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

Abstract

1. (1) The significance of the specific (ouabain-sensitive) 86Rb + or 42K + uptake by cardiac muscle preparations which are not ‘sodium-loaded’ was studied. 2. (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. (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. (4) The effect of the frequency of electrical stimulation on 86Rb + uptake paralleled its influence on the force of contraction, suggesting the involvement of intracellular sodium in both events. 5. (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. (6) When the K + concentration was 1 mM, the resting 42K + uptake was low, and could be enhanced by electrical stimulation.