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Departments of 1 Pediatrics and 2 Internal Medicine, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75235-9063
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 · mm
1 · min
1
in adult tubules and
17.1 ± 23.3 pmol · mm
1 · min
1 in
neonatal proximal tubules (P < 0.01). Chloride
permeability was 10.94 ± 5.21 × 10
5 cm/s in
adult proximal tubules and
1.26 ± 1.84 × 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 × 10
5 vs. 0.93 ± 0.27 × 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
· cm2 in adult tubules and 11.3 ± 1.4
· cm2 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.
chloride transport; renal development; passive transport; tight junction; chloride permeability; bicarbonate permeability
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