CONTROL OF NA+ AND H+ TRANSPORTS BY EXOCYTOSIS ENDOCYTOSIS PHENOMENA IN A TIGHT EPITHELIUM

The relationship linking Na+ and H+ transports and exocytosis/endocyto sis located in the apical membranes of the frog skin epithelium was in vestigated under various conditions of ion transport stimulation. The exocytosis process, indicating insertion of intracellular vesicles, wh ich 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 R inger solution by half-Ringer or 4/5-Ringer), apical PCMPS (10(-3) M) and amphotericin-B (20 mug/ml), were also found to stimulate the exocy totic rates of FD. Acidification of the epithelium by CO2 or post NH4 load, conditions which increase the proton secretion also stimulated t he FD release in the apical bathing solution. On the other hand, alkal ization of the epithelial cells increased the endocytosis rate. Hypoto nic shock, acid load and PCMPS induced an increase in cell calcium whi ch is probably the signal within the cell for exocytosis. In addition, quantitative spectrofluorimetric measurements of F-actin content afte r rhodamine-phalloidin staining, indicated a decrease in the F-actin c ontent as a result of cell acidosis, hypotonic conditions and amphoter icin additions. It is proposed that the insertion/retrieval of intracy toplasmic vesicles containing H+ pumps plays a key role in the regulat ion of proton secretion in tight epithelia. In addition, it is suggest ed that cytoskeleton depolymerization of F-actin filaments facilitates H+ pump insertion. A comparable working hypothesis for the control of Na+ transport is proposed.