Disparity Between Phenol Red and Diodrast Clearances in the Dog

Abstract

The clearance of radioiodinated Diodrast and that of phenol red were compared in anesthetized dogs in an attempt to determine the reason why the former is roughly twice the latter, even though the two substances are secreted by the same mechanism. The infusion of sodium acetate increases both clearances but does not alter the ratio of the two. Hence availability of substrate is not likely to be a significant factor in restricting the clearance of phenol red relative to Diodrast. Replacement of plasma proteins, which bind phenol red, with dextran, which does not, increases the clearance of phenol red relative to Diodrast. This increase in clearance ratio is a result of an increase both in the quantity of dye filtered and in the quantity secreted. These experiments show that either rate of diffusion of free phenol red from peritubular capillaries into tubular cells or rate of dissociation of phenol red bound by plasma proteins is a limiting factor in secretory transport. However, the Diodrast clearance exceeds the phenol red clearance in isolated kidneys perfused with dextran solutions in which all dye is free. Thus, factors other than protein binding play some role in causing disparity of clearances.