Water transport in neonatal and adult rabbit proximal tubules

Abstract

We have recently demonstrated that although the osmotic water permeability (P(f)) of neonatal proximal tubules is higher than that of adult tubules, the P(f) of brush-border and basolateral membrane vesicles from neonatal rabbits is lower than that of adults. The present study examined developmental changes in the water transport characteristics of proximal convoluted tubules (PCTs) in neonatal (9-16 days old) and adult rabbits to determine whether the intracellular compartment or paracellular pathway is responsible for the maturational difference in transepithelial water transport. The permeability of n-butanol was higher in the neonatal PCT than the adult PCT at all temperatures examined, whereas the diffusional water permeability was identical. Increasing the osmotic gradient increased volume absorption in both the neonatal and the adult PCT to the same degree. The P(f) was not different between the neonatal and the adult PCT at any osmotic gradient studied. To assess solvent drag as a measure of the paracellular transport of water, the effect of the osmotic gradient on mannitol and chloride transport were measured. There was no change in chloride or mannitol transport with the increased osmotic gradient in either group, indicating that there was no detectable paracellular water movement. In addition, the mannitol permeability of the neonatal PCT was found to be lower than that of the adult PCT with the isotonic bath (8.97 +/- 4.01 vs. 40.49 +/- 13.89 microm/s, P < 0.05). Thus the intracellular compartment of the neonatal PCT has a lower resistance for water transport than the adult PCT and is responsible for the higher than expected P(f) in the neonatal PCT.

Fig. 1

Osmotic gradient dependence of neonatal and adult proximal convoluted tubule (PCT) volume absorption. Tubules were perfused with a solution adjusted to 300 mosmol/kgH₂O. Bathing solution was identical except for 6 g/dl of albumin and raffinose to increase the osmolality. As can be seen, there was no difference in volume absorption between the adult and neonatal tubules.

Fig. 2

Osmotic gradient dependence of osmotic water permeability (P_f) in adult and neonatal PCT. Data from Fig. 1 were used to calculate the P_f. There was no significant difference between the P_f of the adult and neonatal tubules at any bath osmolality.

Fig. 3

Neonatal and adult PCT butanol permeability (P_DNB). Tubules were perfused with a solution containing ¹⁴C-labeled n-butanol and ³H₂O. P_DNB of the neonatal tubules was higher than that of the adult tubules at each temperature except for 20°C.

Fig. 4

Temperature dependence of neonatal and adult PCT diffusional water permeability (P_DW). There was no difference between the adult and neonatal tubule P_DW at any temperature.

Fig. 5

P_DW of the membrane [P_DW(membrane]. P_DW of the tubule membrane was calculated from the simultaneous measurement of P_DW and P_DNB. P_DW(membrane) of the neonatal tubule was lower than that of the adult at all temperatures studied. Slope of the Arrhenius plot is also greater for the neonatal tubules, indicating that the neonatal membranes have a higher activation energy for water transport than the adult.

Fig. 6

Osmotic gradient dependence of mannitol transport (J_mannitol). Tubules were perfused with a perfusion solution as in Fig. 1 containing [¹⁴C]mannitol. There was no change in J_mannitol as the bath osmolality was increased.

Fig. 7

Developmental changes in mannitol permeability (P_mannitol). Tubules were perfused with [¹⁴C]mannitol to measure the permeability of mannitol with an isosmotic bath. P_mannitol was significantly lower in the neonatal PCT compared with the adult PCT.

Fig. 8

Osmotic gradient dependence of chloride transport. Tubules were perfused as in Fig. 1, and chloride transport was measured. There was a small decrease in chloride transport in the neonatal tubules at the highest osmotic gradient examined.

Fig. 9

Model of proximal tubule water transport. Transcellular transport of water is depicted as the apical and basolateral membranes in series with the intracellular compartment. AQP1, aquaporin-1.