Aims: We investigated the role of the inward rectifier K + channel (K IR ) in the renal interlobular artery (ILA). The ILA supplies the afferent arteriole and ranges in diameter from >100 µm near its origin at the arcuate artery to <30 µm at its most distal segment. Methods and Results : Vasodilatory responses to elevated extracellular K + (15 mmol/L) and vasoconstrictor responses due to K IR blockade by Ba 2+ (10-100 µmol/L) were assessed in in vitro perfused hydronephrotic rat kidneys. The

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... tal ILA (26±1 µm) exhibited K + -induced dilation and Ba 2+ -induced vasoconstriction; whereas, neither response was observed in the proximal ILA (108±3 µm). The intermediate ILA (55±1 µm) exhibited a modest K + -induced vasodilatation, but no Ba 2+ -induced vasoconstriction. The K + -induced dilations were blocked by Ba 2+ , but not by ouabain. Ba 2+ -induced depolarization, measured in ILA segments from normal kidneys, decreased with the increasing diameter. Patch clamp studies demonstrated that the K IR current (I KIR ) density also was inversely correlated with ILA segment diameter. Myocytes from afferent arterioles and distal ILAs exhibited similarly large I KIR , whereas, this current was absent in proximal ILA myocytes. Finally, we found that Ba 2+ attenuated myogenic vasoconstriction, suggesting an involvement of I KIR . The previously shown pattern of myogenic reactivity of the ILA (distal > intermediate>proximal) mirrors the distribution of I KIR reported in the present study, further supporting a role for I KIR . Conclusions: Our findings indicate differences in the magnitude of I KIR along the ILA and suggest that the influence of K IR on reactivity increases as vessel diameter decreases from proximal to distal regions.