BackgroundThe effects of adrenergic agonists, often used as local anesthetic additives or spinal analgesics, on spinal vessels have not been firmly established. The authors investigated the effects of [Greek small letter alpha]2‐and [Greek small letter alpha]1‐adrenergicagonists on spinal and cerebral pial vessels in vivo.MethodsPentobarbital‐anesthetized dogs (n = 28) were prepared for measurement of spinal pial‐vessel diameter in a spinal‐window preparation. The authors applied dexmedetomidine, clonidine, phenylephrine, or epinephrine in three different concentrations (0.5, 5.0, and 50 [micro sign]g/ml; [2.1, 1.9, 2.5, and 2.3] x [10‐6, 10‐5, and 10‐4] M, respectively) under the window (one drug in each dog) and measured spinal pial arteriolar and venular diameters in a sequential manner. To enable the comparison of their effects on cerebral vessels, the authors also administered these drugs under a cranial window.ResultsOn topical administration, each drug constricted spinal pial arterioles in a concentration‐dependent manner. Phenylephrine and epinephrine induced a significantly larger arteriolar constriction than dexmedetomidine or clonidine at 5 [micro sign]g/ml (8%, 11%, 0%, and 1%, respectively). Spinal pial venules tended to be less constricted than arterioles. In cerebral arterioles, greater constrictions were induced by dexmedetomidine and clonidine than those induced by phenylephrine and epinephrine (14%, 8%, 0%, and 1%, respectively). Cerebral pial venules tended to exhibit larger constrictions than cerebral arterioles (unlike in spinal vessels).ConclusionDexmedetomidine and clonidine constricted spinal vessels in a concentration‐dependent manner, but such vasoconstrictions were smaller than those induced by phenylephrine and epinephrine.