Mechanism underlying flow stimulation of sodium absorption in the mammalian collecting duct

Vectorial Na(+) absorption across the aldosterone-sensitive distal nephron plays a key role in the regulation of extracellular fluid volume and blood pressure. Within this nephron segment, Na(+) diffuses from the urinary fluid into principal cells through an apical, amiloride-sensitive, epithelial Na(+) channel (ENaC), which is considered to be the rate-limiting step for Na(+) absorption. We have reported that increases in tubular flow rate in microperfused rabbit cortical collecting ducts (CCDs) lead to increases in net Na(+) absorption and that increases in laminar shear stress activate ENaC expressed in oocytes by increasing channel open probability. We therefore examined whether flow stimulates net Na(+) absorption (J(Na)) in CCDs by increasing channel open probability or by increasing the number of channels at the apical membrane. Both baseline and flow-stimulated J(Na) in CCDs were mediated by ENaC, as J(Na) was inhibited by benzamil. Flow-dependent increases in J(Na) were observed following treatment of tubules with reagents that altered membrane trafficking by disrupting microtubules (colchicine) or Golgi (brefeldin A). Furthermore, reducing luminal Ca(2+) concentration ([Ca(2+)]) or chelating intracellular [Ca(2+)] with BAPTA did not prevent the flow-dependent increase in J(Na). Extracellular trypsin has been shown to activate ENaC by increasing channel open probability, and we observed that trypsin significantly enhanced J(Na) when tubules were perfused at a slow flow rate. However, trypsin did not further enhance J(Na) in CCDs perfused at fast flow rates. Similarly, the shear-induced increase in benzamil-sensitive J(Na) in oocytes expressing protease resistance ENaC mutants was similar to that of controls. Our results suggest the rise in J(Na) accompanying increases in luminal flow rates reflects an increase in channel open probability.