Increased myofilament calcium sensitivity is associated with decreased cardiac troponin I phosphorylation in the diabetic rat heart

Abstract Background The diabetic heart has impaired systolic and diastolic function independent of other comorbidities. The availability of calcium is altered, but does not fully explain the cardiac dysfunction seen in the diabetic heart. Thus, we explored if myofilament protein regulation of contraction is altered. Methods Calcium sensitivity (pCa50) was measured in Zucker Diabetic Fatty (ZDF) rat hearts at the initial stage of diabetes (12-week-old) and after 8 weeks of uncontrolled hyperglycaemia (20-week-old) and in non-diabetic (nDM) littermates. Skinned cardiomyocytes were connected to a capacitance-gauge transducer and a torque motor to measure force as a function of pCa (-log[Ca2+]). Fluorescent gel stain (ProQ Diamond) was used to measure total protein phosphorylation. Specific phospho-sites on cardiac troponin I (cTnI) and total cTnI O-GlcNAcylation were quantified using immunoblot. Results pCa50 was greater in both 12- and 20-week-old diabetic (DM) rats compared to nDM littermates (p = 0.0005). Total cTnI and cTnI serine 23/24 phosphorylation were lower in DM rats (p = 0.003 & p = 0.01, respectively), but cTnI O-GlcNAc protein expression was not different. pCa50 is greater in DM rats and corresponds with an overall reduction in cTnI phosphorylation. Conclusions These findings indicate that myofilament calcium sensitivity is increased and cTnI phosphorylation is reduced in ZDF DM rats, which suggests an important role for cTnI phosphorylation in the DM heart.