The Expression of cAMP-Dependent Protein Kinase Subunits in Primary Rat Hepatocyte Cultures. Cyclic AMP Down-Regulates Its Own Effector System by Decreasing the Amount of Catalytic Subunit and Increasing the mRNAs for the Inhibitory (R) Subunits of cAMP-Dependent Protein Kinase

Cyclic AMP-dependent protein kinase subunit expression was studied during the first 35 h of primary culture of hepatocytes isolated from rats fed a protein restricted diet. In the absence of elevated cAMP the ratio between regulatory (RI + RII) and catalytic (C) subunits was constant. There was an increase of RI and a decrease of RII, the RI/RII ratio rising from 1 to 2.4 during the 35 h of culturing studied. This disproportionate expression of RI was reflected in an increase of RI alpha mRNA relative to RII alpha mRNA. The increase of liver RI previously noted after amino acid feeding of protein starved rats was thus reproduced when hepatocytes from such animals were cultured in an amino acid rich medium. When the cell cAMP level was chronically elevated by adding glucagon/isobutylmethylxanthine at the time of seeding, the C level decreased by more than 50% in a few hours. The concentration of C alpha mRNA was not affected. The elevated cAMP also led to a transient increase of RI alpha- and RII alpha mRNA. The effects of glucagon could be reproduced by cAMP analogs. The cAMP-induced down-regulation of C without concomitant down-regulation of R led to an increased R/C ratio. The decreased C and the increased R/C ratio both ensure that the hepatocytes will show a reduced kinase activation in response to a second challenge with cAMP (i.e. show hysteresis).