Excretion of Phenol Red and Inulin by the Fetal and the Newborn Rabbit

Abstract

Simultaneous clearances of phenol red (PSP) and inulin (in) were measured in fetal, newborn and adult rabbits. The clearances were determined from single injection and single blood and urine collections. This method tends to reduce the numerical values of the clearances but not to change the ratios of the two clearances when compared with clearances obtained by conventional methods in adult rabbits. The CPSP/Cin ratio fell from a fetal value of 0.4 to a newborn level of 0.2 and then rose to approximate the adult ratio of 2.0 by 1 month after birth. The use of plasma PSP concentrations both above and below the adult TmPSP did not influence significantly the clearance ratios in young animals. The percentage of total PSP not bound to plasma proteins, determined by ultrafiltration, was more in the newborn plasma (30%) than in adult plasma (4%). Both are greater than might be predicted from an adsorption isotherm using constants determined in adult blood by others. It was concluded that the rise in clearance ratio with age probably reflected both an increase in filtration with age and a greater increase in tubular secretion with age; and that these factors were more important than corresponding changes in the binding of PSP by plasma proteins in young and adult animals.