On the relative lipid membrane permeability of Na+ and Ca2+. A physical basis for the messenger role of Ca2+.

Calcium ion is demonstrated to bind in the gramicidin A transmembrane channel at a site that is displaced outward about 1.5 A from that of the sodium ion. The calcium ion is also shown to exchange rapidly with the solution such that the impermeability of the channel to calcium ion is due to the proximity of the lipid which limits solvation energy. Also at its binding site, calcium is shown to be able to draw into binding a peptide carbonyl which is not drawn into coordination by the sodium ion. These results demonstrate aspects of calcium ion transport and interaction which are considered central to its role as a cellular messenger.