Rhythmic Ryanodine Receptor Ca ²⁺ Releases During Diastolic Depolarization of Sinoatrial Pacemaker Cells Do Not Require Membrane Depolarization

Abstract

Abstract-Localized, subsarcolemmal Ca 2ϩ release (LCR) via ryanodine receptors (RyRs) during diastolic depolarization of sinoatrial nodal cells augments the terminal depolarization rate. We determined whether LCRs in rabbit sinoatrial nodal cells require the concurrent membrane depolarization, or are intrinsically rhythmic, and whether rhythmicity is linked to the spontaneous cycle length. Confocal linescan images revealed persistent LCRs both in saponin-permeabilized cells and in spontaneously beating cells acutely voltage-clamped at the maximum diastolic potential. During the initial stage of voltage clamp, the LCR spatiotemporal characteristics did not differ from those in spontaneously beating cells, or in permeabilized cells bathed in 150 nmol/L Ca 2ϩ . The period of persistent rhythmic LCRs during voltage clamp was slightly less than the spontaneous cycle length before voltage clamp. In spontaneously beating cells, in both transient and steady states, LCR period was highly correlated with the spontaneous cycle length; and regardless of the cycle length, LCRs occurred predominantly at a constant time, ie, 80% to 90% of the cycle length. Numerical model simulations incorporating LCRs reproduce the experimental results. We conclude that diastolic LCRs reflect rhythmic intracellular Ca 2ϩ cycling that does not require the concomitant membrane depolarization, and that LCR periodicity is closely linked to the spontaneous cycle length. Thus, the biological clock of sinoatrial nodal pacemaker cells, like that of many other rhythmic functions occurring throughout nature, involves an intracellular Ca 2ϩ rhythm.

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

“…In the present context, the evidence indicates that overdrive increases force and rate through the generally agreed [3][4][5][6] increase in cellular calcium. Furthermore, in high [K + ] o overdrive is shown to simultaneously increase the twitch and the size as well as slope of diastolic potential which then subside together during recovery ( Figure 10).…”

“…It is generally agreed that force development is a function of Ca 2+ loading [see 5,34]: indeed, an increase in rate increases cellular Ca 2+ in all cardiac tissues including the SAN (e.g., [3][4][5][6]. Also, overdrive-induced force patterns in SAN are similar to those in other cardiac tissues (e.g., [2,7]).…”

“…It entails an enhanced ion exchange between extraand intra-cellular environments, since the number of action potentials increases in the unit of time. Indeed, a faster rate increases intracellular Na + [1,2] and Ca 2+ [e.g., [3][4][5] in cardiac tissues, including the SAN [6]. Overdrive increases contractile force as well [e.g., 5,7].…”

Cardiac pacemaker cell failure with preserved If, ICaL, and IKr: A lesson about pacemaker function learned from ischemia-induced bradycardia

“…The speed at which the Ca 2+ clock runs is assessed by the LCR period, defined as the time from the AP-induced Ca 2+ transient early in the cycle (Fig.1A, Clock "reset", blue) to LCR emergence during the subsequent DD (Clock "tick"). This Ca 2+ clock's period is slightly shorter than the cycle length (Fig.1A) and approximately coincides with the period of the intrinsic Ca 2+ oscillations under voltage clamp [27,30]. The speed at which Ca 2+ clock ticks is variable, matching the chronotropic demand for a given condition, and is governed by the SR Ca 2+ loading and Ca 2+ release characteristics, which in turn, are governed by the degree of phosphorylation of its aforementioned Ca 2+ cycling proteins.…”

“…The net effects of this phosphorylation create conditions that are required for LCR spontaneous activation during the DD. The LCR activation in rabbit SANC is indeed truly spontaneous as it does not depend on membrane depolarization (but see [29] for cat latent atrial pacemaker cells), and can be observed under voltage clamp or in permeabilized SANC [27,30]. Importantly, it is the spontaneous emergence of LCRs during the late DD that activates I NCX and thus changes the DD dynamics from a linear to a nonlinear, exponentially rising function, culminating in I CaL activation and membrane excitation (Fig.1A) [17,18].…”