We evaluated effects of psoralens on Cl- secretion (short-circuit curr
ent, I-sc) across T84 monolayers. Methoxsalen failed to increase I-sc.
Several observations suggest that psoralens open cystic fibrosis tran
smembrane conductance regulator Cl- channels. I) After activation of t
he Ca2+- dependent basolateral membrane K+ channel (K-Ca) by 1-ethyl-2
-benzimidazolinone or thapsigargin, methoxsalen (10 mu M) further incr
eased I-sc. 2) When added before carbachol (CCh), methoxsalen potentia
ted the I-sc response to CCh, as predicted, if it increased apical Cl-
conductance. 3) After establishment of a mucosal-to-serosal Cl- gradi
ent and permeabilization of basolateral membrane with nystatin, psoral
ens increased Cl- current, which was inhibited by glibenclamide. In co
ntrast, neither TS-TM calix[4]arene nor Cd2+, inhibitors of outwardly
rectifying Cl- channels and the ClC-2 Cl- channel, respectively, inhib
ited psoralen-induced Cl- current. In contrast to their effects on Cl-
conductance, psoralens failed to significantly affect basolateral mem
brane K+ conductance; subsequent addition of 1-ethyl-2-benzimidazolino
ne induced a large increase in K+ conductance. Also, in excised patche
s, methoxsalen failed to activate K-Ca. In addition to potentiating th
e peak response to CCh, psoralens induced a secondary, sustained respo
nse. Indeed, when added up to 60 min after return of CCh-induced I-sc
to baseline, psoralens induced a sustained I-sc. This sustained respon
se was inhibited by atropine, demonstrating the requirement for contin
uous muscarinic receptor activation by CCh. This sustained response wa
s inhibited also by verapamil, removal of bath Ca2+, and charybdotoxin
. These results suggest that return of I-sc to baseline after CCh stim
ulation is not due to downregulation of Ca2+ influx or K-Ca. Finally,
we obtained similar results with psoralens in rat colon and primary cu
ltures of murine tracheal epithelium. On the basis of these observatio
ns, we conclude that psoralens represent a novel class of Cl- channel
openers that can be used to probe mechanisms underlying Ca2+-mediated
Cl- secretion.