Urea transport in the toad bladder; coupling of urea flows

Role of solute drag in intestinal transport

This study presents experiments related to the role of solvent drag and solute drag in the transmembrane movement of nonelectrolytes in a perfused rat intestine preparation. Conditions were chosen to simulate the effects of luminal hyperosmolarity on the permeability of tracer solutes. Data are presented on net water flux, transepithelial potentials, and lumen-to-blood and blood-to-lumen tracer solute movements during control electrolyte perfusion and after making the perfusate hyperosmotic. The results indicate that both solvent drag and solute drag can play significant roles in the transepithelial movement of solute and solute permeabilities in the rat ileum preparation. It is suggested that the potential roles of solvent drag and solute drag should be accounted for or considered during the characterization of the mechanisms of biological membrane function.

The sensitivity of apical Na+ permeability in frog skin to hypertonic stress

Na+ transport across abdominal skins of the frog species Rana esculenta and Rana pipiens was analyzed by recording short-circuit current (Isc), transepithelial conductance (Gt), and the current noise generated by the random blockage of apical Na+ channels by the diuretic, amiloride. Specific Na+ current (INa) and conductance (GNa), as reflected by the amiloride-sensitive part of Isc and Gt, respectively, were markedly depressed after addition of some osmotically active substances, like sugars or alcohols to the mucosal Na+-Ringer solution. These hypertonicity-induced reactions were fast and fully reversible, even at mucosal osmolarities of 1 Osmol. With mucosal solutions of moderate hyperosmolarity a recovery of INa and GNa was observed in presence of the osmotic gradient. This "regulatory" current showed to be carried by Na+ through the Na+-specific apical channels. Contrary to the fast current drop during the initial phase of hyperosmotic shocks, the "osmoregulation" was considerably slower. The recovery of INa was only complete at smaller osmotic gradients but became more and more suppressed at higher osmolarities. Steady-state analysis of the kinetics of the Na+-specific current revealed that the current depression by osmotic shocks obeys Michaelis-Menten kinetics. This current depression at high osmolarities, as well as during the initial phase before "osmoregulation" with small osmotic gradients, can be described in terms of a non-competitive inhibition. This was also suggested by Na+-concentration jump experiments indicating a reduction of the maximal, apical Na+ permeability as mechanism of the hypertonicity-induced drop in INa. The INa kinetics after complete "osmoregulation" were, however, indistinguishable from the isotonic control condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Serosal and mucosal facilitated transport of urea in urinary bladder of of Bufo bufo: evidence for an alleged water uptake

In Bufo bufo urinary bladder an urea facilitated transport has been localised on the luminal membrane. The transport fulfils the criteria for such a mechanism, i.e. is saturable and is inhibited by phloretin, a specific inhibitor for urea transport. Similarly to that of Bufo marinus and Rana esculenta the luminal membrane of Bufo bufo urinary bladder shows an ADH stimulated facilitated transport. Experiments wtih Amphotericin B, serosal phloretin (with and without ADH), have demonstrated the presence of a facilitated urea transport localised on basolateral membrane. Urea uptake on the isolated epithelial cells of Bufo bufo urinary bladder shows a characteristic feature, different from molecules passively transported such as glycerol yet inhibited by phloretin. Allegedly with urea, water flows in to the cells by a dragging or osmotic effect.

Concentration-dependence of nonelectrolyte permeability of toad bladder

A theoretical formulation was derived for the dependence of bulk solute permeability, P, defined as net flux divided by concentration gradient, delta c, across any membrane in which solute concentration is controlling for net flux, delta J. According to this formulation, delta J is stimulated by increments in trans concentration, c2, in the range c2/c1 equals 0.0--0.1. Net flux of urea across toad bladder down concentration gradients was shown to be stimulated threefold by small increments in trans urea concentration. The theory also predicts that, in the absence of concentration gradients, tracer permeability, P, defined as tracer flux divided by tracer concentration, will be independent of c provided that P equals P, but will diminish with increasing c if P/P less than 1.P/P was not significantly different from unity for urea, and both P and P were independent of c in the absence of concentration gradients. However, P/P was significantly less than unity (0.90 and 0.85) for thiourea and mannitol, respectively. In conformity with theory, P (and also P) of these two solutes, measured as c was increased by 3--4 orders of magnitude, diminished progressively. These effects are more consistent with this formulation than with transport via a saturable carrier.

Influence of membrane heterogeneity on kinetics of nonelectrolyte tracer flows

In a composite membrane with heterogeneous channels, prevention of net volume flow with hydrostatic pressure differences and/or impermeant osmotic solutes may induce positive isotope interaction (coupling of isotope flows) consequent to circulation of volume flow. The permeability coefficient for net flow will then exceen the tracer permeability coefficient. A permeant osmotic solute will induce either positive or negative isotope interaction, according to whether membrane heterogeneity is more marked for the test solute or the osmotic solute, respectively. Thus membrane heterogeneity may account for phenomena commonly attributed to "single file diffusion". For sufficiently small flows the general flux ratio relationship for homogeneous membranes will continue to apply.

Amide transport channels across toad urinary bladder

Urea and other small amides cross the toad urinary bladder by a vasopressinsensitive pathway which is independent of osmotic water flow. Amide transport has characteristics of facilitated transport: saturation, mutual inhibition between amides, and selective depression by agents such as phloretin. The present studies were designed to distinguish among several types of transport including (1) movement through a fixed selective membrane channel and (2) movement via a mobile carrier. The former would be characterized by co-transport (acceleration of labeled amide flow in the direction of net flow of unlabeled amide), the latter by counter-transport (acceleration of labeled amide flow in the opposite direction). Mucosal to serosal (M leads to S) and serosal to mucosal (S leads to M) permeabilities of labeled amides were determined in paired bladders. Unlabeled methylurea, a particularly potent inhibitor of amide movement, was added to either the M or S bath, while osmotic water flow was eliminated by addition of ethylene glycol and ethanol could not be demonstrated. Methylurea did not alter water permeability or transmembrane electrical resistance. The demonstration of co-transport is consistent with the presence of ADH-sensitive amide-selective channels rather than a mobile carrier.

Solute flux coupling in a homopore membrane

Our previous studies on solute drag on frog skin and synthetic heteropore membranes have been extended to a synthetic homopore membrane. The 150-A radius pores of this membrane are formed by irradiation and etching of polycarbonate films. The membrane is 6-microm thick and it has 6 x 10(8) pores cm(-2). In this study, sucrose has been used as the driver solute with bulk flow blocked by hydrostatic pressure. As before on heteroporous membranes, the transmembrane asymmetry of tracer solute is dependent on the concentration of the driver solute. Tracer sucrose shows no solute drag while maltotriose shows appreciable solute drag at 1.5 M sucrose. With tracer inulin and dextran, solute drag is detectable at 0.5 M sucrose. These results are in keeping with the previous findings on heteropore membranes. Transmembrane solute drag is the result of kinetic and frictional interaction of the driver and tracer solutes as the driver flows down its concentration gradient. The magnitude of the tracer flux asymmetry is also dependent on the size of the transmembrane pores.