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

Abstract

The fraction of isotonic fluid absorption occurring via a paracellular route was studied 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% O2. At 38.degree. C, net volume absorption (Jv) was 0.22 .+-. 0.01 nl .cntdot. min-1 .cntdot. mm-1, and the transepithelial voltage (Ve, lumen with respect to bath) was 0.9 .+-. 0.1 mV. Cooling to 23.degree. C or 0.1 mM ouabain reduced Jv to zero and increased Ve to 1.3-1.7 mV. The net chloride flux, measured titrametrically, was 26.7 .+-. 2.2 picoequivalents .cntdot. min-1 .cntdot. mm-1, 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 chloride diffusion and the observed values of Jv and Ve, the paracellular volume flow which would be required to produce the solvent drag component of the net chloride flux was computed. The transport parameters for solvent drag under this circumstance were the same as those when NaCl absorption occurred exclusively by passive means, and solvent drag provided the only nondiffusional mode for net NaCl absorption. A major component of proximal fluid absorption appears to occur via a paracellular pathway.