P-aminohippurate transport in the proximal straight tubule: development and substrate stimulation.

Abstract

Summary: p-Aminohippurate transport by the developing kidney has previously been shown to increase as a function of age and to be enhanced by pretreatment with penicillin. The relative contribution of increases in intrinsic cellular transport capacity and in tubular length was assessed in isolated perfused proximal straight tubules obtained from developing rabbits. The intrinsic capacity for transport (10-15 M/min · mm) increased 4.8-fold (y = −315 + 62.0x, r = 0.69, P < 0.005) from 8–19 days of age. Tubular length (microns) also increased (y = −38.5 + 52.3x, r = 0.84, P < 0.001) during the same period. Nine- to 13-day-old animals pretreated with procaine penicillin (400,000 U) divided over 3 days had an intrinsic capacity for transport of 684 ± 88 × 10-15 M/min · mm, which was 89% greater than the 361 ± 84 × 10-15 M/min · mm observed in age-matched, untreated controls (P < 0.02). In these groups tubular length was 630 ±106 μm and 585 ± 37 μm, respectively (P > 0.4). Speculation: Most of the increase in absolute transport (intrinsic capacity × tubular length) seen with age is the result of the increment in the intrinsic transport capacity, while 33% is a consequence of the rise in the surface area of the transport membrane. Substrate stimulation augments the rate of maturation of the intrinsic transport mechanism without altering the age-related change in the length of the proximal straight tubule.