LUMINAL SIGNAL IN TUBULOGLOMERULAR FEEDBACK - WHAT ABOUT POTASSIUM

Evidence suggests that a minimal luminal [K+] is required to elicit a full tubuloglomerular feedback (TGF) response, consistent with transmi ssion of the TGF signal across the macula densa (MD) via the Na+-2Cl(- )-K+ cotransporter. Furthermore, it appears that luminal [K+] at the M D is close to the K+ affinity of the Na+-2Cl(-)-K+ cotransporter and c hanges in response to altering late proximal tubular flow rate (VLP), that is, a maneuver that induces a TGF response. These findings sugges t that luminal [K+] (besides [Cl-]) could be rate limiting in TGF. In the thick ascending limb of Henle's loop (TALH), most of the luminal K + is derived from recycling across the apical tubular membrane. Becaus e changing VLP causes relatively greater alterations in the absolute N a+ and Cl- delivery to Henle's loop than in K+ load, the parallel chan ges of VLP and luminal [K+] at the MD, despite significant alteration in K+-dependent reabsorption of Na+ and Cl- via the Na+-2Cl(-)-K+ cotr ansporter, imply a transport dependent adaptation of K+ recycling in T ALH.