The Na-K-ATPase is a target for an EDHF displaying characteristics similar to potassium ions in the porcine renal interlobar artery

1 The present study was performed to determine the characteristics of the endothelium-derived hyperpolarizing factor (EDHF) that mediates the nitric oxide (NO)- and prostacyclin (PGI(2))independent hyperpolarization and relaxation of porcine renal interlobar arteries. 2 Bradykinin-induced changes in isometric force or smooth muscle membrane potential were assessed in rings of porcine renal interlobar artery preconstricted with the thromboxane analogue U46619 in the continuous presence of N-omega-nitro-L-arginine and diclofenac to inhibit NO synthases and cyclo-oxygenases. 3 Inhibition of NO- and PGI(2)-production induced a rightward shift in the concentration-relaxation curve to bradykinin without affecting maximal relaxation. EDHF-mediated relaxation was abolished by a depolarizing concentration of KCl (40 mM) as well as by a combination of charybdotoxin and apamin (each 100 nM), two inhibitors of calcium-dependent K+ (K-Ca(+)) channels. Charybdotoxin and apamin also reduced the bradykinin-induced, EDHF-mediated hyperpolarization of smooth muscle cells from 13.7 +/- 1.3 mV to 5.7 +/- 1.2 mV. 4 In addition to the ubiquitous alpha1 subunit of the Na-K-ATPase, the interlobar artery expressed the gamma subunit as well as the ouabain-sensitive alpha2, alpha3 subunits. A low concentration of ouabain (100 nM) abolished the EDHF-mediated relaxation and reduced the bradykinin-induced hyperpolarization of smooth muscle cells (13.6 +/- 2.8 mV versus 5.20 +/- 1.39 mV in the absence and presence of ouabain). 5 Chelation of K+, using cryptate 2.2.2., inhibited EDHF-mediated relaxation, without affecting NO-mediated responses. Elevating extracellular KCl (from 4 to 14 mm) elicited a transient, ouabain-sensitive hyperpolarization and relaxation that was endothelium-independent and insensitive to charybdotoxin and apamin. 6 These results indicate that in the renal interlobar artery, EDHF-mediated responses display the pharmacological characteristics of K+ ions released from endothelial K-Ca(+), channels. Smooth muscle cell hyperpolarization and relaxation appear to be dependent on the activation of highly ouabain-sensitive subunits of the Na-K-ATPase.