocardial capillary walls by restricted diffusion. The reasons why some investigators did not find similar evidence of restricted diffusion are discussed.