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 t o characterize further these effects we studied, in neonatal rat ventr icular cardiomyocytes, the effects of several endothelin-receptor anta gonists and the AT(1)-receptor antagonist losartan on AII- and endothe lin-induced inositol phosphate (IP)-formation (assessed as accumulatio n of total [H-3]-IPs in myo-[H-3]-inositol prelabelled cells) and incr ease in rate of protein synthesis (assessed as [H-3]-phenylalanine inc orporation). 2 Endothelin (10 pM-1 mu M) concentration-dependently inc reased IP-formation (max. increase at 100 nM ET-1: 130+/-14% above bas al, n=25) and [H-3]-phenylalanine incorporation (max. increase at 1 mu M: 52+/-4% above basal, n=16) with an order of potency: ET-1>>ET-3. B oth effects were antagonized by the ETA/ETB-receptor antagonist bosent an and the ETA-receptor antagonist BQ-123, but not affected by the ETB -receptor antagonist IRL 1038 and the AT(1)-receptor antagonist losart an. 3 Pretreatment of the cells with 500 ng ml(-1) pertussis toxin (PT X) overnight that completely inactivated PTX-sensitive G-proteins did not attenuate but rather enhance ET-1-induced IP-formation. On the oth er hand. in PTX-pretreated cardiomyocytes ET-1-induced [H-3]-phenylala nine incorporation was decreased by 39+/-5% (n=5). 4. AII (1 nM-1 mu M ) concentration-dependently increased IP-formation (max, increase at 1 mu M: 42+/-7% above basal, n=16) and [H-3]-phenylalanine incorporatio n (max. increase at 1 mu M: 29+/-2%, n=9). These effects were antagoni zed 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 [H-3]-phenylalanine incorpor ation; addition of non-myocyte cells to the cardiomyocytes restored AI I-induced increase in [H-3]-phenylalanine incorporation. 6 We conclude that, in rat neonatal ventricular cardiomyocytes, (a) the ET-1-induce d increase in rate of protein synthesis (through ETA-receptor stimulat ion) involves at least two signalling pathways: one via a PTX-insensit ive G-protein coupled to IP-formation, and the other one via a PTX-sen sitive G-protein. and (b) the trophic effects of AII are brought about via local ET-1 secretion upon AT(1)-receptor stimulation in neonatal rat ventricular non-myocyte cells.