Abstract 1988 Poster Session II, Sunday, 5/2 (poster 178)

Sodium (Na) Absorption in the renal cortical collecting duct (CCD) is mediated by apical amiloride-sensitive ENaCs. ENaC activity is regulated by a variety of intracellular and extracellular factors including hormones, ions, pH, and proteinases. ENaCs share a limited sequence homology to putative mechanosensitive ion channels in C. elegans. Yet, controversy exists as to whether ENaC is stretch-activated, an intriguing possibility for a channel in the mammalian CCD which is exposed in vivo to wide and continuous variations in urinary flow rate. To further investigate this question, we examined the effect of change in "tubular" flow rate and viscosity on net Na absorption (JNa) in isolated microperfused rabbit CCDs and amiloride-sensitive Na current (INa) in Xenopus oocytes expressing α, β, and γ subunits of mouse ENaC. In the CCD, a ∼ 10-fold increase in tubular fluid flow rate (0.3 to 3.0 nl/min.mm) led to a 5-fold increase in JNa (7.3±2.8 to 35.4±3.7 pmol/min.mm; n=10; p<0.05). Similarly, an increase in flow rate across the oocyte membrane, effected by increasing the rate of fluid flow perfusing the chamber from 0 to 5-6 ml/min, led to a reversible stimulation of INa by 4.7±0.5-fold (n=14, p<0.001); flow did not induce any change in current in water injected oocytes. The increase in INa occurred within 1 min after flow was applied, was sustained during the period of high flow, and was partially inhibited when flow was stopped. An increase in viscosity of the perfuse in contact with the luminal membrane of the isolated perfused CCD or oocyte, generated by addition of 1-5% dextran at constant flow, did not alter JNa (13.8±4.0 vs 13.8±2.7 pmol/min.mm; n=4; p=NS) or INa (1.0±0.1 vs 0.8±0.1 µA; p=NS), respectively. In summary, flow activates ENaC by mechanisms apparently unrelated to shear stress. Whether the flow-stimulation of ENaC activity reflects an increase in number of channels at the plasma membrane or an increase in open probability of existing channels remains to be determined.