The role of diastolic voltage oscillations in the initiation and maintenance of pacemaker discharge is studied in guinea pig isolated sino-atrial node (SA node) by means of a microelecrode technique. When [K+]O is suitably increased, the maximum diastolic potential decreases and all action potentials (APs) assume the characteristics of dominant pacemakers (slow responses with U-shaped diastolic depolarization). Subsequently, as the slope and amplitude of diastolic depolarization (DD) decreases, the threshold is missed, unmasking the fused oscillatory potentials Vos and ThVos. As high [K+]O perfusion continues, the oscillatory potentials become separated, Vos following the AP and ThVos appearing later on, when DD enters a less negative voltage range ("oscillatory zone"). ThVos grow in amplitude and attain the threshold, thereby insuring a slow discharge. If [K+]O is further increased, the smaller ThVos miss the threshold and SA node becomes quiescent. On reducing high [K+]o, ThVos reappear, increase in size and initiate spontaneous discharge. As they occur progressively earlier during DD, ThVos eventually fuse with Vos: at that stage, DD appears to continue directly into the upstroke (U-shaped DD) and the oscillations are no longer seen. During recovery in Tyrode solution, size and slope of Vos and of ThVos further increase and cause a faster discharge. When APs assume a subsidiary configuration, their DD (no longer U-shaped) abruptly terminates into the upstroke. In high [K+]O, increasing [Ca2+]O or applying a fast drive increase the size and slope of Vos and of ThVos, which in turn restore or accelerate discharge. In contrast, low [Ca2+]O abolishes Vos and ThVos and causes SA node arrest. Low [Ni2+] (35.5 FM) increases the rate whereas high [Ni2+] (0.73 mM) stops the SA node. Ryanodine eliminates Vos and ThVos and markedly slows or stops discharge. Thus, ThVos and Vos are separate voltage oscillations that play an obligatory role in the initiation and maintenance of SA node discharge, Vos by steepening early DD and ThVos by attaining the threshold in the dominant pacemaker range, either by gradually increasing during late DD at slow rates or by fusing with Vos at fast rates. Both Vos and ThVos are Ca2+-dependent, but apparently in different ways.
As for the mechanisms underlying the after-potential Vos and the pre-potential ThVos, high [K+]O and premature stimuli unmask Vos superimposed on early diastolic depolarization and ThVos within a less negative voltage range ("oscillatory zone"). Sub-threshold stimuli elicit ThVos in the oscillatory zone, but not at more negative values. Drive and caffeine shift the oscillatory zone in a negative direction. Low caffeine concentrations increase the size of Vos and of ThVos, rate and force. High caffeine concentrations suppress Vos but increase the size of ThVos and shift them to more negative values until eventually they miss the threshold. In quiescent SAN in high caffeine, a fast drive enhances ThVos size, thereby initiating a transient spontaneous rhythm ("overdrive excitation"). Adrenergic agonists potentiate caffeine-induced overdrive excitation through an increase in ThVos. In high caffeine, the first twitch after quiescence is not larger, twitch relaxation is slower, Vos is abolished, and the prolonged depolarization Vex is induced, consistent with an impairment of Ca2+ handling by the sarcoplasmic reticulum (SR). The electrical effects of caffeine in Tyrode solution are accounted for by the caffeine-induced changes in the oscillatory potentials in high [K+]O. Tetrodotoxin decreases force and size of both Vos and ThVos. Thus, the mechanism underlying Vos is related to a diastolic release of Ca2+ from a Ca2+-overloaded SR whereas that of ThVos appears to be related ionic events in the resting potential range that can initiate and sustain spontaneous discharge.
Figure 1. Reversal of the pacemaker current IKdd in 5.4 mM [K+]O Tyrode solution in a single cardiac Purkinje cell. The holding potential was -50 mV and steps were applied to -55, -65, -75, -85 and -95 mV. The time-dependent increase of the current was minimal at -55 mV and was clearly apparent at -65 and -75 mV. The pacemaker current reversed at -85 mV.
Sohn, H. G., and Vassalle, M. (1995). Cesium effects on dual pacemaker mechanisms in guinea pig sinoatrial node. J. Mol. Cell. Cardiol. 27, 563-577.
Spiegler, P., and Vassalle, M. (1995). Role of voltage oscillations in the automaticity of sheep cardiac Purkinje fibers. Can. J. Physiol. Pharmacol. 73, 1165-1180.
Vassalle, M., Yu, H., and Cohen, I. S (1995). The pacemaker current in cardiac Purkinje myocytes. J. Gen. Physiol. 106, 559-578.
Satoh, H., and Vassalle, M. (1996). Ca2+-dependence of caffeine modulation of the rate-force relation in canine cardiac Purkinje fibers. J. Pharmacol. Exp. Ther. 278, 826-835.
Kim, E. M., Choy, Y., and Vassalle, M. (1997). Mechanisms of suppression and initiation of pacemaker activity in guinea pig sino-atrial node superfused in high [K+]O. J. Mol. Cell. Cardiol. 29, 1433-1445.
Liu, Y. M., Yu, H., Li, C.-Z., Cohen, I. S., and Vassalle, M. (1998). Cs+ effects on if and iK in rabbit sinoatrial node myocytes, implications for SA node automaticity. J. Cardiovasc. Pharmacol. 32, 783-790.
Zhang, H., and Vassalle, M. (2001). Role of IK and If in the pacemaker mechanisms of sino-atrial node myocytes. Canadian J. Physiol. Pharmacol. 79, 963-976.
Rota, M., and Vassalle, M. (2003). Patch-clamp analysis in canine cardiac Purkinje cells of a novel sodium component in the pacemaker range. J. Physiol. 548, 147-165.
Nett, M. P., and Vassalle, M. (2003). Obligatory role of diastolic voltage oscillations in sino-atrial node discharge. J. Mol. Cell. Cardiol. 35, 1257-1276.
Marcello Rota, Ph.D., Postdoctoral Associate
Council on Basic Sciences of American Heart Association
Miembro Honorario, Sociedad Mexicana de Cardiologia
Editorial Board of Journal of Electrocardiology and of Journal of Biomedical Science
Reviewer for various scientific journals