Na+-dependence of 45Ca2+ uptake in adult rat isolated cardiac cells.

We developed a technique that yields isolated adult rat myocytes, 70% of which are elongated and morphologically similar to intact tissue. Electrophysiological studies showed most of these cells were quiescent, Ca2+-tolerant and exhibited normal action potentials accompanied by contractions. We analyzed 45Ca2+ uptake data in terms of instantaneous, fast and slow compartments. 69% of total exchangeable Ca2+ was found in the slow compartment: the rest was almost equally divided between the instantaneous and fast compartments. Replacement of extracellular Na+ by Li+ or Tris increased 45Ca2+ uptake by the fast compartment; high [K+]o increased this uptake further. These increases appeared to be related also to internal concentrations of Na+. This conclusion was supported by experiments with digitonin-treated cells. Our results indicate that the way Na+-dependent 45Ca2+ uptake is affected by [Na+]o, [Na+]i and [K+]o is compatible with the Na+-Ca2+ exchange mechanism. Our preparation should prove useful in studies of regulation of Ca2+ transport in cardiac muscles.