Cyclic AMP-dependent protein kinase in mammary tissue of the lactating rat. Activity ratio and responsiveness of the target enzymes acetyl-CoA carboxylase and glycogen phosphorylase to β-adrenergic stimulation

Abstract

The role of cyclic AMP in acute regulation of the metabolism of mammary tissue in the lactating rat was examined by measuring the activity ratio of cyclic AMP-dependent protein kinase (A-kinase) and by examining the properties of this enzyme in its two major isoenzymic forms. Isoenzyme II is the major form in soluble extracts of rat mammary tissue. A-kinase activity ratio in such extracts is unaffected by starvation of the lactating rat. Treatment of the intact rat with isoprenaline, or addition of isoprenaline to incubations in vitro of mammary acini, resulted in a major increase in the activity ratio of A-kinase. These treatments equally affected isoenzymes I and II. The treatment in vitro lead to a rapid depletion of A-kinase as subsequently measured in extracts of acini. The degree of activation of the enzymes acetyl-CoA carboxylase and glycogen phosphorylase in extracts of mammary tissue and of acini was assessed as a function of these treatments. The increased activation of A-kinase induced by isoprenaline was unaccompanied by significant changes in the activity of acetyl-CoA carboxylase in acini, although we previously showed that this agent activates acetyl-CoA carboxylase in intact mammary tissue. Contrastingly, isoprenaline-induced enhancement of A-kinase activity was accompanied by an increase in the activity ratio of phosphorylase in acini. These results indicate that: (a) a normal response of expressed A-kinase activity to cyclic AMP operates in mammary acini and mammary tissue from lactating rats; (b) rapid modulation of the total amount of soluble A-kinase is mediated in mammary epithelial cells by cyclic AMP; (c) phosphorylase, an ultimate target of the protein phosphorylation cascade initiated by A-kinase, is activated in acini under conditions where A-kinase activity is enhanced; and (d) mechanisms other than that of the A-kinase phosphorylation/inhibition model for acetyl-CoA carboxylase regulation must operate in mammary tissue preparations and in vivo to account for the response of this enzyme to enhanced A-kinase activity.