Differential phosphorylation-dependent regulation of constitutively active and muscarinic receptor-activated IK,ACh channels in patients with chronic atrial fibrillation

Abstract

Objective: In chronic atrial fibrillation (cAF) the potassium current I K,ACh develops agonist-independent constitutive activity. We hypothesized that abnormal phosphorylation-dependent regulation underlies the constitutive I K,ACh activity. Methods: We used voltage-clamp technique and biochemical assays to study I K,ACh regulation in atrial appendages from 61 sinus rhythm (SR), 11 paroxysmal AF (pAF), and 33 cAF patients. Results: Compared to SR basal current was higher in cAF only, whereas the muscarinic receptor (2 μmol/L carbachol)-activated I K,ACh was smaller in pAF and cAF. In pAF the selective I K,ACh blocker tertiapin abolished the muscarinic receptor-activated I K,ACh but excluded agonist-independent constitutive I K,ACh activity. Blockade of type-2A phosphatase and the subsequent shift to increased muscarinic receptor phosphorylation (and inactivation) reduced muscarinic receptor-activated I K,ACh in SR but not in cAF, pointing to an impaired function of Gprotein-coupled receptor kinase. Using subtype-selective kinase inhibitors we found that in SR the muscarinic receptor-activated I K,ACh requires phosphorylation by protein kinase G (PKG), protein kinase C (PKC), and calmodulin-dependent protein kinase II (CaMKII), but not by protein kinase A (PKA). In cAF, constitutive I K,ACh activity results from abnormal channel phosphorylation by PKC but not by PKG or CaMKII, whereas the additional muscarinic receptor-mediated I K,ACh activation occurs apparently without involvement of these kinases. In cAF, the higher protein level of PKCε but not PKCα, PKCβ 1 or PKCδ is likely to contribute to the constitutive I K,ACh activity. Conclusions: The occurrence of constitutive I K,ACh activity in cAF results from abnormal PKC function, whereas the muscarinic receptormediated I K,ACh activation does not require the contribution of PKG, PKC or CaMKII. Selective drug targeting of constitutively active I K,ACh channels may be suitable to reduce the ability of AF to become sustained.

M2 Muscarinic Receptors Induce Airway Smooth Muscle Activation via a Dual, Gβγ-mediated Inhibition of Large Conductance Ca2+-activated K+ Channel Activity

“…Interestingly, in both of these ion channels, PKC-dependent phosphorylation contributes to the complex regulation (42,43). There are several reports, mainly on arterial smooth muscle cells (44 -46), but also on rat pituitary tumor cells (47), indicating that PKC inhibits BK channels (48).…”

Cellular and Molecular Electrophysiology of Atrial Fibrillation Initiation, Maintenance, and Progression

“…132 The increased single-channel open probability may involve stronger channel dephosphorylation by PP1 and serine/threonine protein phosphatase type 2A in cAF. 133 Agonist-activated I K,ACh is larger in RA than in LA from patients with sinus rhythm, but is decreased in RA of pAF and cAF because of a reduction in underlying Kir3. 135 This Na + -dependent regulation is lost in cAF, possibly because of a more pronounced reduction of the Na + -sensitive subunit Kir3.4 than Kir3.1, and further reduces I K,ACh at fast rates with increased intracellular [Na + ].…”

Cardiac fibrillation: From ion channels to rotors in the human heart

“…The basal current was higher in chronic AF only, whereas the muscarinic receptor-activated I K,ACh was smaller both in paroxysmal and chronic AF, which can be understood as a smaller ACh-sensitive current. 45 This decrease of the response to ACh could also explain the reduced response to ADO observed in persistent AF in patients. 33 The increased basal agonist-independent current would result in rotor stabilization, acceleration of reentry, and spread of the DF sites from the PV to other non-PV locations and disappearance of LA to RA frequency gradients.…”