Physiological regulation of the hepatic circulation

Area of Science:

• Physiology
• Hepatology
• Vascular Biology

Background:

• Liver blood flow is crucial for organ function and is determined by hepatic arterial vascular resistance, preportal resistance, and intrahepatic portal venous resistance.
• Hepatic arterial resistance is modulated by intrinsic and extrinsic mechanisms, including sympathetic innervation and epinephrine, while hormonal roles require further investigation.
• Intrahepatic portal resistance is less influenced by intrinsic factors, with extrinsic mechanisms potentially regulating portal venous pressure.

Purpose of the Study:

• To investigate the mechanisms regulating liver blood flow, focusing on vascular resistance in hepatic and portal circuits.
• To elucidate the transhepatic effects of substances introduced into one liver inflow on the other.
• To determine the specific role of glucagon in modulating hepatic arterial vascular resistance, particularly in response to sympathetic activation.

Main Methods:

• Analysis of factors determining liver blood flow, including hepatic arterial vascular resistance and portal venous vascular resistance.
• Examination of extrinsic regulatory mechanisms, such as sympathetic innervation, epinephrine, and gastrointestinal/pancreatic hormones.
• Investigation of transhepatic mechanisms by which substances in one inflow affect the other.
• Assessment of glucagon's effect on hepatic arterial vasoconstriction induced by sympathetic nerve activation.

Main Results:

• Hormones or drugs introduced into the portal vein can alter hepatic arterial resistance via a transhepatic mechanism independent of recirculation.
• Glucagon, unlike other vasodilator hormones, effectively prevents hepatic arterial vasoconstriction caused by sympathetic nerve activation.
• These glucagon effects are observed at physiologically relevant concentrations.

Conclusions:

• Liver blood flow regulation is complex, involving interplay between arterial and portal circuits.
• A transhepatic mechanism mediates cross-talk between liver inflow circuits.
• Glucagon possesses a unique protective role against sympathetic-mediated hepatic arterial vasoconstriction, with potential pathophysiological significance.