The effects of norepinephrine on active hyperemia in the canine gracilis muscle.

Abstract

The effects of intra-arterial norepinephrine (NE) on skeletal muscle blood flow (BF), O2 consumption (.ovrhdot.VO2) and arteriovenous O2 difference (A-VO2) were studied at rest and during exercise in an autoperfused canine gracilis muscle preparation. Static continuous exercise at a fixed level of maximal developed tension (P0) was induced by gracilis nerve stimulation; developed tension was monitored and used to control stimulation voltage. In 1 group of dogs (no. = 10), data were collected before (rest) and at the end of each of a series of four 2 min periods of exercise (10% P0) 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 .mu.g/min) were made. Control resting data were BF = 5.90 ml/min; A-VO2 = 5.30 vol %; .ovrhdot.VO2 = 0.31 ml/min. NE during rest reduced BF by 39-69%, increased A-VO2 by 79-91% and reduced .ovrhdot.VO2 by an average of 41.9%. Control exercise data were BF = 17.2 ml/min; A-VO2 = 11.2 vol %; .ovrhdot.VO2 = 1.96 ml/min. NE during exercise attenuated BF by 7-65% and widened A-VO2 by 22-35%. .ovrhdot.VO2 was maintained at control exercise levels during lower NE infusion levels but was attenuated by 56% at the highest NE level. In the 2nd group of dogs (no. = 8) data were collected at rest and at 4 times during 10 min of exercise (2.5% P0). NE (0.089, 0.17 and 0.34 .mu.g/min) or saline (control saline) was infused for 2 min each during the final 7 min 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 .ovrhdot.VO2 were attenuated (BF: -22%, .ovrhdot.VO2: -20%) and A-VO2 was unchanged compared to control. The NE-induced attenuation in BF and .ovrhdot.VO2 during exercise may result from a mechanism similar to that which occurs in congestive heart failure in which an exaggerated sympathoadrenal response during exercise and an attenuated exercise-induced rise in forearm .ovrhdot.VO2 occurs.