Potassium secretion by neonatal rat distal colon

Aldosterone increases KCa1.1 (BK) channel-mediated colonic K+ secretion

Mammalian K(+) homeostasis results from highly regulated renal and intestinal absorption and secretion, which balances the unregulated K(+) intake. Aldosterone is known to enhance both renal and colonic K(+) secretion. In mouse distal colon K(+) secretion occurs exclusively via luminal K(Ca)1.1 (BK) channels. Here we investigate if aldosterone stimulates colonic K(+) secretion via BK channels. Luminal Ba(2+) and iberiotoxin (IBTX)-sensitive electrogenic K(+) secretion was measured in Ussing chambers. In vivo aldosterone was augmented via a high K(+) diet. High K(+) diet led to a 2-fold increase of luminal Ba(2+) and IBTX-sensitive short-circuit current in distal mouse colonic mucosa. This effect was absent in BK alpha-subunit-deficient (BK(-/-)) mice. The resting and diet-induced K(+) secretion was stimulated by luminal ionomycin. In BK(-/-) mice luminal ionomycin did not stimulate K(+) secretion. In vitro addition of aldosterone likewise triggered a 2-fold increase in K(+) secretion, which was inhibited by the mineralocorticoid receptor antagonist spironolactone and the BK channel blocker IBTX. Semi-quantification of mRNA from colonic crypts showed up-regulation of BK alpha- and beta(2)-subunits in high K(+) diet mice. The BK channel could be detected luminally in colonic crypt cells by immunohistochemistry. The expression level of the channel in the luminal membrane was strongly up-regulated in K(+)-loaded animals. Taken together, these data strongly suggest that aldosterone-induced K(+) secretion occurs via increased expression of luminal BK channels.

Development of the infant intestine: implications for nutrition support

The incidence of preterm births has continued to increase over the past 25 years, and therefore the optimal feeding of these infants is an important clinical concern. This review focuses on intestinal development and physiology, with a particular emphasis on developmentally immature functions of the preterm intestine and the resulting implications for nutrition therapies used to feed the preterm infant.

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.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.

Ion transport across the normal and CF neonatal murine intestine

Neonatal mice with cystic fibrosis (CF) exhibit a very high mortality due to intestinal obstruction localized primarily to the ileum and colon. It has been hypothesized that lack of Cl(-) secretion and possibly elevated Na(+) absorption contribute to the gut problems in CF neonates. Therefore, intestines (ileum, proximal colon, and distal colon) from normal and CF day-old mouse pups were studied on ultra-small-aperture (0.0135 cm(2)) Ussing chambers. All three regions of the normal neonatal intestine responded to forskolin with an increase in short-circuit current, which was completely absent in the CF intestine. The neonatal distal colon exhibited a high rate of amiloride-sensitive electrogenic Na(+) absorption, which did not differ between the normal and CF preparations. The ileum and proximal colon of both genotypes exhibited a small but significant electrogenic Na(+) absorption. The neonatal proximal colon and ileum also exhibited electrogenic Na(+)-glucose cotransport, which was significantly greater in the normal compared with the CF ileum. In addition, all three intestinal regions exhibited electrogenic Na(+)-alanine cotransport, which was significantly reduced in two of the regions of the CF neonatal intestine. It is speculated that: 1) the reduced rate of Na(+)-nutrient cotransport in the CF intestine contributes to the lower rate of growth in CF pups, whereas 2) the elevated electrogenic Na(+) absorption in the neonatal intestine, coupled with an inability to secrete Cl(-), contributes to the intestinal obstruction in the CF pups.

Aldosterone stimulates K secretion prior to onset of Na absorption in guinea pig distal colon

Distal colon from guinea pig was stimulated in vitro by aldosterone in Ussing chambers that allowed measurement of short-circuit current (Isc) and tissue conductance (Gt). The response to aldosterone was delayed by approximately 20 min and resulted in a negative Isc, consistent with K secretion. Approximately 1 h later the Isc began to increase and eventually became positive, consistent with subsequent stimulation of Na absorption. The Na-absorptive response could be inhibited by mucosal amiloride without altering the rate of K secretion. Similarly, K secretion could be inhibited by serosal bumetanide without altering Na absorption. In the presence of spironolactone, actinomycin D, or cycloheximide, aldosterone failed to stimulate both K secretion and Na absorption. A dose response to aldosterone provided an apparent Kd of 2.6 +/- 0.5 nM, consistent with a high-affinity receptor coupled to this secretory response. Stimulation by the K secretagogue epinephrine did not produce an additive increase in K secretion, suggesting that the same cell type responds to both aldosterone and epinephrine and that the protein induced by aldosterone was not one of the membrane proteins responsible for K secretion.

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.

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.