Characterization of the PGE(2) receptor subtypes and the IP receptor in the rat medullary thick ascending limb.

Abstract

PGE$\sb2$ attenuates AVP-dependent solute transport in the medullary thick ascending limb of Henle (mTAL) and like the PGI$\sb2$ I-prostanoid (IP) receptor inhibits AVP-dependent water flow in the collecting duct. PGI$\sb2$ may play a role similar to PGE$\sb2$ in the mTAL. We therefore further characterize E-prostanoid (EP) receptor subtypes and the IP receptor in a freshly isolated suspension of rat mTAL. Using 10 nM PGE$\sb2,$ we confirmed previous work showing a Gi coupled EP$\sb3$ receptor subtype in rat mTAL that attenuates AVP-dependent cAMP by 70.7%. Pertussis toxin reversed this inhibitory response by 73.2%. In the absence of AVP, micromolar concentrations of PGE$\sb2$ and the EP$\sb2$ receptor subtype specific agonist butaprost produced significant elevations in cAMP levels, indicating the existence of the G$\rm\sb{s}$-coupled EP$\sb2$ receptor subtype. This response was insensitive to the EP$\sb4$ receptor subtype antagonist AH-23848B. Using the PGI$\sb2$ analogs iloprost (ILP) and cicaprost (CCP), we have demonstrated the presence of a Gi-coupled PGI$\sb2$ receptor. AVP-dependent cAMP was reduced 80% and 68.9% by 0.1uM ILP and 1 uM CCP respectively. Pertussis toxin reversed these inhibitory actions by 73%. Neither the EP$\sb3$ receptor subtype nor IP receptor inhibitory responses on AVP-dependent cAMP were sensitive to the EP$\sb1$ receptor subtype specific antagonist AH-6809 and the protein kinase C (PKC) inhibitors bisindolylmaleimide 1 and calphostin C. By associating in situ hybridization for receptor mRNA with TAL-specific Tamm-Horsfall glycoprotein immunostaining on serial sections from rat kidney, we have clearly shown EP$\sb3$ receptor subtype and IP receptor expression in the rat mTAL. These results suggest that a Gi-coupled IP receptor and the EP$\sb3$ receptor subtype play a role in attenuating the AVP-dependent cAMP in the rat mTAL.