Solvent drag component of Cl- flux in superficial proximal straight tubules: evidence for a paracellular component of isotonic fluid absorption.

ANDREOLI, THOMAS E., JAMES A. SCHAFER, SUSAN L. TROUTMAN, AND MARY L. WATKINS. Solvent drag component of Clflux in superficial proximal straight tubules: evidence for a paracellular component of isotonic fluid absorption. Am. J. Physiol. 237(6): F455-F462, 1979 or Am. J. Physiol.: Renal Fluid Electrolyte Physiol. 6(6): F455-F462, 1979.-We have computed the fraction of isotonic fluid absorption occurring via a paracellular route by estimating the solvent drag contribution to net chloride flux. Isolated rabbit superficial proximal straight tubules were perfused and bathed with simplified NaCl solutions; neither solution contained small organic anions or bicarbonate, and both were gassed with 100% 02. At 38”C, net volume absorption (J,) was 0.22 t 0.01 nlmin-‘*mm-‘, and the transepithelial voltage (V,, lumen with respect to bath) was 0.9 t 0.1 mV. Cooling to 23OC or 0.1 mM ouabain reduced J, to zero and increased V, to 1.3-1.7 mV. The net chloride flux, measured titrametrically, was 26.7 t 2.2 peq amin-’ *mm-‘, or approximately 87% of the Na’ flux in an isotonic absorbate; and when Na’ absorption was inhibited with ouabain, the measured net Jcl was indistinguishable from zero. Using the measured electrochemical potential gradient for Cldiffusion and the observed values of J, and V,, we computed the paracellular volume flow which would be required to produce the solvent drag component of the net chloride flux. The transport parameters for solvent drag under this circumstance were the same as those when salt absorption occurred exclusively by passive means, and solvent drag provided the only nondiffusional mode for net salt absorption. We conclude, therefore, that a major component of proximal fluid absorption occurs via a paracellular pathway.