Pathways for movement of ions and water across toad urinary bladder

Abstract

Hypertonicity of the mucosal bathing medium increases the electrical conductance of toad urinary bladder by osmotic distension of the epithelial "tight" or limiting junctions. However, toad urine is not normally hypertonic to plasma. In this study, the transmural osmotic gradient was varied strictly within the physiologic range; initially hypotonic mucosal bathing media were made isotonic by addition of a variety of solutes. Mucosal NaCl increased tissue conductance substantially. This phenomenon could not have reflected soley an altered conductance of the transcellular active transport pathway since mucosal KCl also increased tissue conductance, whether or not Na+ was present in the bathing media. The effect of mucosal NaCl could not have been mediated solely by a parallel transepithelial pathway formed by damaged tissue since mucosal addition of certain nonelectrolytes also increased tissue conductance. Finally, the osmotically-induced increase in conductance could not have occurred soley in transcellular transepithelial channels in parallel with the active pathway for Na+, since the permeability to 22Na from serosa to mucosa (s to m) was also increased by mucosal addition of NaCl; a number of lines of evidence suggest that s-to-m movement of Na+ proceeds largely through paracellular transepithelial pathways. The results thus establish that the permeability of the limiting junctions is physiologically dependent on the magnitude of the transmural osmotic gradient. A major role is proposed for this mechanism, serving to conserve the body stores of NaCl from excessive urinary excretion.

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

“…This suggestion has been verified by the determination of the conductance of the paracellular pathway and of flux rates of the paracellular marker 51 Cr‐EDTA (Schweigel et al 2005). Both parameters are increased upon osmotic challenge, in agreement with results from previous studies of a variety of tight or moderately tight tissues (Ussing, 1965; Civan & DiBona, 1974; Soybel et al 1987). The rumen epithelium is a keratinized multilayered squamous epithelium, so the histological counterpart of the paracellular pathway is more complicated than the corresponding structure (tight junctions) in epithelia of the gut (Graham & Simmons, 2004).…”

“…Hypertonic luminal solutions induce a decrease in cell volume and, hence, an enlargement of the paracellular pathway (Ussing, 1965; Gorodeski et al 1995). Furthermore, the osmolyte (mannitol) diffuses into the space between the epithelial cells, and the subsequent osmotic‐dependent flow of water into this compartment increases the paracellular conductance (DiBona & Civan, 1973; Civan & DiBona, 1974; Gorodeski et al 1995) and, consequently, passive permeability. The osmotic‐dependent increase of J sm Na is significant in tissues of HF sheep and is of the same magnitude in SF600 epithelia, supporting the conclusion that the epithelia become more ‘leaky’ at hyperROP.…”

Interaction between transcellular and paracellular water transport pathways through Aquaporin 5 and the tight junction complex

“…The paracellular and transcellular routes of water transport have been described in the epithelial tissues of a wide range of biological species, including the salamander (9,17), frog (18), toad (19), mouse (12,20), rat (21), rabbit (7,22), and human (23), and appear to have been conserved through evolution.…”

Aldosterone and insulin effects on driving force of Na+ pump in toad bladder

“…Vasopressin has also been found to alter the conductance of the paracellular pathway, but only in the presence of a transepithelial osmotic gradient, where the phenomenon arises from the hormone's hydroosmotic effect (5). Thus, in the presence of isosmotic bathing media (as in the current experiments), it seems reasonable to ascribe vasopressin's natriferic effect to a specific enhancement of Na+ entry.…”