2004 Global Researcher Conference Proceeding
April 09 - 11, 2004
| Conference: | 2004 Global Researcher Conference |
|---|---|
| Title: | Purinergic Control of Medullary Collecting Duct Function: A Novel Vasopressin-independent Regulatory Mechanism |
| Authors: | Kishore, Bellamkonda K.; Nelson, Raoul |
| Institutions: | University of Utah School of Medicine, University of Utah Health Sciences Center |
Arginine vasopressin (AVP) is a potent circulating hormone, which increases osmotic water permeability (Pf) of the collecting duct through cAMP second messenger system. However, it is not the only agent that regulates water transport in the collecting duct. A variety of autacoids and paracrine agents, such as prostaglandin E2 (PGE2) and endothelin acting through their respective receptors and the accompanying phosphoinositide (PI) signaling pathway decrease the Pf of collecting duct. Increased activity of PI signaling pathway brought about by stimulating collecting duct with these agents inhibits the Pf even in the presence of AVP. Thus, in the collecting duct, cAMP and PI systems are mutually opposing signaling pathways. P2Y2 receptor is a G protein-coupled nucleotide receptor, expressed in the medullary collecting duct (Ecelbarger et al, 1994; Kishore et al, 2000). The agonist (ATP/UTP) occupancy of this receptor activates a membrane-bound PI-specific phospholipase C, resulting in enhanced production of inositol triphosphate and the subsequent release of intracellular calcium (Ecelbarger et al, 1994). We demonstrated that agonist activation of P2Y2 receptor in rat medullary collecting duct results in inhibition of AVP-stimulated Pf, by decreasing cellular cAMP levels through protein kinase C-mediated Gi activation (Kishore et al, 1995). Furthermore, since a rise in intracellular calcium, such as the one that occurs following agonist stimulation of P2Y2 receptor, is known to be frequently associated with release of arachidonic acid, the rate-limiting step in the synthesis of PGE2 by cyclooxygenases (COX), we studied the effect of P2Y2 receptor activation on PGE2 production using freshly isolated fractions enriched in rat IMCD. Our results show that in rat IMCD, activation of PI signaling pathway and increased intracellular calcium brought about by the agonist (ATPgS) stimulation of extracellular nucleotide receptor P2Y2 results in enhanced production and release of PGE2 into the medium, in a COX-1-dependent fashion (Welch et al, 2003). PGE2 is known to antagonize the action of AVP on collecting duct water permeability (Nadler et al, 1992), apparently by the activation of AQP2 retrieval from the apical membrane (Zelenina et al, 2000). PGE2 is also known to affect the transport of salt and urea in IMCD (Roman and Lechene, 1981; Rouch and Kudo, 2000). Interestingly, we also observed that in rats subjected to hydration, the expression levels of P2Y2 receptor mRNA and protein are significantly higher in the inner medulla of hydrated rats as compared to the dehydrated rats (Kishore et al, in preparation). To the best of our knowledge, this is the first observation of significant alterations in the expression of a G protein-coupled receptor in the collecting duct in simple conditions of hydration and dehydration. Interestingly, these alterations in the P2Y2 receptor expression are also associated with distinct subcellular redistribution of cytosolic phospholipase A2 (cPLA2) in the IMCD of hydrated and dehydrated rats (work in progress). Cytosolic PLA2 is the major phospholipase in the kidney for the release of arachidonic acid. Taken together, these data indicate that the interaction between the purinergic and prostanoid systems in IMCD expounded by us, represents a novel and complex regulatory mechanism that may open avenues for further research on AVP-independent regulation of IMCD function.
References Cited:
- Ecelbarger CA, Maeda Y, Gibson CC, and Knepper MA. Extracellular ATP increases intracellular calcium in rat terminal collecting duct via a nucleotide rerceptor. Am J Physiol. Renal Fluid Electrolyte Physiol. 267:F998-F1006, 1994
- Kishore BK, Chou C-L, and Knepper MA. Extracellular nucleotide receptor inhibits AVP-stimulated water permeability in inner medullary collecting duct. Am J Physiol. Renal Fluid Electrolyte Physiol. 269:F863-F869, 1995
- Kishore BK, Ginns SM, Krane CM, Nielsen S, and Knepper MA. Cellular localization of P2Y2 receptor in rat inner medulla and lung. Am J Physiol. Renal Physiol. 278:F43-F51, 2000
- Nadler SP, Zimpelmann JA, and Hebert RL. PGE2 inhibits water permeability at a post-cAMP site in rat terminal inner medullary collecting duct. Am J Physiol. Renal Fluid Electrolyte Physiol. 262:F229-F235, 1992
- Roman RJ and Lechene C. Prostaglandin E2 and F2a reduced urea absorption from the rat collecting duct. Am J Physiol. Renal Fluid Electrolyte Physiol. 241:F53-F60, 1981
- Rouch AJ and Kudo LH. Role of PGE2 in a2-induced inhibition of AVP- and cAMP-stimulated H2O, Na+, and urea transport in rat IMCD. Am J Physiol. Renal Physiol. 279:F294-F301, 2000
- Welch BD, Carlson NG, Shi H, Myatt L, and Kishore BK. P2Y2 receptor-stimulated release of prostaglandin E2 by rat inner medullary collecting duct preparations. Am J Physio. Renal Physiol. 285:F711-F721, 2003
- Zelenina M, Christensen BM, Palmer J, Nairn AC, Nielsen S, and Aperia A. Prostaglandin (E2) interaction with AVP: Effects on AQP2 phosphorylation and distribution. Am J Physiol. Renal Physiol. 278:F388-F394, 2000
The antidiuretic hormone, arginine vasopressin (AVP), stimulates an increase in the amount of water flowing through the collecting duct cell. However, it is not the only agent that modulates water flow through these cells. Kishore, et al., found that when a specific extracellular nucleotide receptor, P2Y2, is stimulated, it results in a decrease in the levels of cyclic adenosine monophosphate (cAMP) in the cell, resulting in a decrease in water flow. P2Y2 receptor activation also increases the production and release of prostaglandin E2 (PGE2) in the cell. PGE2 is known to impede AVP’s ability to stimulate increased water flow across the cellular membrane by activating the removal of the water channel AQP2 from the cell membrane.
Kishore’s team also observed that when rats have plenty of water in their system, there are more P2Y2 receptor molecules in their collecting duct cells, and when they are dehydrated, there are less. The team’s work clearly demonstrates that there are ways to regulate inner medullary collecting duct cells that are independent of AVP and the molecular sequence it stimulates.