Effects of rubidium on cardiac tissue: inhibition of Na⁺,K⁺ ATPase and stimulation of contractile force

The effects of rubidium ion on cardiac Na⁺,K⁺ adenosine triphosphatase (ATPase) activity and contractile force were studied in guinea pigs. The activity of partially purified cardiac Na⁺,K⁺ ATPase preparations was assayed in the presence of 5.8 mM KCl and either 20 or 150 mM aCl. The addition of 2 to 5 mM RbCl significantly inhibited the enzyme activity. In guinea pig left atrial strips, which were stimulated with field electrodes at 1 Hz in a medium containing 5.8 mM KCl and 145 mM NaCl, RbCl produced a dose dependent inotropic response reaching a plateau in 2 min. Propranolol pretreatment failed to alter the Rb+ induced positive inotropic response significantly. These data are consistent with the hypothesis that the inhibition of cardiac Na+, k+ ATPase causes a positive inotropic response.