Na,K-ATPase and the development of Na+ transport in rat distal colon

Carbenoxolone accelerates maturation of rat intestine

The rat undergoes profound maturational changes in the intestinal structure and function during the third week of its life. To investigate the role of peripheral glucocorticoid metabolism in this process, we studied the postnatal maturation of intestinal structure and function. The peripheral metabolism of glucocorticoids depends on enzyme 11beta-hydroxysteroid dehydrogenase (11betaHSD), which is responsible for the interconversion of corticosterone to 11-dehydrocorticosterone and thus for the modulation of glucocorticoid access to corticosteroid receptors. The pups were treated with carbenoxolone (CBX), an inhibitor of 11betaHSD, for 10 d during the suckling (days 8-18) or weaning period (days 14-24 or days 20-30), and we determined the parameters of intestinal growth and activities of sucrase, alkaline phosphatase, and Na,K-ATPase. The CBX treatment increased plasma concentrations of corticosterone as a result of a significant reduction of peripheral degradation of corticosterone catalyzed by 11betaHSD. This also stimulated intestinal growth without changing somatic growth. The mucosal cell mass was significantly higher in CBX-treated suckling rats, whereas the effect of this treatment was less obvious in weanling animals. CBX increased the crypt depth and villus height in 18- and 24-d-old pups but not in 30-d-old animals. The small intestinal activities of sucrase, alkaline phosphatase, and Na,K-ATPase were not influenced by CBX. In contrast, colonic Na,K-ATPase was stimulated by CBX. We conclude that the administration of CBX results in acceleration of intestinal growth and structural maturation without any influence on the developmental pattern of brush-border hydrolases. The results indicate an important role of peripheral glucocorticoid metabolism in the regulation of intestinal growth during early postnatal life.

Effects of short chain fatty acids on colonic Na+ absorption and enzyme activity

Short chain fatty acids (SCFA) stimulate colonic Na+ absorption and inhibit cAMP and cGMP-mediated Cl- secretion. It is uncertain whether SCFA have equivalent effects on absorption and whether SCFA inhibition of Cl- secretion involves effects on mucosal enzymes. Unidirectional Na+ fluxes were measured across stripped colonic segments in the Ussing chamber. Enzyme activity was measured in cell fractions of scraped colonic mucosa. Mucosal 50 mM acetate, propionate, butyrate and poorly metabolized isobutyrate stimulated proximal colon Na+ absorption equally (300%). Neither 2-bromo-octanoate, an inhibitor of beta-oxidation, nor carbonic anhydrase inhibition affected this stimulation. All SCFA except acetate stimulated distal colon Na+ absorption 200%. Only one SCFA affected proximal colon cGMP phosphodiesterase (PDE) (18% inhibition by 50 mM butyrate). All SCFA at 50 mM stimulated distal colon cAMP PDE (24-43%) and decreased forskolin-stimulated mucosal cAMP content. None of the SCFA affected forskolin-stimulated adenylyl cyclase in distal colon or ST(a)-stimulated guanylyl cyclase in proximal colon. Na+-K+-ATPase in distal colon was inhibited 23-51% by the SCFA at 50 mM. We conclude that all SCFA (except acetate in distal colon) stimulate colonic Na+ absorption equally, and the mechanism does not involve mucosal SCFA metabolism or carbonic anhydrase. SCFA inhibition of cAMP-mediated secretion may involve SCFA stimulation of PDE and inhibition of Na+-K+-ATPase.

Development of intestinal transport function in mammals

Considerable progress has been made over the last decade in the understanding of mechanisms responsible for the ontogenetic changes of mammalian intestine. This review presents the current knowledge about the development of intestinal transport function in the context of intestinal mucosa ontogeny. The review predominantly focuses on signals that trigger and/or modulate the developmental changes of intestinal transport. After an overview of the proliferation and differentiation of intestinal mucosa, data about the bidirectional traffic (absorption and secretion) across the developing intestinal epithelium are presented. The largest part of the review is devoted to the description of developmental patterns concerning the absorption of nutrients, ions, water, vitamins, trace elements, and milk-borne biologically active substances. Furthermore, the review examines the development of intestinal secretion that has a variety of functions including maintenance of the fluidity of the intestinal content, lubrication of mucosal surface, and mucosal protection. The age-dependent shifts of absorption and secretion are the subject of integrated regulatory mechanisms, and hence, the input of hormonal, nervous, immune, and dietary signals is reviewed. Finally, the utilization of energy for transport processes in the developing intestine is highlighted, and the interactions between various sources of energy are discussed. The review ends with suggestions concerning possible directions of future research.

Permissive effect of thyroid hormones on induction of rat colonic Na+ transport by aldosterone is not localised at the level of Na+ channel transcription

The interrelationship between thyroid hormones and aldosterone has been examined in the regulation of rat colonic amiloride-sensitive Na+ transport which translocates Na+ through apical amiloride-sensitive Na+ channels and basolateral Na+, K+-ATPase. Electrogenic Na+ transport was measured in an Ussing chamber by the short-circuit current and identified by Na+ channel blocker amiloride. Na+-pumping activity of the basolateral Na+,K+-ATPase was investigated in nystatin-treated epithelium by measuring the equivalent short-circuit current after addition of mucosal Na+. The abundance of mRNA coding for alpha, beta and gamma subunits of the Na+ channel (rENaC) was estimated using Northern blot analysis. Hyperaldosteronism was induced by a low-salt diet and hypothyroidism by methimazole. The low-Na+ diet induced electrogenic Na+ transport in euthyroid rats but its effect was almost completely inhibited in hypothyroid animals even if the plasma concentration of aldosterone was high enough to stimulate this transport pathway both in euthyroid and hypothyroid rats. A kinetic study of the basolateral Na+,K+-ATPase revealed a decrease of Na+ transport capacity in hypothyroid rats kept on the low-Na+ diet in comparison with euthyroid animals fed the same diet. No significant differences in steady-state levels of alpha, beta and gamma rENaC mRNA were detected between euthyroid and hypothyroid rats. These data suggest that hypothyroidism decreases the efficacy of the basolateral Na+ pump but fails to inhibit it completely even though it inhibits the transepithelial electrogenic Na+ transport in response to aldosterone. We conclude that the permissive effect of thyroid hormones on the induction of electrogenic Na+ transport by aldosterone is localised beyond the transcriptional step of Na+ channel regulation.

Localization of Na,K-ATPase activity in developing rat distal colon: role of corticosteroids

The role of corticosteroids in the development of Na,K-ATPase and its distribution along the crypt base-colonic surface was investigated in suckling, weanling and adult rats using a biochemical and a histochemical approach. The cerium-based histochemical method for detection of ouabain-sensitive K+-dependent p-nitrophenylphosphatase (K-NPPase) component of the Na,K-ATPase complex was used to localize Na,K-ATPase in the epithelium. The activity of Na,K-ATPase was very low 2 days after birth and increased to a maximum in adulthood. Mature surface colonocytes and immature cells at the crypt base were similarly reactive and the reaction product was decreased by the addition of ouabain and inhibited by omission of K+. Adrenalectomy decreased colonic Na,K-ATPase activity in surface and deep crypt cells of suckling, weanling and adult animals. Mineralocorticoids (deoxycorticosterone acetate, DOCA) restored the Na,K-ATPase activity both in surface and crypt cells of adrenalectomized weanling rats and the effect of DOCA was inhibited by the mineralocorticoid receptor antagonist, spironolactone. Physiological doses of glucocorticoids (dexamethasone) stimulated Na,K-ATPase activity in surface colonocytes of adrenalectomized weanling rats; supraphysiological doses restored Na,K-ATPase probably via cross-over into mineralocorticoid receptors both in surface and crypt cells. High dietary Na+ intake during the weaning period reduced the reaction product to the level detected in adrenalectomized rats. The distribution of Na, K-ATPase activity in the epithelium of adrenalectomized rats with substitutional replacement hormone therapy was the same as in control animals or, in some animals, the surface absorptive epithelium exhibited a stronger reaction than the crypt cells. Similarly, the surface colonocytes of adult rats kept on a low-salt diet showed a stronger reaction than the crypt cells. These data indicate that postnatal development of Na,K-ATPase is regulated predominantly by aldosterone and that both surface and crypt cells are responsive to mineralocorticoids. Surface cells are also responsive to glucocorticoids.

The role of 11 beta-hydroxysteroid dehydrogenase in maturation of the intestine

Glucocorticoids promote the development of many organs including intestine. At the cellular level, the activity of glucocorticoids is regulated by 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) which converts active glucocorticoids to inactive metabolites. As 11 beta HSD is also expressed in the intestine, this enzyme may be an important regulator of intestinal maturation. To investigate this, we have performed the systematic study of the development of intestinal 11 beta HSD activity and its cofactor preference as well as of the effect of 11 beta HSD inhibition by carbenoxolone on postnatal development of sucrase, alkaline phosphatase and Na,K-ATPase in the intestine. The activity of 11 beta HSD was low in ileum of suckling rats and significantly increased during the weaning period. In colon, the activity was already high in suckling rats and gradually rose during the postnatal development. 11 beta HSD activity was undetectable in jejunum both in young and adult rats. At 14.5 nM corticosterone, colonic 11 beta HSD utilized predominantly NAD as a cofactor, but displayed significant sensitivity also to NADP. Ileal 11 beta HSD had similar sensitivity to both cofactors. With NAD as a cofactor, ileal 11 beta HSD had a Km (59 +/- 10 nM) compatible with the colonic enzyme (81 +/- 14 nM). Carbenoxolone administration to suckling and weanling rats in vivo did not result in any changes of sucrase activity in jejunum and ileum, alkaline phosphatase activity in ileum and distal colon or Na,K-ATPase activity in ileum. However, carbenoxolone significantly increased Na,K-ATPase activity in distal colon. Our results indicate that the high-affinity type of 11 beta HSD is expressed not only in colon but also in ileum and that 11 beta HSD is an important factor in the regulation of tissue levels of active glucocorticoids in developing colon but not in the small intestine.

Regulation of apical and basolateral K+ conductances in rat colon

1. Apical administration of an ionophore, nystatin, and basolateral depolarization by K+ were used to investigate the regulation of apical and basolateral electrogenic transport pathways for K+ in the rat proximal and distal colon. 2. Administration of nystatin (100 micrograms ml-1 at the mucosal side), in the presence of Na+ and in the presence of a serosally directed K+ gradient, stimulate a large increase in short-circuit current (ISC) and tissue conductance in both colonic segments. This response was composed of a pump current generated by the Na(+)-K(+)-ATPase and of a current cross a quinine-sensitive basolateral K+ conductance. 3. The pump current, measured as Na(+)-dependent or scilliroside-sensitive current in the absence of a K+ gradient, was significantly greater in the distal than in the proximal colon. The pump current was unaltered by pretreatment of the tissue with forskolin (5 x 10(-6) mol 1(-1)). 4. The current across the basolateral K+ conductance, measured as current in the presence of a serosally directed K+ gradient either in the absence of Na+ or in the presence of scilliroside, was increased by the cholinoreceptor agonist, carbachol (5 x 10(-5) mol 1(-1)), but inhibited by forskolin (5 x 10(-6) mol 1(-1)). 5. Basolateral K+ depolarization induced a negative ISC in both colonic segments, which was inhibited by the K+ channel blocker quinine (10(-3) mol 1(-1)) at the mucosal side), but was resistant to tetraethylammonium (5 x 10(-3) mol 1(-1) at the mucosal side). This K+ current across an apical K+ conductance was stimulated in both colonic segments by carbachol, whereas forskolin had no effect, although control experiments revealed that forskolin was still able to open an apical Cl- conductance under these conditions. 6. These results demonstrate that an increase in intracellular Ca2+ concentration induced by carbachol causes an increase in the basolateral and the apical K+ conductance, thereby inducing K+ secretion in parallel with an indirect support of Cl- secretion due to the hyperpolarization of the cell membrane. In contrast, the dominating effect of an increase in the intracellular cyclic AMP concentration is inhibition of a basolateral K+ conductance; a mechanism which might contribute to the inhibition of K+ absorption.

Corticosteroid induction of renal and intestinal K(+)-dependent p-nitrophenylphosphatase in young and adult rats

The post-natal development of the K(+)-dependent p-nitrophenylphosphatase (K-NPPase) activity of the Na, K-ATPase complex and its regulation by corticosteroids was studied in renal and intestinal epithelia of the rat using the p-nitrophenylphosphatecerium capture method. The distribution of the phosphatase was analysed in detail in the renal epithelia of the medullary thick ascending limb of Henle's loop and distal convoluted tubule and in the surface epithelial cells of the distal colon. The convoluted tubule and Henle's loop segments showed a stronger reaction for K-NPPase than the colon epithelium both in adult and young animals (suckling and weanling pups). The intensity of staining rose progressively in all three epithelia during early postnatal development and reached the highest levels during the weaning period and in adulthood. The most distinct change was observed between days 10 and 16. Adrenalectomy significantly reduced the density of the final reaction product in weanling and adult rats. Replacement hormone therapy of adrenalectomized weanling rats with the glucocorticoid dexamethasone restored the K-NPPase activity in the two renal epithelia, whereas the mineralocorticoid deoxycorticosterone acetate had no effect on the activity in the medullary thick ascending limb, a very slight effect in distal convoluted tubules, and a strong effect on the distal colon epithelial activity. The observed small effect of the mineralocorticoid in distal convoluted tubule activity may reflect a cross-over into glucocorticoid receptors. We conclude that the postnatal development of Na, K-ATPase is regulated by glucocorticoids in nephron epithelia and predominantly by mineralocorticoids in the surface enterocytes of the distal colon.

Sequential alterations in gut mucosal amino acid and glucose transport after 70% small bowel resection

BACKGROUND

Studies in animals with short bowel syndrome (SBS) suggest that up-regulation of nutrient transporter activity occurs as an adaptive response to the loss of absorptive area. It is unclear, however, whether nutrient transport is altered at the cell membrane in SBS. The purpose of this study is to clarify amino acid and glucose transport in small intestinal luminal mucosa after 70% small bowel resection in rabbits.

METHODS

New Zealand white rabbits underwent 70% jejunoileal resection (n = 27) or a sham operation (n = 19). Brush border membrane vesicles were prepared from small intestinal mucosa at 1 week, 1 month, and 3 months by magnesium aggregation-differential centrifugation. Transport of L-glutamine, L-alanine, L-leucine, L-arginine, and D-glucose was assayed by a rapid mixing-filtration technique.

RESULTS

We observed no difference in uptake of all amino acids and glucose at 1 week. The uptake of amino acids and glucose was decreased by 20% to 80% in animals with SBS at 1 month. By 3 months all uptake values except that of glucose returned to normal. Kinetic studies of the system B transporter for glutamine indicate that the decrease in uptake at 1 month was caused by a reduction in the Vmax (1575 +/- 146 versus 2366 +/- 235, p < 0.05) consistent with a decrease in the number of functional carriers on the brush border membrane.

CONCLUSIONS

In addition to the anatomic loss of absorptive area after massive bowel resection, alterations in enterocyte transport function may be responsible for malabsorption in patients with SBS.

Famine, fiber, fatty acids, and failed colonic absorption: does fiber fermentation ameliorate diarrhea?

The salvage function of the colon for absorption of unabsorbed sodium and water from the jejunum and ileum depends upon the metabolic integrity of colonic epithelial cells maintained by luminal short-chain fatty acids. With the depletion of luminal short-chain fatty acids under conditions of starvation, metabolic compensation from vascular substrates is incomplete. Loss of luminal short-chain fatty acids diminishes cell membrane integrity and causes secretion by colonic epithelial cells, leading to starvation diarrhea. Because sodium absorption is dependent upon CO2 production from n-butyrate, no compensatory absorption occurs during starvation. Under conditions in which luminal short-chain fatty acids are depleted, dietary fiber is useful as a low osmolality food constituent and for renewal of short-chain fatty acid levels by bacterial fermentation. The "antisecretory" effect of dietary fiber depends on the degree of the preexisting depletion of short-chain fatty acids and the methodology used to assess absorptive function. Dietary fiber has not been found harmful in refeeding starvation victims for whom it is an essential food constituent.

The influence of high salt intake on intestinal Na,K-ATPase in Wistar and Dahl rats

The Na,K-ATPase of intestinal mucosa was compared in Wistar (W), salt-sensitive (DS) and salt-resistant (DR) Dahl rats fed a low-salt (LS) and high-salt (HS) diet. ATPase activity and the kinetics of its activation by Na+ were determined in three intestinal segments (jejunum, ileum, distal colon). There were demonstrated only small differences in the affinity for Na+ among the strains studied but we found a considerable profile of Na,K-ATPase activity along the intestine in all strains; the activity was higher in jejunum and lower in ileum and distal colon. Chronic salt loading decreased the affinity of Na,K-ATPase for Na+ but had no effect on Vmax. The changes in salt intake were accompanied by different response of plasma aldosterone in particular strains. According to the stimulation of aldosterone level by LS diet the sensitivity of the strains was DR > W > DS. HS diet suppressed aldosterone level to similar values in all strains. The data indicate that the kinetics of intestinal Na,K-ATPase and its response to HS intake is independent of the genotype of the rats.