Ionotropic glutamate receptors in cerebral microvascular endothelium are functionally linked to heme oxygenase

Abstract

Vasodilator effects of glutamate in the cerebral circulation are, in part, mediated by carbon monoxide (CO), which is formed from heme via the heme oxygenase (HO) pathway. The hypothesis addressed was that glutamate receptors (GluRs) in cerebral microvascular endothelium are functionally linked to HO. Using a radioligand binding and immunoblotting, GluRs were characterized in cerebral microvascular endothelial cells (CMVEC) from newborn pigs. High-affinity (80 nmol/L) reversible binding of [3H]glutamate ([3H]Glu) was detected in CMVEC membranes. The N-methyl-D-aspartate (NMDA) receptor ligands - NMDA, quinolinic acid, (±)1-aminocyclopentane-cis-1,3-dicarboxylic acid (cis-ACPD), AP5, 4C3HPG, and CPP - and the (RS)-α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptor ligands - AMPA, kainic acid, quisqualic acid, DNQX, and CNQX - displaced 20% to 30% of bound [3H]Glu in CMVEC membranes. Metabotropic GluRs antagonists (4CPG, PHCC, and CPPG) did not displace bound [3H]Glu. L-Aspartate, an agonist of GluRs and glutamate transporters, displaced 80% or more of bound [3H]Glu. Ionotropic (NR1 and GluR1) and metabotropic (mGluR1α) GluRs were detected in CMVEC by immunoblotting. Glutamate, aspartate, cis-ACPD, AMPA, (RS)-2-amino-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA), and kainate (10-5 mol/L) increased HO-directed CO formation by isolated cerebral microvessels and by cultured CMVEC. These data in newborn pigs suggest that CMVEC express ionotropic GluRs that are functionally linked to HO. GluR-mediated increases in CO formation by vascular endothelium may result in increase in cerebral blood flow.