Use of a miniature GM counter and a whole body counter in the study of potassium transport by the 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 42 K content reached a steady value after 60–90 min of exposure to 42 K 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 42 K 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 42 K 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 42 K 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 42 K 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.