Two types of abnormal automaticity in canine ventricular muscle fibers

When ventricular muscle fibers from dog hearts were exposed to different [Ca2+]0 in K+-free solutions, two types of abnormal automaticity were observed. In the K+-free, Ca2+-free, solution, 9 out of 55 preparations developed spontaneous activity at a reduced membrane potential. The maximum diastolic potential was -47.4 +/- 7.68 mV (n = 9) with slow (less than 20 V/s) upstroke velocity of action potentials. The automaticity was based on membrane oscillations with increasing amplitude to become full-sized responses. It was suppressed after reduction of [Na+]0, by an increase in [Ca2+]0 above 3.6 mmol/l and by application of verapamil. In the K+-free, high-Ca2+, solutions (Ca2+ = 3.6-7.2 mmol/l), oscillatory afterpotentials (OAPs) were observed following the driven action potentials at normal membrane potentials in all 55 preparations. Once the OAPs became large enough to attain threshold, there was the appearance of triggered-automaticity. The automaticity was induced by applying premature stimuli or manipulating [Ca2+]0. It was abolished by application of either verapamil or tetrodotoxin by decreasing the amplitude of the OAPs or the fast Na+ current, respectively, or both in combination. These results indicate that the ventricular muscle fibers can develop abnormal automaticity of two different mechanisms in some conditions, and can be a focus of ectopic impulse formation.