THE ROLE OF CALCIUM IN UROMODULIN EXPRESSION AND SECRETION FROM RENAL MEDULLARY EPITHELIAL CELLS OF HYPERTENSIVE AND NORMOTENSIVE RATS

Abstract

Objective: Uromodulin (UMOD) is the most abundantly secreted protein found within the urine, primarily produced by medullary thick ascending limb (mTAL) epithelial cells of the kidneys. There is accruing genetic evidence implicating UMOD in blood pressure regulation and consequently hypertension. The aim of this study was to investigate the potential role of extracellular calcium and the calcium-sensing receptor (CaSR) on UMOD production and secretion in TAL cells with the hope of defining novel clinical targets for the treatment of hypertension. Design and method: mTAL tubules from Wistar-Kyoto (WKY) and stroke-prone spontaneously hypertensive (SHRSP) female rats were incubated with calcium, nifedipine (calcium channel blocker), CaSR agonist (spermine) and antagonist (NPS2143). UMOD protein levels were detected by Western blot. Gene expression of Umod was determined by qRT-PCR. Results: Calcium increased mTAL UMOD secretion in WKY and SHRSP. Nifedipine slightly decreased UMOD secretion in WKY without calcium. In both strains, NPS2143 increased calcium-induced UMOD secretion, with an enhanced effect in SHRSP. Stimulation of CaSR with spermine decreased UMOD secretion in WKY. Analysis of intracellular UMOD levels in these conditions demonstrated increased accumulation when extracellular secretion was low, and vice versa. Incubation of primary mTAL cells with calcium confirmed increased localisation of UMOD at the membrane compared to cytosol. The Umod mRNA level changes were not statistically significant among conditions. Conclusions: Trafficking of UMOD is influenced by the CaSR ligand and the biased nature of G-protein coupled CaSR signalling. Unravelling the signalling events post-calcium will be necessary for identification of key regulators of UMOD secretion.