PAF increases hepatic vascular resistance and glycogenolysis in vivo.

Administration of platelet-activating factor (PAF) to portal venous circulation of anesthetized fed rats produced decreases in mean arterial pressure and increases in hepatic portal pressure and blood glucose concentration. These responses to PAF were dose dependent with ED50 values of 0.02-0.05 micrograms/kg and specific as lyso- and enantio-PAF did not reproduce effects of PAF. Specific PAF receptor antagonist SRI 63-675 (75 micrograms/kg) inhibited significantly these PAF (0.1 micrograms/kg)-induced responses in rats. Administration of prazosin (500 micrograms/kg) and propranolol (400 micrograms/kg) to rats abolished phenylephrine (50 micrograms/kg)-induced increases in mean arterial pressure, hepatic portal pressure, and blood glucose concentration but did not prevent PAF (1 microgram/kg)-induced alterations in these parameters. Glycogen phosphorylase alpha levels were increased significantly in livers of rats after administration of PAF (1 microgram/kg) or phenylephrine (50 micrograms/kg). Administration of prazosin and propranolol to rats inhibited phenylephrine- but not PAF-induced activation of hepatic glycogen phosphorylase. Hepatic adenosine 3',5'-cyclic monophosphate (cAMP) concentration was increased slightly by PAF, but these increases were eliminated by adrenergic blockade, suggesting that activation of hepatic glycogen phosphorylase by PAF is not dependent on increases in cAMP. Increases in hepatic portal pressure and blood glucose concentration in response to PAF (0.1 micrograms/kg) were not attenuated in adrenalectomized rats. Moreover, PAF (0.1 micrograms/kg) stimulated increases in hepatic portal pressure after administration of the ganglionic blocking agent chlorisondamine (2.5 mg/kg) to adrenalectomized rats. Administration of PAF (0.05 micrograms/kg) to rats instrumented with pulse Doppler flow probes produced decreases in hepatic arterial and portal vein blood flow and increases in hepatic arterial and portal vascular resistance. These observations provide direct evidence that PAF regulates hepatic hemodynamics and glycogenolysis in vivo. It is suggested that PAF plays an important role in regulating hepatic blood flow and supplying extrahepatic tissues with energy substrates by sympathetic-independent mechanism(s) after its release in acute pathophysiological situations.