Regulation of urea permeability in frog urinary bladder by prostaglandin E2

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 Ranatemporaria L. were filled with amphibian Ringer solution containing 370 Bq/ml (0.01 µCi/ml) of [14C]urea, and urea permeability (Purea) 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 Purea [(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 Purea 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). Purea 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 Purea. 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 Purea, 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 Purea 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.