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Isoflurane has been reported to cause dose-dependent constriction in isolated coronary microvessels. However, these results are inconsistent with data from in situ and in vivo heart preparations which show that isoflurane dilates the coronary vasculature. To clarify the direct effects of isoflurane on coronary tone, we measured the response of isolated porcine resistance arterioles (ID, 75 ± 4.0 μm; range, 41–108 μm) to isoflurane in the presence and absence of adenosine triphosphate–sensitive and Ca2+-activated potassium channel blockers and also after endothelial removal.Subepicardial arterioles were isolated, cannulated, and pressurized to 45 mmHg without flow in a 37°C vessel chamber filled with MOPS buffer (pH = 7.4). After all vessels developed spontaneous (intrinsic) tone, dose-dependent (0.17–0.84 mm; approximately 0.5–2.5 minimum alveolar concentration) isoflurane-mediated effects on vessel ID were studied in the presence and absence of extraluminal glibenclamide (1 μm; an adenosine triphosphate–sensitive channel blocker) or iberiotoxin (100 nm; a Ca2+-activated potassium channel blocker) or before and after endothelial denudation using the nonionic detergent CHAPS (0.4%). Vessel ID was measured using an inverted microscope and videomicrometer, and vasomotor responses were analyzed by normalizing changes in arteriole ID to the dilation observed after exposure to 10−4 m sodium nitroprusside, which causes maximal dilation.Isoflurane caused dose-dependent dilation of all coronary arterioles. This vasodilation was 6.0 ± 0.7 μm at an isoflurane concentration of 0.16 mm (approximately 0.5 minimum alveolar concentration) and 25.3 ± 2.1 μm at 0.75 mm (approximately 2.5 minimum alveolar concentration). These values represent 18.1 ± 1.7% and 74.1 ± 3.3%, respectively, of that observed with 10−4 sodium nitroprusside (34 ± 3 μm). Glibenclamide, but not iberiotoxin, exposure affected arteriolar dilation in response to isoflurane. Glibenclamide caused a downward displacement of the isoflurane dose–response curve, reducing isoflurane-mediated dilation by an average of 36%. Denuded arterioles showed a marked (approximately 70%) reduction in their ability to dilate in response to isoflurane.The authors conclude that isoflurane dilates coronary resistance arterioles in a dose-dependent manner, and that this dilation is partially mediated by adenosine triphosphate–sensitive channels and is highly dependent on the presence of a functioning endothelium.