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

Abstract

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.

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

“…The inhibition of the forskolin response observed in Ca2+-free Ringer solution or after exposure to cobalt (Fig. 7) resembles very much that recently reported with vasopressin (Grosso & De Sousa, 1984). The results obtained with manganese are in agreement with those described by Bentley (1967) in toad bladders challenged with oxytocin, an effect attributed to inhibition of adenylate cyclase.…”

“…In seven paired quarter-bladders, cumulative Jw (60 min) was 53-0 + 684 and 20-7 + 9.47 A1 cm-2 (P < 0-001) depending on whether forskolin was present in the inner or the outer solution, respectively. As previously shown with vasopressin (Grosso & De Sousa, 1984), the forskolin effect was anion sensitive. In fact, there was a significant difference between cumulative Jw (60 min) in chloride Ringer and in sulphate Ringer solutions, the values being 27-7 + 7-74 and 15-0 ± 5-82 ul cm-2, respectively (n = 4, P < 0-05).…”

“…It was recently reported that high K+ enhances the effect of submaximal concentrations of vasopressin in toad bladder (Grosso & De Sousa, 1984). In contrast, high K+ markedly inhibited the effect of forskolin as described in the Results.…”

“…Since forskolin potentiates the effects of a variety of hormones (Seamon & Daly, 1981) we looked at the response to vasopressin in epithelia pre-exposed to very low concentrations of forskolin, barely inducing a stimulation of water flow. As shown in the Table, a Both quercetin and high K+ (Grosso & De Sousa, 1984) enhanced the hydrosmotic effect of vasopressin in toad bladder. Interestingly, quercetin also enhanced the forskolin effect (Fig.…”

“…The results obtained with manganese are in agreement with those described by Bentley (1967) in toad bladders challenged with oxytocin, an effect attributed to inhibition of adenylate cyclase. It should be noted however, that manganese, as well as cobalt, are considered as rather specific blockers of voltage-dependent Ca2+ channels that may play a role in the development of the hydrosmotic response (Grosso & De Sousa, 1984). An interplay between cyclic AMP and Ca2+, analogous to that found in the forskolin-induced hydrosmotic response, has been recently documented in other cell systems where the effect of forskolin was also inhibited either by omission of Ca2+ from the bathing media or by the presence of Ca2+ channel blockers.…”