Swelling-activated KCl cotransport in rabbit red cells: flux is determined mainly by cell volume rather than shape.

The effect of cell shape on ouabain-insensitive 86Rb+ fluxes was examined in rabbit red blood cells. The purpose of the study was to assess the role of mechanical deformations of the membrane in the activation of KCl cotransport by cell swelling. Conversion of cells to echinocytes with low concentrations of amphiphilic agents (anionic and cationic detergents and dipyridamole) in an isotonic medium activates KCl cotransport only very slightly. Hypotonic swelling of echinocytes causes a large increase in KCl cotransport flux just as in swollen discocytes; both the rate and the extent of activation are unaffected by the shape change. Stomatocyte (cup cell) formation with 20 microM chlorpromazine in isotonic medium causes slight activation of KCl cotransport. The KCl cotransport flux induced by cell swelling is approximately 20% higher in swollen stomatocytes than in swollen discocytes. It is concluded that major changes in cell shape have only minor effects on the swelling sensor, signal transduction apparatus, and KCl cotransport protein. We interpret these findings as evidence against the idea that the cell detects its volume by way of a membrane-associated mechanical sensor. As an alternative to a mechanical volume sensor, a hypothetical mechanism for swelling activation of transport is presented in which dilution of the cytoplasm, by mass action alone, can have very large effects on KCl cotransport.