Shortening of the cardiac action potential due to a brief injection of KCl following the onset of activity

At 100 C the action potential of a turtle heart has a duration of 3-5 sec. Substances injected into the coronary artery reach the muscle fibres with a delay of the order of 1 sec. It is thus possible to change the extracellular ionic composition during a single action potential, and to study the influence of such changes on the potential-time course. The present results show that a rapid rise of the extracellular potassium concentration shortens the action potential. This finding may be relevant to an interpretation of the normal potential-time course during cardiac activity.