Ca2+ signaling in glutamate‐stimulated CO production by newborn pig astrocytes

Abstract

CO, a product of heme degradation by heme oxygenase (HO), is a vasodilator and a mediator of glutamate‐induced dilation of newborn pig cerebral arterioles. Glutamate rapidly increases CO production by piglet astrocytes via posttranslational activation of constitutive HO‐2. We tested the hypothesis that glutamate‐induced CO production by astrocytes is mediated by intracellular Ca2+ signaling. CO production by freshly isolated and cultured astrocytes from newborn pig brain cortex was detected by gas chromatography/mass spectrometry. Alteration of extracellular Ca2+ did not change astrocyte CO production. Depletion of intracellular Ca2+ with ionomycin in a Ca2+‐free buffer greatly reduced HO activity. Increasing intracellular Ca2+ from 0 to 1 μM in the presence of ionomycin dose‐dependently elevated CO production up to 4‐fold. Glutamate (10−4M) stimulated HO activity in control, but not in Ca2+‐depleted astrocytes. Furthermore, when intracellular Ca2+ was clamped constant (ionomycin and 0–1μM Ca2+), glutamate failed to increase CO production at any level of intracellular Ca2+. Thapsigargin reduced basal CO production and blocked glutamate‐induced CO production, suggesting importance of intracellular Ca2+ stores. These data suggest glutamate‐induced CO production by astrocytes is mediated by Ca2+ signaling involving intracellular Ca2+ stores. Research support by NHLBI/NIH.