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NPPB, 5-nitro-2-(3-phenylpropylamino)-benzoic acid, is a potent inhibitor of chloride channel with IC50 of 80 nM for the short circuit current.[1]

Chloride channel blockers possess several sites of interaction, including the negatively charged carboxylate group, the secondary amine group which probably carries a positive partial charge, and for the very potent agents like NPPB an additional negative partial charge at the -NO2 substituent. In addition, an apolar interaction with a cycloaryl residue is necessary, and this site of interaction has a specific spacing from the secondary amino nitrogen.[1]

NPPB was evaluated for the activity on the equivalent short circuit current, corresponding to the secondary active transport of Cl- and measurements of the voltage across the basolateral membrane. The result revealed that NPPB possessed a good potency with IC50 of 80 nM for inhibiting the short circuit current. Furthermore, NPPB was also tested for its activity on various anion channels. Adopting freshly-isolated cells from the rat portal vein, the effects of NPPB were investigated on evoked and spontaneous currents by use of conventional whole-cell recording and perforated-patch techniques. At a holding potential of -60 mV in potassium-free, caesium-containing solutions, NPPB (10 μM) inhibited Ca-sensitive chloride currents (ICI(Ca)) evoked by caffeine (10 mM) and by noradrenaline (10μM) by the extend of 58% and 96%, respectively. In addition, at a holding potential of -2 mV in potassium -containing solutions, NPPB (10 μM) inhibited charybdotoxin-sensitive potassium-currents (IBK(Ca)) induced by noradrenaline (10 μM) and acetylcholine (10 μM) by approximately 90%. NPPB's inhibitory effects of volume-activated taurine, glucose, and uridine influxes was studied. The IC50 for the inhibition of the volume- activated fluxes by NPPB was around 12 μM. [1-3]

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