J. Ghyselburton

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

1 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 KCl concentration from 2.7 to 12 mm decreased Nai 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 KCl 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 KCl 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.

Competitive and stereoselective histamine H1 antagonistic effect of cicletanide in guinea-pig isolated ileum.

The histamine H1 antagonistic effects of the racemic form and the enantiomers of cicletanide, a new antihypertensive furopyridine derivative, were investigated in guinea-pig isolated ileum. Both the racemic and the (-) enantiomer behaved as competitive histamine antagonists (pA2 values of 6.8 and 7.2, respectively). The (+) enantiomer was at least 100 times less potent than the (-) enantiomer. The H1-blocking effect of cicletanide is the most potent and is the only stereoselective property so far reported for the drug.

Hypothesis for the Mechanism of Stimulation of the Na/K Pump by Cardiac Glycosides—Role of Endogenous Digitalis-Like Factor

Publisher Summary This chapter discusses the role of endogenous digitalis-similar factor and provides hypothesis for the mechanism of stimulation of the Na/K pump by cardiac glycosides. Cardiac glycosides exert a biphasic action on the activity of the Na/K pump in isolated heart preparations. At high concentrations they inhibit the pump, whereas at low concentrations they stimulate it. The stimulation of the pump has been observed in several laboratories using different methodologies and preparations. This action is antagonized by KC1 and requires an unsaturated lactone ring. Indeed, concentrations of glycoside, which stimulate the pump, are without effect or are slightly inhibitory on guinea pig heart Na, K-ATPase preparations. In view of the data that is mentioned in the chapter, the pump could be repressed down to 70% in guinea pig atria. Recent studies show that the endogenous digitalis-like factor can be identified in several tissues. Therefore, the chapter examines the possible existence of this factor in guinea pig heart.

Cardioactive Properties of Sc4453, a Digoxin Analog With a Beta-pyridazine-c-17 Ring

Abstract SC4453 is a digoxin analogue with a pyridazine instead of a lactone ring in C 17β . SC4453 was compared with digoxin with respect to effect on contractility and on activity of the sodium pump in guinea-pig isolated left atria stimulated at 3.3 Hz. The two glycosides stimulated the sodium pump at low concentrations and inhibited at high concentrations. At the time to peak inotropic effect, for a similar inhibition of the Na pump, the increase in systolic tension was higher with SC4453 than with digoxin, whereas the increase in diastolic tension was similar for both. These observations confirm that the inhibition of the Na pump is not the only mechanism responsible for the positive inotropic effect of cardiac glycosides.

Role of Na-H exchange in the inotropic action of Bay K 8644 and of ouabain in guinea-pig isolated atria.

1 The inotropic effects of two concentrations of ouabain and of Bay K 8644 have been studied in isolated left atria of the guinea‐pig in physiological solutions at pH lowered from 7.4 to 6.0 and in the presence of ethylisopropylamiloride (EIPA) an inhibitor of Na+/H+ exchange. The low concentration of ouabain (300 nm) was chosen to saturate the high affinity binding sites (it occupied about 7% of the low affinity sites). The high concentration of ouabain saturated both high and low binding sites. Bay K 8644 evoked a positive inotropic effect of a magnitude similar to ouabain (300 nm). 2 When comparing the positive inotropic effects of equi‐effective concentrations of ouabain (300 nm) and of Bay K 8644 (100 nm), it was observed that extracellular acidification specifically depressed the inotropic effect of ouabain 300nm; the positive inotropic effect of the high concentration of ouabain (3 μm) was barely affected by extracellular acidification. 3 EIPA 10 μm depressed the positive inotropic effect of ouabain 300 nm, but did not affect the peak response to Bay K 8644. The depressant action of EIPA on the positive inotropic effect of ouabain was concentration‐dependent and was much more obvious on the effect of ouabain 300 nm than on ouabain 3 μm. 4 An increase in diastolic tension was evoked by 3 μm but not by 300 nm ouabain. This increase in tone was reduced dose‐dependently by EIPA (10–30 μm). It was also significantly reduced when the extracellular pH was equal to 6.4 or 6.0. 5 Ouabain (300 nm) evoked a gain in tissue Na and an equivalent loss in tissue K. Acidification of the extracellular pH down to pH 6.0 evoked a pH‐dependent reduction of Na gain but left K loss unaltered. EIPA 10, 20 and 30 μm evoked a significant reduction of Na gain without significantly affecting K loss. 6 Ouabain (3 μm) evoked a large gain in tissue Na and an equivalent loss of K. Tissue Ca content was also increased. Acidification of the extracellular pH from pH 7.4 to pH 6.9 evoked a significant reduction of Na and Ca gain; changes in tissue K were not significant. Acidification down to pH 6.0 increased reduction of Na and Ca gain, but not that of K loss which nevertheless became statistically significant. 7 These results show that tissue Na gain observed after inhibition of the sodium pump by ouabain may be related to activation of Na‐H exchange. They also indicate that blockade of Na‐H exchange selectively reduced the inotropic effect of ouabain in guinea‐pig atria resulting from interaction of the glycoside with high affinity binding sites. This confirms previous observations in rat heart.

Contractile-force Effects of Low Concentrations of Ouabain in Isolated Guinea-pig, Rabbit, Cat, and Rat Atria and Ventricles

Publisher Summary This chapter presents the contractile force effects of low concentrations of ouabain in isolated guinea pig, rabbit, cat, and rat atria and ventricles. It is well established that active transport of Na,K-adenosine tri phosphate (ATP)ase across cell membranes is catalyzed by Na,K-ATPase and that the binding of ouabain to isolated Na,K-ATPase and the subsequent inhibition of enzyme activity are directly related. The relationship among these events and a positive inotropic action on heart muscle is still the subject of considerable and increasing controversy. The possibility that two or more sites for digitalis binding exist in cardiac sarcolemma either associated, or not, with the Na,K-ATPase, has also been promulgated for years. Stimulation of Na,K-ATPase, transient decrease in intracellular sodium, stimulation of 42K+ uptake and more negative reversal potentials have been reported with very low concentrations of glycosides. The studies presented in this chapter is found that low concentrations of ouabain have no effect on contractile force of papillary muscles of guinea pig, rabbit, and cat. Higher concentrations of ouabain produce in these ventricular preparations the expected increase in contractile force, with subsequent development of toxicity at concentrations of l0–6 M and higher. In strips prepared from left atria, low doses of ouabain produced apparent negative inotropic effects that were shown to be an extension of a continuous natural decline of contractile force.