AJP - Renal Physiology, Vol 245, Issue 6 707-F715, Copyright © 1983 by American Physiological Society
Role of carboxyl group in Na+-entry step at apical membrane of toad urinary bladder
C. S. Park, J. Kipnowski and D. D. Fanestil
Mucosal addition of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)
and some lipid-soluble carbodiimides, agents which are selective for
carboxyl groups, irreversibly inhibited Na+ transport as measured by
short-circuit current (SCC) in the urinary bladder of the toad. The
inhibition of Na+ transport by EEDQ had the following characteristics: 1)
the inhibition was accompanied by a significant increase in the
transepithelial electrical resistance; 2) the decrease in SCC was accounted
for by a comparable decrease in 22Na+ influx without effect on Na+ efflux;
3) amphotericin B produced complete recovery of SCC inhibited with EEDQ but
not with antimycin A or ouabain; 4) mucosal EEDQ decreased the
amiloride-sensitive reversal of Na+ current that is induced by serosal
nystatin in the absence of mucosal Na+; 5) vasopressin and acid mucosal pH
caused an increase in SCC in proportion to the SCC remaining after EEDQ
inhibition; and 6) Vmax of the SCC was decreased without alteration in the
apparent Km for Na+. Based on these characteristics of EEDQ inhibition of
Na+ transport, we infer that a carboxyl group of the Na+ channel is
involved in the Na+-entry step across the apical membrane of "tight"
epithelia. The inhibition of Na+ transport with EEDQ most likely involves
closing the Na+ channel through a chemical reaction involving a carboxyl
group of the channel.
