Abstract
Absorption of small water-soluble nutrients in vertebrate intestines occurs both by specific, mediated transport and by non-specific, passive, paracellular transport. Although it is apparent that paracellular absorption represents a significant route for nutrient absorption in many birds and mammals, especially small, flying species, its importance in ectothermic vertebrates has not previously been explored. Therefore, we measured fractional absorption (ƒ) and absorption rate of three paracellular probes (arabinose, l-rhamnose, cellobiose) and of 3-O-methyl d-glucose (absorbed by both mediated and paracellular pathways) by the large herbivorous lizard, Uromastyx aegyptia, to explore the relative importance of paracellular and mediated transport in an ectothermic, terrestrial vertebrate. Fractional absorption of 3-O-methyl d-glucose was high (ƒ = 0.73±0.04) and similar to other vertebrates; ƒ of the paracellular probes was relatively low (arabinose ƒ = 0.31±0.03, l-rhamnose ƒ = 0.19±0.02, and cellobiose ƒ = 0.14±0.02), and decreased with molecular mass, a pattern consistent with other vertebrates. Paracellular absorption accounted for approximately 24% of total 3-O-methyl d-glucose uptake, indicating low reliance on this pathway for these herbivorous lizards, a pattern similar to that found in other terrestrial vertebrates, and different from small flying endotherms (both birds and bats).
Highlights
Absorption of small water-soluble nutrients by vertebrates is known to occur by both carriermediated, transcellular pathways, and by passive, paracellular pathways [1]
Whereas carrier-mediated absorption relies on the direct action of specific transporter proteins, paracellular absorption occurs by diffusion and by solvent drag, when water moves through the tight junctions between intestinal enterocytes, carrying water soluble compounds across the epithelium [2]
Experimental animals and husbandry Uromastyx aegyptia occurs in the deserts of the Middle East, from east of the Nile river in Egypt, to Iraq and Iran, and throughout the Arabian peninsula [23], with the subspecies U. a. aegyptia occurring from the Nile river to an area in Jordan just east of the Arava valley that straddles the border between Israel and Jordan
Summary
Absorption of small water-soluble nutrients (e.g. carbohydrates, amino acids) by vertebrates is known to occur by both carriermediated, transcellular pathways, and by passive, paracellular pathways [1]. Whereas carrier-mediated absorption relies on the direct action of specific transporter proteins, paracellular absorption occurs by diffusion and by solvent drag, when water moves through the tight junctions between intestinal enterocytes, carrying water soluble compounds across the epithelium [2]. Transport of molecules via the paracellular pathway is non-specific [2], influenced by charge [4], and limited by molecular size [5], so any water-soluble compound small enough to pass through pores in the tight junctions will be absorbed. Pappenheimer [7] argued that paracellular uptake may be adaptively advantageous because it requires little energy, and provides absorptive capacity that is not limited by the number of available protein carriers (i.e. it is non-saturable). The lack of selectivity and relatively high intestinal permeability associated with high paracellular absorption may result in greater systemic exposure to water-soluble toxins of plant or animal origin found in the diet [8,9]
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