Action of Na⁺, K⁺, and Ca⁺⁺ ions on the duration of the preautomatic pause of the heart

Abstract

The duration of the preautomatic pause arising after rapid electrical stimulation of the Purkinje fibers of the dog's heart, fibers of the frog's sinus venosus, and the ventricles of the isolated rabbit's heart, was investigated. Action potentials of the cardiac pacemakers were recorded with microelectrodes. In response to brief stimulation (10–20 sec) and a lowered K_int/K_ext, Na_ext/Na_int, or Ca_ext/Ca_int ratio, the automatic activity was depressed more strongly, with a consequent lengthening of the preautomatic pause after the end of electrical stimulation. Conversely, in response to prolonged electrical stimulation (3 min) and a lowered K_int/K_ext, Na_ext/Na_int, or Ca_ext/Ca_int ratio, the depression of automatic activity was reduced and the preautomatic pause after the end of stimulation was shortened. These results explain the conflicting data in the literature on the action of these ions on the duration of the preautomatic pause of the heart.