pH gradient effects on chloride transport across basolateral membrane vesicles from guinea-pig jejunum

Mechanisms of net chloride secretion during rotavirus diarrhea in young rabbits: do intestinal villi secrete chloride?

Rotaviral diarrheal illness is one of the most common infectious diseases in children worldwide, but our understanding of its pathophysiology is limited. This study examines whether the enhanced net chloride secretion during rotavirus infection in young rabbits may occur as a result of hypersecretion in crypt cells that would exceed the substantial Cl(-) reabsorption observed in villi. By using a rapid filtration technique, we evaluated transport of (36)Cl and D-(14)C glucose across brush border membrane (BBM) vesicles purified from villus tip and crypt cells isolated in parallel from the entire small intestine. Rotavirus infection impaired SGLT1-mediated Na(+)-D-glucose symport activity in both villus and crypt cell BBM, hence contributing to the massive water loss along the cryptvillus axis. In the same BBM preparations, rotavirus failed to stimulate the Cl(-) transport activities (Cl(-)/H(+) symport, Cl(-)/anion exchange and voltage-activated Cl(-) conductance) at the crypt level, but not at the villus level, questioning, therefore, the origin of net chloride secretion. We propose that the chloride carrier might function in both normal (absorption) and reversed (secretion) modes in villi, depending on the direction of the chloride electrochemical gradient resulting from rotavirus infection, agreeing with our results that rotavirus accelerated both Cl(-) influx and Cl(-) efflux rates across villi BBM.

Rotavirus infection stimulates the Cl- reabsorption process across the intestinal brush-border membrane of young rabbits

Rotavirus is a major cause of infantile gastroenteritis worldwide. However, the mechanisms underlying fluid and electrolyte secretion associated with diarrhea remain largely unknown. We investigated the hypothesis that loss of Cl(-) into the luminal contents during rotavirus infection may be caused by a dysfunction in the chloride absorptive capacity across the intestinal brush-border membrane (BBM). The luminal Cl(-) concentrations in the entire small intestine of young rabbits infected with lapine rotavirus decreased at 1 and 2 days postinfection (dpi), indicating net Cl(-) absorption. At 7 dpi, luminal Cl(-) concentrations were slightly increased, indicating a moderate net Cl(-) secretion. By using a rapid filtration technique, (36)Cl uptake across BBM was quantified by modulating the alkali-metal ion, electrical, chloride, and/or proton gradients. Rotavirus infection caused an identical, 127% +/- 24% increase in all Cl(-) uptake activities (Cl(-)/H(+) symport, Cl(-) conductance, and Cl(-)/anion exchange) observed across the intestinal BBM. The rotavirus activating effects on the symporter started at 1 dpi and persisted up to 7 dpi. Kinetic analyses revealed that rotavirus selectively affected the capacity parameter characterizing the symporter. We report the novel observation that rotavirus infection stimulated the Cl(-) reabsorption process across the intestinal BBM. We propose that the massive Cl(-) reabsorption in villi could partly overwhelm chloride secretion in crypt cells, which possibly increases during rotavirus diarrhea, the resulting imbalance leading to a moderate net chloride secretion.

Heterogeneity of pig intestinal D-glucose transport systems

Heterogeneity of intestinal D-glucose transport is demonstrated using pig jejunal brush-border membrane vesicles in the presence of 100/0 (out/in) mM gradients each of NaCl, NaSCN, and KSCN. Two D-glucose transport systems are kinetically distinguished: high-affinity, low-capacity system 1, which is equivalent to the symporter SGLT1; and low-affinity, high-capacity system 2, which is not a member of the SGLT family but is a D-glucose and D-mannose transporter exhibiting no preference for Na(+) over K(+). A nonsaturable D-glucose uptake component has also been detected; uptake of this component takes place at rates 10 times the rate of components characterizing the classical diffusion marker L-glucose. It is also shown that, in this kinetic work, 1) use of D-glucose-contaminated D-sorbitol as an osmotic replacement cannot cause the spurious appearance of nonexistent transport systems and 2) a large range (>/=50 mM) of substrate concentrations is required to correctly fit substrate saturation curves to distinguish between low-affinity transport systems and physical diffusion.

Direct inhibitory effect of CCCP on the Cl(-)-H+ symporter of the guinea pig ileal brush-border membrane

The effect of carbonyl cyanide-m-chlorophenylhydrazone (CCCP) on Cl- uptake across the brush-border membrane (BBM) was quantified using 36Cl and BBM vesicles from guinea pig ileum. CCCP inhibited only partially both the pH gradient-activated Cl- uptake and Cl-/Cl- exchange activities present in these vesicles. In contrast, CCCP had no effect on the initial (2-30 s) decay rate of an imposed proton gradient, as determined using the pH-sensitive fluorophore pyranine. Taken together, these results strongly indicate that the main action of CCCP does not consist of dissipating any imposed pH gradient but rather in inhibiting directly the pH gradient-activated Cl- uptake and Cl-/Cl- exchange activities characterizing the intestinal BBM. Because these two activities can be explained in terms of a single (homogeneous) random, nonobligatory two-site Cl(-)-H+ symporter, in which Cl-/Cl- exchange occurs by counterflow [F. Alvarado and M. Vasseur. Am. J. Physiol. 271 (Cell Physiol. 40): C1612-C1628, 1996], we developed a new, more general three-site symport model that fully explains the Cl- uptake inhibitions caused by CCCP. This new model postulates the existence of a third, allosteric, inhibitory CCCP-binding site separate from either of the two substrate-binding sites of the Cl(-)-H+ symporter, the Cl(-)-binding and the H(+)-binding sites. Finally, we show that, to explain the partial inhibitions observed, it is necessary to postulate that all the substrate-bound carrier complexes, = C-S, I = C-S, A = C-S, and IA = C-S, where C is carrier, I is inhibitor, S is substrate, and A is activator, can form and be translocated.