Phenol Red Clearances in the Dog

Abstract

Marshall and his coworkers 1 were the first to adduce evidence that phenol red is excreted by the dog's kidney by tubular secretion as well as by glomerular filtration. This evidence was: (1) phenol red injected intravenously accumulated in the cells of the convoluted tubules of the anuric kidney obtained by spinal transection; (2) phenol red is in part bound to plasma colloids and thus rendered non-filtrable, and an insufficient concentration of filtrable phenol red is present in arterial blood to account for the quantity excreted in the urine; (3) in two experiments on anesthetized dogs the rate of excretion of phenol red was not proportional to the concentration in the plasma at all levels of the latter; (4) phenol red clearances in a normal dog were considerably greater than simultaneous creatinine clearances. The last mentioned experiments were done within a restricted range of plasma phenol red (0.21 to 0.54 mg. %) and leave undetermined the question of the relationship of the latter to the phenol red clearance, as well as the relationship of this clearance to the clearances of other urinary constituents.

Simultaneous phenol red and inulin clearances∗ have been determined in normal dogs, with special reference to the effect of the plasma concentration on the former. The results obtained upon one dog are illustrated in Fig. 1.

At plasma concentrations of phenol red from 3.0 to 2.0 mg. % down, the clearance of this substance (calculated upon the total dye in the plasma) has an approximately steady level, and the phenol red/inulin clearance ratio consequently is constant, having a value of about 1.7. As the plasma level of phenol red is increased, the phenol red clearance falls both absolutely and relative to the inulin clearance, until at a plasma level of 40 mg. % it is only 65% of the latter. Only 45% of the total phenol red in this experiment was filtrable through collodion at 38 mm. p CO₂ and 37°C, at this high level, and it is presumed that could the plasma level of dye be raised to a sufficient degree, the phenol red clearance, calculated on the basis of free phenol red, would approach the inulin clearance as a limiting value.