Long term elevation of glucose (24 hours) has been shown to reduce Kv currents in cultured vascular smooth muscle cells (Li et al., 2003). We have examined the acute effects of altering glucose concentration on the activity and on the inhibition by endothelin-1 (ET-1) of Kv and K_ATP currents of rat arterial myocytes. Smooth muscle cells were isolated enzymatically from small mesenteric arteries obtained from humanely killed adult Wistar rats. Currents were recorded using conventional whole-cell recording with physiological K⁺ gradients (6/140mM) for Kv and symmetrical 140mM K⁺ for K_ATP currents. In all experiments the total external concentration of glucose and mannitol was kept at 10mM to maintain osmolarity. Kv currents elicited by pulsing from −65 to +40mV in glucose free solution, were reduced by 61 ± 8% within 2 minutes of adding 4 mM glucose (n = 6 cells). This reduction in current was reversed by removing the glucose. In 4 mM glucose, 10nM ET-1 decreased peak Kv current amplitude by 33 ± 4% (n = 6) and increased the rate of inactivation, decreasing its time constant at +40mV from 1496 ± 90ms in control to 380 ± 80ms in ET-1 (n = 6). Pre-incubation of the cells for 15 min with 50 μM of the membrane permeable myristoylated PKC inhibitor peptide 19-27 (PKC-IP) completely abolished the effects of ET-1 in 6 of 6 cells. Surprisingly, these alterations in Kv current kinetics were not observed when cells were bathed in 10 mM glucose. To test whether the endothelin receptors were still functional we examined the inhibition of K_ATPcurrents by ET-1. K_ATP currents were activated by 100 μM pinacidil and recorded at −60mV. Application of 10nM ET-1 reduced K_ATP current by 81 ± 4% (n = 6). Furthermore, in the presence of PKC-IP, ET-1 inhibited the K_ATP current by only 7 ± 3% (n=6), indicating that the endothelin receptors were still able to activate PKC in 10mM glucose. These results indicate that changes in extracellular glucose concentration within the physiological range can have rapid and profound effects on the ability of vasoactive compounds such as ET-1 to regulate Kv channels. These mechanisms may have an important role in the pathophysiological effects on the vasculature observed in diabetic patients, where glucose concentrations vary much more than in normal circumstances.