Some properties of Ca-binding microsomal subfractions isolated from rabbit colon muscle

From a homogenate of rabbit colon smooth muscle a microsomal fraction was isolated, which was divided into subfractions by centrifugation on a discontinuous sucrose gradient. The Ca-binding properties of the subfractions were investigated under different conditions. In the presence of 0.35 mM ATP the Ca binding of the fractions amounted to 4--8 nmol/mg protein. The 35% fraction bound more Ca per mg protein than the 35--45% fraction. The Ca accumulation was comparatively higher both in the presence of 5 mM ATP and in the presence of 5 mM oxalate. The two fractions showed about the same sensitivity for oxalate. This substance stimulated the Ca uptake at 5 mM but not at lower concentrations. The amount and the rate of Ca binding were more dependent on variations in the exogenous ATP concentration in the 35% fraction than was the case for the 35--45% fraction. The Ca binding was completely inhibited by salyrgan when the microsomal fractions were pretreated with this agent. Sodium azide did not influence the Ca-binding capacity of the fractions. It is suggested that the microsomal subfractions of the rabbit colon muscle represent physiologically important parts of the Ca sequestering system of the muscle, since Ca binding takes place at Ca- and ATP-concentrations which are believed to be present in the myoplasm.