Ontogeny of the Adenosine‐3′:5′‐Phosphate‐Dependent Protein‐Kinase System during Early Uterine Development

The development of protein kinase and estrogen‐binding proteins was studied in the nuclear and cytosol fractions of rat uteri as a function of postnatal age. Analysis of cyclic‐AMP‐dependent protein kinase activity, with protamine sulfate as substrate in the presence of 3 μM cyclic AMP, revealed that the cytosol as well as the nuclear cAMP‐dependent protein kinase activity increased markedly between the third and fifth postnatal days. The total cytosol protein kinase activity remained constant after the age of 5 days (850 pmol [ 32 P]phosphate incorporated × min −1 × mg cytosol protein −1 ), but nuclear cAMP‐dependent protein kinase activity decreased sharply (60%), coinciding with a time period during which nuclei acquire considerable estrogen‐binding activity. Analysis of the multiplicity of cyclic‐AMP‐dependent protein kinases by chromatography of uterine cytosol on DEAE‐cellulose revealed the presence of type I and type II cyclic‐AMP‐dependent protein kinase. Type I isoenzyme levels decreased relative to the type II isozyme from the time of birth to the age of 20 days, type II isozyme did not undergo any major developmental changes. The specific estrogen binding activity in utertine cytosol reached a maximum in 12‐day‐old rats (1.7 pmol of 17β‐estradiol bound/mg protein). It declined thereafter and reached constant levels in 17‐day‐old and older rats. Concomitantly nuclear levels of estrogen binding protein increased progressively from very low levels (0.1 pmol) in 5‐day‐old rats to 1.2 pmol of 17β‐estradiol bound/mg of nuclear protein in 12‐day‐old and 20‐day‐old rats. The results indicate the early existence of a functional uterine cyclic‐AMP/protein kinase system in the neonatal rat and suggest that the protein kinase system and specific estrogen‐binding activity develop in a reciprocal manner.