Effects of luminal hyperosmolality on cellular and paracellular ion transport pathways in necturus antrum

Lateral cell membranes and shunt resistance in rabbit esophageal epithelium

BACKGROUND AND AIMS

The structures that contribute to shunt resistance (Rs) in esophageal epithelium are incompletely understood, with 35-40% of Rs known to be calcium-dependent, reflecting the role of e-cadherin. Two calcium-independent candidates for the remaining approximately 60% of Rs have been identified: the glycoprotein matrix (GPM) within stratum corneum of esophageal epithelium, and the lateral cell membranes (LCMs) from neighboring cells.

METHODS

To determine the contribution of GPM and LCMs to Rs, rabbit esophageal epithelium was mounted in Ussing chambers so that transepithelial resistance (R(T)), a marker of Rs, could be monitored during luminal exposure to either glycosidases for disruption of the GPM or to hypertonic urea for separation of the LCMs.

RESULTS

Glycosidases had no effect on R(T). In contrast, hypertonic urea reduced R(T), increased fluorescein flux and widened the intercellular spaces. That urea reduced R(T), and so Rs, by widening the intercellular spaces, and not by altering the e-cadherin-dependent apical junctional complex, was supported by the ability of: (a) calcium-free solution to reduce R(T) beyond that produced by urea, (b) hypertonic urea to reduce R(T) beyond that produced by calcium free solution, (c) hypertonic sucrose to collapse the intercellular spaces and raise R(T), and (d) empigen, a zwitterionic detergent, to non-osmotically widen the intercellular spaces and reduce R(T).

CONCLUSION

These data indicate that the LCMs from neighboring cells are a major contributor to shunt resistance in esophageal epithelium. As resistor, they are distinguishable from the apical junctional complex by their sensitivity to (luminal) hypertonicity and insensitivity to removal of calcium.

Effects of diet and osmotic pressure on Na+transport and tissue conductance of sheep isolated rumen epithelium

The intention of this study was to determine the effects of mucosal osmotic pressure on transport and barrier functions of the rumen epithelium of sheep, which were fed various diets: hay ad libitum, or 600, 1200 or 1800 g day(-1) of a supplemented diet plus hay ad libitum. The experiments were conducted by using the conventional Ussing chamber technique. Mucosal osmolarity was adjusted to 300 (control), 375 or 450 mosmol l(-1). Feeding of a supplemented diet led to a significant increase of mucosal to serosal Na+ transport and net Na+ transport, probably because of an increase of apical Na+-H+ exchange activity. An increase in mucosal osmotic pressure: (a) reduced net Na+ transport in all feeding groups, the remaining net Na+ transport being higher in tissues of sheep fed a supplemented diet; (b) increased transepithelial tissue conductance, this rise being smallest with a high intake of the supplemented diet; and (c) enhanced the serosal to mucosal Na+ transport in tissues of hay-fed sheep and sheep fed with 600 g day(-1) of the supplemented diet, while higher intakes of the supplemented diet (1200 and 1800 g) did not produce any effect. All these changes indicate a diet-dependent adaptation to luminal hypertonicity.

Nucleotide regulation of paracellular Cl- permeability in natural rabbit airway epithelium

In this study, we demonstrate a novel regulatory mechanism by which mucosal nucleotides via P2Y receptors decrease paracellular Cl(-) ion permeability in natural rabbit airway epithelium (in addition to a decrease in active Na(+) absorption). In contrast to primary cultures, the natural airway epithelium is a low-resistance epithelium, and an equivalent circuit model predicts that changes of more than approximately 12% in transepithelial conductance (G (t)) must include an effect on paracellular conductance (G (s)). Mucosal P2Y receptor stimulation with uridine triphosphate (UTP; 200 microM) decreased G (t) by up to 50% (average, 24%) and simultaneously decreased the paracellular Cl(-) permeability (mucosa-to-serosa Cl(-) flux) by 16%, but had no effect on mannitol permeability. The G (t) response to UTP was mimicked and attenuated by ionomycin (1 microM), suggesting a dependence on Ca(2+) (i). Amiloride (100 microM) and hyperosmolarity (+75 mM mannitol) also decreased G (t), indicating a role of cell shrinkage. Elevation of cAMP with forskolin (8 microM) or isoproterenol (10 microM) increased G (t) by 55 and 32%, and forskolin increased paracellular Cl(-) permeability by 37% without affecting mannitol permeability. The opposite effects of Ca(2+) (i) and cAMP on G (t) suggest an autocrine nucleotide signaling sequence where P2Y-dependent decrease in passive, paracellular Cl(-) transport is succeeded by a reversion of this effect due to P1-receptor-stimulated cAMP formation by adenosine originating from a time-dependent breakdown of mucosal ATP.

Luminal hyperosmolarity decreases Na transport and impairs barrier function of sheep rumen epithelium

The effects of luminal hyperosmolarity on Na and Cl transport were studied in rumen epithelium of sheep. An increase of luminal osmotic pressure with mannitol (350 and 450 mosm/l) caused a significant increase of tissue conductance, G (T), which is linearly correlated with flux rates of (51)Cr-EDTA and indicates an increase of passive permeability. Studies with microelectrodes revealed, that an increase of the osmotic pressure caused a significant increase of the conductance of the shunt pathway from 1.23 +/- 0.10 (control) to 1.92 +/- 0.14 mS cm(-2) (450 mosm/l) without a change of fractional resistance. Hyperosmolarity significantly increased J (sm) and reduced J (net) Na. The effect of hyperosmolarity on J (ms) Na is explained by two independent and opposed effects: increase of passive permeability and inhibition of the Na(+)/H(+) exchanger. Hypertonic buffer solution induced a decrease of the intracellular pH (pH(i)) of isolated ruminal cells, which is consistent with an inhibition of Na(+)/H(+) exchange, probably isoform NHE-3, because NHE-3-mRNA was detectable in rumen epithelium. These data are in contrast to previous reports and reveal a disturbed Na transport and an impaired barrier function of the rumen epithelium, which predisposes translocation of rumen endotoxins and penetration of bacteria.

Anti-neutrophil serum attenuates dextran sulfate sodium-induced colonic damage in the rat

BACKGROUND

The role of neutrophils in experimental colonic damage induced by dextran sulfate sodium is uncertain. We test the hypothesis that neutrophils are of pathogenic significance and anti-neutrophil serum will attenuate the colonic damage induced by oral dextran sulfate sodium in rats.

METHODS

Rabbit anti-rat neutrophil serum (anti-neutrophil serum) or control rabbit serum was administered to, and circulating neutrophil count was monitored in, rats before and during feeding of dextran sulfate sodium or regular rat diet for 2 weeks. Histologic features of mucosal damage were evaluated in hematoxylin and eosin-stained proximal and distal colonic sections by a blinded observer.

RESULTS

Oral dextran sulfate sodium induces weight loss, diarrhea, peripheral neutrophilia, and colonic damage. Anti-neutrophil serum induced neutropenia and significantly attenuated the weight loss, the neutrophil infiltration in the colon, and the mucosal necrosis and pathologic index in the distal colon.

CONCLUSION

The data showing that anti-neutrophil serum attenuates distal colonic mucosal injury induced by dextran sulfate sodium support the hypothesis that neutrophils play a pathogenic role in this model of colonic mucosal damage.