Abstract 509: Role of Kv Channels in Hydrogen Peroxide Induced Coronary Vasodilation
Hydrogen peroxide (H2O2), the dismutated product of superoxide, is increasingly gaining acceptance as an important modulator of vascular tone through its actions as a hyperpolarizing factor. We have found that H2O2 produced dilation via oxidation of thiol groups, because dilation was blocked by the thiol reductant, dithiothreitol. Because Kv channels are reportedly redox sensitive, we proposed that coronary dilation to H2O2 would be prevented by 4-aminopyridine, an antagonist of Kv channels. To test this hypothesis, we studied small coronary arteries (SCA) from the rat heart (diameter from 200 –300 μM) using a Mulvaney myograph. After mounting and establishing optimal loading conditions for tension development, SCA were constricted with either the thromboxane mimetic, U46619 (1 μM) or endothelin-1 (1 nM). H2O2 produced dose-dependent vasodilation with maximal relaxation occurring at 0.1 mM. Administration of 4-AP (10 mM) shifted the dose response curve and decreased the maximal response from 65±14% to 29±8% relaxation (P<0.05); thus suggesting the electrophysiological effector was the Kv channel. To affirm the presence of Kv channels in coronary smooth muscle, Western analysis was performed revealing predominance of the Kv1.5. To unequivocally ascertain that H2O2 produced its effects via Kv activation, we employed patch-clamp technique and found that H2O2 activated a K current that was blocked by 4-AP, and had parameters (current-voltage relationship) consistent with Kv channels. Since in neurons, activation of Kv channels is regulated by p38, and because p38 is reportedly redox sensitive, we further proposed that H2O2-induced dilation would be blocked by p38 inhibition. Accordingly we administered the p38 inhibitor (SB203580, 10 μM) and examined vasodilation in pre-constricted vessels to H2O2. Following p38 inhibition, dilation to H2O2 (0.1 mM) was not changed from that occurring under control conditions (86±12% vs 65±14%); thus refuting our hypothesis. Taken together, our results strongly support the idea that H2O2 produces vasodilation via Kv channel activation and that p38 signaling is not required for channel activation.