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

Stable potassium isotopes (41K/39K) track transcellular and paracellular potassium transport in biological systems

As the most abundant cation in archaeal, bacterial, and eukaryotic cells, potassium (K⁺) is an essential element for life. While much is known about the machinery of transcellular and paracellular K transport–channels, pumps, co-transporters, and tight-junction proteins—many quantitative aspects of K homeostasis in biological systems remain poorly constrained. Here we present measurements of the stable isotope ratios of potassium (⁴¹K/³⁹K) in three biological systems (algae, fish, and mammals). When considered in the context of our current understanding of plausible mechanisms of K isotope fractionation and K⁺ transport in these biological systems, our results provide evidence that the fractionation of K isotopes depends on transport pathway and transmembrane transport machinery. Specifically, we find that passive transport of K⁺ down its electrochemical potential through channels and pores in tight-junctions at favors ³⁹K, a result which we attribute to a kinetic isotope effect associated with dehydration and/or size selectivity at the channel/pore entrance. In contrast, we find that transport of K⁺ against its electrochemical gradient via pumps and co-transporters is associated with less/no isotopic fractionation, a result that we attribute to small equilibrium isotope effects that are expressed in pumps/co-transporters due to their slower turnover rate and the relatively long residence time of K⁺ in the ion pocket. These results indicate that stable K isotopes may be able to provide quantitative constraints on transporter-specific K⁺ fluxes (e.g., the fraction of K efflux from a tissue by channels vs. co-transporters) and how these fluxes change in different physiological states. In addition, precise determination of K isotope effects associated with K⁺ transport via channels, pumps, and co-transporters may provide unique constraints on the mechanisms of K transport that could be tested with steered molecular dynamic simulations.

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.

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.

Movement of thallium (I) ions in vitro

1. Movement of thallium (I) ions across the mucosal epithelium of descending rat colon, stripped of the muscularis externa in vitro was determined under voltage clamp conditions. 2. The unidirectional fluxes of thallium (I) ions from mucosal to serosal side (M leads to S) and in the opposite direction (S leads to M) were measured at 0 mV. A net flux of thallium (I) ions was observed from serosal to mucosal side (0.216 nmoles . cm-2 . h-1) in other words, the mucosal epithelium is apparently asymmetric for the movement of thallium (I) ions. 3. The investigations were carried out on the range of -30mV up to +30 mV from mucosal to serosal side. Thallium (I) ions move from mucosal to serosal side (M leads to S) by diffusion only (Pi = 0.119 cm.h-1), whereas into the opposite direction (S leads to M) thallium (I) ions in addition to a voltage-dependent component are transported against an electrical gradient.

Absorption by the colon during prolonged infusions in conscious, unrestrained rats

1. Colonic absorption was studied in conscious, unrestrained rats during prolonged infusions through implanted cannulae. During infusion of predominantly NaCl-containing solution at rates up to 0.7 ml h-1 per 100 g body wt., absorption of fluid and Na increased considerably without significant change of transmucosal p.d.; diarrhoea did not occur. 2. Exclusion of the distal colon by colostomy showed that the proximal part of the colon was chiefly responsible for the increased absorption and inspection during the infusion showed it to be much dilated. Removal of the caecum showed that it contributed considerably to absorption by the proximal part. The distal colon influenced Na concentration in the faeces but absorbed little volume while direct infusion into this region rapidly produced diarrhoea. 3. The addition of Na deoxycholate (5 mmol/l) to the infusion solution impaired absorption provoking diarrhoea with mucus loss; there was no evidence that fluid secretion was stimulated. Substitution of SO4 for Cl in the infused solution produced only small changes of transmucosal p.d. but considerably impaired absorption and diarrhoea resulted. 4. The findings indicate that the proximal colon and caecum possess a considerable potential for increasing fluid and Na absorption and suggest the possibility that a neutral Na-Cl coupled absorptive mechanism is stimulated by loading the colon with fluid and NaCl.

Epithelial transport pathways of rat colon determined in vivo by impulse response analysis

1. A method is described for studying transepithelial pathways for the movement of different solutes and water. Using the blood and the secretory curves of changing tracer activity following an intravenous bolus, the rate of transit of molecules together with their impulse response functions, which reflect the transfer processes can be examined. 2. Movements of Na, Cl, I, urea and water from blood to lumen across rat colonic epithelium were all consistent with simple diffusion through a paracellular route. Most of the secreted K, however, passed through a K selective route associated with a significant K epithelial pool. 3. Adding cyanide to the luminal solution caused a reversible fall of transepithelial potential difference associated with changes in the impulse response functions of water, urea and K indicating reduction of the restriction on diffusion. Cellular K content was unaffected. 4. K entered the bulk of the epithelial cellular K almost exclusively from the blood side. A small epithelial K pool, identified by studies with a miniature GM counter, had kinetic characteristics like those of the K selective pathway observed in the studies of impulse response functions.

Potassium transport in the human small bowel

Perfusion experiments in the small intestine of normal subjects and in the ileum of patients with ileostomies demonstrated that the distribution of potassium across the mucosa was compatible with a passive process for transport. In the jejunum potassium transport was shown to be markedly influenced by solvent drag and the jejunal mucosa appears to be more permeable to potassium than to sodium. This permeability was apparently unaffected by calcium in the lumen in concentrations of between 1·15 and 7·25 m-equiv/1. In the ileum potassium transport was influenced by changes in luminal sodium concentration and it is suggested that this is a passive consequence of the change in electrical potential which the change in sodium concentration induces. The electrical potential is thought to be a sodium diffusion potential. All the evidence thus points to a purely passive behaviour of the small intestine towards potassium.

Absorption of sodium and water by human rectum measured by a dialysis method

A dialysis method for the study of intestinal absorption is described. Its use has been assessed in animals and normal human subjects and it has been applied to the measurement of rectal transport of sodium and water.When the luminal solution was of high sodium concentration (145 m-equiv/1), the sodium influx rate (lumen to plasma) was about five times greater than the sodium efflux rate (plasma to lumen). The luminal sodium concentration associated with zero net sodium flux was very low (<15 m-equiv/1). As the mucosa was charged with the luminal side negative, the epithelium must therefore possess a powerful sodium absorbing ;pump'. With isotonic solutions in the lumen, the amount of water absorbed depended on the sodium concentration and when this was 30 m-equiv/1 or less, no significant water absorption was detectable. When, however, water absorption was altered by imposing osmotic gradients, sodium absorption was not significantly affected. The luminal solution tended to become issomolar with plasma; osmotic gradients across the epithelium did not develop. The particular transport properties of rectal epithelium enabling it to remove sodium from the lumen against considerable electrochemical gradients are well adapted to its function.

Sodium transport and short-circuit current in rat colon in vivo and the effect of aldosterone

1. A method for measurement of short-circuit current and for applying a voltage clamp to segments of rat colon in vivo is described.2. The mucosa behaved as an ohmic resistor of average resistance 154 Omega/cm(2) although brief transient effects were frequently observed. Tissue resistance was independent of considerable changes in ionic strength and composition of the luminal solution.3. The short-circuit current averaged 120 muA/cm(2) in normal rats. Aldosterone intravenously raised the p.d., short-circuit current rising proportionately and tissue resistance being unchanged. The effects of various modifications of the intraluminal solution in respect to composition, hydrostatic pressure and pH were examined. An increase in the osmolality of the luminal solution sufficient to abolish water absorption did not affect p.d. or short-circuit current.4. The short-circuit current measured with 150 mM-NaCl in the lumen was almost completely accounted for by active Na absorption both in normal and aldosterone-treated rats. The changes in Na efflux rate produced by voltage clamping suggested that only part of Na efflux was due to simple diffusion. With lower [NaCl] in the lumen, the short-circuit current exceeded that atributable to active Na absorption, the discrepancy increasing with reduction of [NaCl].5. The luminal [Na] at which Na efflux and influx rates were equal was reduced by aldosterone, an effect which is probably responsible for the low stool [Na] of aldosterone treated animals. The significance of this finding in terms of the mode of action of aldosterone is discussed.