Protein kinase A-induced myofilament desensitization to Ca(2+) as a result of phosphorylation of cardiac myosin-binding protein C.
In skinned myocardium, cyclic AMP-dependent protein kinase A (PKA)-catalyzed phosphorylation of cardiac myosin-binding protein C (cMyBP-C) and cardiac troponin I (cTnI) is associated with a reduction in the Ca(2+) responsiveness of myofilaments and an acceleration in the kinetics of cross-bridge cycling, although the respective contribution of these two proteins remains controversial. To further examine the relative roles that cTnI and cMyBP-C phosphorylation play in altering myocardial function, we determined the Ca(2+) sensitivity of force (pCa(50)) and the activation dependence of the rate of force redevelopment (k(tr)) in control and PKA-treated mouse myocardium (isolated in the presence of 2,3-butanedione monoxime) expressing: (a) phosphorylatable cTnI and cMyBP-C (wild type [WT]), (b) phosphorylatable cTnI on a cMyBP-C-null background (cMyBP-C(-/-)), (c) nonphosphorylatable cTnI with serines(23/24/43/45) and threonine(144) mutated to alanines (cTnI(Ala5)), and (d) nonphosphorylatable cTnI on a cMyBP-C-null background (cTnI(Ala5)/cMyBP-C(-/-)). Here, PKA treatment decreased pCa(50) in WT, cTnI(Ala5), and cMyBP-C(-/-) myocardium by 0.13, 0.08, and 0.09 pCa units, respectively, but had no effect in cTnI(Ala5)/cMyBP-C(-/-) myocardium. In WT and cTnI(Ala5) myocardium, PKA treatment also increased k(tr) at submaximal levels of activation; however, PKA treatment did not have an effect on k(tr) in cMyBP-C(-/-) or cTnI(Ala5)/cMyBP-C(-/-) myocardium. In addition, reconstitution of cTnI(Ala5)/cMyBP-C(-/-) myocardium with recombinant cMyBP-C restored the effects of PKA treatment on pCa(50) and k(tr) reported in cTnI(Ala5) myocardium. Collectively, these results indicate that the attenuation in myofilament force response to PKA occurs as a result of both cTnI and cMyBP-C phosphorylation, and that the reduction in pCa(50) mediated by cMyBP-C phosphorylation most likely arises from an accelerated cross-bridge cycling kinetics partly as a result of an increased rate constant of cross-bridge detachment.
Department of Physiology and UW Cardiovascular Research Center, University of Wisconsin School of Medicine and Public Health, Madison, WI 53706, USA. peterchen@­wisc.edu
Peter P
PP
Chen
Jitandrakumar R
JR
Patel
Inna N
IN
Rybakova
Jeffery W
JW
Walker
Richard L
RL
Moss
D008841:Actin Cytoskeleton / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology; D009206:Myocardium; D010766:Phosphorylation; D000099163:Myosin Binding Protein C
D002118:Calcium; D002352:Carrier Proteins; D017868:Cyclic AMP-Dependent Protein Kinases; D000099163:Myosin Binding Protein C