Stimulation and Inhibition of the Sodium Pump by Cardioactive Steroids in Relation to Their Binding Sites and Their Inotropic Effect on Guinea‐pig Isolated Atria

Abstract

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.

Stimulation of Monovalent Cation Active Transport by Low Concentrations of Cardiac Glycosides

“…The conditions for assessment of monovalent cation active transport by guinea pig left atria used in these experiments were similar to those employed by Ghysel-Burton and Godfraind (6), except that our tissue samples were quiescent. The low concentration of ouabain selected, 3 nM, was the concentration reported by Ghysel-Burton and Godfraind (6) Fig.…”

“…Nevertheless, a number of investigators using a variety of tissues have shown apparent stimulation of monovalent cation active transport by low concentrations of cardiac glycosides in the nanomolar range (4)(5)(6)(7)(8), with inhibition at higher concentrations (5,6). The mechanism underlying this biphasic effect remains unexplained.…”

Effect of Cardiac Glycosides on Action Potential Characteristics and Contractility in Cat Ventricular Myocytes: Role of Calcium Overload

“…Part of this limitation stems from the fact that the majority of cardiac glycoside structure-activity studies are focused on Na ϩ /K ϩ -ATPase binding (De Pover and Godfraind, 1982;Brown and Erdmann, 1984), and few address physiological actions in intact systems. Notable exceptions include studies in guinea pig atria showing stimulation of the sodium pump at low concentrations of ouabain and ouabagenin, but not DHO (Ghysel-Burton and Godfraind, 1979), and the demonstration of differences in toxic to therapeutic ratios Ramp protocol experiments: steady-state outward current. Representative current traces elicited by a voltage-clamp ramp protocol (inset).…”

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

“…In guinea-pig isolated atria, it has been proposed that the inotropic effect of ouabain is the sum of two processes: one related to the inhibition of the Na+, K+-pump and another related to a still unknown mechanism (Ghysel-Burton & Godfraind, 1979;Noble, 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).…”