VESICULAR PH IS SENSITIVE TO CHANGES IN CELL-VOLUME

As shown previously, osmotic swelling of proximal renal tubules, vascu lar smooth muscle cells and of hepatocytes leads to an increase of the pH in acidic cellular compartments. This vesicular alkalinization is reflected by an increase of acridine orange fluorescence intensity and a corresponding alteration of the FITC-dextran fluorescence ratio. Th e present study has been performed to test whether this phenomenon can be observed in other cell types. Osmotic cell swelling (-30 mOsm) inc reased acridine orange fluorescence intensity in cardiac myocytes as d emonstrated using fluorescence imaging. Microspectrophotometry was use d to demonstrate a similar increase in acridine orange fluorescence in tensity following treatment with hypotonic solution in MDCK cells, mac rophages, brown fat cells, dendritic cells, pancreatic beta-cells, alv eolar cells and normal and ras oncogene expressing fibroblasts. Swelli ng of MDCK and L2 cells, induced by a block of K+ channels with Ba2+ o r the elevation of extracellular K+, also led to vesicular alkalinizat ion. The uptake of various amino acids in MDCK cells, resulting in cel l swelling, similarly increased the acridine orange fluorescence inten sity. Further, the acridine orange results were confirmed and quantifi ed with FITC-dextran in brown fat cells, where treatment with hypotoni c solution increased the vesicular pH by as much as 0.53 +/- 0.04 (n = 4). It is concluded that osmotic swelling of a wide variety of cells increases the pH of acidic cellular compartments, which in turn is exp ected to modify the respective activity of lysosomal proteases and tra fficking of vesicles and receptors.