Noradrenaline-induced increase in protein synthesis in adult rat cardiomyocytes: involvement of only α_1A-adrenoceptors

Abstract.

Adult rat ventricular cardiomyocytes contain α_1A- and α_1B-adrenoceptors (ARs, 20%:80%, assessed by [³H]prazosin binding). We studied which α₁-AR subtype mediates noradrenaline (NA)-induced increase in rate of protein synthesis, and which signalling pathway is involved. NA (10^–9–10^–4 M) concentration-dependently increased inositol phosphate (IP) formation (pEC₅₀-value=6.1±0.1, n=5) and protein synthesis (assessed as [³H]phenylalanine incorporation; pEC₅₀-value=6.6±0.1, n=6). NA-induced IP-formation was partly inhibited by the α_1B-AR antagonist chloroethylclonidine (CEC, 30 µM; 33±9% inhibition, n=5); following CEC-treatment the α_1A-AR-selective 5-methyl-urapidil (5-MU) inhibited NA-induced IP-formation with a pK _i-value of 9.2±0.2 (n=6); the α_1D-AR-selective BMY 7378 was only a weak antagonist (pK _i-value <7). NA-induced increase in protein synthesis was insensitive to CEC whereas 5-MU inhibited it with a pK _i-value of 9.1±0.2 (n=6). NA (1 µM)-induced increase in protein synthesis was inhibited by the protein kinase C (PKC) inhibitor bisindolylmaleimide (IC₅₀-value: 206 nM), the PI 3-kinase inhibitors wortmannin (IC₅₀=3.4 nM) and LY 294002 (IC₅₀=10 µM), and p70^s6-kinase inhibitor rapamycin (IC₅₀=123 pM) but not by the p38 MAP-kinase inhibitor SB 203580 (10 µM) or the MEK-inhibitor PD 98059 (25 µM). Moreover, 5-MU (30 nM) but not CEC inhibited NA-induced activation of p70^s6-kinase. We conclude that, in adult rat cardiomyocytes, α_1A- and α_1B-AR mediate NA-induced IP-formation but only α_1A-ARs mediate increase in protein synthesis. α_1A-AR-mediated increase in protein synthesis involves activation of a PKC, PI 3-kinase and p70^s6-kinase but not of ERK- or p38 MAP-kinase.