Factors Producing Various Responses to Acetylcholine in Dog Hepatic Circulation

Abstract

Abstract-Effect of acetylcholine on hepatic circulation was investigated in 33 mongrel dogs. Mean arterial blood pressure, hepatic artery flow, portal vein pressure and portal vein flow were determined after laparotomy under nembutal anesthesia. In order to analyse the complicated responses in hepatic blood (lows to 3 ,'g of acetylcholine hydrochloride, the peripheral resistance of each vascular system was observed in hy potensive dogs, in vascular obliteration dogs and in portal vein shunting dogs. Ob servation under a hypotensive condition produced by exsanguination revealed that hepatic artery resistance responded to decrease in hepatic artery pressure in different ways depending on whether the systemic blood pressure was above or below 90 mmHg. Experiments in which the hepatic artery or portal vein were obliterated disclosed that there was a certain regularity in the induction of resistance change in the non-injected one of the two hepatic vessels. The result of shunting the portal vein suggested that the complicated change above mentioned would be the expression of the overlapping two sequential responses. Since these results are difficult to explain merely by the direct response of vascular smooth muscle to acetycholine, the possibility that the hypo thetical communication between the hepatic artery and portal vein might be a cholinergic system was discussed.It has long been observed that the hemodynamics of the liver, which is supplied by the two vascular systems, the hepatic artery and portal vein, are somewhat different from those of any other organ (1-7, 10-24). Many authors have discussed the presence or absence of a blood flow regulation mechanism specific to the liver and have estimated the related pro perties, yet did not reach any definite conclusion.On the other hand, the physiologic functions of adrenergic and cholinergic receptors, which are considered to be distributed on the vascular perfusion beds, that is, their phar macologic reactions to noradrenaline, isoproterenol and acetylcholine were also studied by many investigators (6,7,8,15,(21)(22)(23). The diversity among the reports on the distribution and the function of cholinergic receptors in the liver has also attracted the attention of many workers. It has been confirmed that the administraction of a rather large dose of acetyl choline often produced different responses. (8) termed the responses as atypical and discussed the mode of manifestation of the atypical responses. The present author attempted to elucidate the diversity in physiologic roles of cholinergic receptors distributed on the hepatic vascular wall by exploring the mechanism of these various responses to acetylcholine. MATERIALS AND METHODSThirty-three male and female mongrel dogs, weighing 13.0-36.0 kg, were fasted 24 hr prior to the experiment, and were then anesthetized with pentobarital sodium (30 mg/kg,