SUMMARY1. Organic anion permeability in ox red blood cell was measured by studying steady-state self-exchange of oxalate, chosen as a prototypical substrate of the organic anion transport system previously described; chloride self-exchange measured the inorganic anion permeability.2. Carbonic anhydrase inhibitors of the sulphonamide class inhibit both organic anion self-exchange (A-/A-) and chloride self-exchange (Cl-/Cl-) although carbonic anhydrase plays no role in these exchanges. These results confirm the conclusions already published that sulphonamides can act directly on the cellular membrane as specific inhibitors of anion transport.3. There is a correlation between the chemical structure of the sulphonamides and their capacity for inhibiting transmembrane anionic exchange. It is of significance that N-sulphamyl substitution, which abolishes the carbonic anhydrase inhibitory potency, does not destroy anionic inhibitory capacity and may even increase it.4. For each sulphonamide the capacities for inhibiting chloride transport and oxalate transport are strictly identical. Inhibition appears non-competitive.5. The temperature sensitivity of oxalate self-exchange is exactly the same as that of chloride self-exchange. From this, and from the nature of their inhibition by sulphonamides, it is proposed that chloride and organic anions share the same transport mechanism.6. In the light of the present results the chloruretic action of sulphonamides in various tissues, in particular the kidney, is discussed.

Summary. Different techniques were investigated in order to determine the Na, K and C1 concentrations of Dunaliella tertiolecta cells adapted to a large range of salinity (20 to 1640 mM NaCI). The K cell concentrations were 6 to 13 times higher than the K concentration of the external medium (11 raM). The The Na and C1 cell concentrations, on the other hand, were lower than in the external medium at all salinities tested. Considerable differences in the absolute values of Na and C1 were, however, found according to the technique employed. These results are interpreted in terms of compartmentalization of the cells (at least two compartments). It is postulated that the larger compartment regulates its ion concentrations, maintaining low Na and C1 and high K concentrations, whereas the second compartment equilibrates with the external medium. The cation permeability of the membrane limiting the regulating compartment is altered by the antibiotics nystatin and monensin. Incubation of cells in K-free medium leads to a decrease of K and to an increase of the cell Na, this effect being reversed by addition of KC1 to the medium. A good correlation is found between gain of K and loss of Na, suggesting a stoichiometric exchange of these two ions. The magnitude of this apparent Na/K exchange increases as the salinity increases. The external K concentration necessary to mediate half-saturation of the Na/K exchange is a function of the NaC1 concentration of the adaptation medium. This Na/K exchange is partially light-dependant and inhibited by cold, cyanide and DCCD. It is suggested that this mechanism helps in the regulation of the ionic composition of Dunaliella cells.

In human erythrocyte, permeability to the anion is instantaneously, reversibly, and noncompetitively inhibited by the nonsteroidal anti-inflammatory drug, niflumic acid. The active form of this powerful inhibitor (I50 = 6 X 10(-7) M) is the ionic form. We demonstrated that: (i) The binding of niflumic acid to the membrane of unsealed ghosts show one saturable and one linear component over the concentration range studied. The saturable component vanishes when chloride transport is fully inhibited by covalently bound 4-acetamido-4'-isothiocyano stilbene-2,2'-disulfonic acid (SITS). Our estimate of these SITS protectable niflumate binding sites (about 9 x 10(5) per cell) agrees with the number of protein molecules per cell in band 3. These sites are half-saturated with 10(-6) M niflumic acid, a concentration very close to I50. (ii) Niflumic acid inhibits the binding reaction of SITS with anion controlling transport sites. These results indicate that niflumic acid and SITS are mutually exclusive inhibitors, suggesting that niflumic acid interacts with the protein in band 3. Niflumic acid also decreases glucose and ouabain-insensitive sodium permeabilities. However, these effects are produced at a very high concentration of niflumic acid (in millimolar range), suggesting unspecific action, possibly through lipid phase.

SUMMARY1. It is well known that red blood cells suspended in isotonic NH4C1 solution swell because penetration of NH3 induces a transmembrane exchange between Cl-0 and OH-, (or HCO3-j). The rate of swelling thus depends on the speed of the transmembrane exchanges and on the amount of anions available for exchange.2. It has been demonstrated in experiments carried out in a C02-free medium that OH-ions are poorly permeating whereas the permeability for HCO3-is very high. Thus the rate of swelling is largely dependent on the intracellular HCO3-concentration. In this context the well-known inhibitory effect of sulphonamides upon swelling can be interpreted, and always has been until now, as being due to the inhibitory action of the drug on the intracellular carbonic anhydrase. However, this inhibitory effect could also result from a direct action of the drug on the transmembrane exchange; it would explain why under conditions of total carbonic anhydrase inhibition we have shown that the inhibition of swelling is far from maximal.3. A direct experimental evidence of such an effect of carbonic anhydrase inhibitors on the transmembrane exchange of Chwith HCO3-was obtained with benzolamide (Cl 11,366), Cl 13,580 and ethoxzolamide. Surprisingly enough, however, acetazolamide (Diamox) does not affect the transmembrane exchange process.4. The inhibitory effect of sulphonamides on HCO3-transport process is discussed in terms of an interaction of the drug with a transport system common to HCO3-and organic anions.