The slowly exchanging pool of sodium in frog skeletal muscle is confined within a membranous organelle

Abstract

In the sartorius of the frog (Rana pipiens), it was found that 10–15% of the tissue sodium was not lost in sodium-free lithium-substituted solution, and was not exchanged with radiosodium in normal frog Ringer's solution, after 4 h. This sodium is not detected by an intracellular sodium-selective glass microelectrode. Detergent or freezing and thawing, in sodium-free solution, cause the loss of all but about 2% of the tissue sodium. The ionophore monensin causes a similar loss but does not disrupt cellular membranes. It was concluded that the slowly exchanging sodium exists in solution inside an organelle whose membrane has a low sodium permeability relative to the sarcolemma, and that this organelle probably is the sarcoplasmic reticulum.