Urea is transported from mucosa to serosa across the skin of the steno
haline toad, Bufo marinus, studied under short circuit current (SCC) c
onditions. Mucosal to serosal transepithelial urea transport (J(m-->s)
urea) was markedly and asymmetrically enhanced in toads adapted to hyp
ertonic (150 mM) NaCl and showed saturation kinetics with an estimated
K(d) for urea in the bathing solution of -1 mM and a maximal rate of
J(m-->s)urea = 9.4 nmol . cm-2 . hr-1, consistent with a carrier-media
ted transport mechanism. J(m-->s)urea in the skin of 150 mM NaCl-adapt
ed toads was characterized with drugs known to affect transepithelial
urea transport (J(urea)) in the urinary bladder of this species. Amilo
ride (10(-8)-10(-3) M) inhibited J(m--s)urea in a dose-dependent fashi
on, but with a potency only 1/1000th of that for inhibition of SCC in
the same skins. Phloretin (less-than-or-equal-to 5 x 10(-4) M) had no
effect on J(m-->s)urea or SCC; ouabain (5 x 10(-4) M) and NaCN (10(-3)
M) had no effect on J(m-->s)urea but inhibited SCC (indicating inhibi
tion of active sodium transport) by 70 and 67%, respectively and vasop
ressin (10(-8) M) had no effect on J(m-->s)urea, but stimulated SCC 17
9% above base line. The pyrazinoyl amiloride analog, 2-pyrazinoylguani
dine (10(-4) M), reported to inhibit urea transport in mammals, also h
ad no effect on J(m-->s)urea, but inhibited SCC approximately 30%. A 1
.5 unit pH gradient (m-->s or s-->m) had no effect on J(m-->s)urea. Th
ese results indicate that hypertonicity stimulates an amiloride-sensit
ive, phloretin-insensitive, cation-independent, J(urea) mechanism in t
he skin of B. marinus which is entirely distinguished from the J(urea)
mechanism in the urinary bladder of this species. The inhibitory effe
ct of amiloride on J(m-->s)urea may reflect an interaction of its amin
opyrazine structure with the urea transporter.