Distribution of cyclic AMP phosphodiesterase in adipose tissue from trained rats

Fat cells were isolated from sedentary and exercise trained female Sprague-Dawley rats and cyclic AMP phosphodiesterase (cyclic AMP-PDE) activities were determined from crude homogenates of the fat cells in the whole homogenate, P5, P48, and S48 fractions. Exercise training resulted in a significant increase in the mean specific activity of cyclic AMP-PDE (pmol X min-1 X mg-1) from the whole homogenate and S48 fraction at cyclic AMP concentrations of 4, 8, and 16 microM and in the P48 fraction at 8 and 16 microM cyclic AMP. Cyclic AMP-PDE kinetic plots according to Lineweaver-Burk for the calculation of Michaelis constants (Km) and maximum enzyme velocities (Vmax) were nonlinear, indicating both a low and high enzyme form. The Michaelis constants were significantly lower in trained rats than those of its control for the high Km form of cyclic AMP-PDE in the whole and soluble fractions and for the low Km form of the P5 particulate fraction. The Vmax of the high Km form of the P48 particulate fraction from trained animals was also significantly higher than that found in its control. Phosphodiesterase inhibition by methylxanthines in the various fractions was similar in both trained and sedentary animals. These changes in specific activity, Michaelis constants, and Vmax of cyclic AMP-PDE from crude homogenates of isolated fat cells from exercise trained animals may account for the decreased intracellular levels of cyclic AMP following catecholamine stimulation of isolated fat cells from trained rats.