A unitary cause for the exclusion of Na+ and other solutes from living cells, suggested by effluxes of Na+, D-arabinose, and sucrose from normal, dying, and dead muscles.

1. The effluxes of labeled Na+, D-arabinose, and sucrose from normal muscle and muscle poisoned with low concentrations of iodoacetate were studied. The procedure involved repeated loading with isotope, followed by washing of the same muscle while still normal and at different states of dying. 2. The rates of Na+ efflux in both the fast and slow fraction remained either quite constant or showed some unpredictable, minor fluctuations. This was true for both Na+ and the two sugars studied, confirming earlier conclusions that the steady levels of these solutes were not maintained by pumps. 3. In all cases studied, the efflux curves showed at least two fractions. It is the fast-exchanging fraction that steadily and consistently increased in magnitude as the muscles were dying, until finally the concentration of solute in this fraction reached and sometimes surpassed the labeled solute concentrations in the original labeled solutions in which the muscles were equilibrated. The slow fractions showed only a transient increase or none at all. These observations show that it is the fast fraction that represents solute dissolved in cell water and rate-limited by passage through the cell surface and that the partial exclusion of Na+ and the sugars have a unitary cause--a reduced solubility in the cell water which in the presence of ATP exists in the state of polarized multilayers.