Molecule Specific Effects of PKA-Mediated Phosphorylation on Myofibrillar Function

Abstract

Increased cardiac myocyte contractility by the beta-adrenergic system is an important mechanism to elevate cardiac output to meet greater hemodynamic load and this process is often depressed in failing hearts. While increased contractility involves augmented myoplasmic calcium transients, the myofilaments also adapt to boost the transduction of the calcium signal. Accordingly, ventricular contractility is tightly correlated with PKA-mediated phosphorylation of two myofibrillar proteins, cardiac myosin binding protein-C (cMyBP-C) and cardiac troponin I (cTnI), implicating these two proteins as important transducers of hemodynamics to the cardiac sarcomere. Consistent with this, we have previously found that phosphorylation of myofilament proteins by PKA (a downstream signaling molecule of the beta-adrenergic system) increased force, slowed force development rates, sped loaded shortening, and increased power output in rat skinned cardiac myocyte preparations. Here, we sought to define molecule-specific mechanisms by which PKA-mediated phosphorylation modulates these contractile properties. Regarding cTnI, the incorporation of thin filaments with a majority of unphosphorylated cTnI (as observed in some models of late stage heart failure) decreased isometric force production at any given activator [Ca2+] and these changes were reversed by PKA-mediated phosphorylation in skinned cardiac myocytes. In addition, incorporation of unphosphorylated cTnI sped rates of force development, which suggests less cooperative thin filament activation and recruitment of non-cycling cross-bridges into the pool of cycling cross-bridges, a process that would tend to depress myocyte force and power. Regarding cMyBP-C, PKA treatment of slow-twitch skeletal muscle fibers caused phosphorylation of MyBP-C (but not TnI) and yielded faster loaded shortening velocity and an ∼30% increase in power output. These results add novel insight into the molecular specificity by which the beta-adrenergic system controls myofibrillar contractility and how attenuation of PKA-induced phosphorylation of cMyBP-C and cTnI may contribute to ventricular pump failure.