Changes in cyclic adenosine monophosphate-phosphodiesterase activity in nonpregnant and pregnant human myometrium

Cyclic adenosine monophosphate-phosphodiesterase is the enzyme responsible for cyclic adenosine monophosphate degradation. We investigated the kinetic behavior of this enzyme in the myometrium of women who were nonpregnant, pregnant at term not in labor, and pregnant at term in active labor. Phosphodiesterase activity was measured in the 100,000 g supernatant by the two-step isotopic procedure. The Km (Michaelis constant) value remains essentially unchanged from the nonpregnant to the pregnant state and subsequent labor in both the low and the high affinity enzymes. During pregnancy the V max (maximum velocity) is 75% less than in the nonpregnant state (p < 0.005) and remains unchanged during labor. This is true for both the high and the low affinity enzymes. These changes in the kinetic characteristics of the cyclic adenosine monophosphate-phosphodiesterase are indicative of noncompetitive inhibition. We conclude that this inhibition may be interpreted as part of the mechanism for uterine smooth muscle relaxation and pregnancy maintenance.