I The actions of ouabain, ouabagenin and dihydroouabain on the contractility and on the ionic content have been investigated in left guinea-pig atria stimulated at 3.3 Hz. The specific binding of ouabain and its displacement by the other cardenolides have been determined. 2 The action of either ouabain or ouabagenin on Na and K content was qualitatively different according to the concentration employed. Low doses evoked a reduction of Nai whereas high doses produced an increase. Dihydroouabain evoked only a Nai gain.3 The increase of KCI concentration from 2.7 to 12 mm decreased Na, in untreated atria and displaced ouabain dose-effect curves to the right.4 ED50 values for the positive inotropic effect were lower than ED50 values for the inhibition of the pump and were not similarly affected by an increase in KCI concentration. 5 The specific binding of ouabain occurred at high and low affinity sites, related to Na pump stimulation and inhibition respectively. 6 The increase in KCI reduced the affinity of the two groups of sites for ouabain and increased the capacity of the high-affinity sites whereas the capacity of the other sites remained unchanged. 7 The results confirm the existence of two specific binding sites for ouabain in guinea-pig heart and suggest that the inhibition of the Na pump is not the only mechanism responsible for the positive inotropic effect of cardiac glycosides.
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 Na+, K+-pump controls a wide variety of cellular systems and its inhibition by cardiac glycosides modifies important physiological functions and evokes several pharmacological effects (refs 1, 2 and refs therein). However, not all the actions of cardiac glycosides can be attributed to Na+, K+-pump inhibition and several observations show that, at low doses, cardiac glycosides stimulate the pump. It has been proposed that their positive inotropic effect could be the sum of two processes: the inhibition of the pump and a still unknown additional inotropic mechanism. In guinea pig heart, low doses of ouabain interact with high-affinity binding sites, which differ from the lower-affinity sites responsible for Na+, K+-pump inhibition. It has been suggested that ouabain interaction with these high-affinity sites could be responsible for the additional inotropic mechanism. The existence of two classes of ouabain-binding sites has been documented not only in guinea pig heart, but also in dog, rat and human heart. Dihydroouabain, a derivative of ouabain in which the lactone ring is saturated, is about 50-fold less potent than ouabain as an inhibitor of Na+, K+-pump and does not stimulate the pump at low doses. Its inotropic effect can be entirely accounted for by the inhibition of the pump. We have examined the pharmacological action of ouabain in the presence of dihydroouabain and report here that dihydroouabain reduces ouabain inotropic action but not Na+, K+-pump inhibition.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.