USE OF A MINIATURE GM COUNTER AND A WHOLE BODYCOUNTER IN STUDY OF POTASSIUM TRANSPORT BY COLON OF NORMAL, SODIUM-DEPLETED AND ADRENALECTOMIZED RATS IN VIVO

1. A method is described employing a whole‐body counter and a miniature GM counter placed within the lumen of the gut which can be used to study the absorption rates and kinetics of labelled substances in the mucosal epithelium in vivo. The method was applied to the study of K transport by the mucosa of the descending colon of normal, Na‐depleted and adrenalectomized rats. 2. The K influx rate was linearly related to luminal [K] over the range 5–80 m M. NaCN in the lumen produced a fall of electrical potential difference (p.d.) and of K secretion rate but did not reduce K influx rate. The findings were consistent with passive K influx. 3. The mucosal 42K content reached a steady value after 60–90 min of exposure to 42K in the lumen. The value was similar in normal and adrenalectomized rats but was greater in Na‐depleted rats. 4. After replacement of luminal solution by a non‐radioactive solution the fall of mucosal 42K content could be described, in the majority, by a curve having two exponential components with half‐times of 8–10 and 65–106 min. Approximately half of the 42K outflow passed to the lumen. The rate constants were similar in all groups of rats. 5. The results analysed on the basis of representing the mucosal K as in two compartments, indicated that the observed absorption rate could not be accounted for by transport through these compartments alone and that most of the 42K crossed the mucosa by a fast pathway not detectable by the miniature GM counter. Most of the mucosal K was relatively inaccessible from the lumen. 6. Na depletion raised the transmucosal p.d., increased the K secretion rate and appeared to increase K permeability of the mucosa but the kinetics were unaltered. Accumulation of 42K in the mucosa was greater, suggesting that the permeability of the luminal face had increased. Adrenalectomy produced only one change, namely a reduction of K secretion rate. © 1969 The Physiological Society