Membrane pathways for water and solutes in the toad bladder: II. Reflection coefficients of the water and solute channels |
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Authors: | Christos P. Carvounis Sherman D. Levine Nicholas Franki Richard M. Hays |
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Affiliation: | (1) Division of Nephrology, Albert Einstein College of Medicine, 10461 Bronx, New York |
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Abstract: | 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 () of urea and other small molecules under circumstances in which the bladder was transporting urea or water. An osmotic method for the determination of was used, in which the ability of a given solute to retard water efflux from the bladder was compared to that of raffinose (=1.0) or water (=0). When urea transport was activated (low concentration of vasopressin), for urea and other solutes was low, (urea,0.08–0.39;acetamide, 0.55; ethylene glycol, 0.60). When water transport was activated (0.1mm 8 Br-cAMP) urea approached 1.0 urea also approached 1.0 at high vasopressin concentrations. In a separate series of studies, urea was determined in the presence of 2×10–5m KMnO4 in the luminal bathing medium. Under these conditions, when urea transport is selectively blocked, 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. |
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