Showing papers by "Niels Voigt published in 2007"

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 G-protein-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 PKCe but not PKCα, PKCβ1 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 receptor-mediated 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.

Changes in I K, ACh single-channel activity with atrial tachycardia remodelling in canine atrial cardiomyocytes.

Abstract: Aims Although atrial tachycardia (AT) remodelling promotes agonist-independent, constitutively active, acetylcholine-regulated K+-current (IK,ACh) that increases susceptibility to atrial fibrillation (AF), the underlying changes in IK,Ach channel function are unknown. This study aimed to establish how AT remodelling affects IK,ACh single-channel function. Methods and results IK,ACh single-channel activity was studied via cell-attached patch-clamp in isolated left atrial cardiomyocytes of control and AT (7 days, 400 min−1) dogs. Atrial tachycardia prolonged the mean duration of induced AF from 44 ± 22 to 413 ± 167 s, and reduced atrial effective refractory period at a 360 ms cycle length from 126 ± 3 to 74 ± 5 ms ( n = 9/group, P < 0.001). In the absence of cholinergic stimulation, single-channel openings with typical IK,ACh conductance and rectification properties were sparse under control conditions. Atrial tachycardia induced prominent agonist-independent IK,ACh activity because of increased opening frequency ( f o) and open probability ( P o: approximately seven- and 10-fold, respectively, vs. control), but did not alter open time-constant, single-channel conductance, and membrane density. With maximum IK,ACh activation (10 µmol/L carbachol), channel P o was enhanced much more in control cells (∼42-fold) than in AT-remodelled myocytes (approximately five-fold). The selective Kir3 current blocker tertiapin-Q (100 nmol/L) reduced f o and P o at −100 mV by 48 and 51%, respectively ( P < 0.05 for each), without altering other channel properties, confirming the identity of IK,ACh. Atrial tachycardia had no significant effect on mRNA or protein expression of either of the subunits (Kir3.1, Kir3.4) underlying IK,ACh. Conclusion Atrial tachycardia increases agonist-independent constitutive IK,ACh single-channel activity by enhancing spontaneous channel opening, providing a molecular basis for AT effects on macroscopic IK,ACh observed in previous studies, as well as associated refractoriness abbreviation and tertiapin-suppressible AF promotion. These results suggest an important role for constitutive IK,Ach channel opening in AT remodelling and support its interest as a potential target for AF therapy.