Changes in the electrical activity of dog cardiac Purkinje fibres at high heart rates.

Publisher Website
Google Scholar
Abstract
Rate-dependent changes in the electrical activity of dog Purkinje fibers were studied. At high rates of stimulation the rate of repolarization is greater, the action potential is shorter, the maximum diastolic potential is increased, the pace-maker potential is reduced in amplitude and on cessation of rapid stimulation there can be a suppression of spontaneous activity. After an increase of the stimulus frequency there is an abrupt shortening of the action potential, which can be attributed to incomplete recovery of the plateau currents; this is followed by a progressive decline in action potential duration over the next several hundred seconds. The factor responsible for the slow changes in duration could also be responsible for the accompanying increase in maximum diastolic potential because this develops along a similar time course. These slow changes in electrical activity were investigated with the phase-plane technique. They are the result of an increase in the net outward current over a wide range of potentials (.apprx. -10 to .apprx. -90 mV) during the repolarization phase of the action potential. In voltage-clamp experiments background current was observed to be strongly rate dependent: the background current during a test voltage-clamp pulse after a train of action potentials is more outward at higher stimulus frequencies. When the frequency is increased, background current slowly becomes more outward over several hundred seconds and this change therefore occurs along the appropriate time course to explain the slow alteration in electrical activity under these conditions. The extra outward background current at high rates is relatively independent of membrane potential in the range from -110 to -40 mV (more circumstantial evidence indicates that this range may extend to at least +10 mV); this potential dependence is similar to that of the Na-K-pump current. Strophanthidin and ouabain, agents known to block the Na-K pump, alter both the changes in background current and the slow rate-dependent changes in electrical activity. Although after an increase in rate there is a gradual change in background current that can be explained by an increase in electrogenic Na-K-pump activity, the initial effect of switching rate is to produce a change in current that is consistent with an increase of the extracellular K concentration. A transient increase in the K concentration of restricted extracellular clefts has been recorded under these conditions in dog Purkinje strands by Kline et Kupersmith using K-sensitive microelectrodes. The effect on electrical activity of these changes is discussed. The pace-maker current, If, is unchanged at different rates and the decrease in the amplitude of the pace-maker potential at high frequencies is therefore the result of the increase in membrane conductance at diastolic potentials produced by the pump-induced hyperpolarization.