Inorganic phosphate speeds loaded shortening in rat skinned cardiac myocytes

Abstract

Force generation in striated muscle is coupled with inorganic phosphate (P_i) release from myosin, because force falls with increasing P_iconcentration ([P_i]). However, it is unclear which steps in the cross-bridge cycle limit loaded shortening and power output. We examined the role of P_iin determining force, unloaded and loaded shortening, power output, and rate of force development in rat skinned cardiac myocytes to discern which step in the cross-bridge cycle limits loaded shortening. Myocytes ( n = 6) were attached between a force transducer and position motor, and contractile properties were measured over a range of loads during maximal Ca²⁺activation. Addition of 5 mM P_ihad no effect on maximal unloaded shortening velocity ( V_o) (control 1.83 ± 0.75, 5 mM added P_i1.75 ± 0.58 muscle lengths/s; n = 6). Conversely, addition of 2.5, 5, and 10 mM P_iprogressively decreased force but resulted in faster loaded shortening and greater power output (when normalized for the decrease in force) at all loads greater than ∼10% isometric force. Peak normalized power output increased 16% with 2.5 mM added P_iand further increased to a plateau of ∼35% with 5 and 10 mM added P_i. Interestingly, the rate constant of force redevelopment ( k_tr) progressively increased from 0 to 10 mM added P_i, with k_tr∼360% greater at 10 mM than at 0 mM added P_i. Overall, these results suggest that the P_irelease step in the cross-bridge cycle is rate limiting for determining shortening velocity and power output at intermediate and high relative loads in cardiac myocytes.