Role of Na‐H exchange in the inotropic action of Bay K 8644 and of ouabain in guinea‐pig isolated atria

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Abstract
The inotropic effects of two concentrations of ouabain and of Bay K 8644 have been studied in isolated left atria of the guinea‐pig in physiological solutions at pH lowered from 7.4 to 6.0 and in the presence of ethylisopropylamiloride (EIPA) an inhibitor of Na+/H+ exchange. The low concentration of ouabain (300 nm) was chosen to saturate the high affinity binding sites (it occupied about 7% of the low affinity sites). The high concentration of ouabain saturated both high and low binding sites. Bay K 8644 evoked a positive inotropic effect of a magnitude similar to ouabain (300 nm). When comparing the positive inotropic effects of equi‐effective concentrations of ouabain (300 nm) and of Bay K 8644 (100 nm), it was observed that extracellular acidification specifically depressed the inotropic effect of ouabain 300nm; the positive inotropic effect of the high concentration of ouabain (3 μm) was barely affected by extracellular acidification. EIPA 10 μm depressed the positive inotropic effect of ouabain 300 nm, but did not affect the peak response to Bay K 8644. The depressant action of EIPA on the positive inotropic effect of ouabain was concentration‐dependent and was much more obvious on the effect of ouabain 300 nm than on ouabain 3 μm. An increase in diastolic tension was evoked by 3 μm but not by 300 nm ouabain. This increase in tone was reduced dose‐dependently by EIPA (10–30 μm). It was also significantly reduced when the extracellular pH was equal to 6.4 or 6.0. Ouabain (300 nm) evoked a gain in tissue Na and an equivalent loss in tissue K. Acidification of the extracellular pH down to pH 6.0 evoked a pH‐dependent reduction of Na gain but left K loss unaltered. EIPA 10, 20 and 30 μm evoked a significant reduction of Na gain without significantly affecting K loss. Ouabain (3 μm) evoked a large gain in tissue Na and an equivalent loss of K. Tissue Ca content was also increased. Acidification of the extracellular pH from pH 7.4 to pH 6.9 evoked a significant reduction of Na and Ca gain; changes in tissue K were not significant. Acidification down to pH 6.0 increased reduction of Na and Ca gain, but not that of K loss which nevertheless became statistically significant. These results show that tissue Na gain observed after inhibition of the sodium pump by ouabain may be related to activation of Na‐H exchange. They also indicate that blockade of Na‐H exchange selectively reduced the inotropic effect of ouabain in guinea‐pig atria resulting from interaction of the glycoside with high affinity binding sites. This confirms previous observations in rat heart.