Effect of amiloride on sodium and water reabsorption in the rabbit gall‐bladder.

Abstract

SUMMARY1. The effects of the Na+-channel-blocking diuretic agent amiloride were assessed in the rabbit gall-bladder epithelium, a low-resistance epithelium with an isosmotic, coupled NaCl transport mechanism.2. Amiloride caused a rapid, reversible, and dose-dependent decrease in fluid absorption when applied from the mucosal side in concentrations between 8-8 x 10-5 and 1P76 x 10-3 M. These concentrations were without effect from the serosal side, suggesting an action of amiloride in the luminal cell membrane as in high-resistance epithelia.3. Amiloride did not affect the epithelial resistance or the passive serosa-to-mucosa Na+ flux, while net Na+ and water reabsorption were inhibited in parallel. Thus, amiloride did not affect the paracellular tight junction pathway, but inhibited a transcellular, coupled salt and water transport mechanism.4. The kinetics of the amiloride effect were of a Michaelis-Menten type. The dose of amiloride giving 50 % inhibition of fluid absorption (ID50) was 4 x 10-4 M, a value about three orders of magnitude higher than in high-resistance, Na+-retaining epithelia. 5. The percentage inhibitory effect at each concentration of amiloride increased with increasing rate of spontaneous (control) fluid transport, reaching maximal responses fitting a Michaelis-Menten kinetic with an ID50 of 1-5 x 10-4 M.6. No effects of changing the extracellular Na+ concentration between 51 and 145 mequiv/l on the maximal inhibitory effect of amiloride on Na+ and water reabsorption were observed. This suggests a non-competitive type of action of amiloride on a Na+-dependent isosmotic fluid transport mechanism.7. Removal of mucosal Ca2+ did not alter the effect of amiloride. 8. The implications ofthese findings are discussed in relation to concepts concerning the mechanism of isosmotic salt and water transport. The data are compatible with the concept that amiloride interferes with a Na+-dependent formation and transcellular transport of isosmotic fluid volumes in a sequestered compartment in the epithelial cells.

Independence of apical membrane Na+ and Cl- entry in Necturus gallbladder epithelium.

“…This time course is very different from the rapid action of amiloride on membrane potentials in epithelia such as frog skin (Helman and Fisher, 1977 ;Nagel, 1980). In addition, according to the Jv measurements of Frederiksen (1983), at this concentration amiloride would not inhibit fluid transport. Finally, if a reduction of apical membrane PNa took place in Necturus gallbladder after a 2-min delay, it is obvious that such effect could not be responsible for the immediate fall in aNa; (Fig .…”

Effects of amiloride on the endolymphatic sac

“…The ES has been postulated to possess an active transcellular fluid transport in its intermediate portion (Rask-Andersenefa/., 1981, Bagger-Sjoback et al, 1981, Morgensterne/a/., 1982Takumida ef al., 1988). Transcellular transport mechanisms have earlier been thoroughly investigated in several fluid transporting epithelia (Tormey and Diamond, 1967;Frizzel et al, 1979;Frederiksen, 1983). The proposed action is that an initial uptake of ions occurs across the apical cell barrier into the cellular transport compartment in isosmotic transporting epithelia.…”

“…This uptake is governed by an apparently neutral carrier-mediated Na + CI uptake mechanism driven by the electrochemical gradient for Na + . The water shift is a passive consequence of the Na + (orNa + CP) transport (Frizzel etai, 1979;Frederiksen, 1983). Water and ions are transported from the apical to the lateral plasma membrane via an unknown route, possibly via pinocytotic vesicles or TER (Frederiksen, 1983).…”

“…The endolymphatic duct (ED) and sac (ES) have been considered to absorb endolymph (Kimura and Schuknecht, 1965;Lundquist, 1965;Rask-Andersen et al, 1981). It has been suggested that the active mechanisms of this absorption of endolymph resides within the epithelial lining of the ED and ES (Rask-Andersen et al, 1981, Bagger-Sjoback et al, 1981Takumida et al, 1988), Recent morphological and physiological investigations in fluid transporting epithelia such as the renal tubule, liver and gall bladder have revealed several possible mechanisms responsible for the fluid transport (Tormey and Diamond, 1967;Diamond, 1979;Frederiksen, 1983). Concerning the way compounds are removed from a lumen lined by epithelium, two main possible routes prevail the paracellular and the transcellular route (Diamond, 1979).…”

“…The aim of this study is to shed some light upon the transcellular pathways in the ES epithelium using amiloride. Amiloride is a diuretic which is known to specially block the ion and water exchange in the apical plasma membrane without inducing changes in the paracellular fluid transport through the tight junctions (Frederiksen, 1983).…”