Quercetin enhances water transport in toad bladder

Amiloride inhibits the vasopressin-induced increase in epithelial water permeability

The vasopressin (VP)-induced increase in water permeability in high-resistance, amphibian epithelia is not altered by the abolition of net Na+ flux caused by amiloride added to the apical bathing medium. In this work we looked at the effects on water transport of amiloride added to the serosal medium at a concentration (10(-3) M) known to inhibit Na+/H+ exchange. In urinary bladders of Bufo marinus, amiloride partially blocked the hydrosmotic response to VP. A similar inhibition was found with cyclic adenosine 5'-monophosphate (cAMP) or serosal hypertonicity. We hypothesized that this effect of amiloride could be due to an inhibition of Na+/H+ and/or Na+/Ca2+ antiporters present in the epithelial basolateral membrane and looked at the effects of the diuretic in Na(+)-free media. A similar degree of inhibition of water flow was still found, thus showing that amiloride acts on a cell target other than the antiporters. In toad skin, amiloride did not inhibit the hydrosmotic response to VP and to isoproterenol; however the response to high K+ was significantly reduced. Among the amiloride cell targets described so far, adenylate cyclase and protein kinase A appear to be the best candidates to explain the inhibition of the hydrosmotic response reported here. Direct measurements of intracellular cAMP are needed however to substantiate this hypothesis.

Forskolin mimics the hydrosmotic action of vasopressin in the urinary bladder of toads Bufo marinus

Net water flow JW was measured across the urinary bladder of toads Bufo marinus and averaged over periods of 1 min by means of a volumetric, automatic technique. The diterpene forskolin, an activator of adenylate cyclase bypassing the hormonal receptor subunit, induced a rapid, reversible, dose-dependent increase in osmotic water permeability, Pf, very similar to that induced by vasopressin. At 1.1 microM, forskolin induced a half-maximal response. At 5 microM forskolin caused a near maximal response and Pf increased from 1.66 +/- 0.15 to 66.6 +/- 2.99 microns s-1. In bladders pre-exposed to 5 microM-forskolin, further significant increases in Pf were obtained by their subsequent exposure to vasopressin, cyclic AMP, theophylline or serosal hypertonicity. The similarity of the forskolin and vasopressin actions was further demonstrated by the finding that substances causing enhancement (quercetin) or inhibition (trifluoperazine, vanadate, silver, cobalt, manganese and Ca2+-free Ringer solution) of the vasopressin response, induced parallel changes in the forskolin response. Three agents, however, induced dissimilar effects on vasopressin and forskolin: high K+ potentiated vasopressin but inhibited forskolin; methohexital and diamide inhibited vasopressin but had no effect on forskolin. The forskolin-induced hydrosmotic response can be viewed as a new criterion for ascertaining the messenger role of cycle AMP in the the hydrosmotic effect of vasopressin.

High [K+] alters the stimulus-hydrosmotic response coupling in toad bladder

Substitution of K+ for Na+ in the Ringer solution bathing the inner surface of toad urinary bladders (Bufo marinus) had no effect on basal water permeability but significantly altered the stimulus-hydrosmotic response of this epithelium. In chloride-Ringer, high [K+] increased the hydrosmotic responses to submaximal stimulations induced by vasopressin or exogenous cAMP, while the responses to theophylline or serosal hypertonicity were decreased. In sulfate-Ringer, all these responses were enhanced but for that induced by serosal hypertonicity which was actually diminished. As a step towards determining if Ca2+ might mediate the K+-induced effects on water flow, experiments were conducted either in the presence of a Ca2+ "antagonist" (cobalt) or in nominally Ca2+-free Ringer. In both conditions the hydrosmotic effects of vasopressin and cAMP were markedly reduced. The results raise the possibility that a transient Ca2+ influx through voltage-sensitive, Co2+-blockade Ca2+ channels may play a role in the stimulus-hydrosmotic response of toad urinary bladder.

Osmotic water flow across the abdominal skin of the toad bufo marinus: effect of vasopressin and isoprenaline

1. Net water flow J(w), was measured across the abdominal skin of the toad Bufo marinus with a volumetric, automatic technique that allows for averaging J(w) over time intervals as short as 1 sec.2. Basal J(w) was very stable and corresponded to a coefficient of osmotic flow, L(PD), of ca. 15 x 10(-7) cm sec(-1) atm(-1) (or to an osmotic water permeability coefficient, P(f), of 20 mum sec(-1)).3. Both vasopressin and the beta-adrenergic agonist, isoprenaline, triggered high hydrosmotic responses that could lead to P(f) values exceeding 250 mum sec(-1). The effect of isoprenaline was very reproducible while that of vasopressin varied considerably.4. Methohexital and propranolol selectively inhibited the hydrosmotic effects of vasopressin and isoprenaline, respectively, whereas amiloride and ouabain had no effect.5. Mutual inhibition was found between vasopressin and isoprenaline in skins very sensitive to vasopressin. In less sensitive skins isoprenaline further increased J(w) despite exposure of the epithelia to supramaximal concentrations of vasopressin.6. Differential reactivity to vasopressin was found between the skin and the bladder taken from the same toad. In some instances, the bladder responded normally to vasopressin while the skin was totally unresponsive, suggesting the presence of osmoregulatory mechanisms exerting a local modulation of the vasopressin action in different target epithelia of the same animal.