In vitro response of large colon arterial and venous rings to vasodilatingdrugs in horses

Objective-To determine in vitro vasomotor response of equine large colon ar terial and venous rings with and without endothelium to vasodilator drugs, including dopamine (DOP), dopexamine (DPX), acepromazine (ACE), isoxsuprine (ISX), and nifedipine (NFP). Animals-7 adult horses. Procedure-Relaxation of large colon arteries and veins in response to vasod ilating drugs was determined by measuring the change in tension of vessel r ings when exposed to a cumulative concentration range (10(-8) to 10(-4)M) o f each drug. Vessel rings, with and without endothelium, were mounted in or gan baths, attached to a transducer, and contracted with norepinephrine (NE ), Cumulative concentration-response relationships, percentage maximal rela xation, and EC50 (concentration of drug required to relax the NE-induced co ntracted tissue to 50% of its contracted state) values were calculated. Results-There were significant differences among drugs for EC50 (ACE = ISX < NFP) and percentage maximal relaxation (ACE = ISX > NFP = DPX > DOP) valu es in veins. Endothelium removal from veins had no significant effect. Ther e were no differences in EC50 values for arteries; however, percentage maxi mal relaxation was significantly different among drugs (ACE = ISX = NFP > D PX = DOP). Endothelial removal resulted in higher EC50 and lower percentage maximal relaxation values, compared with endothelium-intact arteries. Conclusion and Clinical Relevance-ACE and ISX were the most potent and effi cacious drugs evaluated and could potentially be used to improve blood flow after correction of large-colon volvulus. Dopamine cannot be recommended b ecause of its biphasic response and potential to further decrease blood flo w. Endothelium removal altered the vasodilatory responses of colonic arteri al rings, but did not affect venous rings.