Independence of the positive inotropic effect of ouabain from the inhibition of the heart Na+/K+ pump.

Abstract

Isolated left atria from guinea pigs were stimulated at 3.3 Hz and bathed at 30'C in Tyrode's solution containing 6 mM KCL. After equilibration, this solution was replaced by a low-K solution or by Tyrode's solution containing ouabain or dihydroouabain. These treatments evoked an increase in the contractility of the atria. The time to peak increase was about 30 min, and the inotropic effect was sustained for at least 40 min. After 30 min, 42K was added to the bathing solution in order to estimate the activity of the Na+/K+ pump. A linear relationship was observed between the degree of inhibition of the Na+/K pump and the increase in systolic tension. The regression line was the same for low-K solutions and dihydroouabain but not for ouabain. For a given degree of inhibition of the pump, ouabain evoked a higher increase in contractility.These findings indicate that inhibition of the Na+/K+ pump can be the only mechanism responsible for the positive inotropic effect of dihydroouabain but cannot be the sole mechanism for that of ouabain.The positive inotropic effect evoked by cardiac glycosides has been attributed to inhibition of the Na+/K+ pump (1-5). One of the main supports of this hypothesis is the observation that any intervention that increases intracellular Na+ content enhances the twitch tension in cardiac muscle. Evidence has been presented for the existence of a membrane Na+/Ca2+ exchange operating in such a way that an increase in cytosolic Ca2+ concentration occurs when intracellular Na+ increases (5-7). Because the contractile state of the heart muscle is determined by the concentration of Ca2+ in the cytoplasm, the above hypothesis has received much credit.However, several observations indicate that the mode of action of cardiac glycosides is more complex. It has been reported that low doses of ouabain stimulate the Na+/K+ pump and increase the contractility of the heart (8)(9)(10)(11)(12)(13)(14). This suggests that more than one mechanism could be responsible for the inotropic effect. In order to examine this possibility further, we investigated the relationship between the inhibition of the Na+/K+ pump and the positive inotropic effect evoked by low-potassium solutions, by ouabain, and by dihydroouabain; dihydroouabain is a glycoside with a saturated lactone ring which does not stimulate the Na+/K+ pump (11)(12)(13)(14). The results show that inhibition of the Na+/K+ pump could be the only mechanism responsible for the positive inotropic effect of dihydroouabain but not for that of ouabain.MATERIALS AND METHODS Physiological Solutions. Preparation. Albino guinea pigs weighing approximately 400 g were killed by a blow on the head and bled; the hearts were rapidly removed. The atria were dissected out in physiological solution containing 2.7 mM KC1. The left atria were suspended in an organ bath between platinum electrodes, under a resting tension of 500 mg. The atria were stimulated with rectangular 10-msec pulses (strength at least twice threshold) at a rate of 3.3 Hz. Recordings of the co...

The inotropic effect of ouabain and its antagonism by dihydroouabain in rat isolated atria and ventricles in relation to specific binding sites

“…However, in other tissues, such as sheep Purkinje fibres (Hart, Noble & Shimoni, 1983) and guinea-pig atria (Godfraind & Ghysel-Burton, 1980), it has been proposed that the inotropic effect of ouabain is due to more than one mechanism and the present observations on rat heart are in agreement with this theory.…”

“…This is in agreement with the existence of an antagonism exerted by dihydroouabain to the ouabain inotropic effect. It has been proposed that the ouabain inotropic effect is due to the inhibition of the Na+, K+-pump (Repke, 1963;Langer, 1977), but this theory is not universally accepted (Ghysel-Burton & Godfraind, 1980;Noble, 1980;. In rat ventricles, the range of concentrations at which ouabain inhibits the pump appears to be higher than 10-5 M according to Erdmann, Philipp & Scholz (1980).…”

“…In this preparation, Na+, K+-pump inhibition seems to be the only mechanism responsible for the positive inotropic effect of dihydroouabain, the derivative of ouabain with a saturated lactone ring (Godfraind & Ghysel-Burton, 1980). Furthermore, dihydroouabain has been shown to act as an antagonist to the positive inotropic effect of ouabain that is independent of Na+, K+-pump inhibition (Godfraind, Ghysel-Burton & De Pover, 1982).…”

Ouabain effects on intracellular potassium activity and contractile force in cat papillary muscle.

“…RuizCeretti et al (10) showed that a similar effect occurs in rabbit ventricles. Godfraind (27)(28)(29)(30) showed that low concentrations of ouabain and digoxin that increase contractile force also stimulate 42K uptake by and Na content of guinea pig atria. In sheep Purkinje fibers, 0.1 M ouabain produced depletion of cleft potassium as judged from reversal potential for the iK2 current, again suggesting pump stimulation (20 The indices of Na-K pumilp f'unctioll examined in this study are a, and a'.…”

“…Cats weighing 1.5-3.0 kg were anesthetized with sodium pentobarbital, 30 mg/kg i.p. The hearts were rapidly removed and immersed in Tyrode's solution of the following composition in mM: NaCl, 135.4; KC1, 5.4; NaH2PO4, 1.8; CaCl2, 2.7; MgCl2, 0.5; NaHCO3, 18; and dextrose, 5.5.…”