Abstract
Cyclic AMP metabolism in epididymal adipose tissue of exercise-trained rats was examined to determine if training induced changes in cyclic AMP production or inactivation. Beginning at 7 weeks of age, male rats were physically trained by 12 weeks of treadmill running. Pair-fed control rats remained sedentary in their cages for the duration of the experiment. Tissue levels of cyclic AMP were measured in epididymal adipose tissue slices incubated with norepinephrine. Adenyl cyclase was assayed in adipocyte ghost cell prepartions and low-Km phosphodiesterase was assayed in homogenates of adipose tissue. In response to norepinephrine stimulation, tissue cyclic AMP levels were reduced in trained compared to untrained rats. Training increased the ratio of activity of phosphodiesterase relative to adenyl cyclase. The results of this study indicate that cyclic AMP production in response to norepinephrine stimulation is not increased by training and may even be reduced, implying that adipose tissue cyclic AMP levels may be under a greater degree of control in trained rats. Modulation of adipose tissue cyclic AMP levels may function to regulate more closely the duration of lipolysis in exercise-trained rats.
Highlights
Cyclic AMP metabolism in epididymal adipose tissue of exercise-trained rats was examined to determine if training induced changes in cyclic AMP production or inactivation
The major source of fatty acids oxidized during exercise is adipose tissue, the turnover rate of which is increased by physical training [31], implying that this organ is centrally involved in adaptations of lipid metabolism to exercise training [5]
An increased lipolytic potential of adipose tissue from trained rats [32,33]has been suggested as being a complementary response to oxidative adaptations [33,34,35], some investigators have suggested that the lipolyticcapacity of adipose tissue from sedentary untrained individuals may be adequate to provide sufficient free fatty acids (FFA) to meet the oxidative requirements for this metabolic fuel during exercise [22]
Summary
Cyclic AMP metabolism in epididymal adipose tissue of exercise-trained rats was examined to determine if training induced changes in cyclic AMP production or inactivation. Prolonged physical training increases the sensitivity of isolated fat cells to the lipolytic action of epinephrine [5, 6], agreeing with the suggestion of Ostman and Sjostrand [7] that chronic exercise increases the sensitivity of tissues to catecholamines. Such an adaptation may be important in the mobilization and utilization of energy substrates by exercising animals [7, 8]. Since the levels of cAMP are controlled by both its production and its destruction, we have investigated the effect of exercise training on adipose tissue cAMP levels as well as the activities of two enzymes regulating cAMP levels, adenyl cyclase and phosphodiesterase
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