Inhibition of Urea-Linked Water Flux and [14C]Urea Transport across the Toad Bladder by Amiloride

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Abstract
Urea movement across the toad bladder was measured with [14C]urea and by determining the amount of water flow obligated to urea movement along a urea concentration gradient from mucosa to serosa. This urea-linked volume flux was measured gravimetrically. Both the urea-linked volume flux and the [14C]urea transport were stimulated by vasopressin and reversibly inhibited by 0.1 mM amiloride by .apprx. 50%. Amiloride had no effect on vasopressin-stimulated osmotic water flux across the bladder wall. Calculations on the amount of water obligated to urea movement indicate that the mucosal urea solution becomes concentrated during its passage through the bladder wall in the presence of vasopressin. This concentrating effect of the permeability barrier is less pronounced in the presence of amiloride. Amiloride blocks specific urea channels in the rate-limiting barrier, channels which are capable of selectively enhancing urea movement and retarding water flow. As amiloride is an inhibitor of Na transport in this tissue, whether amiloride acts on 2 different sets of channels or blocks only one which is shared by both Na and urea was postulated.