Calcium-activated force in a turkey model of spontaneous dilated cardiomyopathy: Adaptive changes in thin myofilament Ca²⁺ regulation with resultant implications on contractile performance

Using saponin skinned fibers, we investigated whether decreased myofilament calcium responsiveness and contractile activation may in part contribute to heart failure in an animal model of idiopathic spontaneous cardiomyopathy (SCM). We addressed the question as to whether there are adaptive changes at the level of the thin myofilaments in turkey poults with SCM. The calcium concentration ([Ca²⁺]) required for 50% activation ([Ca²⁺]_50%) was 0.80±0.12 μm (n=12) vs. 0.76±0.08 μm (n=12) and the Hill coefficient was 1.98±0.20 (n=12) vs. 2.14±0.38 (n=12) for control and SCM muscles respectively. Maximal Ca²⁺-activated force was not different between control fibers and fibers from failing hearts (3.83±0.88 g/mm² vs. 3.65±0.39 g/mm²). These data indicate there are no differences in calcium-activation between fibers from control and failing myocardium. The effects of caffeine, an agent that increases myofilament Ca²⁺ sensitivity, were also studied. Addition of 10 mm caffeine resulted in a 0.06 pCa unit leftward shift of the force-pCa relationship in control hearts and 0.14 pCa units in SCM hearts. Caffeine (30 mm) increased force by 26±2.1% (n=7) in control fibers and 44.5±8.7% (n=8) in myopathic fibers at a pCa of 6.0. The increased responsiveness of muscles from failing hearts to caffeine indicates adaptive changes at the level of the thin myofilaments. Addition of dibutyryl-3′,5′-cyclic-Adenosine Monophosphate (D-cAMP) resulted in a 0.21 pCa rightward shift on the calcium axis to higher intracelluar calcium concentrations in control myocardium and 0.38 pCa units in SCM failing myocardium. The muscles were also sinusoidally oscillated at frequencies ranging between 0.01 and 100 Hz. In this analysis the frequency at which dynamic stiffness is minimum is taken as a measure of cross-bridge cycling rate. In control muscles, the frequency of minimum stiffness (f_min) was 1.20±0.11 (n=4) whereas it was 0.71±0.08 Hz (n=4) in myopathic muscles. The addition of 10 μm D-cAMP shifted f_min from 1.20±0.11 Hz to 1.68±0.09 Hz (Δ=0.48±0.06) in control fibers whereas in SCM fibers it caused greater shift of f_min from 0.71±0.08 Hz to 1.73±0.08 Hz (Δ=1.02±0.07). This differential effect of D-cAMP indicates adaptive changes at the level of the myofilaments. Our studies indicate that adaptive changes at the level of the thin myofilaments can potentially alter the "apparent" calcium-force relationship in failing myocardium and contractile performance, and that failing myocardium has the potential to generate similar levels of force as normal non-failing myocardium.