Glycoside inotropy in the absence of an increase in potassium efflux in the rabbit heart.

The inotropic effect of 1.25 times 10(-6)M acetylstrophanthidin (ACS) and the influx and efflux of labeled potassium (42K+) were studied in the arterially perfused rabbit interventricular septum under control conditions and during respiratory acidosis. An increase in the CO2 content of the gas mixture with which the modified Ringer's solution was equilibrated from 5 to 30% reduced the perfusate pH from 7.37 to 6.66. The increment in developed tension in the presence of ACS was 3.0 +/- 0.2 g (n equals 10) under control conditions, but it was greater, 7.1 +/- 0.9 g (N equals 9) during acidosis (P less than less than 0.001). The net K+ loss due to an increase in K+ efflux was 1.8 +/- 0.2 mmoles/kg wet weight in control experiments but only 0.1 +/- 0.1 mmoles/kg net weight under acidotic conitions (P less than less than 0.001); in seven of nine experiments in respiratory acidosis, no increase in K+ efflux occurred despite a marked positive inotropy. In three septums, K+ influx was reduced by ACS during respiratory acidosis. These results demonstrate that during acidosis ACS inhibits sodium-potassium adenosinetriphosphatase (Na+-K+ ATPase) and causes an inotropic effect but does not increase K+ efflux. K+ efflux cannot be linked to calcium (Ca2+) influx or regarded as the controlling factor of glycoside-induced inotropy. The results give further support to the proposal that digitalis-induced inotropy is secondary to an enhancement of a Na+-Ca2+ exchange system.