Is ouabain-sensitive rubidium or potassium uptake a measure of sodium pump activity in isolated cardiac muscle?

(1) The significance of the specific (ouabain-sensitive) 86Rb+ or 42K+ uptake by cardiac muscle preparations which are not 'sodium-loaded' was studied. (2) In left atrial preparations of guinea-pig heart, resting 86Rb+ uptake was relatively low. It was markedly increased by electrical stimulation. This stimulated uptake was further enhanced by isoproterenol and inhibited by verapamil. (3) In rat atria, the resting 86Rb+ uptake was somewhat higher than in guinea-pig atria, and the increase in uptake caused by electrical stimulation was smaller. In guinea-pig right ventricular papillary muscle, the resting uptake was highest among those tissues studied, and the response to electrical stimulation was smallest. In the latter tissue, verapamil produced only a minimal inhibition of the specific 86Rb+ uptake. (4) The effect of the frequency of electrical stimulation of 86Rb+ uptake paralleled its influence on the force of contraction, suggesting the involvement of intracellular sodium in both events. (5) In both left atrial and right papillary muscle preparations of guinea-pig heart, specific 42K+ uptake observed with 5.8 mM K+ was relatively high, and was increased only slightly by electrical stimulation. This electrical stimulation, however, increased ouabain-induced inhibition of 42K+ uptake, suggesting that the stimulation increases the amount of Na+ available to the sodium pump. (6) When the K+ concentration was 1 mM, the resting 42K+ uptake was low, and could be enhanced by electrical stimulation. (7) Thus, in cardiac muscle preparations which are not sodium loaded, the specific 86Rb+ or 42K+ uptake can be used to estimate the rate of sodium influx, which is equivalent to the rate of sodium efflux under steady-state conditions, provided that neither Rb+ nor K+ is in excess compared to the Na+ available to the pump. If Rb+ or K+ is in excess, its specific uptake may not reflect changes in transmembrane Na+ movement.