Methods for expressing the characteristics of transmembrane ion transport systems.

In recent years, many groups of clinical scientists have become interested in measuring transmembrane ion fluxes of various kinds in patients with different diseases. The ion fluxes of interest have included the ouabainsensitive Na+/K+ pump, the loop-diuretic-sensitive Na+/ K+ co-transport system, the phloretin-sensitive Na+/Na+ (or Na+/Li+ ) countertransport system, and the amiloridesensitive Na+/H+ antiport. The diseases of interest have included chronic renal failure, essential hypertension, affective disorders, endocrine disorders, haemolytic anaemias, neuromuscular disorders, cystic fibrosis and obesity, in studies too numerous to refer to individually here (for a general review of some of these see [ 11). In studies of this kind it has become common for workers to measure the intracellular or extracellular concentration of the ion in which they are interested, the rate at which that ion is transported across the cell membrane in one direction or another, and the so-called 'rate constant' of the transport. The 'rate constant' is taken to be a function of the rate of transport and the concentration from which the transport is occurring. It is my purpose in this review to suggest that this 'rate constant' is not an appropriate way of delineating the characteristics of a transmembrane ion transport system. I shall lay the theoretical foundation by discussing the glycoside-sensitive K + influx and Na+ efflux, i.e. the transmembrane inward transport of K+ and the outward transport of Na+ mediated by the action of the Mg2+dependent, Na+, K+-activated adenosine triphosphatase (Na+ /K + -ATPase, EC 3.6.3.37).