trkA Mediates the Nerve Growth Factor-induced Intracellular Calcium Accumulation

Regulation of the cytosolic free Ca concentration by nerve growth factor was investigated in C6-2B glioma cells newly expressing the high affinity nerve growth factor receptor trk A, using Fura-2 fluorescence ratio imaging. In these cells, nerve growth factor (50 ng/ml) evoked a novel 3-fold increase in cytosolic free Ca concentration, while no measurable Ca response was observed in wild type or mock-transfected cells lacking a functional trk A receptor. K-252a, a tyrosine kinase inhibitor which prevents nerve growth factor-mediated responses in C6-2B cells expressing trk A, also blocked the rise in cytosolic free Ca concentration by nerve growth factor. Moreover, basic fibroblast growth factor, which in these cells elicits biochemical changes similar to nerve growth factor, failed to affect cytosolic free Ca concentration, further supporting the specificity of nerve growth factor/ trk A receptor in mediating a Ca response. While insensitive to chelation of extracellular Ca , the response was abolished following depletion of Ca stores or blockade of intracellular Ca release, providing strong evidence that intracellular Ca is the main source for nerve growth factor-evoked cytosolic free Ca concentration increase. Nerve growth factor increased the cytosolic free Ca concentration also in NIH3T3 cells overexpressing trk A but devoid of p75 nerve growth factor receptor. Our data suggest that trk A but not p75 is required for nerve growth factor-evoked Ca signaling.