The Comparative Effect of Administration of Substances Via the Hepatic Artery or Portal Vein on Hepatic Arterial Resistance, Liver Blood Volume and Hepatic Extraction in Cats

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Abstract
Compounds reaching the liver do so via either the hepatic artery or the portal vein. This paper reports on the effectiveness of administration of compounds into these alternate routes for their effects on the hepatic parenchymal cells, the hepatic arterial resistance vessels (blood flow) and hepatic capacitance (blood volume responses). All tests were done on cats under pentobarbital anesthesia. Perfusion of the parenchymal cell mass was assessed by comparing the hepatic elimination of indocyanine green (ICG) administered via the two vascular routes. The ICG uptake was assessed by measuring relative areas under the hepatic venous outflow curve obtained following bolus injections of ICG into the artery and portal vein. In a separate series, using different methods, the hepatic venous levels reached early (2 min) and later (5 min) during a constant infusion were compared during administration via the two routes and found to be equal. Parenchymal cell functions (ICG extraction, bile salt stimulation of bile flow) indicate that blood from the artery and portal vein supplies the hepatic parenchymal cells equally well. This suggests a well-mixed blood supply prior to exposure of either blood stream to parenchymal cells. Substances being processed by the liver are thus equally well handled if reaching the liver via either the arterial or portal blood stream. This has significance in validating the use of some isolated liver perfusion methods that perfuse only via the portal vein. Access of vasoactive compounds in the two blood streams to hepatic arterial resistance vessels was assessed using electromagnetic flow probes. The infusion of adenosine and angiotensin produced dilation and constriction respectively of the hepatic artery with the effect being about 3 times as large at a given dose when infused via the artery. The data show that vasoactive substances in the portal blood can affect the hepatic arterial blood flow by “transvascular” route and that portal blood has quite a high access to hepatic arterial resistance vessels. It does indicate, however, that the arterial resistance sites are not being perfused with mixed portal and arterial blood. The hepatic blood volume changes in responses to intraportal and intraarterial infusions of angiotensin and noradrenaline were measured using an in vivo plethysmograph assembled around the intact liver. Constriction of hepatic blood volume was equal with administration via the two routes indicating that the capacitance vessels are perfused by well-mixed blood and are, therefore, likely to consist mostly of sinusoidal and postsinusoidal vessels with presinusoidal roles being minor or absent.

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