Vasoconstriction plays an important role in the development of acute kidney injury in rhabdomyolysis. We hypothesized that myoglobin enhances the angiotensin II (Ang II) response in afferent arterioles by increasing superoxide and reducing nitric oxide (NO) bioavailability. Afferent arterioles of C57Bl6 mice were isolated perfused and vasoreactivity was analyzed using video microscopy. NO bioavailability, superoxide concentration in the vessels wall, and changes in cytosolic calcium were measured using fluorescence techniques. Myoglobin treatment (10-5M) did not change the basal arteriolar diameter during a 20min period compared to control conditions. L-NAME (10-4M) and L-NAME + myoglobin reduced diameters to 94.7% and 97.9% of the initial diameter, respectively. Myoglobin or L-NAME enhanced the Ang II induced constriction of arterioles compared to control (36.6% and 34.2%, respectively, vs. 65.9%). Norepinephrine responses were not influenced by myoglobin. Combined application of myoglobin and L-NAME further facilitated the Ang II response (7.0%). Myoglobin or L-NAME decreased the NO related fluorescence in arterioles similarly. Myoglobin enhanced the superoxide related fluorescence and tempol prevented this enhancement. Tempol also partly prevented the myoglobin effect on the Ang II response. Myoglobin increased the Fura-2 fluorescence ratio (intracellular calcium) during Ang II application (10-12 to 10-6M). The results suggest that the enhanced afferent arteriolar reactivity to Ang II is mainly due to a myoglobin induced increase in superoxide and associated reduction in the NO bioavailability. Signalling pathways for the augmented Ang II response include enhanced cytosolic calcium transients. In conclusion, myoglobin may contribute to the afferent arteriolar vasoconstriction in this rhabdomyolysis model.