The reaction of ouabain with the sodium pump of guinea-pig myocardium in relation to its inotropic effect.

[3H]ouabain binding (1.85-500 nmol/l) was evaluated in resting guinea-pig papillary muscles at 1.2-12 mmol K/l. The time course of binding was biphasic. This finding excluded a homogeneous population of non-interacting binding sites of ouabain, even though the identical susceptibility of both phases to K suggested the occupation of similarly operative receptors. Concomitant with ouabain binding, intracellular Na ion activity (aiNa) increased in the presence of 2.4 or 12 mmol K/l. Occupation of the receptor molecule by ouabain, therefore, conformed to Na-pump inhibition. Although K antagonized both ouabain binding and its effect on aiNa, the antagonistic effect on aiNa was more pronounced. The reduction of passive Na influx with depolarization as well as the stimulation of the Na pump by K presumably contributed to the antagonistic effect of K. The decrease in aiNa from 8 to 5 mmol/l, when in the absence of ouabain K was raised from 2.4 to 12.0 mmol/l, confirmed the relevance of Na fluxes. Simultaneous changes in aiNa and in the force of rested-state contractions were apparent upon addition of ouabain. At 2.4 mmol K/l, increase in aiNa raised the force of contraction by constant proportions. At 12 mmol K/l, the inotropic effect produced at comparable values of aiNa was approximately tenfold higher and was susceptible to a change in extracellular Ca concentration. Increase in aiNa, however, was differently effective on force of contraction of low as compared with high values of aiNa. The influence of resting membrane potential on electrogenic Na-Ca exchange is supposed to interfere with the inotropic effectiveness of aiNa after the cell membrane depolarized from -102 mV to -65 mV with the increase of K from 2.4 to 12 mmol/l. In view of the role of both membrane potential and aiNa not just a single mechanism appeared to be involved in the control of force of contraction.