Phosphodiesterases 3 and 4 Differentially Regulate the Funny Current, If, in Mouse Sinoatrial Node Myocytes.

Emily Sharpe,Zhandi Liao,Eric Larson,Joshua St. Clair,Catherine Proenza

doi:10.3390/jcdd4030010

Emily Sharpe, Zhandi Liao

Open Access

https://doi.org/10.3390/jcdd4030010

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Abstract

Cardiac pacemaking, at rest and during the sympathetic fight-or-flight response, depends on cAMP (3′,5′-cyclic adenosine monophosphate) signaling in sinoatrial node myocytes (SAMs). The cardiac “funny current” (If) is among the cAMP-sensitive effectors that drive pacemaking in SAMs. If is produced by hyperpolarization-activated, cyclic nucleotide-sensitive (HCN) channels. Voltage-dependent gating of HCN channels is potentiated by cAMP, which acts either by binding directly to the channels or by activating the cAMP-dependent protein kinase (PKA), which phosphorylates them. PKA activity is required for signaling between β adrenergic receptors (βARs) and HCN channels in SAMs but the mechanism that constrains cAMP signaling to a PKA-dependent pathway is unknown. Phosphodiesterases (PDEs) hydrolyze cAMP and form cAMP signaling domains in other types of cardiomyocytes. Here we examine the role of PDEs in regulation of If in SAMs. If was recorded in whole-cell voltage-clamp experiments from acutely-isolated mouse SAMs in the absence or presence of PDE and PKA inhibitors, and before and after βAR stimulation. General PDE inhibition caused a PKA-independent depolarizing shift in the midpoint activation voltage (V1/2) of If at rest and removed the requirement for PKA in βAR-to-HCN signaling. PDE4 inhibition produced a similar PKA-independent depolarizing shift in the V1/2 of If at rest, but did not remove the requirement for PKA in βAR-to-HCN signaling. PDE3 inhibition produced PKA-dependent changes in If both at rest and in response to βAR stimulation. Our results suggest that PDE3 and PDE4 isoforms create distinct cAMP signaling domains that differentially constrain access of cAMP to HCN channels and establish the requirement for PKA in signaling between βARs and HCN channels in SAMs.

Highlights

Cardiac pacemaking, at rest and during the sympathetic fight-or-flight response, depends on cyclic adenosine monophosphate (cAMP) signaling in sinoatrial node myocytes (SAMs)
We found that IBMX shifted the midpoint activation voltage (V1/2) of in SAMs.The “funny current” (If) by ~15 Midpoint Activation Voltage (mV) to more depolarized potentials (Figure 1A,C; Table 1), consistent with an increase in cAMP concentration in the vicinity of the HCN channels
Since we previously found that protein kinase (PKA) activity is required for the cAMP-dependent activation of If by β adrenergic receptors (βARs) in SAMs [11], we asked whether PKA is required for the cAMP liberated by IBMX to activate If

Summary

Introduction

At rest and during the sympathetic fight-or-flight response, depends on cAMP signaling in sinoatrial node myocytes (SAMs). Spontaneous APs in SAMs result from a spontaneous depolarization during diastole that drives the membrane potential to its threshold to initiate the subsequent AP. 3 ,5 -cyclic adenosine monophosphate (cAMP) is a critical regulator of pacemaking in SAMs. The resting cytoplasmic concentration of cAMP is thought to be higher in SAMs than in other cardiac myocytes [5] and sympathetic nervous system stimulation increases heart rate by activating β adrenergic receptors (βARs) and further increasing cAMP in SAMs. The “funny current” (If) is a hallmark of SAMs and is among the many cAMP-sensitive effectors that contribute to spontaneous pacemaker activity in SAMs. If is produced by hyperpolarizationactivated, cyclic nucleotide-sensitive (HCN) ion channels. In accordance with a critical role for If in pacemaking, mutations in HCN4 channels cause sinoatrial node dysfunction in human patients and animal models [12,13,14] and HCN channel blockers decrease the heart rate [15,16]

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