Intracellular Free Calcium Dynamics in Stretch-Injured Astrocytes

Abstract: We have previously developed an in vitro model for traumatic brain injury that simulates a major component of in vivo trauma, that being tissue strain or stretch. We have validated our model by demonstrating that it produces many of the posttraumatic responses observed in vivo. Sustained elevation of the intracellular free calcium concentration ([Ca²⁺]_i) has been hypothesized to be a primary biochemical mechanism inducing cell dysfunction after trauma. In the present report, we have examined this hypothesis in astrocytes using our in vitro injury model and fura-2 microphotometry. Our results indicate that astrocyte [Ca²⁺]_i is rapidly elevated after stretch injury, the magnitude of which is proportional to the degree of injury. However, the injury-induced [Ca²⁺]_i elevation is not sustained and returns to near-basal levels by 15 min postinjury and to basal levels between 3 and 24 h after injury. Although basal [Ca²⁺]_i returns to normal after injury, we have identified persistent injury-induced alterations in calcium-mediated signal transduction pathways. We report here, for the first time, that traumatic stretch injury causes release of calcium from inositol trisphosphate-sensitive intracellular calcium stores and may uncouple the stores from participation in metabotropic glutamate receptor-mediated signal transduction events. We found that for a prolonged period after trauma astrocytes no longer respond to thapsigargin, glutamate, or the inositol trisphosphate-linked metabotropic glutamate receptor agonist trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid with an elevation in [Ca²⁺]_i. We hypothesize that changes in calcium-mediated signaling pathways, rather than an absolute elevation in [Ca²⁺]_i, is responsible for some of the pathological consequences of traumatic brain injury.