Membrane Potentials, Resistances, and Conductances of Toad Bladder during Na+ - H+ Transport and H+ Transport

Abstract This study was undertaken to define the electrical characteristics imparted to toad bladder by H+ secretion in absence of Na+ transport, and to determine Na+ and H+ conductances of the apical bladder cell membrane. Cellular membrane potentials, conductances and resistances, and paracellular resistance were studied with microelectrode methods during Na+ - H+ transport and with H+ secretion after Na+ transport was stopped by withdrawal of mucosal Na+ and addition of mucosal amiloride. Epithelial potential reversed from 26 mV mucosa negative to 14 mV mucosa positive. Apical P.D. reversed from 13.7 mV cell positive to 16 mV cell negative to mucosa. Basal-lateral P.D. decreased to 3.8 mV, but cell remained negative to serosa. With Na+ and H+ transport and residual cationic and anionic conductances (probably K+ and Cl−), voltage divider ratio was 1.90, apical resistance was 2181 ohm·cm2, shunt resistance was 12,387, and basal-lateral resistance was 1162. After stopping Na+ transport, ΔΨa/ΔΨb increased to 3.54, R a to 3397, and R s to 23,963 ohm·cm2. R b was unchanged. A mucosa-serosa H+ gradient of 1000/1 raised R a to 4629 ohm·cm2 which suggested rectification of H+ current. With 100 mM mucosal KCl, ΔΨa/ΔΨb was 5.80 and R a was 6714 ohm·cm2. From these values, apical membrane conductances were calculated. Apical G Na+ of 168 μmhoss·cm2 was only slightly greater than the apical G H + of 151. Results are consistent with an electrogenic apical H+ pump and electrogenic basal-lateral Na+ pump.