Anesthesia-induced alteration of small vessel response to norepinephrine

Closed-circuit television microscopy was used to quantitate the in vivo response of small arteries (approximately 100 micron) and small veins (approximately 150 micron) to topically applied norepinephrine in the rat cremaster muscle. Rats were anesthetized with pentobarbital (50 mg/kg), or urethane (1200 mg/kg) or a combination of urethane (800 mg/kg) and chloralose (60 mg/kg). Complete concentration-response curves were obtained for an artery and vein pair in each rat and pD2 values (-log ED50) were used to evaluate the vascular sensitivity to norepinephrine. Both the artery and the vein in urethane-anesthetized animals had decreased sensitivity to norepinephrine in comparison to the vessels of animals anesthetized with pentobarbital or urethane-chloralose. Pretreatment with cocaine (10(-5) M) significantly increased the sensitivity of both the artery and vein in pentobarbital-anesthetized animals but did not affect the vessels in urethane-chloralose-anesthetized animals. These results are consistent with two opposing effects of the urethane-chloralose combination. The first is an increased sensitivity to norepinephrine via blockade of neuronal uptake and the second is a decreased sensitivity of norepinephrine via a vascular inhibitory effect of urethane.