Intracellular ionic activities and transmembrane electrochemical potential differences in gallbladder epithelium

Abstract

Intracellular ion activities in Necturus gallbladder epithelium were measured with liquid ion-exchanger microelectrodes. Mean values for K, Cl and Na activities were 87, 35 and 22 mM, respectively. The intracellular activities of both K and Cl are above their respective equilibrium values, whereas the Na activity is far below. This indicates that K and Cl are transported uphill toward the cell interior, whereas Na is extruded against its electrochemical gradient. The epithelium transports NaCl from mucosa to serosa. From the data presented and the known Na and Cl conductances of the cell membranes, we conclude that neutral transport driven by the Na electrochemical potential difference can account for NaCl entry at the apical membrane. At the basolateral membrane, Na is actively transported. Because of the low Cl conductance of the membrane, only a small fraction of Cl transport can be explained by diffusion. These data suggest that Cl transport across the basolateral membrane is a coupled process which involves a neutral NaCl pump, downhill KCl transport, or a Cl-anion exchange system.

Mechanism of the effect of cyanide on cell membrane potentials in Necturus gall‐bladder epithelium.

“…In this series, the cell membrane potentials changed from about 67 to about 92 mV, a value close to the potassium equilibrium potential of about 96 mV (Reuss & Weinman, 1979). When ouabain-treated tissues were exposed to cyanide (data not shown) the cells hyperpolarized, but the maximal cell membrane potentials were reduced, as expected from the ouabain-dependent decrease of intracellular potassium activity (Reuss, Weinman & Grady, 1980 hyperpolarization (ca.…”

“…However, if additional effects are present they must be proportionally smaller than the effect on potassium permeability. Increases of sodium or chloride permeability would result in cell depolarization, because intracellular sodium activity is 20 mm or less (Reuss & Weinman, 1979;Graf & Giebisch, 1979) and intracellular chloride activity is 30-35 mmi (Reuss & Weinman, 1979;Reuss & Grady, 1979b), i.e. the intracellular chloride activity is higher than predicted from equilibrium distribution.…”

Active sodium transport and fluid secretion in the gall‐bladder epithelium of Necturus.

“…This is likely to be correct in several tissues (see Walker & Brown, 1977), but no direct information is available for Necturus gall-bladder epithelium. If the Na concentration of 29 mm reported by is compared with the Na activity of 22-2 mm measured by the same group (Reuss & Weinman, 1979) Hill, personal communication) again a minimum cytoplasmic activity coefficient for Na close to that in free solution is obtained. Thus in the absence of other evidence it seems safe to assume that the activity coefficient for Na is the same in the cytoplasm as in the outer solutions.…”

“…The average activities displayed in Table 1 also confirm previous observations with ion-sensitive micro-electrodes. K and Cl were concentrated inside the cells, whereas Na was at a much lower concentration than predicted from a passive distribution (Zeuthen, 1978(Zeuthen, , 1982Reuss & Weinman, 1979;Graf & Giebisch, 1979;Garcia-Diaz & Armstrong, 1980 (Graf & Giebisch, 1979); (2) the final value of aca recorded in low Na was independent of the initial steady-state activity; (3) aca recovered its initial steady-state value after return to normal Ringer. It appears therefore that virtually all the signal of the micro-electrode was related to Na.…”

Bicarbonate effects, electromotive forces and potassium effluxes in rabbit and guinea‐pig gall‐bladder.

“…At the apical membrane it is well documented that both conductive and neutral Na+ pathways exist (Frizzell, Dugas & Schultz, 1975; Henin & Cremaschi, 1975;Cremaschi & Henin, 1975a;Duffey et al 1978;Cremaschi & Meyer, 1982), even though for Necturus gall-bladder Reuss & Finn (1975 b), Reuss & Weinman (1979) and Graf & Giebisch (1979) calculated that the fraction of the transepithelial Na+ flux which could cross the apical barrier through the conductive pathway is only 6-15 00. We have previously demonstrated that Na+ conductance is not affected by HCO3-in the rabbit (Cremaschi & Meyer, 1982) and we can calculate from the data reported here that the net Na+ flux through the apical conductive route is also not modified, in spite of the difference of ENa and Vm in the absence and presence of the anion.…”