Immunocytochemical localization of amiloride-sensitive sodium channels in the lower intestine of the hen

Amiloride-sensitive epithelial Na+channel currents in surface cells of rat rectal colon

Surface cells of the mammalian distal colon are shown to molecularly express the amiloride-sensitive epithelial Na+channel composed of three homologous subunits (α-, β-, and γ-ENaC). However, because basic electrophysiological properties of amiloride-sensitive Na+channels expressed in these cells are largely unknown at the cellular level, functional evidence for the involvement of the subunits in the native channels is incomplete. Using electrophysiological techniques, we have now characterized functional properties of native ENaC in surface cells of rectal colon (RC) of rats fed a normal Na+diet. Ussing chamber experiments showed that apical amiloride inhibited a basal short-circuit current in mucosal preparation of RC with an apparent half-inhibition constant ( Ki) value of 0.20 μM. RT-PCR analysis confirmed the presence of transcripts of α-, β-, and γ-rENaC in rectal mucosa. Whole cell patch-clamp experiments in surface cells of intact crypts acutely isolated from rectal mucosa identified an inward cationic current, which was inhibited by amiloride with a Kivalue of 0.12 μM at a membrane potential of –64 mV, the inhibition being weakly voltage dependent. Conductance ratios of the currents were Li+(1.8) > Na+(1) >> K+(≈0), respectively. Amiloride-sensitive current amplitude was almost the same at 15 or 150 mM extracellular Na+, suggesting a high Na+affinity for current activation. These results are consistent with the hypothesis that a heterooligomer composed of α-, β-, and γ-ENaC may be the molecular basis of the native channels, which are responsible for amiloride-sensitive electrogenic Na+absorption in rat rectal colon.

NaCl uptake by the branchial epithelium in freshwater teleost fish: an immunological approach to ion-transport protein localization

Teleost fishes, living in fresh water, engage in active ion uptake to maintain ion homeostasis. Current models for NaCl uptake involve Na(+) uptake via an apical amiloride-sensitive epithelial Na(+) channel (ENaC), energized by an apical vacuolar-type proton pump (V-ATPase) or alternatively by an amiloride-sensitive Na(+)/H(+) exchange (NHE) protein, and apical Cl(−) uptake mediated by an electroneutral, SITS-sensitive Cl(−)/HCO(3-) anion-exchange protein. Using non-homologous antibodies, we have determined the cellular distributions of these ion-transport proteins to test the predicted models. Na(+)/K(+)-ATPase was used as a cellular marker for differentiating branchial epithelium mitochondria-rich (MR) cells from pavement cells. In both the freshwater tilapia (Oreochromis mossambicus) and rainbow trout (Oncorhynchus mykiss), V-ATPase and ENaC-like immunoreactivity co-localized to pavement cells, although apical labelling was also found in MR cells in the trout. In the freshwater tilapia, apical anion-exchanger-like immunoreactivity is found in the MR cells. Thus, a freshwater-type MR chloride cell exists in teleost fishes. The NHE-like immunoreactivity is associated with the accessory cell type and with a small population of pavement cells in tilapia.

Differences in epithelial morphology correlate to Na(+)-transport: a study of the proximal, mid, and distal regions of the coprodeum from hens on high and low NaCl diet

A study was performed to correlate regional morphology and amiloride inhibitable Na(+)-transport in the coprodeal epithelium in hens, Gallus domesticus, on low-NaCl diet and in controls. Proximal (close to colon), mid and distal (close to urodeum) regions were examined using light microscopy, transmission- and scanning electron microscopy. Na(+)-transport was measured electrophysiologically in Ussing-chambers in the proximal and distal regions. The epithelium, simple and columnar, is composed of absorptive intestinal epithelial cells, goblet cells, brush cells, migrating lymphoid cells, and enteroendocrine cells. Brush cells, identified in avians for the first time, occur in highest number in the proximal part of the coprodeum in low-NaCl hens. Na(+)-transport is high in the low-NaCl hens, ranging from 347 microA/cm2 (proximal) to 187 microA/cm2 (distal). In control hens, which correspond to hens on high-NaCl diet, it is low in all regions (0-4 microA/cm2). Absorptive intestinal epithelial cells as well as brush cells adapt to variations in transepithelial Na(+)-transport by regulating height and packing density of their microvilli, number, size, and localization of apical vesicles, and the width of the intercellular space. Regional differences in the epithelial cell composition and ultrastructure are closely correlated to transepithelial Na(+)-transport but only in low-NaCl hens, as controls do not show these variations.

Na/D-glucose cotransport and SGLT1 expression in hen colon correlates with dietary Na+

We have tested whether separately varying the content of either Na or Cl in diets causes earlier observed increase in Na-coupled sugar and amino acid transport induced by high NaCl diets in hen colon. A comparison was also made between the dependence of the Na-coupled transport on a pure wheat/barley/soya diet against a diet with supplements of essential amino acids, fatty acids, vitamins, and trace elements, as a test for possible elimination of the cotransporters due to a deficient diet. Na/nutrient-coupled transport was measured as changes in short circuit current. The level of expressed Na/glucose cotransporters, SGLT1, due to dietary alterations was followed by quantitative Western blot and immunodetection of SGLT1 in colon, and the dietary effects on plasma aldosterone were assessed as well. An observed switch in transport from amiloride-sensitive electrodiffusive Na transport to phlorizin-sensitive Na/D-glucose cotransport and Na/amino acid-coupled transport is caused solely by increasing Na+ in the diet. Thus, neither dietary Cl- nor the dietary supplements altered the expression of Na(+)-coupled nutrient transport processes. Corroborating these findings, only Na+ in the diet increased the expression of SGLT1 in colon epithelium and suppressed aldosterone level in plasma.

Reconstitution of stretch-activated cation channels by expression of the alpha-subunit of the epithelial sodium channel cloned from osteoblasts

Osteoblasts respond to repetitive strain by activating stretch-activated, nonselective cation channels (SA-CAT) and increasing matrix protein production. SA-CAT channels are thought to be responsible for mechano-transduction in osteoblasts, although the molecular identity of the SA-CAT channel has previously been unknown. We have demonstrated that both the UMR-106 osteoblast-like cell line and human osteoblasts in primary culture express the alpha-subunit of the epithelial sodium channel (alpha-ENaC). The ENaC gene product is closely related to a class of proteins that confer touch sensitivity to Caenorhabditis elegans and are referred to as degenerins. A cDNA clone was obtained of the entire coding region of rat alpha-ENaC (alpha-rENaC). Sequence analysis indicated that the osteoblast clone's sequence was identical to that originally cloned from rat colon. The alpha-rENaC cDNA was cloned into an expression plasmid and transfected into LM(TK-) cells, a null cell for SA-CAT activity. Stable transfectants expressed mRNA and the expected 74-kDa protein corresponding to alpha-rENaC. Reconstitution of alpha-rENaC resulted in the expression of a 24.2 +/- 1.0 psec SA-CAT channel (P(Na):P(K) = 1.1 +/- 0.1). The channel is calcium permeable (P(Na):P(Ca) = 1.4 +/- 0.1) and highly selective for cations over anions (P(Na):P(Cl) >> 20). The channel is only active after negative pressure is applied to cell attached patches, cell swelling, or patch excision. These results represent the first heterologous expression of an SA-CAT channel in a mammalian cell system and provide evidence that the ENaC/degenerin family of proteins are capable of mediating both transepithelial sodium transport and are involved in signal transduction by mechano-sensitive cells such as osteoblasts.