Hydrogen sulfide as an endogenous modulator of biliary bicarbonate excretion in the rat liver

Cystathionine γ-lyase (CSE) is an enzyme catalyzing cystathionine and cysteine to yield cysteine and hydrogen sulfide (H₂S), respectively. This study aimed to examine if H₂S generated from the enzyme could serve as an endogenous regulator of hepatobiliary function. Gas Chromatographic analyses indicated that, among rat organs herein examined, liver constituted one of the greatest components of H₂S generation in the body, at 100 μmol/g of tissue, comparable to that in kidney and 1.5-fold greater than that in brain, where roles of the gas in the regulation of neurotransmission were reported previously. At least half of the gas amount in the liver appeared to be derived from CSE, because blockade of the enzyme by propargylglycine suppressed it by 50%. Immunohistochemistry revealed that CSE occurs not only in hepatocytes, but also in bile duct. In livers in vivo, as well as in those perfused ex vivo, treatment with the CSE inhibitor induced choleresis by stimulating the basal excretion of bicarbonate in bile samples. Transportal supplementation of NaHS at 30 μmol/L, but not that of N-acetylcysteine as a cysteine donor, abolished these changes elicited by the CSE inhibitor in the perfused liver. The changes elicited by the CSE blockade did not coincide with alterations in hepatic vascular resistance, showing little involvement of vasodilatory effects of the gas in these events, if any. These results first provided evidence that H₂S generated through CSE modulates biliary bicarbonate excretion and is thus a determinant of bile salt-independent bile formation in the rat liver.