Asymmetrical response of p38 kinase activation to volume changes in primary rat astrocytes

Activation of p38 kinase by osmotic stress has been documented in many cell s; however, no report has distinguished thee effects of cell volume on p38 activity from the effects of the altered osmotic condition per se. Here we report asymmetrical activation of astrocyte p38 mitogen-activated protein ( MAP) kinase in response to volume increases and volume decreases. We separa te effects of cell volume changes from the effects of osmotic exposure on p 38 activation. Exposure to 400, 500, or 600 mOsm phosphate-buffered saline (PBS) caused cell shrinkage and an osmolailty-dependent increase in p38 act ivity to 175%, 409%, or 518%, respectively, compared with cells maintained in control conditions (290 mOsm). Likewise, hyposmotic conditions ranging f rom 250 to 57 mOsm PBS caused the same activation of p38 (approximately 300 % of the control value within 10 min). The activity in hyposmotic condition s did not diminish over 30 min despite cell volume recovery, indicating a d ependence of extracellular osmolality or ionic strength rather than cell vo lume. Cells that were returned to isosmotic conditions following 30 min in 250, 150, or 57 mOsm PBS shrunk to 73%, 39%, or 26% of the control cell vol ume, respectively. In these cells, the activity of p38 increased further fr om approximately 300% of the control values in each hyposmotic condition to as much as 500% of the control activity as a function of the degree of cel l shrinkage. Thus, p38 may be activated by cell shrinkage in hyperosmotic o r in isoosmotic conditions, indicating reduced cell volume is a more import ant determinant of this enzyme activity than extracellular osmolality. Our results indicate distinct mechanisms of p38 activation in astrocytes expose d to hyperosmotic or hyposmotic PBS.