The NADPH oxidase inhibitor diphenyleneiodonium is also a potent inhibitor of cholinesterases and the internal Ca(2+) pump.
OBJECTIVE
Diphenyleneiodonium (DPI) is often used as an NADPH oxidase inhibitor, but is increasingly being found to have unrelated side effects. We investigated its effects on smooth muscle contractions and the related mechanisms.


METHODS
We studied isometric contractions in smooth muscle strips from bovine trachea. Cholinesterase activity was measured using a spectrophotometric assay; internal Ca(2+) pump activity was assessed by Ca(2+) uptake into smooth muscle microsomes.


RESULTS
Contractions to acetylcholine were markedly enhanced by DPI (10(-4) M), whereas those to carbachol (CCh) were not, suggesting a possible inhibition of cholinesterase. DPI markedly suppressed contractions evoked by CCh, KCl and 5-HT, and also unmasked phasic activity in otherwise sustained responses. Direct biochemical assays confirmed that DPI was a potent inhibitor of acetylcholinesterase and butyrylcholinesterase (IC(50) approximately 8 x 10(-6) M and 6 x 10(-7) M, respectively), following a readily reversible, mixed non-competitive type of inhibition. The inhibitory effects of DPI on CCh contractions were not mimicked by another NADPH oxidase inhibitor (apocynin), nor the Src inhibitors PP1 or PP2, ruling out an action through the NADPH oxidase signalling pathway. Several features of the DPI-mediated suppression of agonist-evoked responses (i.e. suppression of peak magnitudes and unmasking of phasic activity) are similar to those of cyclopiazonic acid, an inhibitor of the internal Ca(2+) pump. Direct measurement of microsomal Ca(2+) uptake revealed that DPI modestly inhibits the internal Ca(2+) pump.


CONCLUSIONS
DPI inhibits cholinesterase activity and the internal Ca(2+) pump in tracheal smooth muscle.
Firestone Institute for Respiratory Health, St. Joseph's Hospital, Hamilton, Ontario, Canada.
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T
Tazzeo
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Worek
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Janssen
D000110:Acetylcholinesterase / Q000378:metabolism; D000818:Animals; D001713:Biphenyl Compounds / Q000494:pharmacology*; D002091:Butyrylcholinesterase / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D002800:Cholinesterase Inhibitors / Q000494:pharmacology*; D005260:Female; D066298:In Vitro Techniques; D016623:Ion Pumps / Q000037:antagonists & inhibitors*; D007537:Isometric Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects* / Q000502:physiology; D019255:NADPH Oxidases / Q000037:antagonists & inhibitors*; D009861:Onium Compounds / Q000494:pharmacology*; D014132:Trachea / Q000187:drug effects / Q000502:physiology
D001713:Biphenyl Compounds; D002800:Cholinesterase Inhibitors; D016623:Ion Pumps; D009861:Onium Compounds; C031291:diphenyliodonium; D019255:NADPH Oxidases; D000110:Acetylcholinesterase; D002091:Butyrylcholinesterase; D002118:Calcium