Trophic effect of angiotensin II in neonatal rat cardiomyocytes: role of endothelin-1 and non-myocyte cells

1. Angiotensin II (AII) and the endothelins (ET) are known to be potent trophic stimuli in various cells including cardiomyocytes. In order to characterize further these effects we studied, in neonatal rat ventricular cardiomyocytes, the effects of several endothelin-receptor antagonists and the AT₁-receptor antagonist losartan on AII- and endothelin-induced inositol phosphate (IP)-formation (assessed as accumulation of total [³H]-IPs in myo-[³H]-inositol prelabelled cells) and increase in rate of protein synthesis (assessed as [³H]-phenylalanine incorporation).
2. Endothelin (10 pM–1 μM) concentration-dependently increased IP-formation (max. increase at 100 nM ET-1: 130±14% above basal, n=25) and [³H]-phenylalanine incorporation (max. increase at 1 μM: 52±4% above basal, n=16) with an order of potency: ET-1>>ET-3. Both effects were antagonized by the ET_A/ET_B-receptor antagonist bosentan and the ET_A-receptor antagonist BQ-123, but not affected by the ET_B-receptor antagonist IRL 1038 and the AT₁-receptor antagonist losartan.
3. Pretreatment of the cells with 500 ng ml⁻¹ pertussis toxin (PTX) overnight that completely inactivated PTX-sensitive G-proteins did not attenuate but rather enhance ET-1-induced IP-formation. On the other hand, in PTX-pretreated cardiomyocytes ET-1-induced [³H]-phenylalanine incorporation was decreased by 39±5% (n=5).
4. AII (1 nM–1 μM) concentration-dependently increased IP-formation (max. increase at 1 μM: 42±7% above basal, n=16) and [³H]-phenylalanine incorporation (max. increase at 1 μM: 29±2%, n=9). These effects were antagonized by losartan, but they were also antagonized by bosentan and BQ-123.
5. In well-defined cultures of cardiomyocytes (not contaminated with non-myocyte cells) AII failed to increase [³H]-phenylalanine incorporation; addition of non-myocyte cells to the cardiomyocytes restored AII-induced increase in [³H]-phenylalanine incorporation.
6. We conclude that, in rat neonatal ventricular cardiomyocytes, (a) the ET-1-induced increase in rate of protein synthesis (through ET_A-receptor stimulation) involves at least two signalling pathways: one via a PTX-insensitive G-protein coupled to IP-formation, and the other one via a PTX-sensitive G-protein, and (b) the trophic effects of AII are brought about via local ET-1 secretion upon AT₁-receptor stimulation in neonatal rat ventricular non-myocyte cells.