L-type Ca2+ current downregulation in chronic human atrial fibrillation is associated with increased activity of protein phosphatases

Abstract

Background-Although downregulation of L-type Ca2+ current (ICa,L) in chronic atrial fibrillation (AF) is an important determinant of electrical remodeling, the molecular mechanisms are not fully understood. Here, we tested whether reduced ICa,L in AF is associated with alterations in phosphorylation-dependent channel regulation. Methods and Results-We used whole-cell voltage-clamp technique and biochemical assays to study regulation and expression of ICa,L in myocytes and atrial tissue from 148 patients with sinus rhythm (SR) and chronic AF. Basal I Ca,L at +10 mV was smaller in AF than in SR (-3.8±0.3 pA/pF, n=138/37 [myocytes/patients] and -7.6±0.4 pA/pF, n=276/86, respectively; P<0.001), though protein levels of the pore-forming α1c and regulatory β2a channel subunits were not different. In both groups, norepinephrine (0.01 to 10 μmol/L) increased ICa,L with a similar maximum effect and comparable potency. Selective blockers of kinases revealed that basal ICa,L was enhanced by Ca2+/calmodulin- dependent protein kinase II in SR but not in AF. Norepinephrine-activated I Ca,L was larger with protein kinase C block in SR only, suggesting decreased channel phosphorylation in AF. The type 1 and type 2A phosphatase inhibitor okadaic acid increased basal ICa,L more effectively in AF than in SR, which was compatible with increased type 2A phosphatase but not type 1 phosphatase protein expression and higher phosphatase activity in AF. Conclusions-In AF, increased protein phosphatase activity contributes to impaired basal ICa,L. We propose that protein phosphatases may be potential therapeutic targets for AF treatment.