Description:
<jats:p>Recently, we described a splice variant of the human natriuretic peptide receptor type B (NPR-Bi) in human proximal tubule cells [immortalized human kidney epithelial cells (IHKE-1) that lacks a functional guanylate cyclase domain (Hirsch JR, Meyer M, Mägert HJ, Forssmann WG, Mollerup S, Herter P, Weber G, Cermak R, Ankorina-Stark I, Schlatter E, and Kruhøffer M. J Am Soc Nephrol 10: 472–480, 1999). Its signaling pathway does not include cGMP, cAMP, or Ca<jats:sup>2</jats:sup><jats:sup>+</jats:sup>but leads to inhibition of K<jats:sup>+</jats:sup>channels. In patch-clamp experiments, effects of tyrosine kinase receptor blockers on C-type natriuretic peptide (CNP)-mediated depolarizations of membrane voltages ( V<jats:sub>m</jats:sub>) of IHKE-1 cells were tested. The epidermal growth factor (EGF) receptor blocker genistein (10 μM) abolished the effect of CNP (0.2 ± 0.4 mV, n = 7), and comparable results were obtained with 10 μM daidzein ( n = 8). Aminogenistein (10 μM, n = 5) and tyrphostin AG1295 (10 μM, n = 5) had no significant effects. EGF (1 nM) hyperpolarized cells by –5.3 ± 0.8 mV ( n = 5). This effect was completely blocked by genistein or daidzein. The Cl<jats:sup>–</jats:sup>channel blocker NPPB (10 μM, n = 5) inhibited the EGF-mediated hyperpolarization. mRNA expression of NPR-B and NPR-Bi shows reversed patterns along the human nephron. NPR-B is highly expressed in glomeruli and proximal tubules, whereas NPR-Bi shows strong signals in the distal nephron. Expression of NPR-Bi in the cortical collecting duct was also confirmed with immunohistochemistry. In other human tissues, NPR-Bi shows strongest expression in pancreas and lung, whereas in the heart and liver NPR-B is the dominating receptor. In conclusion, CNP inhibits an apical K<jats:sup>+</jats:sup>channel in IHKE-1 cells independently of cGMP and so far this effect can only be blocked by genistein and daidzein. Tyrosine phosphorylation might be the missing link in the signaling pathway of CNP/NPR-Bi.</jats:p>