SHRINKAGE-INDUCED ACTIVATION OF NA+ H+ EXCHANGE - ROLE OF CELL-DENSITY AND MYOSIN LIGHT-CHAIN PHOSPHORYLATION/

Previously, we suggested that myosin light chain kinase (MLCK) is invo lved in shrinkage-induced activation of the Na+/H+ exchanger in rat as trocytes. Here we have studied the effects of hyperosmotic exposure in C-6 glioma cells, a common model for astrocytes. Shrinkage-induced ac tivation of the Na+/H+ exchanger in C-6 cells is directly proportional to the degree of shrinkage, results in an alkaline shift in the pK' o f the exchanger, is dependent on ATP, and is inhibited by ML-7 (an MLC K inhibitor) and by various calmodulin inhibitors. Cell shrinkage also results in increased phosphorylation of myosin light chain (MLC). Int erestingly, shrinkage-induced activation of the exchanger does not occ ur in subconfluent C-6 cells. However, phosphorylation of MLC still oc curs in subconfluent cultures of C-6 cells on shrinkage, suggesting th at the lack of activation in these cells occurs at a point between MLC phosphorylation and Na+/H+ exchange activation. The lack of activatio n of Na+/H+ exchange in subconfluent C-6 cells can be utilized to furt her elucidate the shrinkage-induced activation pathway.