Electrically induced automaticity in canine ventricular myocardium

Repetitive spontaneous action potentials (SAP) could be induced in canine ventricular and atrial muscle, although this inhibitory action was antagonized by the pretreatment with voltage range between about -60 mV and 0 mV. The SAP seemed dependent on both slow inward Ca2+ and Na+ currents and was suppressed by verapamil, Mn2+, and diltiazem, but not by tetrodotoxin. The increase of extracellular potassium concentration also suppressed the SAP. Acetylcholine could not block the SAP in ventricular muscle, but inhibited that in atrial muscle, although this inhibitory action was antagonized by the pretreatment with atropine. The automatic activity was attributed to slow inward Ca2+ and Na+ currents modified by decreasing time-dependent K+ outward current and K+ anomalous rectification.