SUMMARY Dietary potassium deficiency, common in Western diets, raises blood pressure and enhances salt sensitivity. Potassium homeostasis requires a molecular switch in the distal convoluted tubule (DCT), which fails in familial hyperkalemic hypertension (pseudohypoaldosteronism type 2), activating the thiazide-sensitive NaCl cotransporter, NCC. Here, we show that dietary potassium deficiency activates NCC, even in the setting of high salt intake, thereby causing sodium retention and a rise in blood pressure. The effect is dependent on plasma potassium, which modulates DCT cell membrane voltage and, in turn, intracellular chloride. Low intracellular chloride stimulates WNK kinases to activate NCC, limiting potassium losses, even at the expense of increased blood pressure. These data show that DCT cells, like adrenal cells, sense potassium via membrane voltage. In the DCT, hyperpolarization activates NCC via WNK kinases, whereas in the adrenal gland, it inhibits aldosterone secretion. These effects work in concert to maintain potassium homeostasis.
brush border microvilli ͉ cytosketal reorganization ͉ fluid shear stress ͉ proximal tubule epithelium ͉ tight junction
In the proximal tubule of the kidney, Na(+) and HCO(3)(-) reabsorption vary proportionally with changes in axial flow rate. This feature is a critical component of glomerulotubular balance, but the basic mechanism by which the tubule epithelial cells sense axial flow remains unexplained. We propose that the microvilli, which constitute the brush border, are physically suitable to act as a mechanosensor of fluid flow. To examine this hypothesis quantitatively, we have developed an elastohydrodynamic model to predict the forces and torques along each microvillus and its resulting elastic bending deformation. This model indicates that: 1) the spacing of the microvilli is so dense that there is virtually no axial velocity within the brush border and that drag forces on the microvilli are at least 200 times greater than the shear force on the cell's apical membrane at the base of the microvilli; 2) of the total drag on a 2.5-microm microvillus, 74% appears within 0.2 microm from the tip; and 3) assuming that the structural strength of the microvillus derives from its axial actin filaments, then a luminal fluid flow of 30 nl/min produces a deflection of the microvillus tip which varies from about 1 to 5% of its 90-nm diameter, depending on the microvilli length. The microvilli thus appear as a set of stiff bristles, in a configuration in which changes in drag will produce maximal torque.
Normal variations in glomerular filtration induce proportional changes in proximal tubule Na ؉ reabsorption. This ''glomerulotubular balance'' derives from flow dependence of Na ؉ uptake across luminal cell membranes; however, the underlying physical mechanism is unknown. Our hypothesis is that flow-dependent reabsorption is an autoregulatory mechanism that is independent of neural and hormonal systems. It is signaled by the hydrodynamic torque (bending moment) on epithelial microvilli. Such signals need to be transmitted to the terminal web to modulate Na ؉ -H ؉ -exchange activity. To investigate this hypothesis, we examined Na ؉ transport and tubular diameter in response to different flow rates during the microperfusion of isolated S2 proximal tubules from mouse kidneys. The data were analyzed by using a mathematical model to estimate the microvillous torque as function of flow. In this model, increases in luminal diameter have the effect of blunting the impact of flow velocity on microvillous shear stress and, thus, microvillous torque. We found that variations in microvillous torque produce nearly identical fractional changes in Na ؉ reabsorption. Furthermore, the flow-dependent Na ؉ transport is increased by increasing luminal fluid viscosity, diminished in Na ؉ -H ؉ exchanger isoform 3 knockout mice, and abolished by nontoxic disruption of the actin cytoskeleton. These data support our hypothesis that the ''brush-border'' microvilli serve a mechanosensory function in which fluid dynamic torque is transmitted to the actin cytoskeleton and modulates Na ؉ absorption in kidney proximal tubules. glomerulotubular balance ͉ flow-dependent transport ͉ Na ϩ -H ϩ exchange
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