Epithelial Na⁺ channels are regulated by flow

Abstract

Na(+) absorption in the renal cortical collecting duct (CCD) is mediated by apical epithelial Na(+) channels (ENaCs). The CCD is subject to continuous variations in intraluminal flow rate that we speculate alters hydrostatic pressure, membrane stretch, and shear stress. Although ENaCs share limited sequence homology with putative mechanosensitive ion channels in Caenorhabditis elegans, controversy exists as to whether ENaCs are regulated by biomechanical forces. We examined the effect of varying the rate of fluid flow on whole cell Na(+) currents (I(Na)) in oocytes expressing mouse alpha,beta,gamma-ENaC (mENaC) and on net Na(+) absorption in microperfused rabbit CCDs. Oocytes injected with mENaC but not water responded to the initiation of superfusate flow (to 4-6 ml/min) with a reversible threefold stimulation of I(Na) without a change in reversal potential. The increase in I(Na) was variable among oocytes. CCDs responded to a threefold increase in rate of luminal flow with a twofold increase in the rate of net Na(+) absorption. An increase in luminal viscosity achieved by addition of 5% dextran to the luminal perfusate did not alter the rate of net Na(+) absorption, suggesting that shear stress does not influence Na(+) transport in the CCD. In sum, our data suggest that flow stimulation of ENaC activity and Na(+) absorption is mediated by an increase in hydrostatic pressure and/or membrane stretch. We propose that intraluminal flow rate may be an important regulator of channel activity in the CCD.

Mechanosensory function of microvilli of the kidney proximal tubule

“…Accordingly, we evaluated whether Na ϩ absorption is altered by increasing the viscosity of the tubular perfusion solution. The flow-dependent transport activities were examined at the same perfusion rates of 5, 10, 15, 20, and 25 nl͞min as the control, and viscosity increased 3.6-fold by addition of dextran-80 (0.075g͞dl) to the tubule perfusate (21). Changing the viscosity did not change the maximal absorption rate, yet the absorption rate increased significantly at perfusion rates of 5, 10, 15, and 20 nl͞min, and the flow-dependent transport curve was shifted to the left.…”

Activation of the Caenorhabditis elegans Degenerin Channel by Shear Stress Requires the MEC-10 Subunit

“…For example, ENaC is activated by shear stress in native kidney tubules, endothelial cells, and heterogeneous expression systems (26,41,42). ENaC gating in response to LSS was altered by mutations introduced at key sites within the large ECD and transmembrane helices (43)(44)(45)(46).…”

Human ADPKD primary cyst epithelial cells with a novel, single codon deletion in the PKD1 gene exhibit defective ciliary polycystin localization and loss of flow-induced Ca2+ signaling

“…Flow sensing has long been thought to contribute to proximal tubular perfusion-absorption balance, to tubuloglomerular feedback (9), to CCD K ϩ secretion (71) and Na ϩ reabsorption (53), and to nitric oxide release by the thick ascending limb of Henle's loop (38) and the inner medullary collecting duct (3). The cilium, with its apparent concentration of receptors and signaling molecules, is an attractive candidate to integrate these signals controlling tubular epithelial cell differentiation and function, perhaps through [Ca 2ϩ ] i -mediated regulation of B-raf (73), regulation of mTOR (54), or other pathways.…”