Induction of heme oxygenase-1 with hemoglobin depresses vasoreactivity in rat aorta

Septic shock induced by lipopolysaccharide (LPS) produces systemic hypotens ion and decreased responsiveness to vasoconstrictors. Recently, intravenous injection of hemoglobin (HGB) into rats was found to be protective from a subsequent lethal dose of LPS and was correlated with induction of the enzy me heme oxygenase-1 (HO-1). To determine whether the HGB modulated the vaso motor tone of systemic arteries, we evaluated the effect of in vivo treatme nt with HGB and LPS on vasoconstrictor responses to phenylephrine (PE) in t he isolated rat aorta. Rats (n = 4, for each group) were injected intraveno usly with rat HGB (200 mg/kg i.v.) or normal saline control (CON) 16 h befo re sacrifice, and/or LPS (20 mg/kg) or CON 4 h before sacrifice. The descen ding aorta was dissected into rings and suspended in a modified Krebs solut ion where vasoconstrictor responses were determined to KCl (60 mM) and PE ( 10(-8) to 10(-5) M). LPS, but not HGB, inhibited the vasoconstrictor respon se to KCI. LPS, HGB, and HGB+LPS inhibited the maximal vasoconstrictor resp onse to PE (PEmax). Induction of HO-1 RNA in the aorta by HGB and by LPS wa s demonstrated by Northern blot analysis. To determine if induction of HO-1 was related to the effect of LPS or HGB on vascular reactivity, vessels we re treated with the HO-1 inhibitor, SnPP9 (30 mu M). PEmax in SnPP9+HGB ves sels was not different from control, whereas SnPP9+LPS vessels had a marked decrease in PEmax. We conclude that induction of HO-1 does not protect the rat aorta from the vasodepressor effects of LPS in vitro. Our results demo nstrate, however, that the induction of HO-1 causes vasodepression, possibl y via increased production of carbon monoxide.