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Regulation of coronary vasomotor tone by serotonin is significantly changed after cardioplegic arrest and reperfusion. The current study investigates whether cardiopulmonary bypass may also affect peripheral arteriolar response to serotonin in patients with or without diabetes.Human peripheral microvessels (90–180 μm diameter) were dissected from harvested skeletal muscle tissues from diabetic and non-diabetic patients before and after cardiopulmonary bypass and cardiac surgery (n=8/group). In vitro contractile response to serotonin was assessed by videomicroscopy in the presence or absence of serotonin alone (10−9-10−5M) or combined with the selective serotonin 1B receptor (5-HT1B) antagonist, SB224289 (10−6M). 5-HT1A/1B protein expression in the skeletal muscle was measured by Western-blot and immunohistochemistry.There were no significant differences in contractile response of peripheral arterioles to serotonin (10−5M) pre-cardiopulmonary bypass between diabetic and non-diabetic patients. After cardiopulmonary bypass, contractile response to serotonin was significantly impaired in both diabetic and non-diabetic patients compared to their pre-cardiopulmonary bypass counterparts (P < .05). This effect was more pronounced in diabetic patients than non-diabetic patients (P < .05 versus non-diabetic). The contractile response to serotonin was significantly inhibited by the 5-HT1B antagonist in both diabetic and non-diabetic vessels (P < .05 versus serotonin alone). There were no significant differences in the expression/distribution of 5-HT1A/1B between non-diabetic and diabetic groups or between pre- versus post- cardiopulmonary bypass vessels.Cardiopulmonary bypass is associated with decreased contractile response of peripheral arterioles to serotonin and this effect was exaggerated in the presence of diabetes. Serotonin-induced contractile response of the peripheral arterioles was via 5-HT1B in both diabetic and non-diabetic patients.