Analysis of the Inotropic Action of Ouabain in Rat Ventricles: Two Apparent Ouabain Inotropic Responses

Abstract

Abstract

Two distinct inotropic effects of ouabain were observed in 68 of 82 right ventricular strips of rats with ED₅₀s of 0.5 and 20 μM referred to as “low-dose” and “high-dose” effects, respectively. The other 14 strips showed monophasic dose-response curves, with an apparent ED₅₀ of 0.3 μM; the inotropic response to ouabain or isoproterenol developed by these atypical strips did not exceed 50% over control. In the strips showing the biphasic inotropic response curves, the proportion of the “low-dose” effect varied from 6 to 89% of the maximum response. After treatment with 160 μM ouabain, followed by 60 min of washout, the maximum developed tension in response to ouabain was unchanged, but the “low-dose” effect was abolished or dramatically reduced. In those cases with a remaining “low-dose” response, the response was only 12% or less of the maximum and the ED₅₀ was shifted from 0.3 to about 3 μM ouabain by the washout. The atypical strips, which showed only the “low-dose” effects during the first ouabain exposure, revealed after washout a consistent “high-dose” effect with an ED₅₀ of about 12 μM ouabain. The data show that the two inotropic responses exist in all ventricular strips and that in some strips, after ouabain washout, a residue of the “low-dose” effect remains.