Abstract

Adaptive changes in the β-adrenergic adenylyl cyclase (AC) system in response to endurance training were studied in heart and adipose tissue. Training was performed by making male Wistar rats run on a motor-driven treadmill. The changes following exercise training were opposite in the two tissues studied. The density of β-adrenergic receptors in left ventricular membranes of trained rats showed a marked decrease. Comparison of AC activities in cardiac membranes prepared from trained and sedentary rats revealed a depressing effect of endurance training on: 1. the β-adrenergic stimulatory pathway and the inhibition of AC via receptor; 2. the G s component and the G s-adenylyl cyclase coupling, as shown by the response of adenylyl cyclase to GppNHp and NaF; and 3. the enzyme catalytic activity in the presence of Mn 2+ or forskolin. The levels of G sα subunits in the left ventricle, as measured in terms of ADP-ribosylated and immunologically reactive proteins, were decreased by endurance exercise, whereas immunodetectable levels of G iα2 increased in the membranes of trained myocardium. In contrast to the diminished sensitivity that characterizes the behavior of the cardiac β-adrenergic-AC system, endurance physical training increased sensitivity of this signal transduction system in adipose tissue. Thus, the density of β-ARs as well as AC activity and the β-adrenergic stimulatory pathway were increased in adipose membranes of trained rats compared with the corresponding sedentary controls. In addition, the levels of G sα subunits were higher in the adipose plasma membranes of trained rats. However, immunodetectable levels of G i1α and G i3α increased with training, whereas the amount of G i2α decreased in membranes of trained rats. In conclusion, the present study shows that chronic exercise is associated with a tissue-specific adaptation of the β-adrenergic AC system.

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