SUMMARY Since the 1980 s nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S), the endogenous gas molecules produced from metabolic pathway, have been realized as signal molecules to be involved in the regulation of body homeostasis and to play important roles under physiological and pathophysiological conditions. The researches on these endogenous gas signal molecules opened a new avenue in life science. To explore the new member of gasotransmitter family, other endogenous gas molecules which have been regarded as metabolic waste up to date, and their biological regulatory effects have been paid close attention to in the current fields of life science and medicine. Sulfur dioxide (SO2) can be produced endogenously from normal metabolism of sulfur-containing amino acids. L-cysteine is oxidized via cysteine dioxygenase to L-cysteinesulfinate, and the latter can proceed through transamination by glutamate oxaloacetate transaminase (GOT) to β-sulfinylpyruvate which decomposes spontaneously to pyruvate and SO2. In mammals, activated neutrophils by oxidative stress can convert H2S to sulfite through a reduced form of nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase-dependent process. The authors detected endogenous production of SO2 in all cardiovascular tissues, including in heart, aorta, pulmonary artery, mesenteric artery, renal artery, tail artery and the plasma SO2 content. As the key enzyme producing SO2, GOT mRNA in cardiovascular system was detected and found to be located enrichedly in endothelial cells and vascular smooth muscle cells near the endothelial layer.When the normal rats were treated with hydroxamate(HDX), a GOT inhibitor, at a dose of 3.7 mg/kg body weight, the blood pressure (BP) went high markedly, the ratio of wall thickness to lumen radius was increased by 18.34%, and smooth muscle cell proliferation was enhanced. The plasma SO2 level in the rats injected with 125 μmol/kg body weight SO2 donor was increased to 721.98±30.11 μmol/L at the end of 30 seconds, while the blood pressure was decreased to the lowest point 65.0± 4.9 mm Hg at the end of 1 minute. The above results showed that endogenous SO2 might be involved in the maintenance of blood pressure and normal vascular structure. In spontaneous hypertensive rat (SHR) animal model, exogenous supplement of SO2 donor decreased the BP, the media cross-sectional area, and pressure of the media and the ratio of wall thickness to lumen radius in the SHR. Moreover, the proliferative index of aortic smooth muscle cells was decreased in the SHR treated with SO2 donor compared with that in SHR. The above data showed that SO2 could prevent the aortic structural remodeling by inhibiting the proliferation of aortic smooth muscle cells.The authors observed the direct vasorelaxant effects of SO2 on the aortic ring pre-treated with norepinephrin (NE). SO2 donor at a concentration of 25—100 μmol/L relaxed the aortic ring temporarily and slightly, but SO2 donor at a concentration of 1—12 mmol/L induced relaxation of the ring in a concentration-dependent manner. Administration with nicardipine, an L-type calcium channel blocker other than glibenclamide, an ATP sensitive potassium channel (KATP channel) blocker or removal of vascular endothelium could decrease the SO2-induced vasorelaxation. In hypoxic pulmonary hypertension animal model, SO2 donor decreased the mean pulmonary artery pressure and the systolic pulmonary artery pressure (P<0.01), respectively as compared with hypoxic group, and alleviated obviously the hypoxic pulmonary vascular structural remodeling. The percentage of muscularized arteries of small pulmonary vessels was significantly decreased in hypoxia+SO2 donor-treated rats compared with that of hypoxic rats (P<0.01), while the percentage of non-muscularized vessels was obviously higher in hypoxia with SO2 donor-treated rats than that of hypoxic rats (P<0.01). Similarly, SO2 obviously decreased relative media area and relative media thickness of small muscularized pulmonary arteries in hypoxic rats (P<0.01). The above data showed that SO2 might play an important role in development of hypoxic pulmonary hypertension.Perfusion with SO2 donor (10-6—10-3 mol/L) to the isolated rat heart obviously inhibited the left ventricular peak rate of contraction ( + LV dp/ dtmax) , peak rate of relaxation ( -LV dp/ dtmax) and difference of left ventricular pressure ( ΔLVP) in a concentration dependent manner. Nicardipine, an L-type calcium channel blocker, could partly antagonize the inhibitory effect of SO2 on the heart function. In a word, SO2 could be endogenously generated in cardiovascular tissues and exert important cardiovascular effects such as vasorelaxant effect and negative inotropic effects. Moreover, SO2 might play considerable roles in the regulation of systemic circulatory pressure, pulmonary circulatory pressure and vascular structural remodeling in the pathogenesis of hypertension and hypoxic pulmonary hypertension. On the basis of the above findings, we presumed that endogenous SO2 might be a novel cardiovascular functional regulatory gasotransmitter. More studies on the significance of endogenous SO2 in cardiovascular system under physiological and pathophysiological conditions need to be investigated.
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