Partition of 125I‐iodoantipyrine among erythrocytes, plasma, and renal cortex in the dog

The tissue/blood partition coefficient, lambda tb, defined as the amount of blood having the same tracer content as one unit of tissue at diffusion equilibrium, was determined for 125I-iodoantipyrine (I-Ap) and tritiated water (THO) in the dog kidney cortex. Measurements were made after in vivo equilibration for 75 to 300 s and with liver circulation excluded. In 18 kidneys, lambda tb for I-Ap averaged 1.38 (S.D. 0.13) w/w (weight/weight), without significant correlation to hematocrit (range: 23-43) or to urine pH (range 5.5-8.6). The lambda tb for THO averaged 0.97 (S.D. 0.06) v/w (volume/weight), close to the relative water contents. Erythrocyte/plasma partition for I-Ap was 0.82 w/w, compared to a water partition of 0.72. Thus, at diffusion equilibrium the apparent I-Ap concentration in renal cortical and red cell water exceeds that of plasma water by 14 and 60%, respectively. It follows that I-Ap cannot be used as a general indicator for total tissue water content. When used for measurement of local blood flow and modum Kety, lambda tb must be determined for each tissue and species.