Diverse toxicity associated with cardiac Na+/K+ pump inhibition: evaluation of electrophysiological mechanisms.

(E,Z)-3-((2-Aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744) is a novel Na(+)/K(+) pump inhibitor with positive inotropic effects. Compared with digoxin in various experimental models, PST2744 was consistently found to be less arrhythmogenic, thus resulting in a significantly higher therapeutic index. The present work compares the electrophysiological effects of PST2744 and digoxin in guinea pig ventricular myocytes, with the aim to identify a mechanism for their different toxicity. The work showed that 1) the action potential was transiently prolonged and then similarly shortened by both agents; 2) the ratio between Na(+)/K(+) pump inhibition and inotropy was somewhat larger for PST2744 than for digoxin; 3) both agents accelerated inactivation of high-threshold Ca(2+) current (I(CaL)), without affecting its peak amplitude; 4) the transient inward current (I(TI)) induced by a Ca(2+) transient in the presence of complete Na(+)/K(+) pump blockade was inhibited (-43%) by PST2744 but not by digoxin; 5) the conductance of Na(+)/Ca(2+) exchanger current (I(NaCa)), recorded under Na(+)/K(+) pump blockade, was only slightly inhibited by PST2744 (-14%) and unaffected by digoxin; and 6) both agents inhibited delayed rectifier current I(Ks) (<or=-21%); delayed rectifier current I(Kr) was inhibited by PST2744 only, but the effect was marginal (-6%). Thus, 1) the higher therapeutic index of PST2744 may be accounted for by inhibition of I(TI), a current directly involved in digitalis-induced arrhythmias. Indeed, the other differences observed concern quantitatively small effects; and 2) I(TI) suppression by PST2744 may be only partly accounted for by inhibition of the Na(+)/Ca(2+) exchanger.