L-Type Ca ²⁺ Current Downregulation in Chronic Human Atrial Fibrillation Is Associated With Increased Activity of Protein Phosphatases

Abstract

Background-Although downregulation of L-type Ca 2ϩ current (I Ca,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,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,L in myocytes and atrial tissue from 148 patients with sinus rhythm (SR) and chronic AF. Basal I Ca,L at ϩ10 mV was smaller in AF than in SR (Ϫ3.8Ϯ0.3 pA/pF, nϭ138/37 [myocytes/patients] and Ϫ7.6Ϯ0.4 pA/pF, nϭ276/86, respectively; PϽ0.001), though protein levels of the pore-forming ␣ 1c and regulatory ␤ 2a channel subunits were not different. In both groups, norepinephrine (0.01 to 10 mol/L) increased I Ca,L with a similar maximum effect and comparable potency. Selective blockers of kinases revealed that basal I Ca,L was enhanced by Ca 2ϩ /calmodulindependent 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 I Ca,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,L . We propose that protein phosphatases may be potential therapeutic targets for AF treatment.

Arrhythmias, elicited by catecholamines and serotonin, vanish in human chronic atrial fibrillation

“…We next compared the role of CaMKII on basal I Ca,L and the activation of I Ca,L by NE in myocytes from patients with SR and patients with AF. As reported previously (13,19), the CaMKII inhibitor KN-93, but not its inactive analog KN-92, reduced basal I Ca,L in myocytes from patients with SR but not in those from patients with AF. In addition, KN-93 reduced the NE-evoked I Ca,L responses in myocytes from patients with SR but not in those from patients with AF, consistent with a loss in AF of the NE-facilitated phosphorylation of the L-type Ca 2+ channel by CaMKII (Fig.…”

Reduced density and altered regulation of rat atrial L-type Ca2+current in heart failure

“…However, although the cAMP-dependent protein kinase, PKA, has also been demonstrated to play an important role in the basal regulation of I CaL in ventricular myocytes (7,8), the constitutive regulation of ventricular I CaL by PKA appears to be increased in a coronary artery ligation model of heart failure (9). In addition, the reduction in atrial I CaL in patients with chronic AF has been associated with a reduction in calciumcalmodulin-dependent protein kinase II (CaMKII) activity and an increase in protein phosphatase activity in AF with no change in the expression of L-type Ca 2ϩ channel subunit protein (14). Changes in the activity of the serine/threonine protein phosphatases, PP1 and PP2A, and the regulatory subunit of PP1, inhibitor-1, have been implicated in the pro-arrhythmic remodeling in AF (25).…”

Anti-adrenergic effects of endothelin on human atrial action potentials are potentially anti-arrhythmic

“…Nevertheless, extrapolation should be made with caution, since it is presently unknown whether such effects of ET-1 would be preserved in patients with AF or CHF. Of note, the modulation of I CaL by PKC, which is involved in ET-1 signalling [30,31], was impaired in chronic AF [32]. It is also unclear whether AF causes ET-1 elevation in the absence of CHF or other myocardial diseases [33].…”