Permeability of the Isolated Toad Bladder to Solutes and Its Modification by Vasopressin

Abstract

A B S T R A C T Measurements have been made of the permeability of the isolated urinary bladder of the toad to a number of small solute molecules, in the presence and absence of vasopressin. Vasopressin has a strikingly specific effect on increasing permeability of the bladder to a group of small, uncharged amides and alcohols while penetration by other small molecules and ions is unaffected. The movement of urea is passive, as indicated by equal flux rates in the two directions. The reflection coefficients for chloride and thiourea indicate a high degree of impermeability of the bladder to these solutes even in the presence of large net movements of water. The low concentration of thiourea in the tissue water when this compound is added to the mucosal bathing medium indicates that the major permeability barrier to thiourea is at the mucosal surface of the bladder. The findings can be accounted for by a double permeability barrier consisting of a fine selective diffusion barrier and a porous barrier in series. The former would constitute the permeability barrier to most small, solutes while the latter would be the rate-limiting barrier for water and the amides. It would be the porous barrier which is affected by vasopressin. Reasons are presented which require both barriers to be contained in or near the plasma membrane at the mucosal surface of the bladder. I N T R O D U C T I O NI n the p r e c e d i n g p a p e r (1) are presented studies on the m o v e m e n t of w a t e r t h r o u g h the isolated u r i n a r y b l a d d e r of the toad a n d the influence of m a m m alian antidiuretic h o r m o n e u p o n this process. T h e picture derived f r o m such studies is t h a t of a p o r o u s m e m b r a n e in w h i c h n e u r o h y p o p h y s e a l h o r m o n e accelerates net w a t e r t r a n s p o r t by increasing the size of pores at the m u c o s a l surface. This i n t e r p r e t a t i o n is in a c c o r d with the results a n d views of K o e f o e d -J o h n s e n a n d Ussing (2).

The effect of antidiuretic hormone on solute flows in mammalian collecting tubules

“…In support of this view, PDi for urea and thiourea in these tubules was indistinguishable from zero and unaffected by ADH (Table V), while the ADH-dependent value of PDW is 14.3 X 10' cm sec'. Thus, in agreement with previous observations on anuran epithelia (18,19) and collecting tubules (13), we find no evidence for molecular seiving (5-8) of either urea or sucrose at the luminal interfaces of these tubules.…”

The Mechanism of Potassium Reabsorption in the Proximal Tubule of the Rat*

“…In the present experiments, bulk water reabsorption in the proximal tubule might conceivably result in passive potassium movement against the opposing electrochemical gradient. 4 Solvent drag has been demonstrated during the increased osmotic water flow produced by vasopressin in the anuran skin (29) and toad bladder (30). In these systems, vasopressin not only increases permeability to water and small solutes such as urea, but also augments active sodium transport (31,32).…”

A Saturable, Vasopressin-Sensitive Carrier for Urea and Acetamide in the Toad Bladder Epithelial Cell

“…It has been known for some time that the effect of vasopressin on the movement of nonelectrolytes across the toad bladder is a selective one, and that factors other than molecular size or lipid solubility are involved in the hormone effect (6,8). Coupled with this observation is the evidence that in the dog (3), the protein-depleted sheep and rat (1,2), and in the elasmobranch (4), renal urea reabsorption appears to proceed against a concentration gradient.…”

“…However, attempts to obtain direct evidence for a carrier by demonstrating saturation kinetics for urea transport were unsuccessful. Kempton was unable to show saturation for urea reabsorption by the dogfish kidney at plasma urea concentrations as high as 750 mM (5); Leaf and Hays (6) concluded that competition between 0.1 M unlabeled urea and isotopic urea, and between 0.1 to 0.6 M acetamide and labeled dimethyl formamide or nicotinamide, could not be demonstrated in the toad bladder. 2 The present experiments were carried out at concentrations of unlabeled acetamide from 25 to 150 mM bathing both sides of the bladder.…”

“…While this suggests carrier-mediated transport, efforts to demonstrate a saturable urea carrier in the dogfish (5) and toad bladder (6) We have recently shown that the movement of urea, other amides, thiourea, and formaldehyde across the toad bladder is sharply inhibited by phloretin in the luminal bathing medium, while the movement of water, sodium, ethanol, and ethylene glycol is unaffected (7). This finding has strengthened the case for carrier-mediated transport across the luminal cell membrane.…”