Synthesis of prostaglandins (PGs), local mediators of salt and water transport in the collecting duct (CD) of the distal nephron, is regulated, in part, by extracellular fluid volume. Extracellular volume expansion promoted by a high Na diet induces local increases in PG synthesis, specifically prostaglandin E2 (PGE2), which is measurable in urine and kidney. PGE2 is a potent inhibitor of Na and water reabsorption in the inner medullary collecting duct (IMCD). Inhibition of the PGE2 synthetic pathway is associated with avid renal Na reabsorption and the development of hypertension, suggesting a critical role for PGE2 in the maintenance of Na balance and blood pressure. In addition, salt-sensitive hypertension is associated with and linked to deficiencies in renal PGE2 synthesis and homeostasis. The physiologic and/or cellular triggers regulating PGE2 production in the distal nephron that maintain precise renal Na homeostasis are unknown. High distal flow rates, as occur in response to water or Na loading, are associated with increases in urinary PGE2 concentration in mice, rodents and humans, which in turn, enhance Na and water excretion. In humans and rodents unilateral nephrectomy also induces increases in distal tubular flow rates and PGE2 production in the solitary kidney, albeit without an alteration in volume status. As expected, inhibitors of PG synthesis reduce urinary Na excretion in these models, suggesting that PGs play an important role post- nephrectomy to maintain Na homeostasis. The common theme among these conditions is that distal tubular flow rate is increased, an observation that leads us to speculate that hydrodynamic forces regulate synthesis of PGE2, and in turn contribute to the final renal regulation of Na balance. Renal tubular epithelial cells respond to hydrodynamic forces associated with increases in urine flow rate, such as laminar shear stress (LSS), with increases in intracellular Ca2+ concentration ([Ca2+]i) which are believed to be transduced by the central cilium, found on the luminal surface of all renal tubular cells, except intercalated cells (though this is controversial). Other investigators have shown that increases in LSS/tubular flow rate regulate nucleotide secretion from renal tubular epithelial cells which, in turn, regulates flow- stimulated [Ca2+]i, suggesting another mechanism by which flow regulates [Ca2+]i. In microperfused cortical CD (CCD), intercalated cells (ICs) release a greater concentration of nucleotides than principal cells (PCs), suggesting the apical cilium is not required for flow-induced nucleotide release. In addition paracrine nucleotide signaling is associated with increased PGE2 production in the renal CD. In conditions of high tubular flow that occur with water loading or lithium ingestion, puringeric signaling and PGE2 production is augmented in CD epithelial isolated from these rodents, suggesting that high tubular flow rates regulate renal purinergic signaling and PGE2 production. However, to date the downstream effects of changes in tubular flow rate (and its hydrodynamic consequences) on intracellular signaling, gene transcription, and protein translation in tubular epithelial cells are largely unknown. Thus, we hypothesize that increases in tubular flow rate trigger nucleotide secretion and purinergic signaling, specifically increasing [Ca2+]i and MAPK activation, in renal tubular epithelia, and that activation of these pathways regulate the synthesis of ptgs-2 mRNA and PGE2 production which influences Na balance. This proposal aims to test this hypothesis by addressing the following specific aims (SAs): SA1: To identify the cellular/molecular mechanisms by which increases in LSS associated with increases in tubular flow rate induce downstream PG synthesis (specifically, PGE2) in vitro in CD cells. SA2: To test whether flow-stimulated transepithelial Na absorption (JNa) is regulated by endogenously produced, flow-stimulated PGE2 synthesis in native CDs isolated from normal and volume expanded mice.
Hypertension is a prevalent medical disorder affecting >30% of the adult U.S. population over the age of 40. It increases a person's risk for kidney disease, stroke, coronary artery disease, congestive heart failure and overall mortality. Reducing blood pressure to normal levels decreases the morbidity and mortality associated with hypertension, but does not bring morbidity and mortality back to control levels. The etiology behind the development of hypertension is unknown, but some investigators have demonstrated that abnormal renal prostaglandin metabolism, which affects Na homeostasis, can lead to Na retention and hypertension. In this research application, we identify a novel physiologic mechanism by which prostaglandin synthesis may be regulated by the kidney, and consequently, renal Na homeostasis. By elucidating the mechanisms by which urine flow rate can activate prostaglandin synthesis in the kidney, we can identify mechanisms which regulate renal Na homeostasis as well as target genes and proteins to prevent the development of hypertension.
