Regulation of urea permeability in frog urinary bladder by prostaglandin E(2)

The present study was performed to investigate the role of prostaglandin E(2) (PGE(2)) 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 microCi/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, PGE(2) (10 nM to 1 microM) caused a dose-dependent increase in P(urea) [(7.2+/-1.8)x10(-6) cm/s in the presence of 0.5 microM PGE(2)versus (1.0+/-0.2)x10(-6) cm/s in control, n=9, P<0.001]. As in response to AVT, the PGE(2)-induced P(urea)reached a maximum in 1-1.5 h after the agonist was added. The stimulatory effects of PGE(2) and AVT applied together were not additive. PGE(2)-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)x10(-6) cm/s (n=12, P<0.001) by butaprost (5 x 10(-6) M), a selective EP(2) receptor agonist, while sulprostone (EP(1)/EP(3) agonist, 10(-6) M) caused no changes in P(urea). PGE(2)dose-dependently increased the content of cAMP in mucosal epithelial cells (control: 18.0+/-1.8; 10(-6) M PGE(2): 74.2+/-9.3 pmol cAMP/mg protein per 30 min, n=7, P<0.001). Phorbol esters did not alter PGE(2)-induced P(urea), whereas H-89 (20 microM), a protein kinase A inhibitor, reduced it by 45.1+/-9.9% ( n=5, P<0.05). PGE(2)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, PGE(2)is a stimulator of phloretin-inhibitable urea transport. Its effect seems to be mediated by EP(2) receptor-coupled generation of intracellular cAMP.