Reduction of the NO-mediated response in the rat aorta by metalloporphyrins

Metalloporphyrins (MPs) have been found to affect the production of carbon monoxide (CO) and nitric oxide (NO). Unlike that for CO, little is known ab out the mechanism of action of MPs on the NO system. We determined the in v itro ability of ferrous protoporphyrin (heme, FePP), zinc protoporphyrin (Z nPP), and bilirubin (BR) to scavenge NO. Heme and ZnPP were studied in the rat aortic ring system for their ability to affect phenylephrine-induced co ntraction and methacholine-stimulated relaxation. Heme was found to be a go od NO scavenger with a k(s) = 0.53 +/- 0.19 x 10(4) M-1.s(-1) (n = 6). ZnPP and BR did not scavenge NO. Neither heme nor ZnPP treatment affected the p henylephrine response as measured by -logEC(50) and the maximal effect. How ever, heme and ZnPP treatments decreased the -logEC(50) and the maximal eff ects of methacholine, therefore decreasing vasorelaxation. We conclude that when ZnPP is administered in vivo blood pressure should be carefully monit ored.