Figure 2.

Characteristics of Ca²⁺ pumps used in model. (A) Schematic of the two-state Ca²⁺ pump model used for SERCA and PMCA. The model consists of two states, one on the myoplasmic side of the membrane (labeled P_M), where Ca²⁺ is collected, and one on the luminal side of the SR membrane for SERCA or t-tubular membrane for PMCA (P_SR), where Ca²⁺ is released. The diagram illustrates the forward and reverse rate constants for transitions between the states (α), Ca²⁺ transport, H⁺ countertransport, and hydrolysis of ATP to provide energy. The rate constants are functions of [ATP] and its metabolites and pH. (B) Dependence on [Ca²⁺] of Ca²⁺ pumping rate of SERCA and PMCA. Curves constructed using the model showing relationship between relative rate of Ca²⁺ pumping (% maximum rate) and [Ca²⁺] using the K_d values for Ca²⁺ dissociation on the myoplasmic side as applied in the current analysis. The maximal rates of pumping (with concentrations expressed relative to myoplasmic water volume; see Table 3) were SERCA, 711 μM s⁻¹; and PMCA, 0.15 μM s⁻¹. Note that expressed relative to JS volume, PMCA maximal Ca²⁺ pumping rate was 705 μM s⁻¹. The Ca₅₀ values were SERCA, 700 nM; and PMCA, 46 nM.