Vasopressin-prostaglandin interactions in isolated tubules from rat outer medulla

Interactions between AVP and prostaglandins were investigated in MAL and MCT microdissected from the rat outer medulla. Incubation of MCT with ¹⁴C-arachidonic acid resulted in the formation of ¹⁴C-PGE₂ and ¹⁴C-PGF_2α; however, when MAL was incubated under the same conditions, only traces of prostaglandins were formed. Prostaglandin synthesis in MCT was inhibited (-50%) by the prostaglandin cyclo-oxygenase inhibitor ibuprofen (10⁻⁶M). Preincubation with ibuprofen enhanced the stimulation of adenylate cyclase by 5 × 10⁻⁹M AVP in MCT but, in contrast, decreased the stimulation of adenylate cyclase by AVP in MAL. The effects of a second PG cyclo-oxygenase inhibitor naproxen (10⁻⁵M) were similar to those of ibuprofen. Ibuprofen did not influence cAMP phosphodiesterase activity in MCT or in MAL. Exogenous PGE₂ or PGF_2α (10⁻⁶M) had no effect on either basal or AVP-stimulated adenylate cyclase activity in MCT. The present results demonstrate that MCT but not MAL is a site of active synthesis and accumulation of prostaglandin. Although both MAL and MCT have AVP-sensitive adenylate cyclase, incubation with prostaglandin cyclo-oxygenase inhibitors have, in the presence of arachidonic acid, an opposite effect on this enzyme in these two segments, resulting in increased AVP stimulation in MCT and decreased stimulation in MAL. Results also suggest that products of prostaglandin synthesis from arachidonic acid inhibit AVP-sensitive adenylate cyclase activity in MCT but not that located in MAL. Although not totally excluding the primary prostaglandins (PGE₂, PGF_2α), these observations suggest that they are not responsible for AVP modulation in MCT.