The uptake of cardiac glycosides in relation to their actions in isolated cardiac muscle

Abstract

Summary1. The uptake of 3H-digitoxin, 3H-ouabain and 3H-dihydro-ouabain by isolated guinea-pig atria has been studied and compared with the inhibition of the sodium pump and with the inotropic effect.2. Analysis of the curve relating the uptake of digitoxin and ouabain at equilibrium to the bath concentration enabled a non-saturable and a saturable binding site to be distinguished. 3. The uptake of inactive doses of dihydro-ouabain was only by a nonsaturable mechanism. 4. The uptake of labelled digitoxin and ouabain was reduced in the presence of another glycoside. The amount of bound glycoside was nearly equivalent to the estimated non-saturable uptake. 5. The uptake was reduced at 40 C to the clearance of the non-saturable site. 6. ED50 of digitoxin and of ouabain for inhibition of the sodium pump were measured and compared to the ED50 for inotropic effect and to the concentrations producing a half-saturation of the saturable binding site. 7. It is concluded that binding to the saturable site may be responsible for the cardiac actions of the glycosides.

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

“…Studies on human heart slices have shown that the sodium pump was inhibited by con-0007-1188/79/060175-10 $01.00 centrations equivalent to those found in the blood of the intoxicated patients, whereas the therapeutic concentrations stimulated the activity of the sodium pump (Godfraind, 1972;. Similar observations have been made with isolated, spontaneously beating atria of the guinea-pig: low concentrations of ouabain near 10-9 mol/l stimulated 42K uptake whereas higher concentrations (from 10-8 mol/l) inhibited (Godfraind & Lesne, 1972). These findings are in agreement with electrophysiological studies on Purkinje fibres showing that low doses of ouabain produce changes in the K gradient that reflect stimulation of the sodium pump (Cohen, Daut & Noble, 1976).…”

“…correction for the content in the inulin space. As shown previously for ouabain concentrations higher than 3 x 10 8 molA (Godfraind & Lesne, 1972), the uptake of ouabain by guinea-pig atria followed the equation: (Godfraind, & Lesne, 1972). The value of parameter a has also been estimated by measuring the residual binding in the presence of a large excess of digitoxin used as a competitor.…”

“…Ouabain-sensitive K content is defined as the cellular K content of guinea-pig atria which is lost during incubation of atria in the presence of ouabain concentrations producing the maximum K loss; this is the same concentration as the one which evoked a maximal inhibition of 42K uptake (Godfraind & Lesne, 1972). Once the equilibrium effect of the glycoside has been reached the changes in cellular K and Na content, expressed as a percentage of the maximal change with the most effective drug, probably reflect the percentage change in the rate of the Na pump.…”

Time Course of Ouabain Uptake in Isolated Myocardial Cells: Dependence on Extracellular Potassium and Calcium Concentration

“…Two different, specific, myocardial binding sites for digitalis glycosides have been suggested (Godfraind & Lesne, 1972). Accordingly, one might suspect that two binding sites would have different rate constants for uptake of ouabain.…”

“…The positive inotropic effect of digitalis is presumably due to an inhibition of the membrane (Na+ + K+) adenosinetriphosphatase (ATPase) (Godfraind & Lesne, 1972;Godfraind & DePover, 1973), but a digitalis-induced enhancement of the calcium transport into the myocardial cell has also been claimed (Bailey & Dresel, 1968;Shine, Serena & Langer, 1971;Langer, 1977).…”