Factors affecting the maturation of renal PAH extraction in the new-born dog.

1. The maturation of renal para-amino-hippurate extraction (E(PAH)) was studied in thirty-nine mongrel new-born dogs aged 2-29 days. E(PAH) was also measured in five adult dogs for comparison.2. There was no apparent change of PAH extraction during the first month of life but E(PAH) was lower in the puppy than in the adult. Under mannitol diuresis E(PAH) averaged 0.49 for the puppies and 0.83 for the adult dogs (P < 0.001). E(PAH) averaged 0.48 for the puppies under nondiuretic conditions, a value not significantly different from that observed during mannitol diuresis. There was no correlation of E(PAH) with mean arterial blood pressure.3. Inulin extraction (E(In)) remained constant during the first month of life and was the same in the new-born and adult dog suggesting that preglomerular shunting of plasma away from functioning nephrons was the same for both the puppy and adult dog.4. E(PAH) did not change in the puppy when plasma concentrations varied from trace (using [(3)H]PAH) to 0.25 mM indicating that excessive PAH load is not a factor in the low E(PAH) of the puppy.5. Diffusion of PAH from the red blood cell was excluded as an explanation for the low E(PAH) since diffusion was the same for both the adult and new-born dog.