Modulation of Sarcoplasmic Reticulum Function by Na+/K+ Pump Inhibitors with Different Toxicity: Digoxin and PST2744 [(E,Z)-3-((2-Aminoethoxy)imino)androstane-6,17-dione Hydrochloride]

Objective: To gain some insight on the lesser arrhythmogenic properties of PST2744 [( E , Z )-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride] compared with digoxin, we compared modulation of intracellular Ca 2+ dynamics by the two agents. Methods: SERCA (sarcoplasmic reticulum Ca 2+ -ATPase) activity and Ca 2+ leak rate were measured in sarcoplasmic reticulum (SR) vesicles from guinea pig ventricles. Membrane current, intracellular Ca 2+ , and twitch amplitude were evaluated in guinea pig ventricular myocytes with or without blockade of the Na + /Ca 2+ exchanger. Results: In SR vesicles, PST2744 (30–300 nM), but not digoxin, increased SERCA activity; digoxin only (≥0.1 nM) increased SR Ca 2+ leak. In myocytes with blocked Na + /Ca 2+ exchanger, Ca 2+ reloading of caffeine-depleted SR was enhanced by PST2744 and slightly inhibited by digoxin. In myocytes with functioning Na + /Ca 2+ exchanger, both agents increased diastolic Ca 2+ , SR Ca 2+ content, the gain of Ca 2+ -induced Ca 2+ release, the rate of cytosolic Ca 2+ decay, twitch amplitude, and relaxation rate. Consistent with the observations in SR vesicles, the effects on SR Ca 2+ content and Ca 2+ decay rate were significantly larger for PST2744 than for digoxin. Conclusions: In isolated SR vesicles, PST2744 and digoxin directly affected SR function in opposite ways; this could be reproduced in myocytes during Na + /Ca 2+ exchanger blockade. Under physiological conditions (functioning Na + /Ca 2+ exchanger), the two agents affected Ca 2+ dynamics in the same direction, as expected by their Na + /K + pump inhibition; however, differential SR modulation was still expressed by quantitative differences. Thus, the more favorable inotropy-to-toxicity ratio previously described for PST2744 appears to be associated with direct SERCA stimulation and/or lack of enhancement of Ca 2+ leak.