Upregulation of heme oxygenase 1 by hemin impairs endothelium-dependent contractions in the aorta of the spontaneously hypertensive rat.

Heme oxygenase converts heme to carbon monoxide, biliverdin (subsequently converted to bilirubin), and free iron. Pharmacological induction of heme oxygenase 1 has an antihypertensive effect in the spontaneously hypertensive rat. The present study investigated whether upregulation of heme oxygenase 1 by hemin reduces endothelial dysfunction in this animal. Thirty-six-week-old rats were divided into a hemin treatment (50 mg/kg, IP injection, once) and a control group. Aortas were isolated for the measurement of isometric tension, production of reactive oxygen species, and heme oxygenase activity, as well as gene and protein expressions. Hemin treatment augmented the expression and activity of heme oxygenase 1. This in vivo induction of heme oxygenase 1, but not in vitro incubation with the heme oxygenase products carbon monoxide or bilirubin, led to an improvement of endothelial function in that acetylcholine-induced relaxations were potentiated and acetylcholine- and calcium ionophore-induced contractions were attenuated. Free radical production was suppressed by hemin treatment, judging from the results of 2',7'-dichlorodihydrofluoresein diacetate staining, dihydroethidium staining, and lucigenin chemiluminescence, which was explained by the decreased expressions of NADPH oxidase 2 and cyclooxygenase 1. The production of prostacyclin was decreased by heme oxygenase 1 induction, which was explained by a lower expression of cyclooxygenase 1. Contractions to vasoconstrictor concentrations of prostacyclin and its mimetic iloprost were attenuated, suggesting that the responsiveness of thromboxane-prostanoid receptors to prostacyclin was decreased in hemin-treated rats. The suppressed production of free radicals and prostacyclin and the decrease of thromboxane-prostanoid receptors sensitivity concur to explain the impairment of endothelium-dependent contractions caused by heme oxygenase 1 induction by hemin.