Frequent stimulation of the guinea-pig myocardium raises the inotropic efficacy of tissue-bound ouabain

Modulation of ouabain binding in beating ventricular myocardium from guinea-pigs: effects of lidocaine and monensin

To further elucidate the interdependence between digitalis sensitivity and the cellular Na+ load, the influence of two Na+ load modifying drugs, monensin and lidocaine, on the concentration-dependence of ouabain binding and ouabain effects was studied in beating ventricular strips from guinea-pig heart. Monensin (3 x 10(-6)M), a Na+ ionophore known to elevate Na+ influx, enhanced 3H-ouabain binding (by approximately 40%) as well as the ouabain effect at non-toxic ouabain concentrations, and it shifted the threshold for toxicity towards threefold lower ouabain concentrations. Lidocaine (2 x 10(-4)M), a Na+ channel blocker, lowered ouabain binding by about one third, and it extended the ouabain concentration range tolerated without toxicity by a factor of three. In the concentrations used, neither compound exerted any direct effect on ouabain binding studied in isolated cardiac membranes. The binding data obtained in the muscle strips were well fitted by a mathematical model which quantifies the dependence of ouabain binding on the underlying (Na+ +K+)-ATPase activity. These findings provide evidence for an indirect drug-induced modulation of ouabain binding via the interference with the cellular Na+ load.

Intracellular sodium affects ouabain interaction with the Na/K pump in cultured chick cardiac myocytes

Whether a given dose of ouabain will produce inotropic or toxic effects depends on factors that affect the apparent affinity (K0.5) of the Na/K pump for ouabain. To accurately resolve these factors, especially the effect of intracellular Na concentration (Nai), we have applied three complementary techniques for measuring the K0.5 for ouabain in cultured embryonic chick cardiac myocytes. Under control conditions with 5.4 mM Ko, the value of the K0.5 for ouabain was 20.6 +/- 1.2, 12.3 +/- 1.7, and 6.6 +/- 0.4 microM, measured by voltage-clamp, Na-selective microelectrode, and equilibrium [3H]ouabain-binding techniques, respectively. A significant difference in the three techniques was the time of exposure to ouabain (30 s-30 min). Since increased duration of exposure to ouabain would increase Nai, monensin was used to raise Nai to investigate what effect Nai might have on the apparent affinity of block by ouabain. Monensin enhanced the rise in Na content induced by 1 microM ouabain. In the presence of 1 microM [3H]ouabain, total binding was found to be a saturating function of Na content. Using the voltage-clamp method, we found that the value of the K0.5 for ouabain was lowered by nearly an order of magnitude in the presence of 3 microM monensin to 2.4 +/- 0.2 microM and the magnitude of the Na/K pump current was increased about threefold. Modeling the Na/K pump as a cyclic sequence of states with a single state having high affinity for ouabain shows that changes in Nai alone are sufficient to cause a 10-fold change in K0.5. These results suggest that Nai reduces the value of the apparent affinity of the Na/K pump for ouabain in 5.4 mM Ko by increasing its turnover rate, thus increasing the availability of the conformation of the Na/K pump that binds ouabain with high affinity.

How fast do cardioactive steroids act independently of diffusion in guinea-pig myocardium?

1 The rate of onset of the inotropic effect of different cardioactive steroids (in order of increasing potency: dihydroouabain, digoxigenin, digoxin, ouabain, digitoxin) was studied in guinea-pig papillary muscles stimulated at 0.5 Hz. For an estimate of diffusion rate the time to defined levels of effect was evaluated in relation to the diameter of cylindrical muscles. 2 The dependence of onset rates on muscle diameter was more pronounced with a highly potent steroid, e.g., digitoxin, than with a less potent compound, e.g., dihydroouabain; that is, the apparent rate of diffusion was inversely correlated with the inotropic potency. Reduction of the ratio of receptor to steroid concentration with increase of ouabain or K+ concentration enhanced the apparent rate of diffusion. 3 Extrapolation to negligibly short diffusion distance indicated that the effects of the various steroids develop faster in the absence of diffusion. The effect of 50 mumol l-1 of dihydroouabain appeared more quickly than with ouabain in the perfused heart. The time courses of the inotropic effect in perfused hearts and in papillary muscles of diameters less than 0.75 mm were superimposable, indicating that the onset of the dihydroouabain effect was not controlled by diffusion. 4 After the interference of diffusion had been excluded, the rates of onset of action correlated inversely with the inotropic potencies of the steroids. The dissociation rate of the drug-receptor complex appeared to be related to the different receptor affinities.