Characterisation of the effects of anthranilic and (indanyloxy) acetic acid derivatives on chloride transport in membrane vesicles

Abstract

The effects of the Cl − channel blockers, NPPB, IAA94/95 and a number of related compounds on 36Cl − transport in membrane vesicles from bovine kidney cortex and rabbit ileum mucosa brush borders have been studied. These vesicles have been previously shown to be enriched in Cl − channel and Cl −/anion cotransport activity, respectively. Chloride transport was assayed in both types of vesicles by measuring the uptake of 36Cl − in response to an outwardly-directed Cl − concentration gradient. In kidney microsomes, a large proportion of the observed 36Cl − uptake was mediated by an electrogenic uniport and could be substantially reduced by clamping the membrane potential at zero mV using K + and valinomycin. Chloride uptake was inhibited by both NPPB and IAA94/95 with apparent IC 50 values of around 10 μM under optimal conditions (i.e., 4 min uptake at 4°C). Under other conditions (e.g., 10 min uptake at 25°C), where uptake had reached a steady-state level, much higher concentrations of inhibitor were required to cause inhibition. Therefore, previous differences in the reported potency of these compounds may, in part, have been due to the conditions under which Cl − uptake was measured. In addition, both NPPB and, to a lesser extent, IAA94/95 were found to have other effects on the vesicles, in that, when added at a concentration of 100 μM, they induced a leakage of pre-accumulated 36Cl −. This was probably caused by either dissipation of membrane potential or damage to the vesicle membranes. The sulphonic acid derivatives of NPPB and IAA94/95 (NPPB-S and ISA94/95, respectively) blocked 36Cl − uptake with around the same potency as NPPB and IAA94/95, but did not cause any non-specific Cl − leakage, when added at concentrations up to 100 μM. Inhibition of 36Cl − uptake by all four compounds was almost completely reversible. However, when vesicles were incubated with the inhibitors in the presence of an outward Cl − concentration gradient, or if vesicles were freeze/thawed in the presence of the compounds, inhibition oculd be only partially reversed. In rabbit brush border membrane vesicles, 36Cl − uptake was not reduced when the vesicles were voltage clamped using valinomycin and K +, and was therefore probably mediated by Cl −/Cl − exchange. However, despite the lack of effect of valinomycin, 36Cl − uptake was inhibited by both NPPB (approx. 80% inhibition at 100 μM) and, to a lesser extent, by IAA94/95 (approx. 30% inhibition at 100 μM). Therefore, although both NPPB and IAA94/95 are able to inhibit electrogenic chloride transport in vesicular systems, they also appear to have additional effects on both the vesicle membranes and on other Cl − transport processes.