Cellular Biology |
From the Abteilung Kardiologie & Pneumologie/Herzzentrum (M.K., T.S., D.Z., N.D., A.S., S.W., L.S.M.), Georg-August-Universität Göttingen, Germany; Department of Pharmacology (T.Z., J.H.B.), University of California San Diego; and Department of Physiology (L.C., D.M.B.), Stritch School of Medicine, Loyola University Chicago, Ill.
Correspondence to Lars S. Maier, MD, Abt. Kardiologie & Pneumologie/Herzzentrum, Georg-August-Universität Göttingen, Robert-Koch-Str. 40, 37075 Göttingen, Germany. E-mail lmaier{at}med.uni-goettingen.de
The predominant cardiac Ca2+/calmodulin-dependent protein kinase (CaMK) is CaMKII
. Here we acutely overexpress CaMKII
C using adenovirus-mediated gene transfer in adult rabbit ventricular myocytes. This circumvents confounding adaptive effects in CaMKII
C transgenic mice. CaMKII
C protein expression and activation state (autophosphorylation) were increased 5- to 6-fold. Basal twitch contraction amplitude and kinetics (1 Hz) were not changed in CaMKII
C versus LacZ expressing myocytes. However, the contractionfrequency relationship was more negative, frequency-dependent acceleration of relaxation was enhanced (
0.5Hz/
3Hz=2.14±0.10 versus 1.87±0.10), and peak Ca2+ current (ICa) was increased by 31% (7.1±0.5 versus 5.4±0.5 pA/pF, P<0.05). Ca2+ transient amplitude was not significantly reduced (27%, P=0.22), despite dramatically reduced sarcoplasmic reticulum (SR) Ca2+ content (41%; P<0.05). Thus fractional SR Ca2+ release was increased by 60% (P<0.05). Diastolic SR Ca2+ leak assessed by Ca2+ spark frequency (normalized to SR Ca2+ load) was increased by 88% in CaMKII
C versus LacZ myocytes (P<0.05; in an multiplicity-of-infectiondependent manner), an effect blocked by CaMKII inhibitors KN-93 and autocamtide-2related inhibitory peptide. This enhanced SR Ca2+ leak may explain reduced SR Ca2+ content, despite measured levels of SR Ca2+-ATPase and Na+/Ca2+ exchange expression and function being unaltered. Ryanodine receptor (RyR) phosphorylation in CaMKII
C myocytes was increased at both Ser2809 and Ser2815, but FKBP12.6 coimmunoprecipitation with RyR was unaltered. This shows for the first time that acute CaMKII
C overexpression alters RyR function, leading to enhanced SR Ca2+ leak and reduced SR Ca2+ content but without reducing twitch contraction and Ca2+ transients. We conclude that this is attributable to concomitant enhancement of fractional SR Ca2+ release in CaMKII
C myocytes (ie, CaMKII-dependent enhancement of RyR Ca2+ sensitivity during diastole and systole) and increased ICa.
Key Words: calcium CaMKII excitationcontraction coupling sarcoplasmic reticulum
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