Abstract
The present study was performed to determine the characteristics of the endothelium-derived hyperpolarizing factor (EDHF) that mediates the nitric oxide (NO)- and prostacyclin (PGI2)-independent hyperpolarization and relaxation of porcine renal interlobar arteries. 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 PGI2-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). 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.