Ouabain sensitivity of Rb+ uptake in canine Purkinje fibre and ventricular muscle

Comparison of ouabain receptors in sheep myocardium and Purkinje fibres

The conducting system of the heart has been reported to be more sensitive to the toxic effects of digitalis than the working myocardium. To investigate the molecular basis of these observations, we have characterized the ouabain receptor in Purkinje fibres and ventricular muscle of the digitalis-sensitive sheep heart using cell membrane preparations, crude homogenates and contracting heart tissues. [3H]-Ouabain binding has the following characteristics: in sheep left ventricular cell membranes, specific binding was of high affinity (KD 1.9 X 10(-9) M at 37 degrees); was co-incident with an inhibition of (Na+ + K+)-ATPase activity; and was inhibited by K+ and unlabelled cardiotonic steroids; in crude homogenates, the maximal binding capacity but not the affinity for ouabain varied in different parts of the sheep heart with Purkinje fibres containing markedly fewer binding sites (0.33 X 10(14)/g wet weight; left ventricle, 1.3 X 10(14)/g wet weight) and in isolated, contracting Purkinje fibres and right ventricular moderator band strips, concentration-response curves for [3H]-ouabain binding, increase in force of contraction and inhibition of [86Rb+]-uptake were co-incident. In both contracting tissues, a ouabain concentration of 3 X 10(-7) M occupied about 50% of the specific binding sites, gave the maximal inotropic effect without toxicity and inhibited [86Rb+]-uptake by about 50%. The maximal binding capacity was lower in contracting Purkinje fibres (2 X 10(14) binding sites/g wet weight) than in contracting moderator band strips (3.9 X 10(14) binding sites/g wet weight). The maximal inotropic effects were reached slightly faster in Purkinje fibres but toxicity also occurred faster in these fibers. We conclude that the specific ouabain binding site is the receptor mediating positive inotropy and inhibition of (Na+ + K+)-ATPase in the sheep heart. Further, this receptor is identical in both the conducting system and working myocardium but the conducting system contains many fewer receptors. This change in receptor number, rather than affinity, may underlie the increased ouabain toxicity observed in Purkinje fibres.

Effects of myocardial ischemia on cardiac contractility and ion transport in working and Langendorff rabbit heart

In order to define the ischemic myocardial injury in terms of the possible alteration of cardiac membrane properties, Rb+ uptake and 3H-ouabain binding to cardiac tissue were compared in normal and ischemic rabbit hearts. In isolated Langendorff or working rabbit hearts, a global ischemia was produced by reducing the coronary flow of oxygenated Krebs-Henseleit solution for various time intervals. The normal or ischemic tissues were subjected to study the uptake of Rb+ and ouabain bindings. In Langendorff hearts there was little or no significant difference in active uptake of Rb+ in normal and ischemic heart. However, in the working heart, there was a significant decrease in Rb+ uptake in the ischemic hearts. As the case of Rb+ uptake ouabain binding was significantly reduced in ischemic working heart in comparison to working normal heart preparations at high concentrations of ouabain, the present study suggests the importance of cardiac contractility for the cellular energy metabolism and membrane transport activity.