Studies performed in vitro suggest that an acute bout of exercise incr
eases the lipolytic response to beta-adrenergic stimulation. We evalua
ted the effect of exercise on lipolytic sensitivity in vivo in five en
durance-trained athletes. The rate of appearance (Ra) of glycerol in p
lasma, an index of whole body lipolysis, was determined during 60 min
of epinephrine infusion (0.015 mu g . kg(-1) . min(-1)) on two occasio
ns: 1) at basal resting conditions and 2) 90 min after completing 1 h
of high-intensity (70% O-2 uptake) cycle ergometer exercise. Total gly
cerol Ra during epinephrine infusion in the basal state (352 +/- 35 mu
mol . kg(-1) . 60 min(-1)) was not significantly different from the v
alue obtained after high-intensity exercise (439 +/- 58 mu mol . kg(-1
) . 60 min(-1)). However, the increase in glycerol Ra above baseline d
uring epinephrine infusion was lower after (30 +/- 16 mu mol . kg(-1)
. 60 min(-1)) than before (148 +/- 28 mu mol . kg(-1) . 60 min(-1)) ex
ercise because of the high postexercise baseline value (P < 0.05). Mea
n plasma free fatty acid (FFA) concentration was lower during exercise
than during epinephrine infusion despite a greater rate of lipolysis
during exercise. The slope of change in plasma FFA with respect to gly
cerol Ra was lower during exercise (0.0171 +/- 0.006) than during epin
ephrine infusion (0.0835 +/- 0.018) (P < 0.05). We conclude that a sin
gle bout of intense exercise does not increase in vivo lipolytic sensi
tivity to beta-adrenergic stimulation in endurance-trained athletes. I
n addition, plasma FFA concentration represents the balance between pl
asma FFA inflow and tissue uptake and cannot be used as an index of li
polytic activity during certain physiological conditions, such as exer
cise.