Norepinephrine mobilization of free fatty acids in endurance trained rats

Wistar rats were trained to run, a male group on 6 days a week for 4 months at 28 m x min-1, and a female group on 5 days a week for 5-6 months at 31 m x min-1. Male rats were given continuous infusion of [1-14C] palmitate-albumin tracer via a jugular catheter at a constant infusion rate of 80 nCI x min-1. The effect of concurrent norepinephrine (NE) infusion on free fatty acid (FFA) mobilization was then studied in this group. NE increased plasma FFA levels similarly in both control and endurance trained male groups. Turnover of FFA, however, increased in response to NE relatively less in the trained group. Furthermore, NE caused the metabolic clearance rate of FFA (MCR) to decrease in control animals but not in runners. Colonic temperature was slightly increased by NE infusion in the control group only. Female rats were given a bolus injection of 5 microCi of [1-14C] palmitate-rat serum through a venous cannula. The disappearance of radioactivity in the circulation was measured. MCR values were higher in the trained group (p = 0.01). The effect of NE infusion on FFA levels was also studied in female animals. NE caused levels to increase in both control and trained rats. FFA concentrations were however more elevated in the control than in the trained group, before and during NE infusion. The relative increase tended to be less extensive in the trained group (p = 0.06). It is concluded that long term endurance training diminishes calorigenic and fat mobilizing actions of NE in the rat in vivo.