[Plethysmographic and in vitro studies of the vasodilator effect of furosemide (Lasix)].

The beneficial effect of furosemide in treatment of chronic and particularly of acute congestive heart failure has been attributed to its potent diuretic action. In recent studies [16], it has been postulated that the effect of this diuretic agent is primarily vascular in origin. The results of venous occlusion plethysmographic experiments (modificated Whitney-gauge technique) carried out in this study on the forearm of 6 healthy men, show that furosemide (40 mg. i.v.) does not influence arterial blood flow (4-5 ml/100 ml tissue - min). In contrast furosemide induces a prompt significant (p less than 0.001) and sustained (about 45 min) increase in venous capacitance (deltaV/100 ml tissue = 15%) and a corresponding decrease in E'. The direct effect of furosemide on vascular muscle tone was studied in vitro on portal vein and aortic strip preparations from 76 male rats. In 18 experiments mechanical and electrical activity (using three pressure electrodes) were simultaneously recorded. The results of the in vitro experiments show: 1. Only extremely high furosemide concentrations (greater than 500 mg/l) induce slight relaxation of aortic strips. 2. Furosemide causes in contrast a marked dose-dependent per cent decrease of the integrated isometric tension in portal vein preparations: 14 mg/l reduce the initial tension (= 100%) to 80%; 35 mg/l to 65%; 70 mg/l to 50% and 100 mg/l to 35%. 3. The venodilating furosemide effect is significantly (p less than 0.001) increased by reduction in external Na+-concentration [Na+]0 to 120 mM though further reduction in the [Na+]0 to 90 mM does not cause additional changes in the relaxation slope. 4. The simultaneous records of electrical activity demonstrate that the relaxing effect of furosemide is based on decreased spike frequency in shortened bursts as well as on pronounced impairment of conduction velocity without relevant changes in pacemaker frequency. These in vitro results are discussed from the point of view that a furosemide induced decrease in Na+-permeability may determine the changes obtained in electrical and mechanical behaviour of portal veins of the rat.