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

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Abstract
Two distinct inotropic effects of ouabain were observed in 68 of 82 right ventricular strips of rats with ED50 of 0.5 and 20 .mu.M 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 .mu.M; 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 6-89% of the maximum response. After treatment with 160 .mu.M 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 .mu.M ouabain by the washout. The atypical strips, which showed only the low-dose effects during the 1st ouabain exposure, revealed after washout a consistent high-dose effect with an ED50 of about 12 .mu.M ouabain. Evidently, the 2 inotropic responses exist in all ventricular strips and in some strips, after ouabain washout, a residue of the low-dose effect remains.