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

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.