The Effects of Norepinephrine on Active Hyperemia in the Canine Gracilis Muscle

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We studied the effects of intra-arterial norepinephrine (NE) on skeletal muscle blood flow (BF), oxygen consumption (VOi), and arteriovenous oxygen difference (A-Voj) at rest and during exercise in an autoperfused canine gracilis muscle preparation.Static continuous exercise at a fixed level of maximal developed tension (P6) was induced by gracilis nerve stimulation; developed tension was monitored and used to control stimulation voltage. In one group of dogs (n − 10), data were collected before (rest) and at the end of each of a series of four 2-minute periods of exercise (10% P) in each preparation. During both the rest and the exercise phases, continuous intra-arterial infusions of isotonic saline alone (control) and saline plus NE (0.11, 0.22, and 0.44 pg/min) were made. Control resting data were: BF-5.90 ml/min; A-VOi-5.30 vol %; Voi-0.31 ml/min. NE during rest reduced BF by 39-69%, increased A-VOi by 79-91%, and reduced VOi by an average of 41.9%. Control exercise data were: BF-17.2 ml/min; A-VO2 = 11.2 vol %; Voi-1.96 ml/min. NE during exercise attenuated BF by 7-65% and widened A-VOi by 22-35%. Vo2 was maintained at control exercise levels during lower NE infusion levels but was attenuated by 56% at the highest NE level. In the second group of dogs (n-8), data were collected at rest and at four times during a 10-minute exercise period (2.5% Po). NE (0.089, 0.17, and 0.34 pg/min) or saline (control saline) was infused for 2 minutes each during the final 7 minutes of exercise. At the lower NE doses, no significant difference was observed relative to the control-saline experiment At the highest NE dose BF and VOi were attenuated (BF: −22%, Vo,: −20%), and A-VOi was unchanged compared to control. The NE-induced attenuation in BF and Voj during exercise may in part result from a mechanism similar to that which occurs in congestive heart failure in which an exaggerated sympathoadrenal response during exercise and an attenuated exerciseinduced rise in forearm Vd occurs. Ore Ret 44: 680-666, 1979

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