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We investigated the effects of cardioplegic arrest and reperfusion on coronary arteriolar responses to endothelium-dependent and -independent vasodilators and associated signaling pathways in uncontrolled diabetic, well controlled diabetic, and case-matched nondiabetic patients undergoing coronary artery bypass graft surgery.Coronary arterioles from harvested right atrial tissues were dissected pre- and post–cardioplegic arrest and reperfusion from uncontrolled diabetic (n = 10; hemoglobin A1c = 9.3 ± 0.3), well controlled diabetic (n = 10; hemoglobin A1c = 6.2 ± 0.2), and nondiabetic patients (n = 10; hemoglobin A1c = 5.1 ± 0.1) undergoing coronary artery bypass graft surgery.The baseline microvascular response to adenosine 5′-diphosphate, substance P, and sodium nitroprusside of arterioles from uncontrolled diabetic patients were decreased compared to the respective response from nondiabetic or well controlled diabetic patients (P < .05). The vasodilatory responses to adenosine 5′-diphosphate and substance P after cardioplegic arrest and reperfusion were significantly decreased in all 3 groups compared to pre–cardioplegic arrest and reperfusion responses (P < .05). However, these decreases were more pronounced in the uncontrolled diabetic group (P < .05). The expression of protein kinase C-α, protein kinase C-β, and protein oxidation in atrial tissues was significantly increased in the uncontrolled diabetic group compared to the nondiabetic or controlled diabetes groups.Uncontrolled diabetes is associated with endothelium-dependent and -independent vascular dysfunction of coronary arterioles. In addition, uncontrolled diabetes worsens the recovery of coronary arteriolar function after cardioplegic arrest and reperfusion. These alterations are associated with an increased expression/activation of protein kinase C-α and protein kinase C-β and enhanced oxidative stress.