The inotropic effects of strophanthidin in Purkinje fibers and the sodium pump

Contribution of the Na+/K+-pump to the membrane potential

The inward movement of sodium ions and the outward movement of potassium ions are passive and the reverse movements against the electrochemical gradients require the activity of a metabolism-driven Na+/K+-pump. The activity of the Na+/K+-pump influences the membrane potential directly and indirectly. Thus, the maintenance of a normal electrical function requires that the Na+/K+-pump maintain normal ionic concentrations within the cell. The activity of the Na+/K+-pump also influences the membrane potential directly by generating an outward sodium current that is larger when the Na+/K+-pump activity is greater. The activity of the Na+/K+-pump is regulated by several factors including the intracellular sodium concentration and the neuromediators norepinephrine and acetylcholine. The inhibition of the Na+/K+-pump can lead indirectly to the development of inward currents that may cause repetitive activity. Therefore, the Na+/K+-pump modifies the membrane potential in different ways both under normal and abnormal conditions and influences in an essential way many cardiac functions, including automaticity, conduction and contraction. Key words. Active transport of ions; cardiac tissues; electroneutral and electrogenic Na+/K/-pump; control of Na+/K+-pump; normal and abnormal electrical events.