Effects of histamine on hepatic volume (outflow block) in anaesthetized dogs.

1. Hepatic volume was recorded by plethysmography in dogs anaesthetized with sodium pentobarbitone. Histamine infusions into the hepatic artery or portal vein increased hepatic volume while hepatic sympathetic nerve stimulation decreased the volume. Simultaneous nerve stimulation and histamine infusion decreased hepatic volume.2. The hepatic volume responses to histamine and nerve stimulation could not be explained on the basis of differences in the responses of the larger hepatic veins and the possibility of a histamine-sensitive sphincter at the junction of the hepatic vein with the inferior vena cava was excluded. Differences in the responses of the hepatic arterial bed contributed to but did not explain the different hepatic volume responses.3. It is suggested that histamine produced an intense and specific constriction of the sublobular veins with passive distension of post-sinusoidal venules, sinusoids and portal venules while hepatic nerve stimulation produced a uniform and generalized constriction of both pre- and post-sinusoidal vessels.4. Intravenous infusions of histamine produced a marked hypotension but small variable effects on hepatic volume. It is concluded that hepatic pooling is not the cause of the hypotension produced by intravenous histamine and that significant hepatic pooling will only be produced by mechanisms which release endogenous histamine within the liver.