On the oxalate stimulation of ATP-dependent calcium accumulation by smooth muscle subcellular membranes
Subcellular distribution of azide-insensitive, ATP-dependent Ca -accumulation in the presence and absence of 5 mM oxalate by fractions isolated from several different types of smooth muscle were studied. Although oxalate stimulated the Ca -accumulation to various degrees depending upon the source of smooth muscle used, there was no clear association between the stimulatory effect of oxalate and the distribution of any of the enzyme activities used as markers for various membranes. The distribution of ATP-dependent Ca -accumulation in the absence of oxalate, on the other hand, correlates relatively well with that of the plasma membrane marker in spite of reduced total recovery of the Ca -accumulation compared to those of the protein content and membrane markers. However, a greater reduction in the total percent recovery of the ATP-dependent Ca accumulation by fraction from three different types of smooth muscle was consistently observed when carried out in the presence compared to the absence of oxalate. The discrepancy in the total percent recovery of the Ca -accumulation in the presence and absence of oxalate poses a major concern on the uses of oxalate stimulated, ATP-dependent Ca accumulations as a subcellular membrane marker.