Cytochemical localization of Ca2+-ATPases and demonstration of ATP-dependent calcium sequestration in giant smooth muscle fibres of Beroe

A cytochemical analysis of the mechanisms underlying cytosolic calcium regulation was undertaken in the giant smooth muscle fibres of the marine invertebrate Beroe. The ability of the sarcoplasmic reticulum to accumulate Ca2+ was demonstrated on living skinned single cells. In the presence of oxalate, and physiological concentrations of Ca2+, calcium oxalate crystals were formed in the lumen of tubules and cisternae of the sarcoplasmic reticulum. The subcellular distribution of Ca2−-ATPase was studied with a cytochemical technique; a dense precipitate resulting from Ca2+-ATPase activity was found on the plasma membrane, on the membranes of tubules and cisternae of the sarcoplasmic reticulum, and in mitochondria.