Mode of action of furosemide on the chloride-dependent short-circuit current across the ciliary body epithelium of toad eyes

The effects of furosemide on the chloride-dependent short-circuit current across the toad ciliary epithelium were examined. Under control conditions, the short-circuit current obeyed Michaelis-Menten kinetics against medium chloride concentration, the Michaelis constant (K_m) for chloride being 90 mm and the maximal short-circuit current (V_max) 128 μA/cm². Furosemide added to the aqueous side of the epithelium rapidly reduced the short-circuit current; the effect was reversible. The effect of furosemide addition to the stromal side was much smaller and slower than that from the aqueous side. The dose-dependent range of furosemide action was from 0.1 μm to 1 mm with 50% inhibition occurring at about 3 μm. Line-weaver-Burk plot of the short-circuit current against the chloride concentration showed that furosemide decreased the value of V_max and increased the K_m; the inhibition being of mixed type. A Hill plot of the dose-response curve yielding a slope of unity suggested one furosemide molecule combines with one chloride transport site. Probenecid, a competitive inhibitor of organic acid transport, reduced the effects of furosemide significantly when added simultaneously. The involvement of organic acid transport system in the mechanism of furosemide action on chloride transport was suggested.