Control of Na+ and H+ transports by exocytosis/endocytosis phenomena in a tight epithelium

Abstract

The relationship linking Na+ and H+ transports and exocytosis/endocytosis located in the apical membranes of the frog skin epithelium was investigated under various conditions of ion transport stimulation. The exocytosis process, indicating insertion of intracellular vesicles, which were preloaded with fluorescent FITC-dextran (FD), was measured by following the FD efflux in the apical bathing solution. Na+ transport stimulators such as serosal hypotonic shock (replacement of serosal Ringer solution by half-Ringer or 4/5-Ringer), apical PCMPS (10(-3) M) and amphotericin-B (20 micrograms/ml), were also found to stimulate the exocytotic rates of FD. Acidification of the epithelium by CO2 or post NH4 load, conditions which increase the proton secretion also stimulated the FD release in the apical bathing solution. On the other hand, alkalization of the epithelial cells increased the endocytosis rate. Hypotonic shock, acid load and PCMPS induced an increase in cell calcium which is probably the signal within the cell for exocytosis. In addition, quantitative spectrofluorimetric measurements of F-actin content after rhodamine-phalloidin staining, indicated a decrease in the F-actin content as a result of cell acidosis, hypotonic conditions and amphotericin additions. It is proposed that the insertion/retrieval of intracytoplasmic vesicles containing H+ pumps plays a key role in the regulation of proton secretion in tight epithelia. In addition, it is suggested that cytoskeleton depolymerization of F-actin filaments facilitates H+ pump insertion. A comparable working hypothesis for the control of Na+ transport is proposed.

Changes in gill H+-ATPase and Na+/K+-ATPase expression and activity during freshwater acclimation of Atlantic salmon (Salmo salar)

“…We also found that H addition, vesicular trafficking of endosomal H + -ATPase stores to the plasma membrane has been described in several epithelia including frog skin (Lacoste et al, 1993), turtle urinary bladder (Cannon et al, 1985) and kidney collecting tubule (Schwartz and Al Awqati, 1985). However, as such post-translational modifications and mobilization of intracellular stores to the plasma membrane surface normally occur over a much shorter time scale (hours), they do not explain the long lag time before a significant increase in activity is seen following freshwater exposure.…”

Apical structures of “mitochondria‐rich” α and β cells in euryhaline fish gill: Their behaviour in various living conditions

“…They are seemingly not involved in endocytosis of cationic ferritin which is always found in small vesicles, the size and shape of which are characteristically difthe fishes to deionized water induces, in contrast, a significant increase of their total surface area which is concomitant with a n increase of the apical surface area of the a cells. It is thus suggested that in these cells the apical structures would participate in the increase of the surface area of the apical plasma membrane thought to contain the H+-ATPase (Brown et al, 1987;Lin and Randall, 1991;Lacoste et al, 1993) which in deionized water would help in the increased retrieval of Na+ from the surrounding medium.…”

Vacuolar and Plasma Membrane Proton-Adenosinetriphosphatases

“…The considerable inhibition of proton secretion by the specific V-ATPase inhibitor bafilomycin and the immunostaining of mitochondria-rich cells with antibodies to the kidney V-ATPase provide evidence for the involvement of V-ATPases (51,106). The intracellular pH, which stimulates the insertion or retrieval of V-ATPase-containing cytoplasmic vesicles, depending on the frog's acid-base status, regulates proton secretion (109). Apically localized V-ATPase pumps the proton outward, hyperpolarizing the cell membrane facing the pond.…”