Abstract

We have investigated the effect of ouabain on the efflux and influx of Ca with a specific pool, Ca_II, known to be involved in the contractile process in the heart. Hearts isolated from cats were perfused by a Langendorff apparatus modified to permit rapid substitution of perfusion media. The hearts were preperfused for 30 minutes with Krebs-Henseleit solution, gas perfused for 30 minutes and washed out with either a Ca-free Krebs-Henseleit solution or with a solution containing 0.5 mEq of Ca per liter until contractile force had decayed to 2 g or less. The hearts were then reperfused with a Krebs-Henseleit solution containing 5 mEq of Ca per liter until contractile force was restored to a steady level. In the treated hearts, ouabain (5 x 10^-8 g/ml) was included in all perfusion media. Ouabain affected neither the efflux of Ca from Ca_II nor the decay of contractile force when the hearts were washed out with Ca-free Krebs-Henseleit solution. The T_1/2 for the decay of contractile force was prolonged during washout with the low Ca solution. Ouabain did not further affect the T_1/2 for the decay of contractile force. Contractile force and Ca_II content were restored by reperfusion with Krebs-Henseleit solution containing 5 mEq of Ca per liter. Ouabain significantly increased the rate of Ca accumulation and the quantity of Ca taken up by the heart. An increase in the rate of restoration of contractile force to a significantly higher level accompanied these effects on Ca uptake. The results suggest that ouabain increases the apparent volume of Ca distribution in the heart by increasing the bulk flow of Ca into the cell from the extracellular space. These effects of ouabain on Ca metabolism in the heart may be related to the positive inotropic effect of the drug.