Effects of sodium nitroprusside, isosorbide dinitrate, isoproterenol, phentolamine and prazosin on hepatic venous responses to sympathetic nerve stimulation in the cat.

Hepatic blood volume was recorded by a plethysmographic technique in cats anesthetized with pentobarbital. The effects of three doses of each vasodilator drug were measured on arterial and portal pressures, hepatic blood volume in the denervated liver and on the portal pressure and hepatic venous responses to sympathetic nerve stimulation. Isosorbide dinitrate caused a small reduction in basal hepatic venous tone increasing hepatic blood volume by up to 15%; it had no effect on the responses to sympathetic nerve stimulation. Isoproterenol increased hepatic venous tone perhaps by stimulation of angiotensin formation and the responses to stimulation of the hepatic nerves were reduced because of this increased basal tone. Sodium nitroprusside produced a small decrease in basal venous tone and only large doses produced any reduction in the venous responses to hepatic nerve stimulation. The evidence that nitroprusside is a venodilator requires reexamination. Phentolamine had no effect on basal venous tone but markedly reduced the responses to sympathetic nerve stimulatiqn. When compared to phentolamine, prazosin produced comparable effects on arterial pressure but much less reduction in the hepatic venous responses to sympathetic stimulation. It is suggested that the alpha receptor blocking action of prazosin is selective for arterioles and this may explain the minor incidence of postural hypotension during clinical use.