Mechanisms for colonic sodium transport during ontogeny: loss of an amiloride-sensitive sodium pathway

SLC9 Gene Family: Function, Expression, and Regulation

The Slc9 family of Na⁺ /H⁺ exchangers (NHEs) plays a critical role in electroneutral exchange of Na⁺ and H⁺ in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins contribute to the transepithelial Na⁺ and water absorption, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. They also influence the function of other membrane transport mechanisms, affect cellular proliferation and apoptosis as well as cell migration, adherence to the extracellular matrix, and tissue repair. Additionally, they modulate the extracellular milieu to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺ /H⁺ exchange is inhibited in selected gastrointestinal diseases, either by intrinsic factors (e.g., bile acids, inflammatory mediators) or infectious agents and associated bacterial toxins. Disrupted NHE activity may contribute not only to local and systemic electrolyte imbalance but also to the disease severity via multiple mechanisms. In this review, we describe the cation proton antiporter superfamily of Na⁺ /H⁺ exchangers with a particular emphasis on the eight SLC9A isoforms found in the digestive tract, followed by a more integrative description in their roles in each of the digestive organs. We discuss regulatory mechanisms that determine the function of Na⁺ /H⁺ exchangers as pertinent to the digestive tract, their regulation in pathological states of the digestive organs, and reciprocally, the contribution of dysregulated Na⁺ /H⁺ exchange to the disease pathogenesis and progression. © 2018 American Physiological Society. Compr Physiol 8:555-583, 2018.

Cytokine expression in rat colon during postnatal development: regulation by glucocorticoids

OBJECTIVES

Cytokine expression and regulation by glucocorticoids and retinoic acid were investigated in the colon during postnatal development.

MATERIALS AND METHODS

Gene expression of the transforming growth factors (TGFs) TGF-beta1, TGF-beta2 and TGF-alpha and the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) was evaluated by reverse transcription-polymerase chain reaction (RT-PCR) in rat colon mucosa during weaning and in adult rats. Protein expression and distribution of TGF-betas was analysed in the colon from 14- and 60-day-old animals. The effect of hydrocortisone administration on mucosal cytokine transcripts (RT-PCR) and of dexamethasone on the expression of cytokines by the epithelial cell line IEC-18 and 2 subepithelial myofibroblasts (MIC 307-1 and 316) was examined.

RESULTS

TGF-beta1 and TGF-beta2 messenger RNAs and proteins decreased in the entire colon from weaning to adult stages, whereas the amount of TGF-alpha messenger RNA increased in the proximal colon and decreased in the distal part of the colon in adult rats in comparison with weanlings. However, proinflammatory cytokines showed no postnatal changes in the proximal colon but decreased in the distal part in comparison with weaning rats. Hydrocortisone treatment did not affect growth factor expression but decreased proinflammatory cytokines. Likewise, dexamethasone decreased TNF-alpha and IL-1beta gene expression but did not affect TGF-betas in either epithelial or myofibroblast cells.

CONCLUSIONS

During postnatal maturation, the expression of growth factors and proinflammatory cytokines decreased in the distal colon, whereas in the proximal colon, a differential maturation occurs with no changes in proinflammatory cytokines, an increase in TGF-alpha and a decrease in TGF-beta. Glucocorticoids may control the developmental profile of proinflammatory cytokines.

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.

Developmental regulation of epithelial sodium channel subunit mRNA expression in rat colon and lung

Na+ absorption via amiloride-sensitive Na+ channels is of critical importance in the transition between fetal and neonatal life in several tissues, including the colon, lung, and kidney. To characterize and contrast the mRNA expression of each of the three epithelial Na+ channel complex (ENaC) subunits, we conducted RNase protection assays (RPA) and in situ hybridization in colon and lung in fetal (17, 19, 20, and 21 days) and postnatal (1, 3, 9, 15, and 30 days) rats (r). In the colon the alpha-, beta-, and gamma-rENaC subunits showed quantitatively different but qualitatively similar expression. All three subunits gradually increased in abundance from fetal day 19 through day 30 of life. The amount of each subunit on day 30 was approximately three times the amount at day 1. In situ hybridization showed that each subunit was localized to the surface epithelial cells with minimal expression in the crypts. The lung showed a completely different pattern. In contrast to the colon, the total amount of alpha-rENaC mRNA (by RPA) in the lung increased dramatically from fetal day 19 to 21, whereas beta- and gamma-rENaC showed modest prenatal increases. The amounts of all three mRNAs fell after birth through day 9 (to about 75% of the day 1 value). On days 15 and 30 the amount of mRNA rose to approach the values on day 1. alpha-rENaC mRNA abundance always exceeded beta- and gamma-rENaC, and the quantitative expression was different for alpha- than for beta- and gamma-rENaC. In situ hybridization studies showed that all three subunits were expressed in epithelial cells of the bronchi, bronchioles, and alveoli and not in blood vessels. These studies show striking developmental heterogeneity in rENaC mRNA expression between lung and colon, probably reflecting different developmental regulatory mechanisms in these organs.

Ontogeny of Na+ transport in rat colon

To obtain information about whether nutrient and hormonal factors are critical for the developmental pattern of electrogenic amiloride-sensitive Na+ transport (NaSCC) in rat distal colon, we studied the effect of adrenalectomy, high dietary Na+ intake, and hypothyroidism on colonic NaSCC in weanling rats. Adrenalectomy and high dietary Na+ intake inhibited NaSCC, decreased plasma level of aldosterone, and did not influence plasma level of thyroxine. Hypothyroidism inhibited NaSCC without significant changes of plasma aldosterone. These results suggest that the high activity of NaSCC during weaning period reflects the relatively low Na+ intake, and that thyroid hormones have an important permissive role in the developmental changes of NaSCC.

Endogenous dopamine modulates jejunal sodium absorption during high-salt diet in young but not in adult rats

BACKGROUND/AIMS

This study was designed to investigate the contribution of endogenous catecholamines to the regulation of small intestinal sodium transport during postnatal development.

METHODS

Jejunal permeability was determined by a constant perfusion, nonabsorbable marker technique in weanling, adolescent, and adult rats fed either a high-salt diet or normal-salt diet. Tissue catecholamine levels were determined by high-performance liquid chromatography with electrochemical detection.

RESULTS

In 20-day-old but not in 40-day-old rats, a significantly lower net sodium absorption was observed during high-salt diet compared with age-matched controls on normal-salt diet. Inhibition of dopamine synthesis significantly increased the net sodium absorption in 20-day-old rats on high-salt diet compared with untreated 20-day-old rats on high-salt diet. The basal levels of dopamine in 20-day-old rats were twofold higher than in 40-day-old rats. During high-salt diet, both age groups responded with an increase in dopamine production. Norepinephrine levels were significantly higher (30-fold) in 20-day-old rats than in 40-day-old rats, but norepinephrine content was not significantly changed during high-salt diet in either groups.

CONCLUSIONS

The results indicate that weanling animals have a greater jejunal sodium absorption than older animals, probably because of higher noradrenergic tonus. A challenge with a high-salt diet results in a decrease of the intestinal sodium absorption in weaning rats but not in adult rats; endogenous dopamine appears to play an important role in this regulation.

Eicosanoid production by a differentiated canine colonic epithelial cell line, VNCC

BACKGROUND/AIMS

The lack of pure, proliferative, but not transformed intestinal epithelial cells has impeded progress in understanding their role in chronic intestinal inflammation. To clarify that role, the present study characterized the epithelial cell line VNCC, derived from normal adult dog distal colon.

METHODS

Cells were cultured on plastic and permeable supports for analysis of eicosanoid production (by radioimmunoassay and high-performance liquid chromatography) and transport characteristics (by Ussing chamber short-circuit determinations).

RESULTS

In culture, VNCC formed confluent monolayers and domes, suggesting formation of tight junctions and active solute absorption. When cultured on permeable supports, VNCC developed modest, but variable, transepithelial resistances (563 +/- 94 omega/cm2) with a spontaneous short-circuit current of 5.0 +/- 0.4 microA/cm2. Forskolin caused a prolonged increase in the short-circuit current, inhibited by amiloride but not bumetanide, suggesting that VNCC display 5'-cyclic adenosine monophosphate-stimulated sodium absorption. VNCC incubated with arachidonic acid released a variety of eicosanoids including 6-keto-prostaglandin (PG)F1 alpha, PGE2, thromboxane B2, and PGF2 alpha, but no hydroxyarachidonate metabolites. Bradykinin stimulated VNCC eicosanoid release.

CONCLUSIONS

The ability of VNCC to divide and differentiate in culture, to form polarized monolayers capable of active sodium absorption, and to respond to inflammatory mediators with eicosanoid release makes them a unique tool for the study of the interactions of inflammation on colonocyte function.

Liquid flow across the epithelium of the artificially perfused lung of fetal and postnatal sheep

1. The lungs of five fetal (133-140 days gestation) and thirty-four postnatal (2-240 days) sheep were artificially perfused in situ with warmed and oxygenated sheep blood. In postnatal animals the airspace of the lung was filled with liquid similar in composition to fetal lung liquid. In fetal and postnatal animals luminal liquid volume was measured by the impermeant tracer technique. 2. Under resting conditions the pulmonary epithelium of fetal animals secreted liquid at a mean (+/- S.E.M.) rate of 2.0 (+/- 0.4) ml (kg body weight)-1 h-1, those of postnantal animals absorbed liquid at -1.8 (+/- 0.2) ml (kg body weight)-1 h-1. 3. Addition of 2,4-dinitrophenol to achieve a concentration of 1.5 x 10(-3) M in the perfusing blood in postnatal animals caused complete cessation of liquid absorption. 4. Light and electron microscopic examination of the lung after periods of up to 6 h of artificial perfusion showed no evidence of epithelial damage. From 3 h onwards, liquid accumulation was evident in the perivascular spaces. 5. Addition of adrenaline to the perfusate in fetal animals caused absorption of liquid to occur at a mean rate of -2.9 (+/- 1.3) ml (kg body weight)-1 h-1. In postnatal animals adrenaline caused the rate of liquid absorption to increase from a mean rate of -1.4 (+/- 0.2) to -2.2 (+/- 0.3) ml (kg body weight)-1 h-1. 6. In the fetus addition of amiloride (0.8 x 10(-4) M) to the luminal fluid blocked adrenaline-induced liquid absorption and caused secretion to occur at 1.3 (+/- 0.3) ml (kg body weight)-1 h-1. 7. In postnatal animals the response to amiloride was age dependent. In newborn lambs (2-14 days) amiloride blocked liquid absorption and caused secretion of liquid to occur in seven out of eight animals at a mean rate of 0.9 (+/- 0.3) ml (kg body weight)-1 h-1 (n = 8). In older animals (15-240 days) the characteristic response to amiloride was slowing of the rate of liquid absorption (mean rate of absorption,-0.2 (+/- 0.09) ml (kg body weight)-1 h-1, n = 18) with liquid secretion being seen in only three of eighteen animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of colonic sodium transport in early childhood and its regulation by aldosterone

Aldosterone is important in the regulation of sodium conservation by both kidney and colon. In the very preterm neonate marked urinary salt wasting occurs because of immature renal tubular function, but little is known of the ontogeny of colonic transport processes. Using an in vivo rectal dialysis technique, we have shown that in the human infant the colon has well developed salt conserving mechanisms from early in the last trimester of gestation and that aldosterone is an important regulatory hormone. Sodium transport mechanisms in the colon appear to develop before those in the kidney and it is possible that the colon is the major organ of sodium conservation in the preterm neonate.