Parasympathetic Cholinergic Vasodilator Mechanism in the Terminal Liver Microcirculation in Rats

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Abstract
Changes in the diameter of liver sinusoids were studied by an intravital television microscope method in pentobarbital-anesthetized rats. Dilatation of liver sinusoids was observed during parasympathetic neural stimulation and during acetylcholine administration. Frequency-dependent stimulation-effect relationships were obtained by electrical excitation of intact vagus nerves at supramaximal intensity from 2-8 Hz. Acetylcholine concentration-effect relationships were obtained by intraportal venous infusions of acetylcholine 30 .mu.l for 5 s from 10-9 to 10-2 mol/l. Systemic cholinergic receptor blockade with atropine (1 mg/kg) markedly reduced dilatation of liver sinusoids produced by both vagus nerve stimulation and acetylcholine administration. Changes in diameter of liver sinusoids with frequency of neural stimulation and with concentration of administered acetylcholine were expressed as percentage of observed maximum effect and the respective stimulation-effect curves were constructed such that at a certain percentage of diameter change, the equivalent level of vagus nerve activity was represented by a given concentration of administered acetylcholine. Liver plasma concentration of acetylcholine presumably released during electrical vagal stimulation and reaching liver sinusoids were estimated and was within physiological range. Rat liver sinusoids may have the capacity for parasympathetic cholinergic vasodilatation.