Regulation of urea permeability in frog urinary bladder by prostaglandin E₂

Abstract.

The present study was performed to investigate the role of prostaglandin E₂ (PGE₂) in the regulation of urea transport in the frog urinary bladder, which is known to occur via a specialized arginine-vasotocin- (AVT-) regulated urea transporter. The bladders isolated from Rana temporaria L. were filled with amphibian Ringer solution containing 370 Bq/ml (0.01 µCi/ml) of [¹⁴C]urea, and urea permeability (P _urea) was determined by sampling the serosal and mucosal bathing medium at 30-min intervals for measurement of radioactivity. It was found that, from the serosal side, PGE₂ (10 nM to 1 µM) caused a dose-dependent increase in P _urea [(7.2±1.8)×10^–6 cm/s in the presence of 0.5 µM PGE₂ versus (1.0±0.2)×10^–6 cm/s in control, n=9, P<0.001]. As in response to AVT, the PGE₂-induced P _urea reached a maximum in 1–1.5 h after the agonist was added. The stimulatory effects of PGE₂ and AVT applied together were not additive. PGE₂-induced urea transport was strongly inhibited by nearly 75% in the presence of mucosal or serosal phloretin (10^–4 M). P _urea was enhanced up to (4.7±0.8)×10^–6 cm/s (n=12, P<0.001) by butaprost (5×10^–6 M), a selective EP₂ receptor agonist, while sulprostone (EP₁/EP₃ agonist, 10^–6 M) caused no changes in P _urea. PGE₂ dose-dependently increased the content of cAMP in mucosal epithelial cells (control: 18.0±1.8; 10^–6 M PGE₂: 74.2±9.3 pmol cAMP/mg protein per 30 min, n=7, P<0.001). Phorbol esters did not alter PGE₂-induced P _urea, whereas H-89 (20 µM), a protein kinase A inhibitor, reduced it by 45.1±9.9% (n=5, P<0.05). PGE₂ did not change the AVT-stimulated P _urea measured in isoosmotic conditions, but inhibited the last one in the presence of a serosa-to-mucosa osmotic gradient. The data obtained show that, in the frog urinary bladder, PGE₂ is a stimulator of phloretin-inhibitable urea transport. Its effect seems to be mediated by EP₂ receptor-coupled generation of intracellular cAMP.