Peculiarities of the prostaglandin E2 effect on the urinary bladder osmotic permeability in active and hibernating frogs Rana temporaria

In active summer and hibernating frogs, effects of arginine-vasotocin (AVT) and prostaglandin E-2 (PGE(2)) on osmotic permeability of isolated urinary bladder have been studied. Despite that the initial osmotic permeability level in the summer frogs was higher than in the winter animals, sensitivity to action of AVT used in the 10(-12)-10(-10) M concentration range turned out to be very similar. Both in the winter and in the summer frogs, PGE(2) at a concentration from 10(-9) to 10(-6) M inhibited the AVT-stimulated increase of osmotic permeability; however, in the summer frogs the inhibitory effect of PGE(2) was much lower. The IC25 value in the winter and summer frogs was 1 and 40 nM, respectively. High doses of PGE(2) (10(-7) and 10(-6) M) in the winter frogs practically blocked the AVT action, while in summer their effect was not higher than 35%. In the winter frogs the inhibitory PGE(2) effect was considerably eliminated in the presence of an antagonist of EP1-receptors, SC-19220, while in the summer frogs the effect of this antagonist was absent. In the summer frogs, unlike the winter frogs, PGE2 at 10(-8) and 10(-7) M concentrations did not affect the dibutyryl-cAMP-stimulated rise of the osmotic permeability. The obtained results indicate the existence of a seasonal dependence of sensitivity of urinary bladder to PGE(2) action and allow suggesting that the fluid reabsorption in summer frogs can be increased due to a decrease of the PGE(2)-dependent counteraction to the effect of AVT. It is possible that the low sensitivity of summer frogs to the PGE(2) action is based on reduction of the number of the EP1 type receptors.