The Pharmacology of the Cardiac Muscle of the Great Veins of the Rat

Preparations of rat superior and inferior vena cava and pulmonary vein behave essentially as preparations of atrial and ventricular muscle in response to electrical stimulation and a variety of drugs. In addition, rat inferior vena cava has spontaneous rhythmic activity. Epinephrine, norepinephrine, isoproterenol, and phenylephrine had positive inotropic effects in all three preparations and a positive chronotropic effect in inferior vena cava. All these responses were blocked by propranolol but unaffected by tolazoline. Acetylcholine and carbachol produced a negative inotropic effect in all three preparations and a negative chronotropic effect in inferior vena cava. These responses were blocked by atropine but unaffected by hexamethonium. Angiotensin, histamine, serotonin, and pitocin were without effect on the three preparations studied although slight positive inotropic effects were observed from time to time with angiotensin and serotonin. Transmembrane action potentials of the pulmonary vein and inferior vena cava were compared with those of rat ventricular strips and isolated atria in their response to acetylcholine. The response of preparations of vein was similar to that of atrial muscle in that acetylcholine reduced the duration of the action potentials.