UREA TRANSPORT IN TOAD SKIN (BUFO-MARINUS)

Urea is transported from mucosa to serosa across the skin of the stenohaline toad, Bufo marinus, studied under short circuit current (SCC) conditions. Mucosal to serosal transepithelial urea transport (J(m-->s)urea) was markedly and asymmetrically enhanced in toads adapted to hypertonic (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-mediated transport mechanism. J(m-->s)urea in the skin of 150 mM NaCl-adapted toads was characterized with drugs known to affect transepithelial urea transport (J(urea)) in the urinary bladder of this species. Amiloride (10(-8)-10(-3) M) inhibited J(m--s)urea in a dose-dependent fashion, 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 inhibition of active sodium transport) by 70 and 67%, respectively and vasopressin (10(-8) M) had no effect on J(m-->s)urea, but stimulated SCC 179% above base line. The pyrazinoyl amiloride analog, 2-pyrazinoylguanidine (10(-4) M), reported to inhibit urea transport in mammals, also had 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. These results indicate that hypertonicity stimulates an amiloride-sensitive, phloretin-insensitive, cation-independent, J(urea) mechanism in the skin of B. marinus which is entirely distinguished from the J(urea) mechanism in the urinary bladder of this species. The inhibitory effect of amiloride on J(m-->s)urea may reflect an interaction of its aminopyrazine structure with the urea transporter.