11 beta-Hydroxysteroid dehydrogenase in developing rat intestine

The enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta-OHSD) prevents the binding of corticosterone to mineralocorticoid receptors by reversible conversion of biologically active corticosterone to inactive 11-dehydrocorticosterone. To clarify the relationship between high plasma concentrations of corticosterone during weaning and high activity of intestinal transport pathways that are induced by aldosterone in immature intestine, we have studied the distribution, developmental pattern and regulation of 11 beta-OHSD in intestinal segments that possess mineralocorticoid target epithelium. Dehydrogenase activity was already high in the caecum, and the proximal and distal colon on the second postnatal day and altered little until adulthood. In contrast, the activity in the ileum was low during the first two weeks of life, rose more than 5-fold in the next 20 days to attain a peak in 30-day-old rats, and thereafter declined to the values of adult animals. There was no significant reductase activity (conversion of 11-dehydrocorticosterone to corticosterone) in any intestinal segment of young and adult rats. The regulation of intestinal 11 beta-OHSD by corticosteroids and thyroid hormones was studied in the ileum and distal colon. In weanling rats, adrenalectomy or a high-salt diet decreased 11 beta-OHSD activities in both intestinal segments whereas dexamethasone administration prevented this decline in adrenalectomized rats and administration of deoxycorticosterone acetate led to a significant increase of intestinal 11 beta-OHSD activities in rats kept on a high-salt diet. Dexamethasone administration to intact adult rats also stimulated 11 beta-OHSD activity in the ileum and distal colon. The changes in thyroid status of weanling rats did not change the 11 beta-OHSD activities. We conclude that (1) the developmental patterns of 11 beta-OHSD activity in the small and large intestine are not identical and this discrepancy may facilitate the maturation effect of glucocorticoids in the small intestine and the stimulatory effect of aldosterone in the large intestine and (2) corticosteroids but not thyroid hormones can modulate 11 beta-OHSD activity in the developing intestine.

Regulation of amiloride-sensitive Na+ transport in immature rat distal colon by aldosterone

The effects of dietary changes and plasma aldosterone levels on channel-mediated electrogenic amiloride-sensitive Na+ transport were examined in the distal colon of immature and adult rats. The decrease of the short-circuit current after amiloride addition (ISCamil) was used as a measure of electrogenic Na+ absorption. Plasma aldosterone levels were six to eight times higher between d 15 and 30 than in younger suckling or prepubertal rats. In adult rats, the plasma aldosterone was approximately 30 times lower than in young animals kept on the same standard diet. ISCamil followed the developmental profile of plasma aldosterone. ISCamil increased between d 10 and 20, reached a plateau between d 20 and 25, then decreased and entirely disappeared after d 30. ISCamil was zero in adult distal colon but was induced if dietary Na+ intake decreased below 100 mumol Na+/(100 g body wt.d) and plasma aldosterone increased above 200-300 pg/mL. Adrenalectomy, or high Na+ intake, inhibited ISCamil and significantly depressed plasma aldosterone in 20- and 25-d-old rats. Premature weaning decreased ISCamil without appreciable changes in plasma aldosterone in 18- and 20-d-old rats, prolonged suckling inhibited ISCamil and caused a significant depression of plasma aldosterone. We conclude from these results that the postnatal changes of distal colonic Na+ transport are regulated predominantly by circulating aldosterone and dietary Na+ intake.

Relationship between dietary Na+ intake, aldosterone and colonic amiloride-sensitive Na+ transport

The effect of changes in dietary Na+ intake on plasma aldosterone levels and electrogenic amiloride-sensitive Na+ transport (Iamil) was studied in the rat distal colon. Five groups of rats were fed on diets containing different amounts of Na+. Estimated Na+ intake ranged from about 400-80,000 mu equiv Na+/kg body weight (BW) per d. Both variables investigated, Iamil and plasma aldosterone, depended non-linearly on Na+ intake. Reduction of the daily Na+ intake increased plasma aldosterone levels and if these levels reached the value 200 pg/ml or more then Iamil was induced. The corresponding Na+ intake was 1300 mu equiv Na+/kg BW per d. Iamil was not observed at lower aldosterone levels and higher Na+ intakes. Aldosterone infusion for 7 d produced similar changes in Iamil compared with dietary Na(+)-depleted animals and made the estimation of maximum transport capacity of Iamil possible. We conclude that Iamil operates only if Na+ intake decreases below minimal Na+ requirement in growing rats and that the maximum transport capacity of this pathway is reached only after very severe Na+ deprivation.

Distribution of 11 beta-hydroxysteroid dehydrogenase along the rat intestine

Aldosterone selectivity of mineralocorticoid target tissues has been suggested to be due to the inactivation of glucocorticoids in the target tissue by 11 beta-hydroxysteroid dehydrogenase (11 beta-OHSD). The distribution of 11 beta-OHSD was studied in the intestine which is composed of aldosterone-sensitive and insensitive segments. The activity of the enzyme was high in distal colon and medium in ileum, cecum, and proximal colon. Zero activity was found in duodenum and jejunum. Carbenoxolone completely blocked the enzyme. Low-salt diet increased the activity in proximal colon and decreased in ileum. Adrenalectomy decreased the activity in ileum and proximal colon. The existence of segmental differences in the distribution of 11 beta-OHSD and the hormonal effect on the activity of the enzyme suggest a physiological role of 11 beta-OHSD in the intestine.

Regulation of Na channels of the rat cortical collecting tubule by aldosterone

The activity of apical membrane Na channels in the rat cortical collecting tubule was studied during manipulation of the animals' mineralocorticoid status in vivo using a low-Na diet or the diuretic furosemide. Tubules were isolated and split open to expose the luminal membrane surface. Induction of Na channel activity was studied in cell-attached patches of the split tubules. No activity was observed with control animals on a normal diet. Channel activity could be induced by putting the animals on the low-Na diet for at least 48 h. The mean number of open channels per patch (NPo) was maximal after 1 wk on low Na. Channels were also induced within 3 h after injection of furosemide (20 mg/kg body wt per d). NPo was maximal 48 h after the first injection. In both cases, increases in NPo were primarily due to increases in the number of channels per patch (N) at a constant open probability (Po). With salt depletion or furosemide injection NPo is a saturable function of aldosterone concentration with half-maximal activity at approximately 8 nM. When animals were salt repleted after 1-2 wk of salt depletion, both plasma aldosterone and NPo fell markedly within 6 h. NPo continued to decrease over the next 14 h, while plasma aldosterone rebounded partially. Channel activity may be dissociated from aldosterone concentrations under conditions of salt repletion.

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.

Identification of apamin binding sites in rat intestinal mucosa

Apamin, a specific blocker of one class of Ca(2+)-activated K+ channes, was used to detect the apamin receptors associated with K+ channels in the mucosa of the rat jejunum and colon. Two receptor sites for 125I-apamin have been identified. These sites differed in their affinity for apamin (jejunum: KD1 = 1.1 nM and KD2 = 170 nM; colon: KD1 = 0.5 nM and KD2 = 1.1 nM and KD2 = 140 nM) and the maximum number of sites (jejunum: B(max1) = 111 and B(max2) = 4030; colon: B(max1) = 187 and B(max2) = 7550 fmol/mg of protein). 125I-apamin binding was stimulated by K+ ions with K0.5 = 1.0 mM and inhibited by the neuromuscular blocker tubocurarine (KI = 50 microM). We interpret these data to demonstrate that the high-affinity, low-capacity binding sites reflect the existence of apamin-sensitive K+ channels in the intestinal mucosa.

Apical maxi K channels in intercalated cells of CCT

High-conductance (maxi) K channels in the apical membrane of rat and rabbit cortical collecting tubules (CCT) were studied using the patch-clamp technique. Principal cells (PC) and intercalated cells (IC) were distinguished with Hoffman modulation optics in split-open tubules. IC were further identified by staining tubules with the fluorescent mitochondrial dye, rhodamine 123. Maxi-K channels were distinguished by their high conductance (greater than 80 pS) and voltage-dependent kinetics. In CCT of rats on a low-Na diet, maxi K channels were observed in 11% of the cell-attached patches on PC and 79% of patches on IC. In rats on a normal diet, the channels were seen in 23 and 79% of patches on PC and IC, respectively. In the rabbit CCT, maxi K channels were observed in 12% (4 of 32) of the patches on PC and 82% (122 of 148) of the patches on IC. The greater abundance of channels in IC was confirmed in rat CCT using the whole-cell clamp technique. Current through the maxi K channels (IK) was measured as the tetraethylammonium (TEA)-sensitive (2.5 mM) outward current in cells equilibrated with 115 mM K and 10(-5) M Ca2+ in the pipette solution. When the cell was clamped to an internal potential of +40 mV, the average IK per cell was -4 +/- 5 pA in PC and 290 +/- 90 pA in IC. Lowering cytoplasmic Ca2+ from 10(-5) M to 10(-7) M reduced IK to 32 +/- 21 pA. Neither single Na channels nor amiloride-sensitive whole-cell currents were seen in IC. Finally, maxi K channels could be activated by pipette suction (10-40 cm H2O) in either cell-attached or inside-out patches on IC from rabbit CCT. This mechanosensitivity was observed even after chelation of free Ca2+ with ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) in the pipette or the bath solutions, implying that stretch activation of these channels was not mediated by increased Ca2+ entry into the cell. The IC maxi K channel may play a role in cell volume regulation or in K secretion during elevation of luminal hydrostatic pressure.

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

Na,K-ATPase function was studied in order to evaluate the mechanism of increased colonic Na+ transport during early postnatal development. The maximum Na(+)-pumping activity that was represented by the equivalent short-circuit current after addition of nystatin (ISCN) did not change during postnatal life or after adrenalectomy performed in 16-day-old rats. ISCN was entirely inhibited by ouabain; the inhibitory constant was 0.1 mM in 10-day-old (young) and 0.4 mM in 90-day-old (adult) rats. The affinity of the Na,K pump for Na+ was higher in young (11 mM) than in adult animals (19 mM). The Na,K-ATPase activity (measured after unmasking of latent activity by treatment with sodium dodecylsulfate) increased during development and was also not influenced by adrenalectomy of 16-day-old rats. The inhibitory constant for ouabain (KI) was not changed during development (0.1-0.3 mM). Specific [3H]ouabain binding to isolated colonocytes increased during development (19 and 82 pmol/mg protein), the dissociation constant (KD) was 8 and 21 microM in young and adult rats, respectively. The Na+ turnover rate per single Na,K pump, which was calculated from ISCN and estimated density of binding sites per cm2 of tissue was 500 in adult and 6400 Na+/min.site in young rats. These data indicate that they very high Na+ transport during early postnatal life reflects an elevated turnover rate and increased affinity for Na+ of a single isoform of the Na,K pump. The development of Na+ extrusion across the basolateral membrane is not directly regulated by corticosteroids.

Corticosteroid regulation of Na+ and K+ transport in the rat distal colon during postnatal development

To study the role of corticosteroids in the regulation of colonic electrogenic amiloride-sensitive Na+ absorption (ISCNa) and barium-sensitive K+ secretion (ISCK) during development, we investigated suckling (10-day old), weanling (25-day old) and adult (90-day old) adrenalectomized rats after they had received aldosterone, dexamethasone or corticosterone. Adrenalectomy reduced markedly ISCNa in suckling rats and completely inhibited ISCNa in weanling animals; the ISCNa was absent in intact adult rats. The doses of aldosterone, corticosterone and dexamethasone estimated to be equivalent to the endogenous production rate of aldosterone and corticosterone restored ISCNa after 1 day in both suckling and weanling rats. Compared with aldosterone, glucocorticoids produced a greater increase in ISCNa. Concurrent spironolactone treatment (a mineralocorticoid antagonist) completely prevented the effect of aldosterone but had no effect in dexamethasone-treated rats. The glucocorticoid antagonist RU 38 486 inhibited the dexamethasone-induction of ISCNa but had no effect on aldosterone. The response to corticosteroids, measured as the increase of ISCNa, declined from suckling to adult rats. In contrast to ISCNa, the same time of treatment and the same doses of corticosteroids did not influence ISCK. ISCK was stimulated only after chronic treatment (4 days). These findings suggest that, in the distal colon of young rats, (1) both corticosteroids may regulate amiloride-sensitive Na+ absorption and barium-sensitive K+ secretion, (2) different receptors mediate the colonic effects of glucocorticoids and mineralocorticoids, (3) immature rats are more sensitive to corticosteroids than adult animals, and (4) the acute effect of corticosteroids is an increase in Na+ absorption which is followed by delayed stimulation of K+ secretion.

Potassium secretion by neonatal rat distal colon

Electrogenic K+ secretion across the distal colon of young rats was investigated by measuring the sensitivity of the short-circuit current to Ba2+ added to the mucosal side of the tissue. Ba2+-sensitive short-circuit current (IBasc) was high during the suckling and weaning periods but very low in adult animals. Increasing the mucosal K+ concentration was accompanied by the inhibition of the serosa-to-mucosa IBasc and the induction of the mucosa-to-serosa IBasc. The IBasc was decreased by serosal omission of either Na+ or Cl- as well as by serosal addition of furosemide or ouabain. Mucosal omission of Na+ did not change IBasc. By increasing the plasma level of aldosterone (low-sodium diet) IBasc rose by 95% whereas treatment decreasing this level (high-sodium diet) reduced IBasc by 76%. Bilateral adrenalectomy lowered IBasc by 59% and treatment of adrenalectomized rats with deoxycorticosterone acetate prevented the reduction of IBasc. Tetraethylammonium and quinidine had similar effects on Isc as Ba2+. These data are consistent with the presence of a high level of K+ secretion in the distal colon of neonatal rats. This secretory pathway is electrogenic and independent of Na+ absorption. It appears to be mediated by the Na-K-ATPase as well as a furosemide-sensitive Na-Cl or Na-Cl-K cotransport on the basolateral side and by Ba2+-sensitive K+ conductive pathways on the mucosal side. The results suggest that this K+ secretion can be regulated by mineralocorticoids. The mineralocorticoids are necessary for "stimulated" K+ secretion but they are not essential for maintaining "basal" K+ secretion.

Amiloride-sensitive sodium transport of the rat distal colon during early postnatal development

To evaluate developmental changes in colonic sodium transport, the sensitivity of the transepithelial potential and short-circuit current to amiloride was investigated. The amiloride-sensitive short-circuit current (IscNa), which represents the electrogenic sodium transport through Na+ channels, rose significantly from day 5, reached a peak on day 10, and entirely disappeared after weaning. The maximum rate of electrogenic, amiloride-sensitive sodium transport was 12.0 microEq/cm2 X h. The IscNa was suppressed by adrenalectomy and/or premature weaning but not by a mineralocorticoid antagonist, spironolactone. On the contrary, treatments which increase aldosterone levels in vivo (low-sodium diet, furosemide-induced natriuresis, high dietary intake of potassium) stimulated the IscNa. The effect of adrenalectomy increased during postnatal development. The sensitivity of IscNa to aldosterone was highest at the end of the weaning period. High-sodium diet, which causes a decrease in circulating aldosterone, was associated with a partial inhibition of IscNa (P less than 0.016). These data suggest that the distal colon of neonatal rats can transport sodium via an electrogenic, amiloride-sensitive mechanism and that adrenocortical hormones exert the main regulatory control of this pathway.

Sodium transport and electrical properties of the chick chorioallantoic membrane

Transport and electrical properties of the chick chorioallantoic membrane (CAM) were studied in order to find the osmoregulatory organ which helps to compensate the renal filtration-reabsorption disbalance of chick embryos. It could be shown that CAM resembles Na+ transporting epithelia in that active Na+ absorption is responsible for the potential difference and short circuit current, which could be abolished by ouabain on the ectodermal and amiloride on the endodermal side. The transepithelial conductance rose with increasing sodium concentration in accordance with the Michaelis-Menten kinetics. The allantoic sac thus plays a role similar to the toad urinary bladder despite the low potential difference and resistance which indicate that CAM is a leaky epithelium. CAM is therefore not only a respiratory but also an osmoregulatory organ.