title: Frequency-dependent myofilament Ca2+ desensitization in failing rat myocardium.
abstract: The positive force-frequency relation, one of the key factors modulating performance of healthy myocardium, has been attributed to an increased Ca(2+) influx per unit of time. In failing hearts, a blunted, flat or negative force-frequency relation has been found. In healthy and failing hearts frequency-dependent alterations in Ca(2+) sensitivity of the myofilaments, related to different phosphorylation levels of contractile proteins, could contribute to this process. Therefore, the frequency dependency of force, intracellular free Ca(2+) ([Ca(2+)](i)), Ca(2+) sensitivity and contractile protein phosphorylation were determined in control and monocrotaline-treated, failing rat hearts. An increase in frequency from 0.5 to 6 Hz resulted in an increase in force in control (14.3 +/- 3.0 mN mm(-2)) and a decrease in force in failing trabeculae (9.4 +/- 3.2 mN mm(-2)), whereas in both groups the amplitude of [Ca(2+)](i) transient increased. In permeabilized cardiomyocytes, isolated from control hearts paced at 0 and 9 Hz, Ca(2+) sensitivity remained constant with frequency (pCa(50): 5.55 +/- 0.02 and 5.58 +/- 0.01, respectively, P>0.05), whereas in cardiomyocytes from failing hearts Ca(2+) sensitivity decreased with frequency (pCa(50): 5.62 +/- 0.01 and 5.57 +/- 0.01, respectively, P<0.05). After incubation of the cardiomyocytes with protein kinase A (PKA) this frequency dependency of Ca(2+) sensitivity was abolished. Troponin I (TnI) and myosin light chain 2 (MLC2) phosphorylation remained constant in control hearts but both increased with frequency in failing hearts. In conclusion, in heart failure frequency-dependent myofilament Ca(2+) desensitization, through increased TnI phosphorylation, contributes to the negative force-frequency relation and is counteracted by a frequency-dependent MLC2 phosphorylation. We propose a novel role for PKC-mediated TnI phosphorylation in modulating the force-frequency relation.
authors.affiliation: Department of Anesthesiology, Institute for Cardiovascular Research (ICaR-VU), VU University Medical Center (VUMC), 1081 BT Amsterdam, The Netherlands. r.lamberts@vumc.nl
authors.forename: Regis R
authors.initials: RR
authors.lastname: Lamberts
authors.forename: Nazha
authors.initials: N
authors.lastname: Hamdani
authors.forename: Tenoedj W
authors.initials: TW
authors.lastname: Soekhoe
authors.forename: Nicky M
authors.initials: NM
authors.lastname: Boontje
authors.forename: Ruud
authors.initials: R
authors.lastname: Zaremba
authors.forename: Lori A
authors.initials: LA
authors.lastname: Walker
authors.forename: Pieter P
authors.initials: PP
authors.lastname: de Tombe
authors.forename: Jolanda
authors.initials: J
authors.lastname: van der Velden
authors.forename: Ger J M
authors.initials: GJ
authors.lastname: Stienen
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002303:Cardiac Output, Low / Q000139:chemically induced / Q000378:metabolism / Q000503:physiopathology*; D003285:Contractile Proteins / Q000378:metabolism; D006321:Heart / Q000503:physiopathology*; D006352:Heart Ventricles; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D016686:Monocrotaline; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000378:metabolism; D018994:Myosin Light Chains / Q000378:metabolism; D010766:Phosphorylation; D011312:Pressure; D051571:Protein Kinase C-alpha / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D019210:Troponin I / Q000378:metabolism
chemical_list: D003285:Contractile Proteins; D018994:Myosin Light Chains; D019210:Troponin I; D016686:Monocrotaline; D051571:Protein Kinase C-alpha; D002118:Calcium