Crosslinked hemoglobin inhibits endothelium-dependent relaxations in isolated canine arteries

Several previous in vivo studies demonstrated that crosslinked hemoglobin is a potent vasoconstrictor capable of significantly increasing arterial blood pressure following systemic administration. The precise mechanisms underlying the vascular effects of crosslinked hemoglobin are not clear. The present study was designed to determine the effect of crosslinked hemoglobin on the endothelial L-arginine-nitric oxide biosynthesis pathway in isolated canine arteries. Isolated femoral and renal arteries were suspended in organ chambers for isometric tension recordings. Endothelium-dependent relaxations to acetylcholine and calcium ionophore A23187 were studied in the absence or in the presence of crosslinked hemoglobin or hemoglobin. A radioimmunoassay technique was used to determine levels of guanosine 3',5'-cyclic monophosphate (cyclic GMP) and adenosine 3',5'-cyclic monophosphate (cyclic AMP). A nitric oxide synthase inhibitor L-NAME (10^-4M) selectively inhibited endothelium-dependent relaxations to acetylcholine and calcium ionophore A23187. The inhibitory effect of L-NAME was reversed by L-arginine (3x10^-4M). Crosslinked hemoglobin (10^-7, 10^-6 and 10^-5M) inhibited endothelium-dependent relaxations to acetylcholine (10^-9-10^-5M) or A23187 (10^-9-10^-6M). In the same concentration range, purified bovine hemoglobin exerted a similar inhibitory effect on relaxations mediated by activation of endothelial cells. Crosslinked hemoglobin (10^-6M) significantly reduced basal production of cyclic GMP, but did not affect production of cyclic AMP. Acetylcholine (10^-6M) stimulated production of cyclic GMP. This effect of acetylcholine was abolished in the presence of crosslinked hemoglobin. These studies demonstrate that crosslinked hemoglobin impairs endothelium-dependent relaxations in isolated large conduit arteries. This effect appears to be mediated by the chemical antagonism of crosslinked hemoglobin against nitric oxide released from the endothelium. Inhibition of the endothelial L-arginine-nitric oxide biosynthesis pathway, with subsequent decrease of cyclic GMP in smooth muscle, may help to explain vasoconstrictor and pressor effects of crosslinked hemoglobin.