Manganese and electrogenic phenomena in canine Purkinje fibers

Studies were performed on canine cardiac Purkinje fibers to evaluate the effects of manganese on membrane electrogenesis. The results indicate that manganese has a calciumlike effect on the excitatory sodium current and inhibitory effects on potassium conductance and slow inward current. The calciumlike effect of manganese on sodium current was reflected through a leftward (toward less negative potentials) and downward shift in the curve relating maximum upstroke velocity to membrane potential. The inhibitory action of manganese on potassium conductance was suggested by the following observations. Manganese caused an initial increase in action potential duration largely due to a lengthening of the plateau and decreases in the rates of phase 3 and terminal repolarization. Manganese increased the rate of diastolic depolarization. Manganese blocked the initial fall in maximum diastolic potential accompanying rapid stimulation. Manganese in high concentrations caused generalized depolarization which was reversed by rapid stimulation and by increased extracellular potassium concentrations. The action of manganese to block slow inward current was indicated by the eventual shortening of the plateau and by the elimination of responses initiated from low levels of membrane potential (<-55 mV). In addition to these effects, manganese also reduced membrane excitability, eliminated arrhythmic beats occurring during low frequency electrical stimulation, and caused membrane hyperpolarization which was blocked by tetrodotoxin.