Protein Kinase A Modifies Skeletal Muscle Contractile Function Via Modification Of Mybp-C

Abstract

Human aging is associated with decreased skeletal muscle (SM) contractile velocity and powerproduction. Interestingly, older muscle exhibits greater tension (force per cross sectional area)compared to younger muscle when velocity is zero (isometric contraction), suggesting age-related contractile deficits are velocity-dependent and distinct from changes in muscle size.Mechanisms explaining this phenomenon are unknown, but evidence in cardiac myocytessuggests that Myosin Binding Protein-C (MyBP-C) has similar, phosphorylation-dependenteffects on maximal velocity (Vmax), power (Pmax), and tension (Tmax). However, this potentialhas not been explored in human SM. Therefore, the PURPOSE of our study is to examine ifphosphorylation of skeletal MyBP-C in older muscle produces contractile outcomes emulatingthose reported in younger muscle. METHODS: Measures of contractile performance, Tmax, Vmax and Pmax, were collected during maximal activation (pCa 4.5) of thirteen SM fibers fromtwo males (41 and 74 yrs.). Measures were acquired before and after incubation with ProteinKinase A (PKA), a known phosphorylating agent of MyBP-C, or relaxing solution without PKA(Control). Phospho-specific changes to MyBP-C were confirmed in parallel experiments on fiberbundles exposed to identical incubation conditions and assessed via SDS-PAGE followed byProQ Diamond and SYPRO Ruby staining. RESULTS: Phospho-specific staining revealed thatPKA selectively modified MyBP-C under these conditions without altering Regulatory LightChain or Troponin I. Following PKA incubation, MHC I fibers produced greater Vmax (0.46 ± 0.29 vs. 0.37 ± 0.25 ML/s, p < 0.05) and Pmax (1.73 ± 0.68 vs. 1.46 ± 0.54 mN/mm2·ML-s,p < 0.05) and lower Tmax (83.72 ± 11.44 vs. 92.19 ± 14.39 mN/mm2, p < 0.05). In contrast, neitherMHC II nor Control MHC I fibers demonstrated significant contractile changes after incubation. CONCLUSION: These exciting pilot data suggest that PKA selectively phosphorylates MyBP-C, resulting in functional differences in older SM fiber contractile performance that mirror theyouthful contractile phenotype.