Assessment of body potassium stores

Abstract

The variety of methods used for the assessment of potassium status in man are evidence of the difficulties which are involved. For simple negative balance of potassium, there seem to be few qualitative differences between the methods, with the solitary exception of the data derived from anemic red cells. For pure loss of LBM without any other defect, only whole body counting, or balance data, will demonstrate potassium deficiency. Pure loss of potassium capacity rarely occurs, in that whole tissue is rarely lost. For instance, in muscle wasting the supporting collagen framework tends to remain, so that muscle biopsy shows a reduced potassium, expressed as per 100 g of FFDS. Marked wasting will eventually lead to a low TBK/kg of body wt because of the increasing importance of the skeleton. In more prolonged and metabolically complex potassium deficiency, plasma potassium is very unreliable, and attempts to measure intracellular potassium concentrations by combination of whole body counting and measurement of the ECF space are dependent upon the distribution volume of the marker, which in disease may measure much larger volumes. Potassium content based on 40K measurements remains a precise measurement, but its interpretation is dependent upon the method used to define the normal range and the assumption that all the tissues of the body behave similarly. The equations used to predict the normal range may not all be applicable to disease states. Equations based on height are the safest. Unfortunately, the predicted normal range for a given individual is about ± 10%. Isolated cell preparations offer the only currently feasible method of measuring the physiology of potassium transport in disease. These methods, however, are essentially 'in vitro' measurements, and the effects of hormones and other short half-life substances which interact with the transport processes are lost. Muscle appears to be the most representative tissue to biopsy, in that most of the body potassium is in muscle, but essential organs may well behave differently. The variation between muscle groups and the problem of measuring the extracellular space also present technical and interpretive problems. A combination of these various methods seems to be the most sensible approach for the present, with the hope that measurement of membrane potentials and the development of electron probes may give us more precise tools in the future. In the interval some more cadaver studies with careful discrimination between the contributions of different tissues would be immensely valuable.