Calcium Sensitivity, Force Frequency Relation and Cardiac Troponin I: Critical Role of PKA and PKC Phosphorylation Sites

Abstract

Transgenic models with mutants of cardiac troponin I, cTnI-PKA S22,23D (cTnIDD22,23) or cTnI-PKC S22A,S23D,S42,S44D (cTnI PKA/PKC) displayed differential force-frequency relationships (FFR) in vivo. We hypothesized that these cTnI phospho-mimics would impact cardiac muscle force development and Ca2+ sensitivity in opposite directions in a rate dependent fashion. Our study shows that cTnIDD22,23 increases (while cTnI PKA/PKC decreases) its ability to generate normal force per unit of [Ca2+]i when stimulation frequency increases. Force-[Ca2+]I hysteresis-loops revealed that cTnIDD22,23 shows an increased calcium sensitivity in the activation phase of force-[Ca2+]i loops at 1 to 4 Hz when compared to NTG (Figure1E). An integrated computational model that encompasses electrophysiology, Ca2+ dynamics, contractile and mitochondrial activity (ECME model) indicates that these cTnI mutants might change the association-dissociation constants for Ca2+ binding, both the low- and/or high-affinity binding sites, of troponin complex. Our data indicate that cTnI phosphorylation at PKA sites is a crucial mediator of the FFR by increasing the frequency-dependent myofilament sensitivity; which might be achieved by adaptive changes on association-dissociation constants for Ca2+ binding of the troponin complex.