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

1. LND 623 and LND 796 are two aminosteroid derivatives which exert similar positive inotropic effects to digitalis. Their electrophysiological, toxic and inotropic effects were investigated in both normal and partially K+-depolarized ventricular muscle. 2. In guinea-pig myocardial fibres, LND 623 and LND 796 required tenfold higher concentrations than digoxin to induce the same signs of toxicity; e.g. triggered activities generated from delayed afterdepolarizations, leading to the marked depression of action potential characteristics and inexcitability. These abnormal rhythms and delayed afterdepolarizations were abolished by 1 mM caffeine. The toxic effects were reversed by washout, particularly in the case of LND 796. 3. In normal-K+ solution, LND 623 and LND 796 exhibited concentration-dependent positive inotropic effects on guinea-pig papillary muscle and increased concomitantly resting membrane potential and action potential amplitude. The range of active concentrations (0.1 to 1 microM) of LND 623 was larger than that of digoxin (0.3 to 1 microM). Like digoxin, LND 796 exerted negative inotropic effects at the lowest concentrations (0.01 to 0.03 microM) and positive inotropic effects at high concentrations (1 and 3 microM). 4. In partially K+-depolarized papillary muscle, in the presence of 2 microM histamine, LND 623 (3 and 10 microM) and LND 796 (10 and 30 microM) enhanced the two components P1 and P2 of the contraction and increased slow action potential amplitude, resting potential and maximal rate of depolarization. Low concentrations (0.03 to 0.3 microM) of LND 796 induced negative inotropic effects. beta-Adrenoceptor blockade with atenolol (1 microM) did not modify the activity of LND 623 but significantly enhanced the negative inotropic effect on P2 induced by 1 and 3 microM LND 796 and reduced the positive inotropic effect on P1 and P2 of the highest concentration (30 microM) studied. 5. In the presence of either caffeine (1 mM) or Ca2+-free, Sr2+-rich (3.6 mM) solution, LND 623 and LND 796 produced a positive inotropic effect which was stronger with LND 623. 6. It is suggested that two mechanisms are involved in the inotropic effects of these aminosteroids: (i) an enhanced Ca2 + entry via the slow calcium channels partially brought about by a local release of endogenous catecholamines in the case of LND 796, (ii) an inhibitory effect on Na+-K+ ATPase which, at the highest concentrations, lead to similar signs of cellular toxicity to those described for digitalis drugs. Because of their enlarged positive inotropic range, both aminosteroids may be of interest in the treatment of congestive heart failure.