title: The contributions of cardiac myosin binding protein C and troponin I phosphorylation to β-adrenergic enhancement of in vivo cardiac function.
abstract: β-adrenergic stimulation increases cardiac myosin binding protein C (MyBP-C) and troponin I phosphorylation to accelerate pressure development and relaxation in vivo, although their relative contributions remain unknown. Using a novel mouse model lacking protein kinase A-phosphorylatable troponin I (TnI) and MyBP-C, we examined in vivo haemodynamic function before and after infusion of the β-agonist dobutamine. Mice expressing phospho-ablated MyBP-C displayed cardiac hypertrophy and prevented full acceleration of pressure development and relaxation in response to dobutamine, whereas expression of phosphor-ablated TnI alone had little effect on the acceleration of contractile function in response to dobutamine. Our data demonstrate that MyBP-C phosphorylation is the principal mediator of the contractile response to increased β-agonist stimulation in vivo. These results help us understand why MyBP-C dephosphorylation in the failing heart contributes to contractile dysfunction and decreased adrenergic reserve in response to acute stress. β-adrenergic stimulation plays a critical role in accelerating ventricular contraction and speeding relaxation to match cardiac output to changing circulatory demands. Two key myofilaments proteins, troponin I (TnI) and myosin binding protein-C (MyBP-C), are phosphorylated following β-adrenergic stimulation; however, their relative contributions to the enhancement of in vivo cardiac contractility are unknown. To examine the roles of TnI and MyBP-C phosphorylation in β-adrenergic-mediated enhancement of cardiac function, transgenic (TG) mice expressing non-phosphorylatable TnI protein kinase A (PKA) residues (i.e. serine to alanine substitution at Ser23/24; TnI(PKA-)) were bred with mice expressing non-phosphorylatable MyBP-C PKA residues (i.e. serine to alanine substitution at Ser273, Ser282 and Ser302; MyBPC(PKA-)) to generate a novel mouse model expressing non-phosphorylatable PKA residues in TnI and MyBP-C (DBL(PKA-)). MyBP-C dephosphorylation produced cardiac hypertrophy and increased wall thickness in MyBPC(PKA-) and DBL(PKA-) mice, and in vivo echocardiography and pressure-volume catheterization studies revealed impaired systolic function and prolonged diastolic relaxation compared to wild-type and TnI(PKA-) mice. Infusion of the β-agonist dobutamine resulted in accelerated rates of pressure development and relaxation in all mice; however, MyBPC(PKA-) and DBL(PKA-) mice displayed a blunted contractile response compared to wild-type and TnI(PKA-) mice. Furthermore, unanaesthesized MyBPC(PKA-) and DBL(PKA-) mice displayed depressed maximum systolic pressure in response to dobutamine as measured using implantable telemetry devices. Taken together, our data show that MyBP-C phosphorylation is a critical modulator of the in vivo acceleration of pressure development and relaxation as a result of enhanced β-adrenergic stimulation, and reduced MyBP-C phosphorylation may underlie depressed adrenergic reserve in heart failure.
authors.affiliation: Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, USA.
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Gresham
authors.affiliation: Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, USA.
authors.forename: Julian E
authors.initials: JE
authors.lastname: Stelzer
mesh_terms: D058665:Adrenergic beta-1 Receptor Agonists / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure; D006332:Cardiomegaly / Q000473:pathology / Q000503:physiopathology*; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000494:pharmacology; D004280:Dobutamine / Q000494:pharmacology; D005260:Female; D006321:Heart / Q000503:physiopathology; D008297:Male; D008822:Mice, Transgenic; D009206:Myocardium / Q000473:pathology; D009210:Myofibrils / Q000378:metabolism; D010766:Phosphorylation; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D019210:Troponin I / Q000235:genetics / Q000502:physiology*; D000099163:Myosin Binding Protein C
chemical_list: D058665:Adrenergic beta-1 Receptor Agonists; D002352:Carrier Proteins; D017868:Cyclic AMP-Dependent Protein Kinases; D004280:Dobutamine; D011943:Receptors, Adrenergic, beta; D019210:Troponin I; D000099163:Myosin Binding Protein C