Carbon dioxide influence on nitric oxide production in endothelial cells and astrocytes: Cellular mechanisms

Cerebral vessels may regulate cerebral blood flow by responding to changes in carbon dioxide (CO2) through nitric oxide (NO) production. To better determine the role of NO production by human adult cerebral microvascular endothelial cells and human fetal astrocytes under different CO2 conditions, we studied endothelial cell and astrocyte production of NO under hypo-, normo- and hypercapnic conditions. Human cerebral endothelial cell and fetal astrocyte cultures were exposed to hypocapnic (pCO2 21.7 ± 6.7 mmHg), normocapnic (pCO2 40.1 ± 0.9 mmHg) and hypercapnic (pCO2 56.3 ± 8.7 mmHg) conditions. NO production was recorded continuously over 24 hours with stable pH. N-nitro-l-arginine [NLA; a nitric oxide synthase (NOS) inhibitor] and l-arginine (substrate for NO production via NOS) were used to further define the role of NOS in chemoregulation. NO levels in endothelial cells increased during hypercapnia by 36% in 8 hours and remained 25% above baseline. NO increase in astrocytes was 30% after 1 hour but returned to baseline at 8 hours. NLA blocked NO increase in endothelial cells under hypercapnia. During hypocapnia, NO levels in the endothelial cells decreased by 30% at 8 hours but were unchanged in astrocytes. l-arginine prevented NO decrease in endothelial cells under hypocapnia. NO changes in the endothelial cells correlated with changes in pCO2 (R = 0.99) and were independent of pH. This study suggests that cerebral endothelial cells and astrocytes release NO under normocapnic conditions and NO production is increased during hypercapnia and decreased during hypocapnia independent of pH. Further, this demonstrates that endothelial cells may play a pivotal role in chemoregulation by modulating NOS activity.

Research highlights
► Cerebral vessels may regulate CBF by responding to changes in CO2 via NO production.
► Cerebral endothelial cells and astrocytes release NO under normocapnic conditions.
► NO production is increased during hypercapnia and decreased during hypocapnia.
► This NO production occurs independent of pH.
► Endothelial cells may play a role in chemoregulation by modulating NOS activity.