Multiple Actions of Phencyclidine and (+)MK-801 on Isolated Bovine Cerebral Arteries
This study examines the direct effects of 3 noncompetitive N-methyl-D-aspartate receptor antagonists, phencyclidine (PCP), (+)MK-801, and (−)MK-801, on bovine middle cerebral arteries (BMCA). Rings of BMCA were mounted in isolated tissue chambers equipped with isometric tension transducers to obtain pharmacologic dose-response curves. In the absence of endogenous vasoconstrictors, the 3 N-methyl-D-aspartate antagonists each produced direct constriction of BMCA. The thromboxane A2 receptor antagonist SQ-29,548, the TxA2 synthase inhibitor furegrelate, the calcium antagonist nimodipine, and calcium-deficient media all inhibited maximal phencyclidine or (+)MK-801-induced constriction. Direct constriction by PCP or (+)MK-801 was independent of the presence of endothelium. When BMCA were preconstricted with potassium-depolarizing solution, PCP, (+)MK-801, and (−)MK-801 each produced only concentration-dependent relaxation. When BMCA were preconstricted with the stable TxA2 analog U-46,619 and exposed to increasing concentrations of PCP, (+)MK-801, or (−)MK-801, tension increased. Thromboxane A2 may contract BMCA by acting as a potassium channel blocker; iberiotoxin and tetraethylammonium both constrict BMCA. In Ca2+-deficient media containing either potassium or U-46,619, phencyclidine and (+)MK-801 each produced competitive inhibition of subsequent Ca2+-induced constriction. In additional experiments, arterial strips were mounted in isolated tissue chambers to directly measure calcium uptake, using 45Calcium as a radioactive tracer. Both phencyclidine and (+)MK-801 blocked potassium-stimulated or U-46,619-stimulated 45Ca uptake into arterial strips. These results suggest that phencyclidine and (+)MK-801 have 2 separate actions on BMCA. They may constrict arterial rings by releasing TxA2 from cerebrovascular smooth muscle, and relax arterial rings by acting as calcium antagonists.