[Relation between sodium pump activity and the osmotic pressure of the medium]

The dependence of electrogenic sodium pump activity on the tonicity of the surrounding medium was studied on snail giant neurons. It has been shown that the activity of electrogenic sodium pump in hypertonic media increases as compared to that in hypotonic media in neurons with low Na content, and decreases in neurons with high Na content. It has been found that the number of ouabain molecules bound with the membrane increases in hypotonic media as compared to its binding in iso- and hypertonic media. Existence of two mechanisms are suggested which are involved in the influence of cellular volume changes on the activity of the sodium pump. One of the mechanisms depends on the level of intracellular sodium, another on the number of functioning Na,K-ATPase molecules (pump units in the membrane).