Uterine Adenylate Cyclase in the Rat: Responses to a Decidual-Inducing Stimulus1

The properties of uterine adenylate cyclase (AC) in the rat were studied after the application of a decidual-inducing stimulus on Day 4 of progesterone (Day 0 = estrus). Within 60 s after the application of a gentle massage to the uterus, activity of AC increased from 6 to 16 pmoles/min X mg protein. After the trauma to the uterus, no changes were measured in the apparent Michaelis constant (Km) for adenosine 5'-triphosphate (ATP, 0.3 mM), in the Hill coefficient (2.73 vs. 2.74), or in the temperature optimum for the enzymatic reaction (30-40 degrees C). However, the maximum velocity (Vmax) was increased from about 7 to about 16 pmoles/min X mg protein and the pH optimum was broadened from 7.5-8.0 to 6.5-9.5 after trauma. Calculations of the apparent energy of activation (Ea) revealed an increase in Ea (20-30 degrees C) from 4.1 +/- 0.8 to 13.0 +/- 1.8 kcal/mol X deg after trauma. A similar but less dramatic increase in the Q10 (20-30 degrees C) was measured. After trauma, AC increased nineteen fold in resistance to inhibition by calcium ion. The inhibition concentration (IC50) for CaCl2 was 0.3 mM before and 5.8 mM after trauma. The data suggested that a decidual-inducing stimulus resulted in an activation of AC followed by an alteration in the microenvironment of the cyclase, which protected it from additional external influences.