Electron microscopic localization of calcium in vascular smooth muscle

Potassium pyroantimonate has been employed in this study to localize calcium in the vascular smooth muscle of the thoracic aorta of the rabbit. The pyroantimonate ion precipitates sodium, magnesium and calcium. Incubation of the isolated thoracic aorta in a high potassium bathing medium which does not contain sodium, magnesium or calcium depletes the tissue of sodium. Addition of 10.8 mM CaCl2 to the incubation medium results in welllocalized depositions of reaction product, presumably that of calcium pyroantimonate, in mitochondria, sarcoplasmic reticulum, and at the plasma membrane. Some or all of these organelles may, therefore, play a vital role in the contractionrelaxation cycle of vascular smooth muscle.