Developmental regulation of peptidylglycine alpha-amidating monooxygenase (PAM) in rat heart atrium and ventricle. Tissue-specific changes in distribution of PAM activity, mRNA levels, and protein forms

The high levels of peptidylglycine alpha-amidating monooxygenase (PAM, EC 1.14.17.3) found in adult rat atrium led us to examine PAM expression in rat atrium and ventricle from embryonic day 14 through adulthood. Immunocytochemical studies using antisera to PAM identified cardiocytes as the major site of PAM expression in atrium and ventricle throughout development. Levels of PAM mRNA and PAM activity exhibited distinctly different developmental profiles in atrium and ventricle. Ventricular PAM mRNA and PAM activity were highest from embryonic days 14 through 18, declined at the time of birth, rose slightly during the first postnatal week, and declined toward adult levels. Atrial PAM mRNA and PAM activity were low at embryonic day 14, rose to a peak immediately before birth, declined at the time of birth, and then rose after birth. Levels of atrial PAM mRNA and PAM activity were not directly correlated at all developmental stages. Two major forms of PAM mRNA (4.2 +/- 0.1 and 3.8 +/- 0.1 kilobase(s] were identified in atrium and ventricle throughout development. The prevalence of the two forms varied with developmental stage, with atrium and ventricle containing similar forms at each stage. Western blots of atrial and ventricular membranes revealed the existence of a developmental stage-specific distribution of PAM protein among forms ranging in mass from 125 to 94 kDa. In both atrium and ventricle PAM activity was primarily soluble from embryonic days 14 through 16 and primarily particulate after birth. The role of PAM in the heart is not yet clear, but the presence of tissue-specific and developmentally regulated alterations in PAM mRNA, PAM protein, and PAM activity suggests that this peptide processing enzyme plays a key role in the heart.