Factors controlling the washout of the interstitial space of the isolated, perfused rat heart.

1. The time course of the washout of the extracellular markers, inulin, raffinose, sucrose and sorbitol, has been determined for the isolated rat heart perfused for 20 min with Krebs bicarbonate medium containing the marker, and washed out with marker-free perfusate.2. The filtration problem for the perfused rat heart was found to be due to omission of the gassing of the concentrated sodium bicarbonate solution with CO(2) in the preparation of the perfusate. This probably caused calcium phosphate to be precipitated. The reduced contractility of the heart was reflected in the washout of the extracellular markers.3. The logarithmic plot of the heart content of the marker against time showed two components. The slower was an exponential process, the rate of which was linearly related to the diffusion coefficient for the various markers.4. The faster component was absent in the quiescent heart, and in the beating heart its contribution to the washout of the interstitial space was related to the strength of contraction.5. Similar washout curves were obtained for Evans Blue-albumin conjugate, which readily penetrates the interstitial space of the isolated rat heart.6. It is suggested that the two components of efflux are a diffusion process limited by perfusate flow, and bulk movements of fluid.