Calcium incorporation by canine aortic smooth muscle microsomes

The accumulation of calcium (Ca2+) and factors influencing Ca2+ incorporation in vascular smooth muscle were examined in a microsomal fraction prepared from canine aortae. The accumulation of Ca2+ required the presence of ATP and increased over time. Furthermore, the incorporation of 45Ca into the microsomal preparation was stimulated both in the presence of oxalate and when the Ca2+ concentration of the bathing media was elevated from 20 micron to 40 micron. Calcium uptake (with oxalate) was temperature-dependent; uptake was unaffected by azide. Both lanthanum (La3+) and phosphatidyl serine (PS) inhibited ATP-dependent uptake in a concentration-dependent manner. In the presence of ATP, calcium binding (without oxalate) was slightly inhibited by La3+, whereas PS had no effect under these conditions. When ATP was removed from the incubation medium, the presence of PS stimulated both the uptake and binding of 45Ca, whereas La3+ decreased both uptake and binding. These findings support the idea that La3+ binds on the surface of the vesicles and, in this manner, decreases available Ca2+ sites, whereas PS appears to increase the number of Ca2+ sites by either entering the membranes or adsorbing on it.