Comparative species studies on the effect of monovalent cations and ouabain on cardiac Na+, K+-adenosine triphosphatase and contractile force.

The effects of ouabain, Rb+ and Tl+ on Na+, K+-adenosine triphosphatase (Na+,K+-ATPase; Mg++-dependent, Na+,K+-activated ATP phosphohydrolase, EC 3.6.1.3) and contractile force were compared in guinea-pig and rat hearts. Although ouabain produced a dose-dependent positive inotropic effect in rat as well as in guinea-pig atrial preparations, concentrations of ouabain needed to produce comparable positive inotropic effects were more than an order of magnitude higher in rats than in guinea pigs. Additionally, the time to reach the plateau of the inotropic response was significantly shorter in rat than in guinea-pig atrial preparations. Concentrations of ouabain needed to produce comparable inhibition of cardiac Na+, K+-ATPase in vitro observed with partially purified cardiac enzyme preparations were also more than an order to magnitude higher in rats than in guinea pigs.