Different roles of α₁‐adrenoceptor subtypes in mediating cardiomyocyte protein synthesis in neonatal rats

Abstract

Summary 1. Three different α 1 ‐adrenoceptor subtypes, designated α 1A , α 1B and α 1D , have been cloned and identified pharmacologically in cardiomyocytes. In vitro studies have suggested that α 1 ‐adrenoceptors play an important role in facilitating cardiac hypertrophy. However, it remains controversial as to which subtype of α 1 ‐adrenoceptors is involved in this response. In the present study, we investigated the different role of each α 1 ‐adrenoceptor subtype in mediating cardiomyocyte protein synthesis, which is a most important characteristic of cardiac hypertrophy in cultured neonatal rat cardiomyocytes. 2. Cardiomyocyte hypertrophy was monitored by the following characteristic phenotypic changes: (i) an increase in protein synthesis; (ii) an increase in total protein content; and (iii) an increase in cardiomyocyte size. 3. The role of each α 1 ‐adrenoceptor subtype in mediating cardiomyocyte protein synthesis was investigated by the effect of specific α 1 ‐adrenoceptor subtype‐selective antagonists on noradrenaline‐induced [ 3 H]‐leucine incorporation. In addition, pK B values for α 1 ‐adrenoceptor subtype‐selective antagonists were calculated and compared with the corresponding pK i values to further identify their effects. 4. Activation of α 1 ‐adrenoceptors by phenylephrine or noradrenaline in the presence of propranolol significantly increased [ 3 H]‐leucine incorporation, protein content and cell size. 5. Pre‐incubating cardiomyocytes with 5‐methyl‐urapidil, RS 17053 or WB 4101 significantly inhibited noradrenaline‐induced [ 3 H]‐leucine incorporation. However, there was no effect when cardiomyocytes were pre‐incubated with BMY 7378. The correlation coefficients between pK B values for α 1 ‐adrenoceptor subtype‐selective antagonists and pK i values obtained from cloned α 1A ‐, α 1B ‐ or α 1D ‐adrenoceptors were 0.92 ( P < 0.01), 0.66 ( P > 0.05) and 0.24 ( P > 0.05), respectively. 6. Our results suggest that the α 1 ‐adrenoceptor is dominantly responsible for adrenergic hypertrophy of cultured cardiomyocytes in neonatal rats. The efficiency in mediating cardiomyocyte protein synthesis is α 1A > α 1B ≫ α 1D .