Changes in intracellular Na⁺ during Na,K pump inhibition in sheep cardiac tissues

The differences in inotropic and toxic sensitivities of cardiac ventricular tissue to cardiac glycosides were investigated in order to determine whether or not the reportedly greater sensitivity of Purkinje fibers compared to myocardium is the result of a fundamentally different response to Na,K pump inhibition. I measured the changes in intracellular Na⁺ activity (a_Naⁱ) in the two tissue types simultaneously during exposure to actodigin (4.0 μm) and ouabain (0.5 μm) under quiescent conditions. A sheep papillary muscle and a Purkinje fiber from the same heart were placed in an experimental chamber and measurements of a_Naⁱ from both were obtained with Na⁺-sensitive microelectrodes. In five experiments in which all electrode impalements were successfully maintained, actodigin caused similar changes in a_Naⁱ in the two tissues (from 7.2 ± 1.0 mm in control to 12.9 ± 1.9 mm in the presence of drug in papillary muscles compared to 7.3 ± 0.3 mm in control and 13.2 ± 1.0 mm in Purkinje fibers; means ± s.e.m.). After washout, exposure to ouabain increased a_Naⁱ in both papillary muscles and in Purkinje fibers (from 7.2 ± 0.7 mm in control and 16.2 ± 1.4 mm during exposure to drug in papillary muscles compared to 7.4 ± 0.3 mm and 14.9 ± 0.8 mm in Purkinje fibers). In fact, ouabain caused a greater increase of a_Naⁱ in papillary muscles than in Purkinje fibers (P<0.01). The differences in tissue sensitivity to digitalis, therefore, are probably the result of other physiological characteristics of the tissues such as the dissimilar rates of Ca²⁺ influx that may result from the differences in ionic current and transmembrane potentials that exist during their characteristic action potentials. These results demonstrate that despite profound differences in cell ultrastructure, tissue geometry and electrophysiology, Purkinje fibers and myocardium share a strikingly similar ability to regulate Na⁺_i during moderate Na,K pump inhibition.