Evidence for Phosphatidic Acid as the Sodium Carrier

Abstract

THE mechanism of active transport of ions is one of the basic problems of cell physiology. For example, most cells extrude sodium against a concentration gradient, and the system which brings this about has been termed the ‘sodium pump’. The biochemical mechanism of the ‘sodium pump’ has remained a mystery. In view of the fact that the turnover of certain phosphatides is concerned in the active extrusion of organic molecules from endocrine and exocrine glands¹, we have recently investigated the possibility that this turnover may also be involved in the secretion or active transport of sodium ions. The salt glands of marine birds are particularly suitable for studying this problem, since they are capable of secreting an apparently² pure solution of sodium chloride in concentrations as high as 0.84 M. Furthermore, the secretory activity of the gland can be stimulated by cholinergic agents. The secretion of sodium chloride by the salt gland is normally regulated by the activity of that branch of the facial nerve (cholinergic) which innervates it, and which in turn appears to be regulated by osmoreceptors.