Effects of nitric oxide on platelet-activating factor- and alpha-adrenergic-stimulated vasoconstriction and glycogenolysis in the perfused rat liver.

Effects of nitric oxide (NO) on hemodynamic and glycogenolytic responses to platelet-activating factor (PAF) and phenylephrine were investigated in perfused livers derived from fed rats. Infusion of NO (34 microM) into perfused livers inhibited PAF (0.22 nM)-induced increases in hepatic glucose output and portal pressure approximately 90 and 85%, respectively, and abolished effects of PAF on hepatic oxygen consumption. NO attenuated PAF-stimulated increases in glucose output and portal pressure, the latter indicative of hepatic vasoconstriction, with a similar dose dependence with an IC50 of approximately 8 microM. In contrast to its effects on PAF-induced responses in the perfused liver, NO inhibited increases in hepatic portal pressure in response to phenylephrine (10 microM) approximately 75% without altering phenylephrine-stimulated glucose output and oxygen consumption. Similarly, infusion of NO into perfused livers significantly inhibited increases in hepatic portal pressure but not in glucose output in response to a submaximal concentration of phenylephrine (0.4 microM). Like NO, sodium nitroprusside (83 microM) significantly inhibited hemodynamic but not glycogenolytic responses to phenylephrine in perfused livers. However, PAF (0.22 nM)-stimulated alterations in hepatic portal pressure, glucose output, and oxygen consumption were unaffected by infusion of sodium nitroprusside (83 microM) into perfused livers. These results provide the first evidence for regulatory effects of NO in the perfused liver and support the contention that PAF, unlike phenylephrine, stimulates glycogenolysis by mechanisms secondary to hepatic vasoconstriction. These observations raise the intriguing possibility that NO may act in liver to regulate hemodynamic responses to vasoactive mediators.