Functional evidence for an inward rectifier potassium current in rat renalafferent arterioles

An inward rectifier potassium current, Kir, has been identified in cerebral and coronary resistance vessels, where it is considered to be an important determinant of resting membrane potential (RMP) and to play a role in bloo d flow regulation. We investigated the functional role of Kir in the renal afferent arteriole using the in vitro-perfused hydronephrotic rat kidney. I ncreasing external KCI from 5 to 15 mmol/L induced afferent arteriolar vaso dilation. This response was inhibited by 10 to 100 mu mol/L Ba2+, concentra tions selective for blockade of Kir, and by chloroethylclonidine (100 mu mo l/L) but was not blocked by glibenclamide (10 mu mol/L) or ouabain (3 mmol/ L). Reducing external KCl from 5 to 1.5 mmol/L to enhance rectification of Kir caused vasoconstriction at low renal arterial pressure (40 mm Hg) and v asodilation during myogenic vasoconstriction (120 mm Hg), suggesting that t his current dominates RMP at low perfusion pressures. When administered to kidneys perfused at 40 mm Hg renal arterial pressure, 30 mu mol/L Ba2+ elic ited afferent arteriolar depolarization, reducing RMP from -47+/-2 to -34+/ -2 mV (n=10, P<0.0001), and vasoconstriction, reducing diameters from 14.5/-1 to 10.9+/-0.8 <mu>m (n = 10, P=0.0016). Although Ba2+ reduced resting d iameter, blockade of Kir did not prevent myogenic signaling in this vessel. Our findings thus demonstrate the presence of Kir in rat renal afferent ar terioles and suggest that this current is an important determinant of RMP i n situ.