Na(+)-channel activators increase intracellular Na+ and thereby enhance the transport rate of sarcolemmal Na+,K(+)-ATPase. We investigated the interaction of the new Na(+)-channel activator BDF 9148 (BDF) with the cardiac glycoside ouabain (OUA) in human myocardium. The influence of OUA (0.01-0.1 microM) and of OUA after prestimulation with BDF (0.1 microM, 1 microM; BDF+OUA) on isometric force of contraction (FOC, force of contraction; +T/-T, peak rate of tension increase/decay) of electrically driven (1 Hz, 37 degrees C) papillary muscle strips from terminally failing [New York Heart Association classification IV (NYHA IV) heart transplants, n = 19] human myocardium was studied. We also examined the effects of BDF and OUA on nonfailing human myocardium (brain death resulting from traumatic injury, n = 5). 0.01 microM OUA enhanced FOC only after prestimulation with BDF (NYHA IV+2.9 +/- 0.4 mN; p less than 0.01). The time until maximal (Tmax: BDF+OUA 117 min, OUA 166 min), half-maximal (T1/2max: BDF+OUA 47 min, OUA 85 min) inotropic effects and time until toxic signs (contracture, extrasystoles) occurred were significantly shorter with BDF+OUA as compared with OUA alone. BDF influenced Tmax, T1/2max, and time until toxic side effects occurred (Ttox) of the OUA-mediated inotropism in a concentration-dependent manner. Both OUA and BDF enhanced +T and -T. The effectiveness of OUA and BDF in increasing FOC was similar to that of Ca2+ (1.8-15 mM) but significantly (p less than 0.01) higher as compared with the beta-adrenoceptor-agonist isoprenaline in NYHA IV. In myocardial membranes, [3H]ouabain binding (Bmax, Kd) was not affected by BDF.(ABSTRACT TRUNCATED AT 250 WORDS)