EFFECT OF EXERCISE ON LIPOLYTIC SENSITIVITY IN ENDURANCE-TRAINED ATHLETES

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.