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

1. Three different alpha(1)-adrenoceptor subtypes, designated alpha(1A), alpha(1B) and alpha(1D), have been cloned and identified pharmacologically in cardiomyocytes. In vitro studies have suggested that alpha(1)-adrenoceptors play an important role in facilitating cardiac hypertrophy. However, it remains controversial as to which subtype of alpha(1)-adrenoceptors is involved in this response. In the present study, we investigated the different role of each alpha(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 alpha(1)-adrenoceptor subtype in mediating cardiomyocyte protein synthesis was investigated by the effect of specific alpha(1)-adrenoceptor subtype-selective antagonists on noradrenaline-induced [H-3]-leucine incorporation. In addition, pK(B) values for alpha(1)-adrenoceptor subtype-selective antagonists were calculated and compared with the corresponding pK(i) values to further identify their effects. 4. Activation of alpha(1)-adrenoceptors by phenylephrine or noradrenaline in the presence of propranolol significantly increased [H-3]-leucine incorporation, protein content and cell size. 5. Pre-incubating cardiomyocytes with 5-methyl-urapidil, RS 17053 or WB 4101 significantly inhibited noradrenaline-induced [H-3]-leucine incorporation. However, there was no effect when cardiomyocytes were pre-incubated with BMY 7378. The correlation coefficients between pK(B) values for alpha(1)-adrenoceptor subtype-selective antagonists and pK(i) values obtained from cloned alpha(1A)-, alpha(1B)- or alpha(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 alpha(1)-adrenoceptor is dominantly responsible for adrenergic hypertrophy of cultured cardiomyocytes in neonatal rats. The efficiency in mediating cardiomyocyte protein synthesis is alpha(1A) > (alpha(1B) much greater than alpha(1D).