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.
Subject(s)
Kidney Tubules, Collecting/physiology , Potassium Channels, Inwardly Rectifying , Sodium Channels/metabolism , Amiloride/pharmacology , Animals , Biological Transport/physiology , Diuretics/pharmacology , Epithelial Sodium Channels , Gene Expression/physiology , Hydrostatic Pressure , Membrane Potentials/drug effects , Membrane Potentials/physiology , Nephrons/physiology , Oocytes/physiology , Patch-Clamp Techniques , Potassium/metabolism , Potassium Channels/metabolism , Rabbits , Sodium/metabolism , Sodium Channels/genetics , Stress, Mechanical , Transfection , Xenopus laevisABSTRACT
K+ secretion by the cortical collecting duct (CCD) is stimulated at high flow rates. Patch-clamp analysis has identified a small-conductance secretory K+ (SK) and a high-conductance Ca(2+)-activated K+ (maxi-K) channel in the apical membrane of the CCD. The SK channel, encoded by ROMK, is believed to mediate baseline K+ secretion. The role of the stretch- and Ca2+-activated maxi-K channel is still uncertain. The purpose of this study was to identify the K+ channel mediating flow-dependent K+ secretion in the CCD. Segments isolated from New Zealand White rabbits were microperfused in the absence and presence of luminal tetraethylammonium (TEA) or charybdotoxin, both inhibitors of maxi-K but not SK channels, or apamin, an inhibitor of small-conductance maxi-K+ channels. Net K+ secretion and Na+ absorption were measured at varying flow rates. In the absence of TEA, net K+ secretion increased from 8.3 +/- 1.0 to 23.4 +/- 4.7 pmol. min(-1). mm(-1) (P < 0.03) as the tubular flow rate was increased from 0.5 to 6 nl. min(-1). mm(-1). Flow stimulation of net K+ secretion was blocked by luminal TEA (8.2 +/- 1.2 vs. 9.9 +/- 2.7 pmol. min(-1). mm(-1) at 0.6 and 6 nl. min(-1). mm(-1) flow rates, respectively) or charybdotoxin (6.8 +/- 1.6 vs. 8.3 +/- 1.6 pmol. min(-1). mm(-1) at 1 and 4 nl. min(-1). mm(-1) flow rates, respectively) but not by apamin. These results suggest that flow-dependent K+ secretion is mediated by a maxi-K channel, whereas baseline K+ secretion occurs through a TEA- and charybdotoxin-insensitive SK (ROMK) channel.
