Regulation of beta-adrenoceptor density and mRNA levels in the rat heart cell-line H9c2.

The regulation of the expression of beta-adrenoceptor (beta-ARs) is not thoroughly understood. We demonstrate that the rat heart cell-line H9c2 expresses both beta 1- and beta 2-ARs. In radioligand-binding experiments, the maximal binding capacity of (-)-[125I]-iodocyanopindolol was determined as 18 +/- 0.6 fmol/mg of protein with a KD of 35.4 +/- 4.1 pM. Competitive radioligand-binding experiments with subtype-specific beta-antagonists reveal a subtype ratio of beta 1- to beta 2-ARs of 29%: 71%. With competitive reverse-transcriptase PCR we found beta 2-mRNA to be up to 1600 times more frequent than beta 1-mRNA. Treatment of the H9c2 cell-line with the beta-adrenergic agonist (-)-isoproterenol (10(-6) M), the antagonist (-)-propranolol (10(-6) M) and the glucocorticoid dexamethasone (500 nM) induces regulatory effects on both the beta-AR protein and mRNA level. Isoproterenol treatment leads to down-regulation of the total receptor number by 56 +/- 4%, due to a decrease in beta 2-ARs, while maintaining the beta 1-AR number constant. On the transcription level, both beta 1-and beta 2-mRNAs are decreased by 30% and 42% respectively. mRNA stability measurements reveal a reduced half-life of beta 2-mRNA from 9.3 h to 6.5 h after isoproterenol treatment. Incubation of cells with (-)-propranolol does not affect the amounts of beta-ARs and their mRNAs. Dexamethasone induces a 1.8 +/- 0.2-fold increase in beta-AR number over the basal level as well as a 1.9 +/- 0.2-fold increase in the amount of beta 2-mRNA. Because the half-life of beta 2-mRNA was unaffected by dexamethasone, the increased beta 2-mRNA level must be due to an enhanced transcription rate. The beta 1-mRNA levels are unchanged during dexamethasone-incubation of the cells. Our data clearly demonstrate that treatment of H9c2 rat heart cells with isoproterenol and dexamethasone induces alterations in the level of RNA stability as well as gene transcription, leading to altered receptor numbers.