Sarcolemma-bound calcium. Its importance for cell viability.

We report the presence of a sarcolemma-associated Ca2+-pool in the intact myocardium and isolated ventricular myocytes of the rat. Ca2+-deposits, which are visualized in the electron microscope as 20 nm thick particles, could be demonstrated when fixation of the tissue was performed in the presence of high inorganic phosphate and most probably represent stable Ca2+-acidic phospholipid-phosphate complexes. Various pathophysiological conditions all leading to intracellular Ca2+-overload and compromising myocardial cell viability were imposed and the distribution of Ca2+ was assessed. It was found that in all these conditions cellular Ca2+-overload was preceded or at least accompanied by a loss of sarcolemma-associated Ca2+-deposits. It is concluded that this Ca2+, possibly bound to acidic phospholipids of the sarcolemmal bilayer, plays a role in the maintenance of sarcolemmal integrity. These cytological observations support a concept previously proposed by Langer and his group on the controlling role of membrane Ca2+ in the overall cellular Ca2+-homeostasis.