RESUMO
This pharmacologic study examines the direct cerebrovascular effects of N-methyl-D-aspartate (NMDA) receptor agonists and antagonists to determine whether large cerebral arteries have NMDA receptors. Bovine middle cerebral arteries were cut into rings to measure isometric tension development in vitro. Two competitive agonists, L-glutamate and NMDA, each had negligible effects on ring tension in the absence of exogenous vasoconstrictors. L-glutamate (in high concentrations) produced direct relaxation of potassium (K+)-constricted arteries, but the relaxation was not selective for L-glutamate, D-glutamate, or mannitol. Relaxation with L-glutamate was abolished when it was isosmotically substituted in the K(+)-rich medium. NMDA (in the absence or presence of glycine) and two competitive antagonists, 2-amino-5-phosphopentanoic acid (AP5) and (+/-)-3-(s-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), each had little effect on the tone of arteries preconstricted with potassium or the stable thromboxane A2 analog U-46,619. Three noncompetitive antagonists (S(+)-ketamine, dizocilpine, and dextrorphan) and their steroeisomers (R(-)-ketamine, (-)MK-801, and levorphanol) each produced dose-dependent relaxation of K(+)- or U-46,619-constricted arteries; relaxation was not selective for the (+) or (-) stereoisomers. These results suggest that large cerebral arteries lack NMDA receptors mediating constriction or relaxation. All noncompetitive antagonists dilated cerebral arteries, but by mechanisms that were not stereospecific.