Abstract
Net inward flux of mannitol across toad skin induced by making the outside solution hypertonic with urea has been investigated. No significant relation between net mannitol flux and net Na flux could be detected when both fluxes were measured simultaneously. In addition, the net mannitol flux caused by hypertonic solution was not altered by inhibition of Na transport with ouabain or by replacement of all Na in the bathing solutions by choline. The rate of net mannitol flux was dependent on the magnitude of the urea concentration difference across the skin and the direction of net flux could be reversed by reversing the direction of the urea concentration difference. These observations suggest that the mannitol transfer is the result of a coupling between the flows of urea and mannitol.
Highlights
ABsTRACw Net inward flux of mannitol across toad skin induced by making the outside solution hypertonic with urea has been investigated
When the solution bathing the outside of the frog skin is made hyperosmotic with urea, there is a discrepancy between short-circuit current and net Na transport [1]
Toad skin was used in these studies because in preliminary experiments hypertonic outside solutions caused a greater asymmetry in unidirectional mannitol fluxes in this tissue than in frog skin
Summary
ABsTRACw Net inward flux of mannitol across toad skin induced by making the outside solution hypertonic with urea has been investigated. Ussing [1] has recently reported an unexpected effect on solute transfer across the isolated frog skin when the outside bathing solution is m a d e hypertonic with urea or other solutes Under these conditions, net inward flow of sucrose was observed when both bathing solutions contained equal concentrations of sucrose. Franz and Van Bruggen [2] have reported a similar effect on several organic solutes but have questioned the role of Na transport in the phenomenon They suggested that the net solute transfer could be caused by a drag arising from the diffusion of the hypertonic agent itself from a high concentration in the outside solution to a low one in the inside solution. Relations between urea concentration in the outside solution and mannitol fluxes and between urea fluxes and mannitol fluxes were examined
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