Anion-induced dynamic behavior of apical water channels in vasopressin-sensitive epithelia exposed to mercury

Vascular aspects of water uptake mechanisms in the toad skin: perfusion, diffusion, confusion

Blood cell flow (BCF) in the water absorbing "seat patch" region of toad skin was measured with laser Doppler flow cytometry. BCF of dehydrated toads increased by a factor of 6-8 when water contact was made and declined gradually as toads rehydrated. Water absorption was initially stimulated and declined in parallel with BCF. Water absorption measured during the initial rehydration period did not correlate with BCF and hydrated toads injected with AVT increased water absorption without an increase in BCF indicating the lack of an obligate relation between blood flow and water absorption. Aquaporins 1-3 were characterized by RT-PCR analysis of seat patch skin. AQP 1 was localized in the endothelium of subepidermal capillaries and serves as a pathway for water absorption in series with the apical and basolateral membranes of the epithelium. Dehydrated toads rehydrated more rapidly from dilute NaCl solutions than from deionized water despite the reduced osmotic gradient. BCF of toads rehydrating on 50 mM NaCl was not different than on deionized water and blocking Na+ transport with 100 microM amiloride did not reduce water absorption from 50 mM NaCl. Thus, neither circulation nor solute coupling explains the greater absorption from dilute salt solutions. Rehydration from 10 mM CaCl2 was stimulated above that of DI water by a similar degree as with 50 mM NaCl suggesting the anion might control water permeability of the skin.

Effect of mercuric chloride on electrical parameters and anion fluxes in the toad skin

The amphibian skin, widely used for studying the transepithelial passage of electrolytes, exhibits anion pathways relatively specific for Cl(-). We studied the effect of HgCl(2), 1.0 x 10(-4) M on its electrical parameters and unidirectional anion fluxes. In the presence of Cl(-), the transepithelial conductance (G) of the isolated skin of the Bufo arenarum toad increased considerably following exposure to HgCl(2), whereas short-circuit current (SCC)--reflecting transepithelial Na(+) transport-underwent only slight stimulation. Following the blockade of Na(+) intake by amiloride, 1.0 x 10(-4) M, the removal of Cl(-) from the solution bathing the epidermal border of the skin brought about a decrease in G, and gave rise to a gradient-induced SCC (SCCg) consistent with transepithelial passage of Cl(-) along its gradient. Addition of mercaptoethanol, 5.0 x 10(-3) M to the bath containing Hg(2+) fully reversed these effects. The increase in G was accompanied by an increase in the unidirectional (epidermal to dermal) fluxes of (36)Cl(-) and (131)I(-), and a decrease in the passage of (99m)TcO(4)(-). These results show the effects of HgCl(2) to be similar to those of theophylline, although exhibiting a different selectivity. Our data suggest that anion passage following exposure to HgCl(2) is, like that stimulated by theophylline, predominantly if not exclusively transcellular, and does not involve a significant opening of the tight junctions.

Interaction between ion transporters and the mucociliary transport system in dog and baboon

To gain insight into the role of epithelial ion channels, pumps, and cotransporters in regulating airway water and mucociliary transport, we administered inhibitors of the Na+ channel (amiloride), 3Na-2K-adenosinetriphosphatase (acetylstrophanthidin), and Na-K-2Cl cotransporter (furosemide) to anesthetized dogs and/or baboons. Tracheal ciliary beat frequency was measured by using heterodyne laser light scattering. Tracheal mucus velocity (TMV) and bronchial mucociliary clearance (BMC) or lung mucociliary clearance were measured by using radioaerosols and nuclear imaging. Respiratory tract fluid output was collected by using a secretion-collecting endotracheal tube. In six dogs, amiloride aerosol -lung deposition, 96 +/- 11 microg (means +/- SE)- had minimal effect, whereas acetylstrophanthidin aerosol (lung deposition, 71 +/- 9 microg) increased BMC, and furosemide (40 mg iv) markedly increased TMV. In five baboons, TMV increased after iv furosemide administration (2 mg/kg) as well as by aerosol (lung deposition, 20 +/- 3 mg), coincident with increases in ciliary-mucus coupling from 11.5 +/- 0. 1 to 29.5 +/- 0.4 and 46.5 +/- 0.7 microm/beat, respectively. Furosemide also increased lung mucociliary clearance in baboons. In dogs, respiratory tract fluid output increased after intravenous furosemide from 2.2 +/- 0.5 to 6.8 +/- 1.7 mg/min. When combined with dry-air inhalation, furosemide failed to stimulate TMV and reversed the inhibition of BMC by dry air. Thus pharmacological manipulation of the Na-K-2Cl cotransporter and the 3Na-2K-adenosinetriphosphatase pump may provide increases of clinical relevance in airway hydration and mucociliary transport.