ETA receptors are the primary mediators of myofilament calcium sensitization induced by ET-1 in rat pulmonary artery smooth muscle: a tyrosine kinaseindependent pathway

1 We have investigated the possibility that ET-1 can induce an increase in myofilament calcium sensitivity in pulmonary artery smooth muscle. Arterial rings were permeabilized using a-toxin (120 mu g ml(-1)), in the presence of A23187 (10 mu M) to 'knock out' Ca2+ stores, and pre-constricted with pC a 6.8 (buffered with 10 mM EGTA). In the presence of this fixed Ca2+ concen tration, 1 mu M ET-I induced a sustained, reversible constriction of 0.15 m N. 2 Pulmonary arterial rings were freeze-clamped at the peak of the induced c onstriction (time matched). Subsequent densitometric analysis revealed that ET-I (1 mu M) increased the level of phosphorylated myosin light chains by 34% compared to an 11% increase in the presence of pCa 6.8 alone. 3 In contrast to ET-1, the selective ETB receptor agonist Sarafotoxin S6C ( 100 nh I) failed to induce a significant constriction. 4 The constriction induced by 1 mu M ET-I was reversibly inhibited when the preparation was preincubated (15 min) with the ETA receptor antagonist BQ 123 (100 mu M). The constriction measured 0.13 mN in the absence and 0.07 m N in the presence of 100 mu M BQ 123. 5 In contrast, the constriction induced by 1 mu M ET-I measured 0.19 mN in the absence and 0.175 mN following a 15 min pre-incubation with the ETB ant agonist BQ 788 (100 mu M). 6 The constriction induced by 1 mu M ET-I measured 0.14 mN in the presence and 0.13 mN following pre-incubation with the tyrosine kinase inhibitor Tyr phostin A23 (100 mu M). 7 We conclude that ET-1 induced an increase in myofilament calcium sensitiv ity in rat pulmonary arteries via the activation of ETA receptors and by a mechanism(s) independent of tyrosine kinase.