From two competing oscillators to one coupled-clock pacemaker cell system

Abstract

At the beginning of this century, debates regarding "what are the main control mechanisms that ignite the action potential (AP) in heart pacemaker cells" dominated the electrophysiology field. The original theory which prevailed for over 50 years had advocated that the ensemble of surface membrane ion channels (i.e., "M-clock") is sufficient to ignite rhythmic APs. However, more recent experimental evidence in a variety of mammals has shown that the sarcoplasmic reticulum (SR) acts as a "Ca(2+)-clock" rhythmically discharges diastolic local Ca(2+) releases (LCRs) beneath the cell surface membrane. LCRs activate an inward current (likely that of the Na(+)/Ca(2+) exchanger) that prompts the surface membrane "M-clock" to ignite an AP. Theoretical and experimental evidence has mounted to indicate that this clock "crosstalk" operates on a beat-to-beat basis and determines both the AP firing rate and rhythm. Our review is focused on the evolution of experimental definition and numerical modeling of the coupled-clock concept, on how mechanisms intrinsic to pacemaker cell determine both the heart rate and rhythm, and on future directions to develop further the coupled-clock pacemaker cell concept.

Keywords: arrhythmias; coupled-clock pacemaker system; heart rate variability; mathematical modeling; sinoatrial node.

Figure 1

Coupled-clock molecules and brain-heart signaling receptors that drive basal automaticity of SANC. (i) The neurotransmitters noradrenaline (NE) and acetylcholine (ACh) released from sympathetic or parasympathetic nerve terminals bind to β-adrenergic receptors (β-AR) or cholinergic receptors (CR), respectively. Autonomic receptor signaling couples to G-proteins (GPCR) and leads to modulation of the same coupled-clock molecules that drive basal automaticity of SANC. Basal Ca²⁺-calmodulin activation of adenylyl cyclases (AC), which produce cAMP-PKA-dependent phosphorylation and calmodulin-dependent kinase II (CaMKII)-dependent phosphorylation signaling. cAMP positively shifts the f-channel activation curve. Phosphodiesterases (PDE) degrade cAMP production, while protein phosphatase (PPT) degrades phosphorylation activity. PKA and CaMKII signaling phosphorylate SR Ca²⁺ cycling proteins (RyR, phospholamban, which bind to and inhibit SERCA) and surface membrane ion channels.^*The values are for I_NCX amplitude (within the cycle) achieved during systole, however the diastolic amplitude is almost an order of magnitude lower. (ii) Numerical model simulations of membrane potential (blue), I_NCX (green) and subspace Ca²⁺ (Ca_sub, red) in response to reduction in NCX expression. As NCX expression becomes reduced the spread of Ca²⁺ release between RyRs via Ca²⁺ induced Ca²⁺ release is enhanced, resulting in a more effective activation of the remaining NCX molecules by LCRs. Further reduction in NCX uncoupled (partially or fully) LCR from AP generation. Specifically, NCX current becomes too small to depolarize the membrane and AP generation fail (modified from Maltsev et al., 2013).

Figure 2

Reduced synchronization of coupled-clock mechanisms prolongs AP beating interval variability and LCR period variability. To unravel clock-crosstalk effects on AP BIV and LCR period variability, clock function was perturbed by directly inhibiting either the M or Ca²⁺-clock. To inhibit the M-clock, a range of concentrations of ivabradine (IVA), an I_f inhibitor were employed. To inhibit the Ca²⁺-clock, a range of concentrations of cyclopiazonic acid (CPA), a SR Ca²⁺ pump inhibitor were employed. Poincaré plots of the beating interval in control and in response to (A) IVA or (B) CPA. Poincaré plots of LCR period in control and in response to (C) IVA or (D) CPA. (E) The relationship between the average AP BIV, quantified by coefficient of variation to the LCR period prior to and in response to different concentrations of either IVA or CPA (modified from Yaniv et al., 2014b).