ADH-sensitive cAMP system in papillary collecting duct: Effect of osmolality and PGE₂

The papillary collecting duct (PCD) is considered to be of major importance in the final elaboration of the urine, but the metabolism of cyclic adenosine 3',5'-monophosphate (cAMP) has not yet been directly studied in the PCD. Therefore, in the present study the authors examined the basic properties of the cAMP system in isolated PCD microdissected from rat kidney. Vasopressin (VP) caused a marked (5- to 10-fold) stimulation of adenylate cyclase (AdC) but parathyroid hormone, calcitonin, isoproterenol, and bradykinin were without effect. A gradual increase in osmolality from 200 mosM had a biphasic effect on AdC, first enhancing (at 800 mosM) then inhibiting AdC activity at 2,000 mosM. cAMP-phosphodiesterase activity was inhibited as osmolality was increased from 200 to 800 mosM and the inhibition remained constant to 2,000 mosM. Incubation of intact PCD with VP resulted in a threefold increase in cAMP levels. As the osmolality of the incubation medium was increased from 300 to 2,000 mosM, both basal and VP-stimulated cAMP levels continued to increase. Prostaglandin E₂ (PGE₂) (10^-5 M) alone (in the absence of VP) caused an increase in AdC activity, but the same dose of PGE₂ had no effect on AdC activity stimulated by submaximal or maximal doses of VP. PGE₂ (10^-5 M) caused a small increase in cAMP levels in intact PCD. On the other hand, PGE₂ inhibited VP-stimulated cAMP levels by 50%. Incubation of PCD with PGE₂ had no effect on cAMP-phosphodiesterase activity. The results demonstrate that osmolality in the physiologic range has a major influence on cAMP metabolism in the PCD and document an antagonism between PGE₂ and VP at the level of cAMP accumulation in the PCD.