Acetyl Phosphate as Substrate for Ca²⁺ Uptake in Skeletal Muscle Microsomes

Abstract

In skeletal muscle microsomes, Ca²⁺ uptake activated by acetyl phosphate is inhibited by alkali ions. The inhibitory activity of these ions depends on the acetyl-P concentration in the assay medium. For 0.2 mm acetyl-P the pattern of inhibition is Li⁺ > Na⁺ > K⁺ = Rb⁺ = Cs⁺ and for 2.0 mm acetyl-P it is Na⁺ > Li⁺ ≥ K⁺ = Rb⁺ = Cs⁺.

In assay media containing 7.1 µm Ca²⁺ and either 20 mm KCl, 120 mm KCl, or 120 mm NaCl, the plot of the initial rate of Ca²⁺ uptake against acetyl-P concentration yields hyperbolic saturation curves. In an assay medium containing 120 mm LiCl, this saturation curve has a sigmoidal shape.

In the presence of 2 mm acetyl-P, the initial rate of Ca²⁺ uptake measured as a function of the Ca²⁺ concentration in the assay medium displays a saturation curve with a sigmoidal shape. However, if ATP is used as substrate, a hyperbolic saturation curve is obtained. For 2 mm acetyl-P, the inhibitory activity of the alkali ions depends on the Ca²⁺ concentration in the assay medium. For Ca²⁺ concentrations ranging from 1 to 2 µm, the pattern of inhibition is Li⁺ ≥ Na⁺ > K⁺, and for Ca²⁺ concentration of 7.1 µm, the pattern of inhibition is Na⁺ > Li⁺ ≥ K⁺.