ION SECRETION AND ISOTONIC TRANSPORT IN FROG-SKIN GLANDS

The aim of this study was to clarify the mechanism of isotonic fluid t ransport in frog skin glands. Stationary ion secretion by the glands w as studied by measuring unidirectional fluxes of Na-24(+), K-42(+), an d carrier-free Cs-134(+) in paired frog skins bathed on both sides wit h Ringer's solution, and with 10(-5) M noradrenaline on the inside and 10(-4) M amiloride on the outside. At transepithelial thermodynamic e quilibrium conditions, the Cs-134(+) flux ratio, J(Cs)(out)/J(Cs)(in), varied in seven pairs of preparations from 6 to 36. Since carrier-fre e Cs-134(+) entering the cells is irreversibly trapped in the cellular compartment (Ussing & Lind, 1996), the transepithelial net flux of Cs -134(+) indicates that a paracellular flow of water is dragging Cs-134 (+) in the direction from the serosal- to outside solution. From the m easured flux ratios it was calculated that the force driving the secre tory flux of Cs+ varied from 30 to 61 mV among preparations. In the sa me experiments unidirectional Na+ fluxes were measured as well, and it was found that also Na+ was subjected to secretion. The ratio of unid irectional Na+ fluxes, however, was significantly smaller than would b e predicted if the two ions were both flowing along the paracellular r oute dragged by the flow of water. This result indicates that Na+ and Cs+ do not take the same pathway through the glands. The flux ratio of unidirectional K+ fluxes indicated active secretion of K+. The time i t takes for steady-state K+ fluxes to be established was significantly longer than that of the simultaneously measured Cs+ fluxes. These res ults allow the conclusion that - in addition to being transported betw een cells - K+ is submitted to active transport along a cellular pathw ay. Based on the recirculation theory, we propose a new model which ac counts for stationary Na+, K+, Cl- and water secretion under thermodyn amic equilibrium conditions. The new features of the model, as compare d to the classical Silva-model for the shark-rectal gland, are: (i) th e sodium pumps in the activated gland transport Na+ into the lateral i ntercellular space only. (ii) A barrier at the level of the basement m embrane prevents the major fraction of Na+ entering the lateral space from returning to the serosal bath. Thus, Na+ is secreted into the out side bath. It has to be assumed then that the Na+ permeability of the basement membrane barrier (P-Na(BM)) is smaller than the Na+ permeabil ity of the junctional membrane (P-Na(JM)), i.e., P-Na(JM)/P-Na(BM) > 1 . The secretory paracellular flow of water further requires that the N af reflection coefficients (sigma(Na)) of the two barriers are governe d by the conditions, sigma(Na)(Bm) > 0, and sigma(Na)(BM) > sigma(Na)( JM). (iii) Na+ channels are located in the apical membrane of the acti vated gland cells, so that a fraction of the Na+ outflux appearing dow nstream the lateral intercellular space is recirculated by the gland c ells. Based on measured unidirectional fluxes, a set of equations is d eveloped from which we estimate the ion fluxes flowing through major p athways during stationary secretion. It is shown that 80% of the sodiu m ions flowing downstream the lateral intercellular space is recycled by the,oland cells. Our calculations also indicate that under the cond itions prevailing in the present experiments 1.8 ATP molecule would be hydrolyzed for every Na+ secreted to the outside bath.