Control of ionic permeabilities in normal and ischemic heart.

The major ionic conductances underlying electrical activity in cardiac tissues are described. The participation of electrogenic active transport in electrical phenomenon and the influence of metabolic inhibition on cardiac action potentials are briefly summarized. Some electrophysiological effects of lactate and acidosis, such as might be induced by ischemia, are described. In dog Purkinje fibers, lactate (20 mM pH 7.0) may induce transient periods of arrhythmias. Acidosis decreases rapid sodium conductance, slow calcium-sodium conductance, and anomalous and delayed rectifications in frog atrial fibers. CO2-induced acidosis (20% CO2, pH 6.6) may alter the repolarization phase of the action potential in dog Purkinje fibers, presumably because it decreases potassium conductance. Alterations consist of partial depolarizations (humps) that result in reexcitation of the fibers and lead to a maintained depolarization. It is proposed that acidosis induces a decrease in potassium conductance that can be responsible for ectopic foci causing arrhythmias during ischemia.