Strontium accumulation by sarcoplasmic reticulum and mitochondria in vascular smooth muscle

Electron-opaque deposits of strontium were observed in the sarcoplasmic reticulum and in mitochondria of spontaneously contracting vascular smooth muscles that had been incubated in a strontium-containing solution prior to fixation. The deposits were present in those elements of the sarcoplasmic reticulum that are in close contact with the surface membrane and also in more centrally located portions. In vascular smooth muscle that does not contract spontaneously, similar deposits of strontium were only seen if the muscle was depolarized during or glycerinated before exposure to the strontium-containing solution. Strontium was also deposited in the sarcoplasmic reticulum of the endothelium. It is suggested that translocation of calcium from the sarcoplasmic reticulum that is in close contact with the surface membrane, and now shown to accumulate divalent cations, is responsible for the action potential-triggered contractions of rabbit and guinea pig mesenteric veins. Strontium may also be a suitable marker for identifying sites that accumulate calcium in other types of cells in which translocation of calcium plays a major regulatory function.