Studies of nitric oxide over the past two decades have highlighted the fundamental importance of gaseous signaling molecules in biology and medicine. The physiological role of other gases such as carbon monoxide and hydrogen sulfide (H 2 S) is now receiving increasing attention. Here we show that H 2 S is physiologically generated by cystathionine γ-lyase (CSE) and that genetic deletion of this enzyme in mice markedly reduces H 2 S levels in the serum, heart, aorta, and other tissues. Mutant mice lacking CSE display pronounced hypertension and diminished endothelium-dependent vasorelaxation. CSE is physiologically activated by calcium-calmodulin, which is a mechanism for H 2 S formation in response to vascular activation. These findings provide direct evidence that H 2 S is a physiologic vasodilator and regulator of blood pressure.Nitric oxide (NO) and carbon monoxide (CO) are established physiologic messenger molecules, and NO has an important role as an endothelial cell-derived relaxing factor (EDRF) and regulator of blood pressure (1,2). Indirect evidence has implicated another endogenous gasotransmitter, hydrogen sulfide (H 2 S), in similar functions (3-7). H 2 S can be produced by cystathionine γ-lyase (CSE) or cystathionine β-synthase (CBS) (3,4), but definitive evidence for either of these enzymes in the physiologic formation of H 2 S is lacking.To investigate the role of H 2 S as a physiologic vasorelaxant and determinant of blood pressure, we generated mice with a targeted deletion of the gene encoding CSE (8) (fig. S1, A to C). The homozygous (CSE −/− ) and heterozygous (CSE −/+ ) mutant mice were viable, fertile, and indistinguishable from their control wild-type littermates (CSE +/+ ) in terms of growth pattern.

Hydrogen sulfide (H2S) has been shown recently to function as an important gasotransmitter. The present study investigated the vascular effects of H2S, both exogenously applied and endogenously generated, on resistance mesenteric arteries of rats and the underlying mechanisms. Both H2S and NaHS evoked concentration-dependent relaxation of in vitro perfused rat mesenteric artery beds (MAB). The sensitivity of MAB to H2S (EC50, 25.2 +/- 3.6 microM) was about fivefold higher than that of rat aortic tissues. Removal of endothelium or coapplication of charybdotoxin and apamin to endothelium-intact MAB significantly reduced the vasorelaxation effects of H2S. The H2S-induced relaxation of MAB was partially mediated by ATP-sensitive K+ (KATP) channel activity in vascular smooth muscle cells. Pinacidil (EC50, 1.7 +/- 0.1 microM, n=6) mimicked, but glibenclamide (10 microM, n=6) suppressed, the vasorelaxant effect of H2S. KATP channel currents in isolated mesenteric artery smooth muscle cells were significantly augmented by H2S. L-cysteine, a substrate of cystathionine-gamma-lyase (CSE), at 1 mM increased endogenous H2S production by sixfold in rat mesenteric artery tissues and decreased contractility of MAB. DL-propargylglycine (a blocker of CSE) at 10 microM abolished L-cysteine-dependent increase in H2S production and relaxation of MAB. Our results demonstrated a tissue-specific relaxant response of resistance arteries to H2S. The stimulation of KATP channels in vascular smooth muscle cells and charybdotoxin/apamin-sensitive K+ channels in vascular endothelium by H2S represents important cellular mechanisms for H2S effect on MAB. Our study also demonstrated that endogenous CSE can generate sufficient H2S from exogenous L-cysteine to cause vasodilation. Future studies are merited to investigate direct contribution of endogenous H2S to regulation of vascular tone.