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J Pharmacol Exp Ther. 1982 Oct;223(1):25-9.

Nature of the inhibition of Cl- transport by furosemide: evidence for competitive inhibition of active transport in toad cornea.


Furosemide inhibits short-circuit current (active Cl- transport) in isolated toad cornea, a model of the thick ascending limb of the loop of Henle. The relationship between furosemide and Cl- was assessed in corneas exposed to different concentrations of Cl-. Furosemide (2.2 x 10(-5) M) or drug vehicle was added to both sides of the corneas. Double-reciprocal plots of short-circuit current vs. Cl- concentration with or without furosemide revealed an interaction between furosemide and Cl- that appeared to be competitive in nature. That is, the 1/short-circuit current x 10(3) intercepts (short-circuit currentmax) of the lines were not significantly different (54 with furosemide and 40 for the control, P = .765), whereas the slopes of the lines were significantly different (7.9 with furosemide and 2.1 for the control, P = .037). Km values calculated from the slopes and intercepts with and without furosemide were 145 and 53 mM, respectively. The relationship between external Cl- and furosemide was unique in that a similar competitive relationship was not apparent for either external Na+ and furosemide or external Cl- and ouabain (another inhibitor of Cl- transport in amphibian cornea). These findings support the hypothesis that furosemide inhibits active Cl- transport by competitively blocking the access of Cl- to some component of the active transport mechanism, possibly the Na+-Cl- cotransport mechanism.

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