Context: Triglyceride (TG) deposits in skeletal muscle (SM) are an important energy reservoir, and increased im TG content is associated with muscle insulin resistance.
Objective: The objective of the study was 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 abdominal sc adipose tissue during a hyperinsulinemic, hypoglycemic clamp (n = 13) and in response to in situ perfusion of epinephrine and norepinephrine (10−10 to 10−5M) (n = 12). Local tissue blood flow was monitored with the ethanol perfusion technique.
Setting: This was an experimental study.
Participants: The study population consisted of 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 10−7M of either epinephrine and norepinephrine (P < 0.0001) in AT. The maximum stimulation was seen at 10−6M norepinephrine (2-fold increase) and 10−5M epinephrine (3-fold increase). In SM, tissue glycerol increased at 10−7M epinephrine and 10−6M norepinephrine, respectively (P < 0.0001); the maximum increase of glycerol values (at 10−6M) 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 norepinephrine stimulation.