Electrical and mechanical effects of new aminosteroids on guinea-pig isolated ventricular muscle

Functional changes in isolated guinea-pig papillary muscle induced by monensin and digoxin

The effects of digoxin and monensin on contraction force (CF), initial contraction velocity (ICV), average contraction velocity (ACV), initial relaxation velocity (IRV) and stimulus to response time (ST) in 'fatigued' (tired) and 'non-fatigued' (fresh) guinea-pig papillary muscles were investigated. 'Fatigued' muscles had lost 30% of their original CF with the elapse of time before they were treated. The 5 h of measurement were divided into five periods (T0 was equilibration, T1, T2, T3 and T4 were, respectively, 1, 2, 3 and 4 h after drug administration). It was found that both monensin and digoxin increased the CF, ICV and ACV at T1 and increased the IRV at T2. Digoxin lost its effect with the elapse of time while monensin did not. Digoxin also decreased the ST at T2, T3 and T4. However, monensin did not change the ST. It was also found that 'fatigued' and 'non-fatigued' guinea-pig papillary muscles did not respond to the drug treatment differently. It was concluded that the initial effects of these two drugs on guinea-pig papillary muscles are similar regarding contractility but in time digoxin loses its effect while monensin does not.

Cordil reversibly inhibits the Na,K-ATPase from outside of the cell membrane. Role of K-dependent dephosphorylation

Cordil-LND796 is a new cardiotonic glycoside under development. In rat brain microsomes where three isoforms of the Na,K-ATPase with differential affinities for cardiac glycosides have been identified, Cordil had higher affinity for the alpha 3 (IC50 = 0.02 microM) than for the alpha 2 (IC50 = 0.6 microM) and the alpha 1 (IC50 = 30 microM) isozymes. Cordil is potentially a selective inhibitor for both alpha 2 and alpha 3 Na,K-ATPase isoforms. Using inside out vesicles we have shown that Cordil binds to and inhibits Na,K-ATPase at an extracellular site. The dissociation kinetic rates (k-1) from the ATPase and the phosphatase activity (K-dependent dephosphorylation) of the Na,K-ATPase were similar for Cordil. Despite these similarities to ouabain comparison of the kinetics of the Na,K-ATPase inhibition by ouabain and Cordil revealed marked differences in their association rates (k+1 = 0.7 l mol-1 min-1 and k+1 = 6 x 10(-3) l mol-1 min-1 respectively) and their dissociation rates (k-1 = 1.3 +/- 0.2 x 10(-4) s-1 and k-1 = 69 +/- 7 x 10(-4) s-1 respectively). Both binding association and dissociation rates were enhanced for Cordil. These data are compatible with a stabilizing effect of Cordil on the E2P conformational state of Na,K-ATPase.

Tissue localization of active Na,K-ATPase isoenzymes by determination of their profile of inhibition with ouabain, digoxin, digitoxigenin and LND 796, a new aminosteroid cardiotonic

Recently, Na,K-ATPase isoforms with differential affinities for digitalis have been identified that may contribute to different toxicity profiles. Our objectives were to localize them and to define tissue receptor patterns by examining the effect of different glycosides on the Na,K-ATPase activity. The digitalis derivatives used exhibit variation in lipophilicity and rate of enzyme inhibition. Membrane fractions enriched in Na,K-ATPase were prepared from canine heart, brain, aorta and peripheral nerves. The inhibition of enzyme activities indicates a pattern of differential sensitivities with IC50 values starting from 3 nM in heart and 30 nM in brain. Therefore, high and low affinity active forms of the Na,K-ATPase enzyme coexist in these tissues. The data also suggest the existence of two Na,K-ATPase isoforms in aorta and peripheral nerves as identified by the action of digitoxigenin and LND 796 where the predominant expression is that of a high affinity form. The comparison of the patterns of digitalis sensitivities in these different tissues, suggests a more complex molecular interaction than that which can be explained by the presence of only two forms.

Positive inotropic and vasoconstrictor responses to 14 beta-aminopregnane derivatives in isolated tissues from the guinea-pig

1. The objective of this study was to compare the positive inotropic and vasoconstrictor responses in guinea-pig isolated tissues of the aminosteroid derivatives LND 623 and LND 796 and the cardiac glycoside ouabain. 2. Ouabain (-log EC50, 6.66 +/- 0.04, n = 11) was more potent as a positive inotropic agent on guinea-pig right ventricular papillary muscles than either LND 623 (-log EC50, 6.26 +/- 0.08, n = 6) or LND 796 (-log EC50 5.66 +/- 0.05, n = 7). All compounds gave similar maximal increases in force of contraction. However, the ratio of the maximally effective concentration to the concentration giving 25% of the maximal response was approximately doubled for the aminosteroids compared with ouabain. 3. Ouabain (-log EC50 6.27), LND 623 (-log EC50 6.24) and LND 796 (-log EC50 5.43) produced slowly developing contractions of guinea-pig thoracic aortic rings. 4. These results indicate that, while the aminopregnane derivatives and ouabain produce qualitatively similar results, the therapeutic range for the positive inotropic responses is greater for the aminosteroids.