Because cytoplasmic cAMP has been reported to be the secondary messenger me
diating K+ transport in marginal cells of freshly isolated stria vascularis
, the possible role of cAMP in ion transport processes of an immortalized m
arginal cell line (MCPV-8) showing evidence of K+ and Na+ reabsorption was
evaluated in this study. Confluent MCPV-8 monolayers were mounted into Ussi
ng chambers and perfused on both sides with perilymph-like Ringer's solutio
n. Transepithelial short-circuit current (I-SC), resistance (R-T) and open-
circuit voltage (V-T) were measured using voltage clamp technique. The foll
owing results were obtained. (1) Addition of forskolin (10(-4) M) to the ba
solateral perfusate increased I-SC to 311 +/- 42%; no significant change in
R-T was observed. Addition of BaCl2 (2 mM) to the apical perfusate at the
maximal response of forskolin blocked 50-60% of I-SC and subsequent additio
n of amiloride (10(-5) M) to the apical perfusate further blocked I-SC to a
value close to 0. (2) To evaluate the effect of cellular cAMP on Ba2+-sens
itive K+ current, amiloride-sensitive Na+ current was blocked first by addi
tion of amiloride (10(-5) M) to the apical perfusate; subsequent addition o
f 3-isobutyl-1-methylxanthine (IBMX, 1 mM) or N-6,2'-O-dibutyryladenosine 3
',5'-cyclic monophosphate (dbcAMP, 1 mM) to the basolateral perfusate incre
ased I-SC to 175 +/- 13% and 411 +/- 32%, respectively. The stimulated Ise
was blocked to close to 0 by addition of BaCl2 (2 mM) to the apical perfusa
te. N-2,2'-O-Dibutyrylguanosine 3',5'-cyclic monophosphate (dbcGMP, 1 mM) h
ad no effect on I-SC. (3) To assess the effect of cellular cAMP on amilorid
e-sensitive Na+ current, Ba2+-sensitive K+ current was blocked in advance b
y addition of BaCl2 to the apical perfusate; subsequent addition of IBMX or
dbcAMP to the basolateral perfusate increased I-SC to 219 +/- 21% and 388
+/- 39%, respectively. The stimulated I-SC was blocked to close to 0 by add
ition of amiloride to the apical perfusate. dbcGMP had no effect on I-SC. H
ence, these results suggest that cellular cAMP is the secondary messenger t
hat mediates the transepithelial transport of both K+ and Na+ in MCPV-8 mon
olayers. (C) 1999 Published by Elsevier Science B.V. All rights reserved.