Effects of Anions on Cellular Volume and Transepithelial Na+ Transport Across Toad Urinary Bladder

The effects of complete substitution of gluconate for mucosal and/or serosal medium Cl- on transepithelial Na+ transport have been studied using toad urinary bladder. With mucosal gluconate, transepithelial potential difference (VT) decreased rapidly, transepithelial resistance (RT) increased, and calculated short-circuit current (Isc) decreased. Calculated ENa was unaffected, indicating that the inhibition of Na+ transport was a consequence of a decreased apical membrane Na+ conductance. This conclusion was supported by the finding that a higher amiloride concentration was required to inhibit the residual transport. With serosal gluconate VT decreased, RT increased and Isc fell to a new steady-state value following an initial and variable transient increase in transport. Epithelial cells were shrunken markedly as judged histologically. Calculated ENa fell substantially (from 130 to 68 mV on average). Ba2+ (3 mM) reduced calculated ENa in Cl- Ringer's but not in gluconate Ringer's. With replacement of serosal Cl- by acetate, transepithelial transport was stimulated, the decrease in cellular volume was prevented and ENa did not fall. Replacement of serosal isosmotic Cl- medium by a hypo-osmotic gluconate medium (one-half normal) also prevented cell shrinkage and did not result in inhibition of Na+ transport. Thus the inhibition of Na+ transport can be correlated with changes in cell volume rather than with the change in Cl-per se. Nystatin virtually abolished the resistance of the apical plasma membrane as judged by measurement of tissue capacitance. With K+ gluconate mucosa, Na+ gluconate serosa, calculated basolateral membrane resistance was much greater, estimated basolateral emf was much lower, and the Na+/K+ basolateral permeability ratio was much higher than with acetate media. It is concluded the decrease in cellular volume associated with substitution of serosal gluconate for Cl results in a loss of highly specific Ba2+-sensitive K+ conductance channels from the basolateral plasma membrane. It is possible that the number of Na+ pump sites in this membrane is also decreased.