Overexpression of a Neuronal Type Adenylyl Cyclase (Type 8) in Sinoatrial Node Markedly Impacts Heart Rate and Rhythm

Abstract

Heart rate (HR) and HR variability (HRV), predictors of over-all organism health, are widely believed to be driven by autonomic input to the sinoatrial node (SAN), with sympathetic input increasing HR and reducing HRV. However, variability in spontaneous beating intervals in isolated SAN tissue and single SAN cells, devoid of autonomic neural input, suggests that clocks intrinsic to SAN cells may also contribute to HR and HRV in vivo . We assessed contributions of both intrinsic and autonomic neuronal input mechanisms of SAN cell function on HR and HRV via in vivo , telemetric EKG recordings. This was done in both wild type (WT) mice, and those in which adenylyl cyclase type 8 (ADCY8), a main driver of intrinsic cAMP-PKA-Ca 2+ mediated pacemaker function, was overexpressed exclusively in the heart (TG AC8 ). We hypothesized that TG AC8 mice would: (1) manifest a more coherent pattern of HRV in vivo , i.e., a reduced HRV driven by mechanisms intrinsic to SAN cells, and less so to modulation by autonomic input and (2) utilize unique adaptations to limit sympathetic input to a heart with high levels of intrinsic cAMP-Ca 2+ signaling. Increased adenylyl cyclase (AC) activity in TG AC8 SAN tissue was accompanied by a marked increase in HR and a concurrent marked reduction in HRV, both in the absence or presence of dual autonomic blockade. The marked increase in intrinsic HR and coherence of HRV in TG AC8 mice occurred in the context of: (1) reduced HR and HRV responses to β-adrenergic receptor (β-AR) stimulation; (2) increased transcription of genes and expression of proteins [β-Arrestin, G Protein-Coupled Receptor Kinase 5 (GRK5) and Clathrin Adaptor Protein (Dab2)] that desensitize β-AR signaling within SAN tissue, (3) reduced transcripts or protein levels of enzymes [dopamine beta-hydorxylase (DBH) and phenylethanolamine N -methyltransferase (PNMT)] required for catecholamine production in intrinsic cardiac adrenergic cells, and (4) substantially reduced plasma catecholamine levels. Thus, mechanisms driven by cAMP-PKA-Ca 2+ signaling intrinsic to SAN cells underlie the marked coherence of TG AC8 mice HRV. Adaptations to limit additional activation of AC signaling, via decreased neuronal sympathetic input, are utilized to ensure the hearts survival and prevent Ca 2+ overload.

Acclimation to a thermoneutral environment abolishes age-associated alterations in heart rate and heart rate variability in conscious, unrestrained mice

“…The double autonomic blockade ECG protocol was then repeated then at 30°C. ECG time-series analysis ECG time-series were analyzed using Labchart software (version 7.3.7), and HRV was calculated using custombuilt python 3.5 software as previously described in detail by Moen et al (2019). Because of marked heart rate responses to ambient temperatures, cut-off parameters for frequency domain HRV categories were adjusted for changes in the beating rate (Behar et al 2018): high frequency power spectral density (HF.PSD) was defined as 1.50-5.0 Hz at 20°C and 0.75-3.0 Hz at 30°C…”

“…A coherent rhythm has low beat-to-beat variability and is associated with a high sympathetic input to the SAN, or a high level of adenylyl cyclase signaling intrinsic to SAN cells (Moen et al 2019). More complex rhythms have a higher beat-to-beat variability, conferred by a high parasympathetic input or lower intrinsic SAN cell adenylyl cyclase activity (Thayer et al 2010;Moen et al 2019) and are considered to be indicative of a more balanced rhythm, reflecting heart homeostatic adaptations to the body's physiologic rhythms, e.g., breathing (Shaffer et al 2014); more coherent heart rhythms, on the other hand, are associated with a myriad of poor health outcomes, such as increased mortality following myocardial infarction (MI) or post-traumatic stress disorder (PTSD) (Bigger Jr et al 1992a, b, 1993Cohen et al 2000;Kleiger et al 1987;La Rovere et al 1998;Thayer et al 2010;Thayer et al 2010) HR, HRV, and ANS modulation of these are all profoundly affected by age, even in the absence of disease, consistent with the idea that aging itself is a disease (Lakatta 2015). Maximum HR in humans has been found to linearly decrease with advancing age, and is independent of physical fitness (Tanaka et al 2001;Londeree and Moeschberger 1984;Christou and Seals 2008;Higginbotham et al 1986).…”

“…There is also an age-associated loss of beta-adrenergic receptor (BAR)-induced vasorelaxation (Pan et al 1986;Schutzer et al 2011) and evidence of decreased postsynaptic efficiency (Fleg and Strait 2012). But the age-associated decline in HRV not only results from a reduction in the effectiveness of postsynaptic autonomic receptor responses of pacemaker cells residing in SAN tissue but also to deterioration of intrinsic pacemaker clock functions of SAN cells (Yaniv et al 2014;Liu et al 2014;Moen et al 2019).…”

Verification of a Central Pacemaker in Brain Stem by Phase-Coupling Analysis Between HR Interval- and BOLD-Oscillations in the 0.10–0.15 Hz Frequency Band

“…However, it is still unknown how they are generated. Baroreflex mechanisms and intrinsic activity of sinoatrial node cells may play a dominant role ( Van Roon et al, 2004 ; Moen et al, 2019 ), but also a central (neural) pacemaker may contribute independently to their generation ( Preiss and Polosa, 1974 ; Zhang et al, 1998 ; Eckberg et al, 2016 ; Ghali and Ghali, 2020 ). This has been controversely discussed a couple of years ago ( Eckberg and Karemaker, 2009 ).…”

Neurohumoral Control of Sinoatrial Node Activity and Heart Rate: Insight From Experimental Models and Findings From Humans

“…Catecholamines (NE and epinephrine) are released from sympathetic neurons of the ANS (Gordan et al, 2015) (although both are also produced by the adrenal gland and can act as circulating hormones, or can be intrinsically generated by cells within the SAN) (Marin-Garcia, 2011; Moen et al, 2019). NE and epinephrine bind to αand β-adrenergic receptors (AR).…”