Changes in cell volume measured with an electrophysiologic technique.

Epithelial cells of the gallbladder of Necturus maculosus were loaded with tetramethylammonium (Me4N+) by transient exposure of the apical (lumen-facing) surface to a solution of high Me4N+ concentration containing also the polyene antibiotic nystatin. Upon removal of nystatin, in the continued presence of Me4N+, spontaneous restoration of the native ionic permeability of the apical cell membrane was observed. At this time, external Me4N+ was removed; intracellular [Me4N+] measured with ion-sensitive microelectrodes was 2-15 mM and remained unchanged for several hours. Changes in cell volume were estimated from the changes in intracellular [Me4N+] produced by alterations in the osmolality of the mucosal bathing solution. Assuming that such changes are caused entirely by water fluxes across the apical membrane, the minimum value of its hydraulic permeability coefficient (Lp) was 1-3 X 10(-3) cm.sec-1.(osmoles/kg)-1, suggesting that an osmolality difference across the apical membrane as small as 1-3 milliosmoles/kg could explain the average rate of transepithelial water transport. These results agree with optical measurements [Persson, B. O. & Spring, K. R. (1982) J. Gen. Physiol. 79, 481-505]. The effective thickness of the apical unstirred layer was estimated from the time courses of both the apical membrane voltage and the response of an extracellular K+-sensitive microelectrode to an increase in [K+] in the mucosal bath. Since changes in concentration of the osmotically active solute at the membrane surface were thus shown to be significantly delayed by diffusion, the Lp value, calculated assuming a step-change in osmolality, is an underestimate.