Context: Triglyceride (TG) deposits in skeletal muscle (SM) is an important energy reservoir and increased intramuscular TG-content is associated with muscle insulin resistance.

Objective: To investigate the effect of endogenous catecholamines on TG-lipolysis in human SM in vivo. Adipose tissue (AT) was studied for comparison.

Design and Main Outcome Measures: Glycerol levels (index of lipolysis) were measured using microdialysis in the gastrocnemius muscle and in abdominal sc adipose tissue during a hyperinsulinemic, hypoglycaemic clamp (n = 13), and in response to in situ perfusion of epinephrine and norepinephrine (10^-10 M - 10^-5 M) (n = 12). Local tissue blood flow was monitored with the ethanol perfusion technique.

Setting: Experimental study.

Participants: Healthy subjects.

Results: Plasma epinephrine increased 10-fold and plasma norepinephrine 2-fold in response to insulin-induced hypoglycemia. In parallel, the fractional glycerol release (difference between tissue and arterial glycerol) increased 2-fold in both tissues (P < 0.0001). No changes in AT and SM blood flow were registered. When the catecholamines were perfused in situ, tissue glycerol increased significantly at 10^-7 M of either epinephrine and norepinephrine (P < 0.0001) in AT. The maximum stimulation was seen at 10^-6 M norepinephrine (2-fold increase) and 10^-5 M epinephrine (3-fold increase). In SM, tissue glycerol increased at 10^-7 M epinephrine and 10^-6 M norepinephrine, respectively (P < 0.0001); the maximum increase of glycerol values (at 10^-6 M) was 2.5 times for epinephrine and 1.6 times for norepinephrine, respectively (P < 0.01).

Conclusions: The lipolytic activity of SM is increased by endogenous catecholamines in vivo and appears to be more responsive to epinephrine than to norepinephrine stimulation.