Flow-induced prostaglandin E2 release regulates Na and K transport in the collecting duct

Abstract

Fluid shear stress (FSS) is a critical regulator of cation transport in the collecting duct (CD). High-dietary sodium (Na) consumption increases urine flow, Na excretion, and prostaglandin E2 (PGE2) excretion. We hypothesize that increases in FSS elicited by increasing tubular flow rate induce the release of PGE2 from renal epithelial cells into the extracellular compartment and regulate ion transport. Media retrieved from CD cells exposed to physiologic levels of FSS reveal several fold higher concentration of PGE2 compared with static controls. Treatment of CD cells with either cyclooxygenase-1 (COX-1) or COX-2 inhibitors during exposure to FSS limited the increase in PGE2 concentration to an equal extent, suggesting COX-1 and COX-2 contribute equally to FSS-induced PGE2 release. Cytosolic phospholipase A2 (cPLA2), the principal enzyme that generates the COX substrate arachidonic acid, is regulated by mitogen-activated protein-kinase-dependent phosphorylation and intracellular Ca2+ concentration ([Ca2+]i), both signaling processes, of which, are activated by FSS. Inhibition of the ERK and p38 pathways reduced PGE2 release by 53.3 ± 8.4 and 32.6 ± 11.3%, respectively, while antagonizing the JNK pathway had no effect. In addition, chelation of [Ca2+]i limited the FSS-mediated increase in PGE2 concentration by 47.5 ± 7.5% of that observed in untreated sheared cells. Sheared cells expressed greater phospho-cPLA2 protein abundance than static cells; however, COX-2 protein expression was unaffected ( P = 0.064) by FSS. In microperfused CDs, COX inhibition enhanced flow-stimulated Na reabsorption and abolished flow-stimulated potassium (K) secretion, but did not affect ion transport at a slow flow rate, implicating that high tubular flow activates autocrine/paracrine PGE2 release and, in turn, regulates flow-stimulated cation transport. In conclusion, FSS activates cPLA2 to generate PGE2 that regulates flow-mediated Na and K transport in the native CD. We speculate that dietary sodium intake modulates tubular flow rate to regulate paracrine PGE2 release and cation transport in the CD.