Factors Controlling Pacemaker Action in Cells of the Sinoatrial Node

Abstract

Rapid drive of isolated pacemaker tissues from cats resulted in a post-drive depression followed by a late acceleration to supernormal rates of pacemaker activity. These effects were similar to those occurring after drive of the pacemaker in situ . Lower SA nodal pacemakers discharged more slowly and irregularly than did upper SA nodal pacemaker cells. They were more readily depressed by rapid imposed drive. The balance between depression and acceleration varied in different preparations. Drive at only slightly above the intrinsic rate resulted frequently in acceleration not preceded by depression. Within limits, the greater the frequency and duration of drive, the greater the intensity and duration of both the depression and the late acceleration. Prostigmin augmented and atropine reduced post-drive depression. Cocaine potentiated the late acceleration. Excess potassium reduced post-drive depression and, in concentrations used, caused some acceleration. Pacemaker cells could be driven less rapidly than could other SA nodal cells. Drive generally shifted pacemaker action to a distant site; the first post-drive propagated responses originated from other pacemaker cells and dominance by the original unit was reestablished slowly. Rapid drive reduced amplitudes of action potentials and prepotentials. It also raised threshold potentials and during the post-drive period the durations of pacemaker cell action potentials were temporarily prolonged. In some preparations membrane potentials remained at a subnormal value after drive. Subthreshold potentials occurred at a somewhat subnormal rhythm but gradually developed an effective amplitude. Conduction block was observed in isolated SA nodal tissue. This was augmented during the post-drive period of depression. This work lends support to the hypothesis 1 that dominating action by pacemaker cells influences the pacemaker activity in other potential pacemaker tissues.

Mechanisms underlying overdrive suppression and overdrive excitation in guinea pig sino-atrial node

“…In Tyrode solution, both dominant and subsidiary pacemaker action potentials are present [8,[11][12][13].…”

“…The consequent enhancement of the diastolic potentials leads to overdrive excitation. Instead, overdrive suppresses the discharge of cat surgically separated SAN subsidiary pacemakers (see [8]), since the latter are fast responses (enhancement of the electrogenic Na + extrusion by overdrive) and their DD is too negative for ThV os to be present [20]. In intact SAN, the reduced slope of DD of subsidiary pacemakers by Na + extrusion would electrotonically antagonize overdrive excitation of dominant pacemakers.…”

“…Overdrive of cardiac pacemakers is followed by a temporary suppression of discharge which is much shorter in the sino-atrial node [8,9] than in Purkinje fibers [10]. The reason for this difference has not been determined.…”

“…In dominant pacemaker cells, a prevalent extrusion of Ca 2+ would steepen the slope of diastolic depolarization whereas in subsidiary pacemakers a prevalent extrusion of Na + would decreases it. In fact, overdrive of the cat SAN subsidiary pacemaker region surgically separated from the dominant region is followed by a prolonged inhibition [8], as in Purkinje fibers (''overdrive suppression'', [10]). Conversely, when in high [K + ] o SAN action potentials acquire the characteristics of dominant responses [16][17][18][19][20], overdrive may enhance or induce spontaneous discharge [18,20] (''overdrive excitation'', [21]).…”

“…Since the electrical changes associated with overdrive have been reported [8,18,20], the main focus was on force behavior, as it gives essential insights on both Ca 2+ changes on a beat to beat basis and simultaneous rate changes.…”