Effect of changes in osmolarity on sodium transport across isolated toad bladder.

LIPTON, PETER. E$ect of changes in osmolarity on sodium transport across isolated toad bladder. Am. J. Physiol. 222(4) : 82 l-828. 1972. -Reducing the osmolarity of the solution bathing the serosal surface of the isolated toad bladder by one-third (hypotonicity) doubled short-circuit current (SCC) across the tissue (osmotic response). 22Na tracer studies showed that in the steady state this was due to an increase in active transepithelial Na transport. Increasing serosal osmolarity by 20yo halved transepithelial Na transport. Hypotonically induced alterations in cell volume and electrolyte concentration were complete well before the SCC was maximal, indicating that the increase did not result from a direct effect of altered environment upon the transport mechanism itself. Hypotonicity reduced epithelial cell Na concentration by ~25%. If this reduction occurs in the Na pool participating in the osmotic response, then, assuming the twobarrier transport model, hypotonicity must lead to a stimulation of the basally located Na pump which is not a result of increased availability of Na. Hypotonicity caused an ouabain-inhibitable increase in respiration which rose faster than the SCC. Insofar as the early rise in SCC represents a rise in active Na transport, the data suggest that hypotonicity acts by increasing energy metabolism with a resultant activation of the transport mechanism via increased supply of high-energy substrate.