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

Background-Although downregulation of L-type Ca2+ current (I-Ca,I-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 I-Ca,I-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 I-Ca,I-L in myocytes and atrial tissue from 148 patients with sinus rhythm (SR) and chronic AF. Basal I-Ca,I-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 alpha(1c) and regulatory beta(2a) channel subunits were not different. In both groups, norepinephrine (0.01 to 10 mu mol/L) increased I-Ca,I-L with a similar maximum effect and comparable potency. Selective blockers of kinases revealed that basal I-Ca,I-L was enhanced by Ca2+/calmodulin-dependent protein kinase II in SR but not in AF. Norepinephrine-activated I-Ca,I-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 I-Ca,I-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 I-Ca,I-L. We propose that protein phosphatases may be potential therapeutic targets for AF treatment.