cAMP-dependent protein kinase responses of S49 cells are reduced by growth in low epinephrine concentrations

S49 wild-type mouse lymphoma cells grown in 3 nM epinephrine are extensively desensitized. Cellular cAMP responses to subsequent challenges with 100 nM epinephrine are reduced by as much as 80-90%. In this report, we document that protein kinase activity ratios were also attenuated. For example, the activity ratios in naive cells were increased from 0.26 +/- 0.02 to 0.72 +/- 0.04 by incubation with 100 nM epinephrine for 2 min, whereas in cells grown in 3 nM epinephrine for 24 hr before the experiment they were 0.19 +/- 0.02 and 0.29 +/- 0.03. Attenuated protein kinase activity ratios were obvious at epinephrine challenge concentrations ranging from 10 to 1000 nM. Three kinds of experiments provided evidence that the reduced ratios in desensitized cells were secondary to diminished cAMP responses rather than to changes in the cAMP-dependent protein kinase itself. Firstly, when protein kinase activity ratios were plotted against cAMP levels in naive and desensitized cells, the points fell along a common line. Secondly, cell-free cAMP-dependent protein kinase preparations from naive or epinephrine-treated cells had similar activities in the presence of maximal exogenous cAMP and similar half-maximal cAMP concentrations. Finally, the levels of cAMP-binding proteins in extracts from naive and desensitized cells were essentially identical. We conclude that desensitization of S49 cells by very low levels of epinephrine significantly reduced cAMP-dependent protein kinase responses to much higher concentrations of the catecholamine.