Developmental changes in rabbit proximal straight tubule paracellular permeability

Abstract

The early proximal tubule preferentially reabsorbs organic solutes and bicarbonate over chloride ions, resulting in a luminal fluid with a higher chloride concentration than that in blood. From this late proximal tubular fluid, one-half of NaCl reabsorption by the adult proximal tubule is active and transcellular and one-half is passive and paracellular. The purpose of the present in vitro microperfusion study was to determine the characteristics of passive chloride transport and permeability properties of the adult and neonatal proximal straight tubules (PST). In tubules perfused with a late proximal tubular fluid, net passive chloride flux was 131.7 +/- 37.7 pmol x mm(-1) x min(-1) in adult tubules and -17.1 +/- 23.3 pmol x mm(-1) x min(-1) in neonatal proximal tubules (P < 0.01). Chloride permeability was 10.94 +/- 5.21 x 10(-5) cm/s in adult proximal tubules and -1.26 +/- 1.84 x 10(-5) cm/s in neonatal proximal tubules (P < 0.05). Thus neonatal PST have a chloride permeability not different from zero and have no net passive chloride transport. Bicarbonate permeability is also less in neonates than adults in this segment (-0.07 +/- 0.03 x 10(-5) vs. 0.93 +/- 0.27 x 10(-5) cm/s, P < 0.01). Neonatal PST have higher sodium-to chloride and bicarbonate-to-chloride permeability ratios than adult PST. However, mannitol and sucrose permeabilities were not different in adult proximal tubules and neonatal PST. Transepithelial resistance was measured using current injection and cable analysis. The resistance was 6.7 +/- 0.7 Omega x cm(2) in adult tubules and 11.3 +/- 1.4 Omega x cm(2) in neonatal PST (P < 0.01). In conclusion, there are significant maturational changes in the characteristics of the PST paracellular pathway affecting transport in this nephron segment.

Fig. 1

Passive chloride transport in isolated perfused adult and neonatal proximal straight tubules (PST). PST were perfused in vitro at 20°C to inhibit active transport. Net chloride transport (J_Cl; top) was measured using the microtitration technique of Ramsay et al. (24). Inulin was used as a volume marker (n = 7 for both groups). P_Cl, chloride permeability.

Fig. 2

Mannitol permeability (P_mann) measured in neonatal and adult isolated perfused PST using luminal [¹⁴C]mannitol (n = 9 for both groups).

Fig. 3

Relative sodium-to-chloride permeability ratio (P_Na/P_Cl) measured at 37°C using dilution potentials in neonatal (n = 6) and adult (n = 5) isolated perfused PST.

Fig. 4

Relative bicarbonate-to-chloride permeability ratio (P_HCO3/P_Cl) measured at 37°C using dilution potentials in neonatal (n = 6) and adult (n = 5) isolated perfused PST.

Fig. 5

Neonatal and adult PST specific resistance (R_m) measured at 37°C using current injection and cable analysis (n = 8 neonatal PST and 7 adult PST).