Permeability and model testing of heart capillaries by osmotic and optical methods

Rabbits were anesthetized with Nembutal; their hearts were removed and perfused with Ringers solution. Osmotic weight transients were then produced by test solute infusion into the perfusate. The rate constant for organ weight change was used to predict test solute venous concentration (Cv) and the distribution volume (V) during an osmotic transient by use of the Johnson and Wilson (6) model for capillary tissue exchange. By use of Cr EDTA, strongly purple in solution, we compared the above predictions with a value of Cv obtained directly by optical measurement of outflow venous concentration and values of V calculated from our measurements of organ weight and the known sucrose distribution volume. The close agreement between both sets of values with a permeability coefficient of 5 x 10-5 cm/s and a volume of distribution of 3.2 ml/10 g heart lead us to conclude that the model used closly represents the conditions of the isolated perfused heart, and that both the osmotic transient and the extraction measurements provide good estimates of organ capillary permeability.