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Paracellular Filtration Secretion Driven by Mechanical Force Contributes to Small Intestinal Fluid Dynamics.

ABSTRACT: Studies of fluid secretion by the small intestine are dominated by the coupling with ATP-dependent generation of ion gradients, whereas the contribution of filtration secretion has been overlooked, possibly by the lack of a known mechanistic basis. We measured apical fluid flow and generation of hydrostatic pressure gradients by epithelia of cultured mouse enterocytes, Caco-2 and T-84 cells, and fibroblasts exposed to mechanical force provided by vigorous aeration and in response to ion gradients, inhibitors of ion channels and transporters and in vitro using intact mouse and rat small intestine. We describe herein a paracellular pathway for unidirectional filtration secretion that is driven by mechanical force, requires tight junctions, is independent of ionic and osmotic gradients, generates persistent hydrostatic pressure gradients, and would contribute to the fluid shifts that occur during digestion and diarrhea. Zinc inhibits the flow of fluid and the paracellular marker fluorescein isothyocyanate conjugated dextran (MW = 4 kD) across epithelia of cultured enterocytes (>95%; p < 0.001) and intact small intestine (>40%; p = 0.03). We propose that mechanical force drives fluid secretion through the tight junction complex via a "one-way check valve" that can be regulated. This pathway of filtration secretion complements chloride-coupled fluid secretion during high-volume fluid flow. The role of filtration secretion in the genesis of diarrhea in intact animals needs further study. Our findings may explain a potential linkage between intestinal motility and intestinal fluid dynamics.

SUBMITTER: Buddington RK

PROVIDER: S-EPMC7931054 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Lacking an effective mechanism to safely and consistently enhance macromolecular uptake across the intestinal epithelium, prospects for successful development of oral therapeutic peptide drugs remain unlikely. We previously addressed this challenge by identifying an endogenous mechanism that controls intestinal paracellular permeability that can be activated by a peptide, termed PIP 640, which can increase cellular levels of phosphorylated myosin light chain at position S19 (MLC-pS19). Apical application in vitro or luminal application in vivo was shown to increase macromolecular solute transport within minutes that recovered completely within a few hours after removal. We now examine the nature of PIP 640-mediated permeability changes. Confluent Caco-2 cell monolayers treated with PIP 640 enhanced apical-to-basolateral (AB) transport of 4-kDa, but not 10-kDa, dextran. Expression and/or cellular distribution changes of tight junction (TJ) proteins were restricted to increased claudin-2 over a time course that correlated with an apparent shift in its distribution from the nucleus to the membrane fraction of the cell. PIP 640-mediated epithelial changes were distinct from the combined actions of the pro-inflammatory cytokines tumor necrosis factor alpha (TNF-?) and interferon gamma (IFN-?). While TNF-?/IFN-? treatment also increased MLC-pS19 levels, these cytokines enhanced AB transport for 70-kDa dextran and decreased occludin expression at TJs. Claudin-2-dependent changes induced by PIP 640 resulted in an AB transport bias for positively-charged macromolecules demonstrated in vitro using charge variants of 4-kDa dextrans and by comparing transport of salmon calcitonin to exenatide. Comparable outcomes of increased TJ localization of claudin-2 and enhanced transport of these therapeutic peptides that biased toward cationic characteristics was demonstrated in vivo following after intra-luminal injection into rat jejunum. Together, these data have shown a potential mechanism for PIP 640 to enhance paracellular permeability of solutes in the size range of small therapeutic peptides that is biased toward positively-charged solutes.

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Paracellular Filtration Secretion Driven by Mechanical Force Contributes to Small Intestinal Fluid Dynamics.

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