Bradykinin acutely inhibits activity of the epithelial Na+ channel in mammalian aldosterone-sensitive distal nephron

摘要

Activation of the renal Kallikrein-kinin system results in natriuresis and diuresis, suggesting its possible role in renal tubular sodium transport regulation. Here, we used patch-clamp electrophysiology to directly assess the effects of bradykinin (BK) on the epithelial Na+ channel (ENaC) activity in freshly isolated split-opened murine aldosterone-sensitive distal nephrons (ASDNs). BK acutely inhibits ENaC activity by reducing channel open probability (P_o) in a dose-dependent and reversible manner. Inhibition of B2 receptors with icatibant (HOE-140) abolished BK actions on ENaC. In contrast, activation of Bl receptors with the selective agonist Lys-des-Arg~9-BK failed to reproduce BK actions on ENaC. This is consistent with B2 receptors playing a critical role in mediating BK signaling to ENaC. BK has little effect on ENaC P_o when G_(q/11) was inhibited with Gp antagonist 2A. Moreover, inhibition of phospholipase C (PLC) with U73122, but not saturation of cellular cAMP levels with the membrane-permeable nonhydrolysable cAMP analog 8-cpt-cAMP, prevents BK actions on ENaC activity. This argues that BK stimulates B2 receptors with subsequent activation of G_(q/11)-PLC signaling cascade to acutely inhibit ENaC activity. Activation of BK signaling acutely depletes apical PI(4, )P_2 levels. However, inhibition of Ca~(2+) pump SERCA of the endoplasmic reticulum with thapsigargin does not prevent BK signaling to ENaC. Furthermore, caffeine, while producing a similar rise in [Ca~(2+)]i as in response to BK stimulation, fails to recapitulate BK actions on ENaC. Therefore, we concluded that BK acutely inhibits ENaC P_o in mammalian ASDN via stimulation of B2 receptors and following depletion of PI(4, )P_2, but not increases in [Ca~(2+)]i.