RECEPTOR MECHANISM OF THROMBIN-MEDIATED PULMONARY VASODILATION IN NEONATES

A recently identified peptide sequence exposed after proteolytic cleav age of the NH2-terminus of the thrombin receptor mimics some cellular effects of alpha-thrombin. To determine whether a proteolytic action o f thrombin is required for vasoactivity, we examined the vascular effe cts of modified thrombins and synthetic NH2-terminus peptide sequences of the thrombin receptor (TRPs) in isolated piglet lungs. Lungs of pi glets 1-6 days old were perfused with recirculating Ringer-albumin sol ution at a constant flow of 60 ml/min. We measured the pulmonary arter y pressure (P(pa)) and segmental distribution of pulmonary vascular re sistance (using the double occlusion method) in response to injections of human alpha-thrombin, modified thrombins, and TRP-14 and TRP-7 (i. e., 14 and 7 amino acid NH2-terminus peptides of the cleaved thrombin receptor). Alpha-Thrombin produced a rapid and transient decrease in P (pa); the magnitude and duration [time for one-half recovery (t1/2R)] of the vasodilation responses were concentration dependent [t1/2R valu es of 1.4 +/- 0.1 and 3.3 +/- 2.4 min (mean +/- SE) at concentrations of 10(-10) and 10(-9) M, respectively]. The vasodilation was due prima rily to a decrease in precapillary resistance. Proteolytically active, but binding-impaired gamma-thrombin was a less potent vasodilator and proteolytically inactive D-phenylalanyl-prolyl-arginine-chloromethyl ketone (PPACK)-alpha-thrombin did not induce vasodilation. TRP-14 was also a pulmonary vasodilator with a t1/2R value of 0.8 +/- 0.09 min at a concentration of 10(-7) M; both TRP-14 and TRP-7 were approximately 3-log less potent than equimolar alpha-thrombin. The vasodilator resp onse to alpha-thrombin, but not to TRP-14, was inhibited by hirudin. A cetylation of TRP-14 or omission of NH2-terminus serine from the hepta peptide caused loss of vasodilator activity. The results suggest that proteolytic cleavage of the thrombin receptor's NH2-terminus and expos ure of a tethered ligand sequence cause receptor autoactivation, there by initiating a pulmonary vasodilation response in the neonate.