Effect of sodium transport inhibition on active phosphate transport by toad bladder

Summary The relationship between active Na transport (estimated by the short-circuit (SCC)) and active inorganic phosphate (P_i) transport was studied in the toad bladder. When SCC was inhibited by amiloride, ouabaim, or removal of K from the serosal bathing solution, active P_i transport was totally inhibited. When Na was replaced isotonically by choline in either the mucosal bathing solution or both the mucosal and serosal bathing solutions, there was no measurable SCC or active P_i transport. These experiments are compatible with the hypothesis that active P_i transport occurs only in the presence of active Na transport.     

Dobutamine inhibits vasopressin-mediated water transport across toad bladder epithelium

This study aimed to investigate the effect of dobutamine on water transport across toad bladder epithelium. Water flow through the membrane was measured gravimetrically in bladder sac preparations. Dobutamine had no effect on basal water transport, but partially inhibited transport stimulated by vasopressin. Similarly, dobutamine exerted no influence on the hydrosmotic response to 8-chlorophenylthio-cAMP, but interfered with the response to phosphodiesterase inhibitor 1-methyl-3-isobutyl-xanthine. These results demonstrate that this catecholamine may inhibit vasopressin-stimulated water transport at a site prior to cAMP formation. The use of propranolol was ineffective in blocking the effect of dobutamine on transport stimulated by vasopressin, indicating that beta-adrenoceptors play no role in this effect. On the other hand, phentolamine significantly reduced the effect of dobutamine, indicating the involvement of alpha-adrenoceptors in such event. Rauwolscine also inhibited the effect of dobutamine, pointing to the specific contribution of the alpha(2)-adrenoceptors to this effect. Taken together, the results of this study demonstrate that dobutamine inhibits vasopressin-stimulated water transport in toad bladders through a mechanism mediated by the stimulation of alpha(2)-adrenoceptors, thus suggesting that such a drug may exert a direct cellular effect on membrane permeability to water in transporting epithelia. The current study may provide a better understanding of the effects of dobutamine on renal function by contributing towards the elucidation of its action mechanism.     

Effect of colchicine on the osmotic water flow across the toad urinary bladder

Osmotic water movement across the toad urinary bladder in response to both vasopressin and cyclic AMP was inhibited by 10^?5 to 10^?4 M colchicine on the serosal but not on the mucosal side. This inhibitory effect was found to be time- and dose-dependent. Colchicine alone did not change basal osmotic flow and a baseline of the short-circuit current (I_sc) and also did not affect a vasopressin-induced rise of the I_sc. The inhibitory effect was not prevented by the addition of pyruvate. The osmotic water movement produced by 360 mM Urea (mucosal), 360 mM mannitol (serosal) or 2 μg/ml amphotericin B (mucosal), was not affected by 10^?4 M colchicine. These results suggest that colchicine inhibits some biological process subsequent to the formation of cyclic AMP except a directional cytoplasmic streaming process where microtubules may be involved.     

Surface hydrophobicity and water transport of the toad urinary bladder: Effects of vasopressin

Summary The present study investigated whether the hydrophobic properties (wettability) of the luminal surface of the toad urinary bladder might play a role in modulating water transport across this epithelium. In the absence of vasopressin (ADH), water transport across the tissue was low, while luminal surface hydrophobicity (water contact angle) was relatively high. Following stimulation by ADH, water transport increased and surface hydrophobicity decreased. The addition of indomethacin to inhibit ADH-induced prostaglandin synthesis did not reduce these actions of ADH. In an attempt to alter water transport in this tissue, a liposomal suspension of surface-active phospholipids was administered to the luminal surface. This addition had no detectable influence on the low basal rates of water transport, but blocked the ADH-induced stimulation of water transport. We suggest that surface-active phospholipids on the toad bladder luminal membrane may contribute to the hydrophobic characteristics of this tissue. ADH may act to decrease surface hydrophobicity, facilitating the movement of water molecules across an otherwise impermeable epithelium. This surface alteration may be associated with the appearance of water channels in the apical membrane.     

Effect of colchicine on the osmotic water flow across the toad urinary bladder.

Osmotic water movement across the toad urinary bladder in response to both vasopressin and cyclic AMP was inhibited by 10(-5) to 10(-4) M colchicine on the serosal but not on the mucosal side. This inhibitory effect was found to be time- and dose-dependent. Colchicine alone did not change basal osmotic flow and a baseline of the short-circuit current (Isc) and also did not affect a vasopressin-induced rise of the Isc. The inhibitory effect was not prevented by the addition of pyruvate. The osmotic water movement produced by 360 mM Urea (mucosal), 360 mM mannitol (serosal) or 2 mug/ml amphotericin B (mucosal), was not affected by 10(-4) M colchicine. These results suggest that colchicine inhibits some biological process subsequent to the formation of cyclic AMP except a directional cytoplasmic streaming process where microtubules may be involved.     

Effect of distension on ADH-induced osmotic water flow in toad urinary bladder

Summary We recently described a method by which the resistance to water flow of the luminal membrane of ADH-stimulated toad bladder can be quantitatively distinguished from that of barriers lying in series with it. This method requires estimates of both total bladder water permeability (assessed by transbladder osmotic water flow at constant gradient) and luminal membrane water permeability (assessed by quantitation of the frequency of ADH-induced luminal membrane particle aggregates). In the present study we examined the effect of bladder distension on transepithelial osmotic water flow before and during maximal ADH stimulation. Base-line water flow was unaffected by bladder distension, but hormonally stimulated flow increased systematically as bladders became more distended. Distension had no effect on the frequency of ADH-induced intramembranous particle aggregates. By comparing the relationships between aggregate frequency and hormonally induced water permeability in distended and undistended bladders, we found that distension appeared to enhance ADH-stimulated water flow by decreasing the resistance of the series permeability barrier while the apparent water permeability associated with each single luminal membrane aggregate was unaffected. In that bladder distension causes tissue thinning, the series resistance limiting ADH-stimulated water flow appears to be accounted for by deformable barriers within the bladder tissue itself, probably unstirred layers of water.     

Mechanism of inhibition by lithium of sodium transport in the toad bladder

Lithium transport across the urinary bladder of Bufo marinus has been studied by means of the short-circuit current technique, as well as unidirectional ion flux measurements. Exposure to lithium of the epithelial (mucosal) surface of this preparation led to a slow, progressive decrease of ion transport, with increasing discrepancy between short-circuit current and lithium influx; in fact there was still an appreciable lithium influx across bladder exposed to amiloride even though short-circuit current was suppressed. Ohmic conductance and sodium efflux barely increased under these circumstances. Upon replacement of lithium by sodium on the epithelial side, the preparations recovered slowly indeed, and residual lithium could be detected in bladder tissue for more than 2 hr while the rate of sodium extrusion at the basal-lateral cell border was slowed down. Recovery from exposure to lithium was accelerated by vasopressin and amphotericin, both of which facilitate sodium entry at the apical border of the epithelium. Thus the lasting deleterious influence of lithium on sodium transport might result from the fact that this ion, once trapped in the cytoplasm, closes the sodium channels.     

Insulin-stimulated sodium transport in toad urinary bladder

Mammalian and teleost insulins increase active sodium transport by the toad urinary bladder at subnanomolar concentrations. This stimulation is evident within 15 min and persists for hours. Porcine proinsulin and a cross-linked derivative of bovine insulin are less effective than porcine insulin in stimulating the short-circuit current (SCC), indicating the specificity appropriate for activation of sodium transport through an insulin receptor. The initial stimulation by insulin of the SCC is not blocked by pretreatment with actinomycin D, puromycin, cycloheximide, or tunicamycin. However, in the presence of any one of these inhibitors the sustained increase in SCC is blocked and the rise is short-lived, lasting only 45 to 90 min. In amphotericin-treated bladders, the addition of insulin did not further stimulate SCC.     

Effect of carbenoxolone sodium on steroid-induced sodium transport in the toad bladder: Further studies

The licorice derivative, carbenoxolone sodium, is a potent inhibitor of the enzyme 11β-hydroxysteroid dehydrogenase. When this enzyme is suppressed or is absent, endogenous glucocorticoids induce mineralocorticoid-like sodium retention by the kidney. Carbenoxolone sodium administered in vivo to an adrenalectomized rat has also recently been shown to enhance the mineralocorticoid response to submaximal concentrations of aldosterone, deoxycorticosterone (DOC) and 11-dehydrocorticosterone (compound A). In the present studies conducted on the urinary bladder isolated from the Dominican toad, Bufo marinus, a concentration of carbenoxolone sodium shown previously to increase glucocorticoid-induced sodium transport (2.5 × 10⁻⁵ M) did not appear to alter the response to submaximal concentrations of aldosterone 10⁻⁸ M, DOC 10⁻⁷ M, or compound A 10⁻⁵ M. These findings are consistent with the view that in the whole animal carbenoxolone sodium may modify additional steroid metabolic pathways and/or physiological processes in several organs to produce the enhanced renal response to mineralocorticoids and compound A.     

The effect of catecholamines on ion transport in the toad bladder

Noradrenalin (8 · 10⁻⁶ M) and adrenalin (6 · 10⁻⁶ and 6 · 10⁻⁷ M) were found to cause marked stimulation of short-circuit current (S.C.C.) in isolated toad bladder, but isoprenalin (8 · 10⁻⁷ M) was found to be without effect. The percentage rise in S.C.C. due to noradrenalin was found to be inversely proportional to the initial S.C.C. or total conductance of the bladder. Again in the case of noradrenalin the rise in S.C.C. was almost completely abolished by α-adrenergic blockade but not by β-blockade. This rise in S.C.C. was found not to be significantly different from the rise in net Na⁺ flux. Bidirectional Cl⁻ fluxes were estimated using ⁸²Br as a companion radionuclide to ³⁶Cl. No significant net Cl⁻ flux was apparent, either before or after addition of any of the three catecholamines tested. However, in some cases the unidirectional Cl⁻ fluxes rose markedly following addition of noradrenalin or of adrenalin and this change was not reflected in a change in total conductance. This anomaly was noted to occur in bladders whose initial conductance was of the order of 0.5 kΩ⁻¹ · cm⁻² or greater. The evidence presented suggests that two actions of catecholamines on ion transport in toad bladder are (a) to increase Na⁺ transport via stimulation of α-adrenergic sites and (b) at the concentrations tested to cause an increase in passive Cl permeability in bladders whose initial conductance is high.