CALCIUM REMOVAL KINETICS OF THE SARCOPLASMIC-RETICULUM ATPASE IN SKELETAL-MUSCLE

The models of the sarcoplasmic reticulum (SR) Ca pump used to simulate Ca kinetics in muscle fibers are simple but inconsistent with data on Ca binding or steady-state uptake. We develop a model of the SR pump that is consistent with data on transient and steady-state Ca removal and has rate constants identified under near-physiological conditions. We also develop models of the other main Ca-binding proteins in skele tal muscle: troponin C and parvalbumin. These models are used to simul ate Ca transients in cut fibers during and after depolarizing pulses. Simulations using the full SR pump model are contrasted with simulatio ns using a Michaelis-Menten (MM) approximation to SR pump kinetics. Th e MM pump underestimates the amount of Ca released during depolarizati on, underestimates the initial rate of Ca binding by the pump, and ove restimates the later rate of Ca pumping. These errors are due to fast initial binding by the SR pump, which is neglected in the MM approxima tion.