The Prevalence of K

THE PREVALENCE OF K H. BURR STEINBACH* The earth comprises the lithosphère, the hydrosphere, the atmosphere, and the biosphere, all in dynamic balance with exchanges ofmaterial and energy taking place at variable but finite rates. The rapidity ofthese exchanges in the biosphere results in its acting as a catalytic agent for many of the exchanges between the other subdivisions. The biosphere is composed of the same elements found in the other subdivisions, its distinctiveness lying in the relative amounts ofthe elements, the special arrangements of the elements into compounds, and the constancy of composition despite the very rapid turnover ofthe elements. Sodium and potassium are widely spread throughout all subdivisions of the earth (i). The other alkali metals—cesium, rubidium, and lithium—are quantitatively ofless general importance and show no special properties as normal components ofliving cells (2). The stability ofthe amounts ofsodium and potassium in animal bodies has been recognized for many years, balance studies in adults having shown that for each metal the amount excreted is equal to the amount ingested. In man, and terrestrial and aerial animals in general, intake ofalkali metal occurs orally, the amount tolerated in the diet varying between fairly well defined physiological limits (3). These limits, in man at least, show a definite ontogenetic development, being less precise and fixed in the newborn. In aquatic and semi-aquatic forms other devices occur. Most amphibians obtain sodium from the aqueous environment by active uptake through the skin and potassium by ingestion offood. Marine forms also have both food and environmental sources; some, such as marine fishes, have had to develop special sodium extrusion tissues or organs. Thus the biosphere rep- * Department ofZoology, University ofChicago. The original investigationson which this paper is based were aided by National Science Foundation Grant G12449 and by the Wallace G. and Clara A. Abbott Fund. 338 H. Burr Steinbach · The Prevalence ofK Perspectives in Biology and Medicine · Spring 1962 resents a mass ofsodium and potassium which is relatively fixed in total amount but which exchanges with hydrosphere and lithosphère at a rather rapid rate. As an example, the human population of 3.109 people would contain roughly 6.108 kilograms ofsodium and almost the same amount of potassium. Figured on the basis ofa mixed diet, this specific biomass has passing through it some 6.106 kilograms each of sodium and potassium daily; smaller animals, many plants, and microorganisms would have a higher proportional rate ofturnover. As noted by Fenn (4), interesting similarities exist between lithosphère and protoplasm, both rich in potassium, and between body fluids and hydrosphere, both rich in sodium: "Potassium is ofthe soil and not the sea; it is ofthe cellbutnotthe sap." Betweenlithosphèreandhydrospherethere exists a balance of reactions leading to high-potassium soils and highsodium oceans; between body fluids and cells ofanimals, there also exists a balance ofreactions leading to high-potassium cells and high-sodium body fluids. The purpose ofthis essay is to re-emphasize two points especially: (a) the relative constancy of the potassium concentration of all protoplasms , and (b) the remarkable ability ofliving cells ofhigher animals to regulate the sodium content of their body fluids while at the same time preserving a constant internal cellular potassium concentration. The physiology ofalkali metals in animals has been well detailed elsewhere . The 1940 review by Fenn can be read with much profit by anyone interested, especially in potassium. It suffices for the purposes ofthis discussion to point out the ubiquity ofthe problems involved. Current information indicates that potassium is a normal universal constituent ofall cells, plant and animal, and is a required nutrient (5). Sodium, on the other hand, seems a very variable constituent ofprotoplasm and is a required nutrient only for a very few bacteria and for some animals maintaining body fluids. There is recent evidence that a minimal amount ofsodium, acting with calcium, may be essential for the growth offresh water Hydra (6). In the present discussion, the presence ofthe metabolic systems will be assumed, as will their ability to provide the organic components and energy which may be required for the regulation of protoplasmic constituents in general. I. Distribution ofSodium and Potassium in Body Fluids ofAnimals It is a familiar...