SUGGESTIVE EVIDENCE FOR THE ACTIVATION OF AN ELECTROGENIC SODIUM-PUMP IN STIMULATED RAT ATRIA - APPARENT DISCREPANCY BETWEEN THE PUMP INHIBITION AND THE POSITIVE INOTROPIC RESPONSE INDUCED BY OUABAIN

Rat left atria superfused at 35°C were submitted to stimulation trains of varying frequency (0.2 to 5 Hz). During stimulation the diastolic potential depolarized and later on repolarized to a level near that of the prestimulation potential. At the end of the stimulation period the membrane potential transiently hyperpolarized. These phenomena were magnified by an increase in the frequency. The stimulation repolarization phase and the rate of rise of the action potential upstroke were similarly reduced by a 50% NaCl solution, and unchanged by an acetylcholine addition. The membrane resistance decreased regularly to a steady value through the period of stimulation. From these experiments it is assumed that stimulation repolarization reflects the activation of an electrogenic sodium pump and that the pump activity increases with frequency. At 4 Hz, a noticeable correlation appeared between the positive inotropic effect and the inhibition of stimulation repolarization induced by different doses of ouabain. However, inotropism is not always followed by changes of resting or action potential (0.2 Hz) and reverses much more rapidly than the electrical alteration (4 Hz). Consequently, it is proposed that there is an independence between the sodium pump inhibition and the positive action of ouabain in rat myocardium in spite of the apparent correlation mentioned above. © 1979.