Na, K-activated ATPase activity during and after arrhythmic response to AY-22-241 in the canine heart

Relationship between the positive inotropic effect of ouabain and its inhibitory effects on Na+, K+ -ATPase and active transport of Rb+ in the dog heart

Two previous reports have indicated that in the dog if sustained positive inotropy without arrhythmias is induced by ouabain infusion, inhibition of myocardial Na+, K+-activated adenosine triphosphatase is not observed. Another previous study has shown that in similar experiments induction of sustained inotropy is accompanied by inhibition of the active uptake of Rb+. To resolve the apparent inconsistencies between these findings, we repeated the experiments and measured the inhibition of the enzyme and the inhibition of Rb+ uptake after the induction of sustained inotropy with ouabain. Utilizing three different ouabain dosage regimens in open-chested dogs, we obtained three different quasi-steady state plasma levels of ouabain. At the highest level, ouabain toxicity accompanied by enzyme inhibition and inhibition to RB+ uptake was observed. With the intermediate levels, positive inotropy without significant toxicity was also accompanied by enzyme inhibition and the inhibition of Rb+ uptake. At the lowest ouabain plasma levels, sustained positive inotropy without significant inhibition of either enzyme activity or +b+ uptake was obtained. The data indicate that in this preparation either a small degree of enzyme inhibition, which is difficult to detect is sufficient to produce pronounced positive inotropy, or that the inhibition of the enzyme is not required for the induction of positive inotropy.

Cardenolide analogs. An explanation for the usual properties of AY 22241

AY 22241 (AY) was compared with digitoxigenin with respect to effects on myocardial contractility and inhibition of Na+, K+-ATPase. Ay was less potent than digitoxigenin but otherwise showed the same type of rapid onset and rapid reversal of activity usually associated with genins. Since AY is a glycoside, its behaviour was regarded as anomalous since glycosides usually show both slow onset and slow reversal of activity. Attempts were made to account for the genin-like properties of AY in terms of the model for the digitalis receptor proposed by Thomas et al. (1974a,b). It is suggested that the low potency and genin-like properties of AY may be due to the fact that the steroid is attached to the lactone through the carbon atom that is alpha to the carbonyl group. This feature could result in non-alignment of the sugar residue with the sugar binding site and might also reduce the effectiveness of the interaction of the steroid moiety with its binding site. A notable feature of AY toxicity was its extremely rapid progression to irreversible fibrillation.

The intracellular sodium activity of cardiac Purkinje fibres during inhibition and re-activation of the Na-K pump

1. The intracellular Na activity, aiNa, of sheep heart Purkinje fibres was continuously monitored using Na+-sensitive glass micro-electrodes. The effects of removal and restoration of external K, and of application and removal of various cardioactive steroids, were investigated. 2. The aiNa increased in K-free solutions and rapidly recovered on addition of external K. The rate of this recovery depended on both the external K concentration, [K]o, and the aiNa. The rate of aiNa recovery was found to be half maximally activated at a [K]o of about 10 mM. If corrections are applied to allow for changes in the net passive Na influx at various [K]o, then this value is increased to approximately 12.5 mM. 3. At a given [K]o, there appeared to be a linear relationship between the rate of aiNa recovery and the level to which aiNa had increased in K-free solution (over the range of aiNa from 7.5 to 31 mM). 4. Addition of the cardioactive steroids strophanthidin, acetylstrophanthidin, actodigin (AY 22,241) or dihydro-ouabain produced rapid changes of aiNa. At low concentrations, these compounds sometimes produced a small decrease in aiNa, while at concentrations above 10(-7) M they produced a dose-dependent increase. 5. The effects on aiNa of both low and high concentrations of all these cardioactive steroids were readily reversible within 120 min. The time course of the aiNa recovery mainly depended on the concentration of the cardioactive steroid applied, and on the level to which aiNa had increased. 6. Upon addition of a cardioactive steroid (above 10(-7) M, aiNa at first increased almost linearly with time. The rates of such an increase were measured during this period at various cardioactive steroid concentrations and used to produce dose-response curves. The concentrations that produced a half-maximum rate of aiNa increase were near to 10(-6) M for strophanthidin and acetylstrophanthidin, but near to 10(-5) M for actodigin and dihydro-ouabain. 7. The mean maximum rate of aiNa increase produced by the addition of a high cardioactive steroid concentration was 0.49 +/- 0.17 mM/min (+/-S.D., n = 21). This would indicate a net passive Na influx into the cells of approximately 2.8 p-mole/cm2sec. 8. This maximum rate of aiNa increase could be achieved by the addition of 10(-5) M-strophanthidin or acetylstrophanthidin, but 10(-4) to 10(-3) M-actodigin or dihydro-ouabain was required to produce a similar rate of increase. 9. The addition of these high cardioactive steroid concentrations produced an initially rapid increase of aiNa. After 15-30 min this aiNa increase slowed considerably. The aiNa appeared to reach a 'plateau' within 2-4 hr at levels much below those predicted for a Na electrochemical equilibrium across the cell membrane.

Correlation between inhibition of (Na+, K+)-membrane-ATPase and positive inotropic activity of cardenolides in isolated papillary muscles of guinea pig

Concentrations of 17 cardenolides, cardenolide glucuronides and sulfates producing half-maximal inhibition of (Na+, K+)-membrane-ATPase from different organs and animal species were determined in vitro. In addition the concentrations that increased the contractility of guinea pig isolated papillary muscles to a particular level were investigated. Comparisons between ATPase-inhibiting and positive inotropic cardiac activities showed extensive parallelism: the correlation coefficients after log/log transformation were between 0.92 and 0.97. The same close correlations are found if dissociation constants of cardenolide receptor complexes and concentrations causing 86Rb-uptake inhibition in human erythrocytes are examined. The concentrations necessary for inhibition of (Na+, K+)-membrane-ATPase of the guinea pig heart and the concentrations required to achieve a defined positive inotropic effect in guinea pig papillary muscle showed a log/log correlation coefficient of 0.97 (P less than 0.001). In both tests the potencies covered more than three orders of magnitude. The results support Repke's hypothesis on the digitalis receptor.

The relationship between Na+, K+-ATPase inhibition and cardiac glycoside-induced arrhythmia in dogs

In order to determine if there is a relationship between Na+, K+-ATPase inhibition and cardiac glycoside-induced arrhythmia, the time course of the onset and offset of the arrhythmia induced by the semi-synthetic glycoside, actodigin, and the enzyme activity during arrhythmia and following reversion to normal sinus rhythm was studied in the intact, anethetized dog. An infusion of actodigin(AY22,241) at the rate of 0.1 micronmol/kg/min for 30 min induced a severe and persistent arrhythmia within 13.1 +/- 192 min of 9 dogs. Upon termination of the actodigin infusion, the arrhythmia spontaneously converted to sinus rhythm within 17.5 +/- 2.3 min. Left ventricular tissue was taken from dogs sacrificed at the peak of the actodigin-induced arrhythmic periods or from the dogs that were allowed to recover from the actodigin-induced arrhythmia. These samples were homogenized and the membrane-containing fraction was passed through a Millipore filter. The membrane fraction trapped in the filter was then assayed for Na+ + K+ stimulated, Mg2+ dependent ATPase acctivity. The results showed that, in comparison to the time matched control dogs, the cardiac microsomes prepared from the arrhythmic dogs had a markedly reduced Na+, K+-ATPase activity. On the other hand, actodigin-treated dogs that were allowed to recover from the arrhythmic episode had Na+, K+-ATPase activity that was not significantly different from the control values. The amount of 3H-actodigin bound by the cardiac muscle microsomal fraction was also investigated. The microsomes from left ventricle were isolated with a slight modification of the method of Dutta et al. (1968). The microsomal binding of 3H-actodigin was maximum at 30 min (26.6 mol/mg protein) when the sample was prepared from the dogs at the peak of the arrhythmic effect. However, the binding was significantly reduced (11.5pmol/mg protein) in the microsomal fraction from hearts that had returned to sinus rhythm. These data provide direct evidence that inhibition of Na+, K+-ATPase and cardiac glycosideinduced arrhythmia may have some cause and effect relationship.