Beta-adrenergic activation of solute coupled water uptake by toad skin epithelium results in near-isosmotic transport

Transepithelial potential (VT), conductance (GT), and water flow (JV) were measured simultaneously with good time resolution (min) in isolated toad (Bufo bufo) skin epithelium with Ringer on both sides. Inside application of 5 μM isoproterenol resulted in the fast increase in GT from 1.2 ± 0.3 to 2.4 ± 0.4 mS·cm− 2 and slower increases in equivalent short circuit current, ISCEqv = − GT × VT, from 12.7 ± 3.2 to 33.1 ± 6.8 μA cm− 2, and JV from 0.72 ± 0.17 to 3.01 ± 0.49 nL cm− 2 s− 1. Amiloride in the outside solution abolished ISCEqv ( −  1.6 ± 0.1 μA cm− 2) while JV decreased to 0.50 ± 0.15 nL cm− 2·s− 1, which is significantly different from zero. Isoproterenol decreased the osmotic concentration of the transported fluid, Cosm ≈ 2 × ISCEqv/JV, from 351 ± 72 to 227 ± 28 mOsm (Ringer's solution: 252.8 mOsm). JV depicted a saturating function of [Na+]out in agreement with Na+ self-inhibition of ENaC. Ouabain on the inside decreased ISCEqv from 60 ± 10 to 6.1 ± 1.7 μA cm− 2, and JV from 3.34 ± 0.47 to 1.40 ± 0.24 nL cm− 2·s- 1. Short-circuited preparations exhibited a linear relationship between short-circuit current and JV with a [Na+] of the transported fluid of 130 ± 24 mM ([Na+]Ringer's solution = 117.4 mM). Addition of bumetanide to the inside solution reduced JV. Water was transported uphill and JV reversed at an excess outside osmotic concentration, ΔCS,rev = 28.9 ± 3.9 mOsm, amiloride decreased ΔCS,rev to 7.5 ± 1.5 mOsm. It is concluded that water uptake is accomplished by osmotic coupling in the lateral intercellular space (lis), and hypothesized that a small fraction of the Na+ flux pumped into lis is recirculated via basolateral NKCC transporters.