The Autonomic Nervous System Regulates the Heart Rate through cAMP-PKA Dependent and Independent Coupled-Clock Pacemaker Cell Mechanisms

Abstract

Sinoatrial nodal cells (SANCs) generate spontaneous action potentials (APs) that control the cardiac rate. The brain modulates SANC automaticity, via the autonomic nervous system, by stimulating membrane receptors that activate (adrenergic) or inactivate (cholinergic) adenylyl cyclase (AC). However, these opposing afferents are not simply additive. We showed that activation of adrenergic signaling increases AC-cAMP/PKA signaling, which mediates the increase in the SANC AP firing rate (i.e., positive chronotropic modulation). However, there is a limited understanding of the underlying internal pacemaker mechanisms involved in the crosstalk between cholinergic receptors and the decrease in the SANC AP firing rate (i.e., negative chronotropic modulation). We hypothesize that changes in AC-cAMP/PKA activity are crucial for mediating either decrease or increase in the AP firing rate and that the change in rate is due to both internal and membrane mechanisms. In cultured adult rabbit pacemaker cells infected with an adenovirus expressing the FRET sensor AKAR3, PKA activity and AP firing rate were tightly linked in response to either adrenergic receptor stimulation (by isoproterenol, ISO) or cholinergic stimulation (by carbachol, CCh). To identify the main molecular targets that mediate between PKA signaling and pacemaker function, we developed a mechanistic computational model. The model includes a description of autonomic-nervous receptors, post- translation signaling cascades, membrane molecules, and internal pacemaker mechanisms. Yielding results similar to those of the experiments, the model simulations faithfully reproduce the changes in AP firing rate in response to CCh or ISO or a combination of both (i.e., accentuated antagonism). Eliminating AC-cAMP-PKA signaling abolished the core effect of autonomic receptor stimulation on the AP firing rate. Specifically, disabling the phospholamban modulation of the SERCA activity resulted in a significantly reduced effect of CCh and a failure to increase the AP firing rate under ISO stimulation. Directly activating internal pacemaker mechanisms led to a similar extent of changes in the AP firing rate with respect to brain receptor stimulation. Thus, Ca2+ and cAMP/PKA-dependent phosphorylation limits the rate and magnitude of chronotropic changes in the spontaneous AP firing rate.

Eliminating contraction during culture maintains global and local Ca2+ dynamics in cultured rabbit pacemaker cells

“…We used our computational model [20] to gain insight into the experimental results and to predict Ca 2+ kinetics in the cytosol and the SR, the latter of which is challenging to measure. Fig.…”

“…In parallel to the Ca 2+ , the phosphorylation status determines the degree of clock coupling and thus the spontaneous AP firing rate [8,20]. Thus, we checked whether eliminating contraction also maintains phosphorylation status.…”

“…To show the importance of phosphorylation status in maintaining pacemaker function, we used our computational model to examine the effect of reduced phosphorylation on SERCA activity, on pacemaker beating rate, and on Ca 2+ kinetics in the cytosol and SR. (The effect of reduced phosphorylation on SERCA activity was simulated by reducing K PLBp by 10%, 20%, 30% and 40%; see [20] for parameter values and equations.) Fig.…”

“…Our computational modeling is based on [20]. The model describes the coupled-clock function that includes different compartments of the cells (cytosol, submembrane, and SR), the major ion channels that constitute the membrane clock, and the proteins on the SR that constitute the Ca 2+ clock.…”

Modeling pacemaker deterioration with age

“…To explore how SAN function deteriorates in aged cells, Behar and Yaniv based their model on an earlier one that takes into account both plasma membrane and SR pacemaker activity (2, 3). The new model additionally incorporated nervous system inputs and integrated in vivo observations about how SAN function changes in aged mice (4, 5). …”