Mingone, Christopher J., Sachin A. Gupte, Joseph L. Chow, Mansoor Ahmad, Nader G. Abraham, and Michael S. Wolin. Protoporphyrin IX generation from ␦-aminolevulinic acid elicits pulmonary artery relaxation and soluble guanylate cyclase activation. Am J Physiol Lung Cell Mol Physiol 291: L337-L344, 2006; doi:10.1152/ajplung.00482.2005.-Protoporphyrin IX is an activator of soluble guanylate cyclase (sGC), but its role as an endogenous regulator of vascular function through cGMP has not been previously reported. In this study we examined whether the heme precursor ␦-aminolevulinic acid (ALA) could regulate vascular force through promoting protoporphyrin IX-elicited activation of sGC. Exposure of endothelium-denuded bovine pulmonary arteries (BPA) in organoid culture to increasing concentrations of the heme precursor ALA caused a concentration-dependent increase in BPA epifluorescence, consistent with increased tissue protoporphyrin IX levels, associated with decreased force generation to increasing concentrations of serotonin. The force-depressing actions of 0.1 mM ALA were associated with increased cGMP-associated vasodilator-stimulated phosphoprotein (VASP) phosphorylation and increased sGC activity in homogenates of BPA cultured with ALA. Increasing iron availability with 0.1 mM FeSO 4 inhibited the decrease in contraction to serotonin and increase in sGC activity caused by ALA, associated with decreased protoporphyrin IX and increased heme. Chelating endogenous iron with 0.1 mM deferoxamine increased the detection of protoporphyrin IX and force depressing activity of 10 M ALA. The inhibition of sGC activation with the heme oxidant 10 M 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) attenuated the force depressing actions of an NO donor without altering the actions of ALA. Thus control of endogenous formation of protoporphyrin IX from ALA by the availability of iron is potentially a novel physiological mechanism of controlling vascular function through regulating the activity of sGC.guanosine 3Ј,5Ј-cyclic monophosphate; heme metabolism; iron; vasodilation THE REGULATION OF THE SOLUBLE FORM of guanylate cyclase (sGC) by nitric oxide (NO) has been extensively studied for its role in promoting relaxation in the pulmonary and systemic vasculature through increasing production of cGMP (2,12,23,33). The bovine pulmonary arteries (BPA) examined in this study show an endothelium-dependent relaxation to NO, which is mediated through the stimulation of sGC (17,18). NO relaxes BPA through stimulating sGC in a manner that is controlled by a thiol redox mechanism regulated by cytosolic NADPH and glutathione redox (25). Under hypoxia the mechanism of relaxation to NO appears to shift to a cGMPindependent mechanism stimulating calcium-reuptake by the sarco(endo)plasmic reticulum Ca 2ϩ -adenosinetriphosphatase (SERCA) pump (26). NO activates sGC by binding a ferrous heme group, which appears to be a cofactor that is normally bound to sGC when it is isolated from tissues (6, 9, 34). Early studies investigating how NO activate...

This study examines in endothelium-denuded bovine pulmonary arteries the effects of increasing heme oxygenase-1 (HO-1) activity on relaxation and soluble guanylate cyclase (sGC) activation by nitric oxide (NO). A 24 hour organ culture with 0.1 mM cobalt chloride (CoCl 2 ) or 30 μM Coprotoporphyrin IX was developed as a method of increasing HO-1 expression. These treatments increased HO-1 expression and HO activity by ~2-4 fold, and lowered heme levels by 40-45%. Induction of HO-1 was associated with an attenuation of pulmonary arterial relaxation to the NOdonor spermine-NONOate. The presence of a HO-1 inhibitor 30 μM chromium mesoporphyrin during the 24 hour organ culture (but, not acute treatment with this agent) reversed the attenuation of relaxation to NO seen in arteries co-cultured with agents that increased HO-1. Relaxation to isoproterenol, which is thought to be mediated through cAMP, was not altered in arteries with increased HO-1. Inducers of HO-1 did not appear to alter basal sGC activity in arterial homogenates or expression of the β1-subunit of sGC. However, the increase in activity seen in the presence of 1 μM spermine-NONOate was attenuated in homogenates obtained from arteries with increased HO-1. Since arteries with increased HO-1 had decreased levels of superoxide detected by the chemiluminescence of 5 μM lucigenin, superoxide did not appear to be mediating the attenuation of relaxation to NO. These data suggest that increasing HO-1 activity depletes heme, and this is associated with an attenuation of pulmonary artery relaxation and sGC activation responses to NO.

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