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

View Article: PubMed Central - PubMed

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.

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Related in: MedlinePlus

Schematic diagram of sinoatrial node mechanisms. Autonomic regulation via adenylyl cyclase-cyclic adenosine monophosphate-Protein kinase A (AC-cAMP-PKA) signaling: the internal pacemaker mechanisms are tightly coupled with cAMP/PKA signaling through the stimulation of G-protein-coupled receptors that activate (adrenergic receptors, β-AR) or inactivate (cholinergic receptors, ChR) AC as well as Ca2+-calmodulin activated AC. Only the main ion channels modulated in our model by AC-cAMP-PKA signaling are represented in the figure. SERCA, sarcoplasmic reticulum Ca2+ ATPase; PLB, phospholamban; RyR, ryanodine; PDE, phosphodiesterase; PPT, protein phosphatase; P, phosphate.
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Figure 1: Schematic diagram of sinoatrial node mechanisms. Autonomic regulation via adenylyl cyclase-cyclic adenosine monophosphate-Protein kinase A (AC-cAMP-PKA) signaling: the internal pacemaker mechanisms are tightly coupled with cAMP/PKA signaling through the stimulation of G-protein-coupled receptors that activate (adrenergic receptors, β-AR) or inactivate (cholinergic receptors, ChR) AC as well as Ca2+-calmodulin activated AC. Only the main ion channels modulated in our model by AC-cAMP-PKA signaling are represented in the figure. SERCA, sarcoplasmic reticulum Ca2+ ATPase; PLB, phospholamban; RyR, ryanodine; PDE, phosphodiesterase; PPT, protein phosphatase; P, phosphate.

Mentions: The sinoatrial node function, the heart's primary pacemaker, is regulated by two main signaling cascades: the autonomic nervous system (ANS) and the internal coupled-clock system. In particular dysregulations in the ANS control plays a critical role in coronary artery disease and in the development of ventricular arrhythmias (Schwartz and Priori, 1990; Kjellgren and Gomes, 1993). The control of SANC function by the ANS depends on the balance between the sympathetic and parasympathetic stimulation of G-protein-coupled receptors. In general, adrenergic stimulation (i.e., sympathetic) causes the action potential firing rate to increase and cholinergic stimulation (parasympathetic) causes the action potential firing rate to decrease. However, there is also a non-additive sympathetic-parasympathetic stimulation interaction (Grodner et al., 1970; Levy, 1971). In addition, it has been shown that phasic changes in SANC cycle length occur with respect to the timing, amplitude and duration of the stimulation of the vagus nerve (Jalife and Moe, 1979). Stimulation of G-protein-coupled receptors activate [the β-adrenergic receptor (β-AR)] or inactivate [the cholinergic receptor (ChR)] adenylyl cyclase (AC; Figures 1, 2). The AC generates a high cAMP level, which controls protein kinase A (PKA) activity. Another kind of AC expressed in pacemaker cells is regulated by calmodulin, which is activated by Ca2+ cycling. Ca2+ cycling is balanced by internal pacemaker mechanisms that are part of the coupled-clock system: interaction between membrane channels, exchangers and pumps (membrane clock, M clock), and internal Ca2+ storage (Ca2+ clock) (Yaniv et al., 2015b; Figure 1). The internal clock mechanisms interact even without autonomic modulation via a range of node mechanisms: voltage-dependent channels (e.g., via Ca2+-dependent inactivation of L-type Ca2+ channels; Mangoni et al., 2003), ensembles of local subsarcolemmal Ca2+ releases (LCR; Bogdanov et al., 2001), and protein kinase-A (PKA), and calmodulin-dependent kinase II (cAMKII) dependent protein phosphorylation (Trautwein et al., 1987; Takasago et al., 1989; Freeman et al., 1992; Toyofuku et al., 1993). In short, the ensemble of local submembrane Ca2+ releases from the sarcoplasmic reticulum (SR) activates the INCX exchanger (demonstrating the interaction between the Ca2+ and M clocks), the inward current of which has been shown to be one of the main contributors to the diastolic depolarization (DD) (Bogdanov et al., 2006). The M clock also acts on the Ca2+ clock through the resetting and refueling of the SR via ICaL influx (Maltsev and Lakatta, 2009) (Figure 1). Finally, PKA and CaMKII signaling phosphorylate different targets on the M and Ca2+ clocks (Figure 1), resulting in changes in the spontaneous action potential (AP) firing rate (Yaniv et al., 2015a) of SANCs. The interaction between these two clocks forms a robust, stable, coupled-clock system that is responsible for SANC automaticity (Yaniv et al., 2015c). Although, there have been many studies of the node mechanisms responsible for the coupling between intrinsic pacemaker molecules of both the M and Ca2+ clocks, there is still only a limited understanding of the underlying ionic and molecular mechanisms involved in the crosstalk between the ANS membrane receptors (β-AR or ChR in Figure 1) and the internal mechanisms intrinsic to SANCs. We showed that activation of adrenergic signaling increases AC-cAMP/PKA signaling, which mediates the increase in the SANC AP rate (i.e., positive chronotropic modulation) (Yaniv et al., 2015a). 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 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 based mechanisms.


The Autonomic Nervous System Regulates the Heart Rate through cAMP-PKA Dependent and Independent Coupled-Clock Pacemaker Cell Mechanisms
Schematic diagram of sinoatrial node mechanisms. Autonomic regulation via adenylyl cyclase-cyclic adenosine monophosphate-Protein kinase A (AC-cAMP-PKA) signaling: the internal pacemaker mechanisms are tightly coupled with cAMP/PKA signaling through the stimulation of G-protein-coupled receptors that activate (adrenergic receptors, β-AR) or inactivate (cholinergic receptors, ChR) AC as well as Ca2+-calmodulin activated AC. Only the main ion channels modulated in our model by AC-cAMP-PKA signaling are represented in the figure. SERCA, sarcoplasmic reticulum Ca2+ ATPase; PLB, phospholamban; RyR, ryanodine; PDE, phosphodiesterase; PPT, protein phosphatase; P, phosphate.
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Figure 1: Schematic diagram of sinoatrial node mechanisms. Autonomic regulation via adenylyl cyclase-cyclic adenosine monophosphate-Protein kinase A (AC-cAMP-PKA) signaling: the internal pacemaker mechanisms are tightly coupled with cAMP/PKA signaling through the stimulation of G-protein-coupled receptors that activate (adrenergic receptors, β-AR) or inactivate (cholinergic receptors, ChR) AC as well as Ca2+-calmodulin activated AC. Only the main ion channels modulated in our model by AC-cAMP-PKA signaling are represented in the figure. SERCA, sarcoplasmic reticulum Ca2+ ATPase; PLB, phospholamban; RyR, ryanodine; PDE, phosphodiesterase; PPT, protein phosphatase; P, phosphate.
Mentions: The sinoatrial node function, the heart's primary pacemaker, is regulated by two main signaling cascades: the autonomic nervous system (ANS) and the internal coupled-clock system. In particular dysregulations in the ANS control plays a critical role in coronary artery disease and in the development of ventricular arrhythmias (Schwartz and Priori, 1990; Kjellgren and Gomes, 1993). The control of SANC function by the ANS depends on the balance between the sympathetic and parasympathetic stimulation of G-protein-coupled receptors. In general, adrenergic stimulation (i.e., sympathetic) causes the action potential firing rate to increase and cholinergic stimulation (parasympathetic) causes the action potential firing rate to decrease. However, there is also a non-additive sympathetic-parasympathetic stimulation interaction (Grodner et al., 1970; Levy, 1971). In addition, it has been shown that phasic changes in SANC cycle length occur with respect to the timing, amplitude and duration of the stimulation of the vagus nerve (Jalife and Moe, 1979). Stimulation of G-protein-coupled receptors activate [the β-adrenergic receptor (β-AR)] or inactivate [the cholinergic receptor (ChR)] adenylyl cyclase (AC; Figures 1, 2). The AC generates a high cAMP level, which controls protein kinase A (PKA) activity. Another kind of AC expressed in pacemaker cells is regulated by calmodulin, which is activated by Ca2+ cycling. Ca2+ cycling is balanced by internal pacemaker mechanisms that are part of the coupled-clock system: interaction between membrane channels, exchangers and pumps (membrane clock, M clock), and internal Ca2+ storage (Ca2+ clock) (Yaniv et al., 2015b; Figure 1). The internal clock mechanisms interact even without autonomic modulation via a range of node mechanisms: voltage-dependent channels (e.g., via Ca2+-dependent inactivation of L-type Ca2+ channels; Mangoni et al., 2003), ensembles of local subsarcolemmal Ca2+ releases (LCR; Bogdanov et al., 2001), and protein kinase-A (PKA), and calmodulin-dependent kinase II (cAMKII) dependent protein phosphorylation (Trautwein et al., 1987; Takasago et al., 1989; Freeman et al., 1992; Toyofuku et al., 1993). In short, the ensemble of local submembrane Ca2+ releases from the sarcoplasmic reticulum (SR) activates the INCX exchanger (demonstrating the interaction between the Ca2+ and M clocks), the inward current of which has been shown to be one of the main contributors to the diastolic depolarization (DD) (Bogdanov et al., 2006). The M clock also acts on the Ca2+ clock through the resetting and refueling of the SR via ICaL influx (Maltsev and Lakatta, 2009) (Figure 1). Finally, PKA and CaMKII signaling phosphorylate different targets on the M and Ca2+ clocks (Figure 1), resulting in changes in the spontaneous action potential (AP) firing rate (Yaniv et al., 2015a) of SANCs. The interaction between these two clocks forms a robust, stable, coupled-clock system that is responsible for SANC automaticity (Yaniv et al., 2015c). Although, there have been many studies of the node mechanisms responsible for the coupling between intrinsic pacemaker molecules of both the M and Ca2+ clocks, there is still only a limited understanding of the underlying ionic and molecular mechanisms involved in the crosstalk between the ANS membrane receptors (β-AR or ChR in Figure 1) and the internal mechanisms intrinsic to SANCs. We showed that activation of adrenergic signaling increases AC-cAMP/PKA signaling, which mediates the increase in the SANC AP rate (i.e., positive chronotropic modulation) (Yaniv et al., 2015a). 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 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 based mechanisms.

View Article: PubMed Central - PubMed

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.

No MeSH data available.


Related in: MedlinePlus