Membrane pathways for water and solutes in the toad bladder: II. Reflection coefficients of the water and solute channels

Summary

Urea and water transport across the toad bladder can be separately activated by low concentrations of vasopressin or 8 Br-cAMP. Employing this method of selective activation, we have determined the reflection coefficient (σ) of urea and other small molecules under circumstances in which the bladder was transporting urea or water. An osmotic method for the determination of σ was used, in which the ability of a given solute to retard water efflux from the bladder was compared to that of raffinose (σ=1.0) or water (σ=0). When urea transport was activated (low concentration of vasopressin), σ for urea and other solutes was low, (σ_urea,0.08–0.39;σ_acetamide, 0.55; σ_{ethylene glycol}, 0.60). When water transport was activated (0.1mm 8 Br-cAMP) σ_urea approached 1.0 σ_urea also approached 1.0 at high vasopressin concentrations. In a separate series of studies, σ_urea was determined in the presence of 2×10⁻⁵ m KMnO₄ in the luminal bathing medium. Under these conditions, when urea transport is selectively blocked, σ_urea rose from a value of 0.12 to 0.89. Thus, permanganate appears to “close” the urea transport channel. These findings indicate that the luminal membrane channels for water and solutes differ significantly in their dimensions. The solute channels, limited in number, have relatively large radii. They carry a small fraction (approximately 10%) of total water flow. The water transport channels, on the other hand, have small radii, approximately the size of a water molecule, and exclude solutes as small as urea.