Kinetics of potassium in colonic mucosa of normal and sodium-depleted rats

Colonic Potassium Absorption and Secretion in Health and Disease

The colon has large capacities for K⁺ absorption and K⁺ secretion, but its role in maintaining K⁺ homeostasis is often overlooked. For many years, passive diffusion and/or solvent drag were thought to be the primary mechanisms for K⁺ absorption in human and animal colon. However, it is now clear that apical H⁺ ,K⁺ -ATPase, in coordination with basolateral K⁺ -Cl^- cotransport and/or K⁺ and Cl^- channels operating in parallel, mediate electroneutral K⁺ absorption in animal colon. We now know that K⁺ absorption in rat colon reflects ouabain-sensitive and ouabain-insensitive apical H⁺ ,K⁺ -ATPase activities. Ouabain-insensitive and ouabain-sensitive H⁺ ,K⁺ -ATPases are localized in surface and crypt cells, respectively. Colonic H⁺ ,K⁺ -ATPase consists of α- (HKC_α ) and β- (HKC_β ) subunits which, when coexpressed, exhibit ouabain-insensitive H⁺ ,K⁺ -ATPase activity in HEK293 cells, while HKC_α coexpressed with the gastric β-subunit exhibits ouabain-sensitive H⁺ ,K⁺ -ATPase activity in Xenopus oocytes. Aldosterone enhances apical H⁺ ,K⁺ -ATPase activity, HKC_α specific mRNA and protein expression, and K⁺ absorption. Active K⁺ secretion, on the other hand, is mediated by apical K⁺ channels operating in a coordinated way with the basolateral Na⁺ -K⁺ -2Cl^- cotransporter. Both Ca²⁺ -activated intermediate conductance K⁺ (IK) and large conductance K⁺ (BK) channels are located in the apical membrane of colonic epithelia. IK channel-mediated K⁺ efflux provides the driving force for Cl^- secretion, while BK channels mediate active (e.g., cAMP-activated) K⁺ secretion. BK channel expression and activity are increased in patients with end-stage renal disease and ulcerative colitis. This review summarizes the role of apical H⁺ ,K⁺ -ATPase in K⁺ absorption, and apical BK channel function in K⁺ secretion in health and disease. © 2018 American Physiological Society. Compr Physiol 8:1513-1536, 2018.

Absorption of electrolytes and volatile fatty acids in the hind-gut of the rabbit. Circadian rhythm of hind-gut electrolytes and plasma aldosterone

Interrelationships between the amount of water, solute concentrations in the hind-gut, plasma aldosterone concentrations, and the excretory cycle in the rabbit were studied as well as the effects of hormones generally implicated in electrolyte regulation. It appeared that the levels of water, electrolytes and volatile fatty acids (VFA) in the large intestine varied cyclicly with the excretion pattern: during the soft-phase, water, sodium, chloride and VFA were absorbed and potassium secreted as in other mammals; during the hard-phase two opposite phenomena occurred: secretion in the ascendant colon and absorption in the rest of the colon. Absorption of water, Na, K, chloride and VFA was increased and by this means only material without nutritional value was lost. Plasma levels of aldosterone showed a circadian rhythm parallel to hind-gut changes, i.e. the higher values were found during the hard-phase. Exogenous aldosterone enhanced secretive and absorptive processes, which occurred normally when hard faeces were elaborated.

Potassium transport by rabbit descending colon

Unidirectional mucosal-to-serosal (Jm leads to s) and serosal-to-mucosal (Js leads to m) fluxes of potassium and sodium were determined simultaneously on paired sections of descending colon from the same rabbit under short-circuit conditions. In 13-16 pairs of tissues, net potassium secretion and sodium absorption averaged 0.49 +/- 0.08 and 4.0 +/- 0.8 mueq.cm-2.h-1, respectively. Short-circuit current (Jsc) averaged 3.7 +/- 0.4 mueq.cm-2.h-1 and was approximately equal to the algebraic sum of net potassium and sodium fluxes. Treatment of both sides of the colon with 10(-4) M ouabain reduced the Jsc and transmural potential difference to near zero. Ouabain abolished net potassium secretion by reducing JKs leads to m and abolished net sodium absorption by inhibiting JNam leads to s. In the presence of ouabain, net potassium absorption averaging 0.15 +/- 0.07 mueq.cm-2.h-1 (n = 11) was observed. In the presence of 10(-3) M 2,4-dinitrophenol, both net potassium and net sodium fluxes were abolished, primarily as a result of a reduction in JKs leads to m and JNam leads to s without altering JKm leads to s and JNas leads to m. These results suggest that the rabbit descending colon has the capacity to secrete and possibly to absorb potassium by active mechanisms requiring metabolic energy. Comparison of potassium and sodium fluxes suggest that the paracellular pathway in the rabbit colon is not potassium selective.

The effect of raising the concentration of potassium in the blood or intestinal lumen on the transport of potassium in the dog intestine

The rates of transport of potassium, sodium and water across the intestinal mucosa of Thiry-Vella fistulas of colon and ileum, were studied in dogs in two experimental situations; (1) following intravenous infusion of potassium (15 mEg) over 30 minutes and (2) during variations in the intraluminal concentration of potassium in the range 0.8-16.0 mEg/l. Following intravenous infusion of potassium the rates of potassium secretion into the colon were usually increased; the rates of sodium and water absorption were either unchanged or decreased. The unidirectional rate of movement of potassium into the lumen increased as the concentration difference across the mucosa increased; the movement of potassium in the opposite direction remained relatively constant. The undirectional movement of sodium and water into the lumen of the colon also increased but to a lesser extent. The effect on potassium transport in the ileum was not so marked. With increase in the intraluminal concentration of potassium there was a decrease in the rate of net potassium secretion in both colon and ileum. The rates of net absorption of sodium and water were not significantly altered. Unidirectional potassium movement into the body increased in all the ileal fistulas.

Kinetics of potassium transport across single distal tubules of rat kidney

1. The transport of potassium across the distal tubular epithelium was studied in vivo in rats on a normal potassium intake and in rats in which distal tubular potassium secretion was either stimulated by potassium loading or the I.V. administration of a 5% sodium bicarbonate solution or in which potassium secretion was suppressed by dietary deprivation of potassium or sodium.2. (42)K was used to measure unidirectional fluxes across the luminal and peritubular cell membranes and to assess the magnitude of cellular potassium partaking in the transport process. This was accomplished by the simultaneous perfusion of the peritubular capillary network with (42)K-Ringer and of the distal tubular lumen with initially tracer-free solution. From the steady-state flux and the time course of tracer washout into the lumen after discontinuing the peritubular perfusion, unidirectional fluxes, rate coefficients of ion transfer and cellular transport pools could be measured.3. Transepithelial movement of potassium involves mixing with a variable cellular potassium transport pool. The latter is significantly elevated in conditions of enhanced distal tubular potassium secretion; cellular potassium labelling is reduced in conditions in which potassium secretion has been suppressed by potassium deprivation.4. Evidence is presented that changes in the peritubular transport pattern are primarily responsible for modifications of potassium translocation. Thus, stimulation of potassium secretion is associated with increased peritubular potassium uptake; a reduced potassium uptake across the peritubular cell membrane accounts for the fall in potassium secretion in potassium-depleted animals. Whereas passive entry of potassium across the peritubular membrane is augmented in potassium-loaded animals, the induction of metabolic alkalosis by the administration of 5% sodium bicarbonate stimulates active potassium uptake across the peritubular cell membrane. Sodium deprivation stimulates active reabsorptive transfer of potassium from the tubular lumen.

Potassium and sodium transport across single distal tubules of Amphiuma

The transport properties of potassium (K) and sodium (Na) were studied in single distal tubules of Amphiuma using free-flow micropuncture techniques and stationary microperfusion methods. The transepithelial movement of labeled potassium was measured utilizing a three-compartment system in series in which the time course of tracer disappearance from the lumen was followed. Under control conditions, in blood- and doubly-perfused kidneys, extensive active net reabsorption of sodium and potassium obtains along single distal tubules. Tubular potassium reabsorption is abolished by ouabain at a concentration of 5 x 10(-6)M. Significant net secretion of K can be induced by exposing Amphiuma to a high K environment (100 mM KCl) or by adding acetazoleamide (1 x 10(-4)M) to the perfusion fluid. Transepithelial movement of potassium involves mixing with only a small fraction of total distal tubular cell potassium. This transport pool of potassium increases significantly with the transition from tubular net reabsorption to net secretion. Indirect evidence is presented which indicates that increased active K uptake across the peritubular cell boundary may be of prime importance during states of net K secretion.

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 mM. 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.