Inotropic effects of endothelin compared to noradrenaline in porcine myocardial trabeculae

Isolated porcine myocardial trabeculae from the right atria and left ventri cles were paced at 1.5 Hz in tissue baths, and changes in isometric contrac tile force upon exposure to noradrenaline (NA), endothelin-1 (ET-1) and end othrlin-3 (ET-3) were studied. The endothelin-B- (ETB) receptor agonists IR L 1620 and sarafotoxin S6c (S6c), and the endothelin-A- (ETA) receptor anta gonist FR139317 were used to assess the functional involvement of ETA- and ETB-receptors. NA, ET-1 and ET-3 induced strong increases in contractile am plitude of all trabeculae. In both atrial and ventricular trabeculae the in creases in contractile amplitudes induced by ET-1 and ET-3 were significant ly lower than those induced by noradrenaline, whereas the potencies for ET- 1 and ET-3 were significantly higher than those for noradrenaline (p < 0.05 , n = 6-10 in each group). The positive inotropic effect of ET-1 was antago nized by preincubation with FR139317 (10(-6) hi). IRL 1620 had a positive i notropic effect in only a few of the ventricular but in none of the atrial trabeculae, and S6s had no positive inotropic effect in either atrial or ve ntricular trabeculae (n = 6-9 in each group). These results suggest that po sitive inotropic responses can be mediated by both ETA- and ETB-receptors.