CYCLIC-AMP-DEPENDENT PROTEIN-KINASE IN RAT MAMMARY TISSUE - EXPRESSION OF CATALYTIC AND REGULATORY SUBUNITS THROUGHOUT PREGNANCY AND LACTATION

'Expressed' and 'total' activities of cyclic AMP-dependent protein kin ase (PK-A) were measured in extracts of rat mammary tissue sampled thr oughout pregnancy and lactation. Expression of the genes encoding the catalytic subunit (C-subunit) isoforms C-alpha and C-beta was examined by Northern blotting, as a function of mammary development, to determ ine relative levels of their respective mRNAs. The content of C-subuni t protein (all isoforms) was estimated immunochemically and related to levels of C-subunit catalytic activity and of mRNAs. It was found tha t C-subunit isoform mRNAs are expressed co-ordinately during mammary d evelopment and that a marked decline in expression, per cell, at aroun d parturition is paralleled by a fall in 'total' PK-A activity. The 'e xpressed' activity of PK-A activity underwent characteristic changes t hroughout pregnancy and lactation, reaching a peak late in pregnancy. The PK-A activity ratio reached a peak in early lactation. C-subunit p rotein mass closely paralleled 'total' PK-A activity throughout pregna ncy and lactation, thereby demonstrating the constancy of C-subunit sp ecific catalytic activity during these developmental events. Regulator y subunits (R-subunits) were probed with the photoaffinity label 8-azi do-[P-32]cAMP. The abundance of R-II as a proportion of total R-subuni t increased throughout pregnancy and lactation, and quantitative analy sis of the photoaffinity labelling suggested inconstancy in the ratio of R:C subunits, with highest values occurring in late pregnancy/early lactation.