Passive ionic permeabilities of synaptosomal membranes from rat brains

Summary

Passive ionic permeabilities of synaptosomal membranes from rat brains were measured under various ionic conditions of intra- and extrasynaptosomal solutions by means of radioisotopic tracers, and were treated quantitatively. The efflux rates of Na⁺ and K⁺ ions were observed to be affected by the concentrations of permeable monovalent cations inside and outside the membranes, and not by other factors such as the osmotic pressure or the concentrations of anions. The rates increased with the outer concentrations of permeable monovalent cations, and decreased with the inner ones. Na-efflux was most efficiently accelerated by Na⁺ ions in the outer solution, and K-efflux by K⁺ ions. At the optimal conditions, K⁺ ions traversed the membranes much faster than Na⁺ ions. Cl-efflux was observed to be independent of the ionic condition of the outer solution.

These observations could not be explained by simple electrochemical diffusions, and were discussed in relation to an ionic permeation model assuming exchange permeations at certain sites.