Receptor mechanism of thrombin-mediated pulmonary vasodilation in neonates
Pinheiro, J.M.; Andersen, T.T.; Malik, A.B.
American Journal of Physiology 265(4 Pt 1): L355-L359
1993
ISSN/ISBN: 0002-9513 PMID: 8238369 Document Number: 416242
A recently identified peptide sequence exposed after proteolytic cleavage of the NH-2-terminus of the thrombin receptor mimics some cellular effects of alpha-thrombin. To determine whether a proteolytic action of thrombin is required for vasoactivity, we examined the vascular effects of modified thrombins and synthetic NH-2-terminus peptide sequences of the thrombin receptor (TRPs) in isolated piglet lungs. Lungs of piglets 1-6 days old were perfused with recirculating Ringer-albumin solution at a constant flow of 60 ml/min. We measured the pulmonary artery pressure (P-pa) and segmental distribution of pulmonary vascular resistance (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 NH-2-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 (t-1/2R)) of the vasodilation responses were concentration dependent (t-1/2R values 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 primarily 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 t-12R value of 0.8 +- 0.09 min at a concentration of 10-7 M; both TRP-14 and TRP-7 were apprx 3-log less potent than equimolar a-thrombin. The vasodilator response to alpha-thrombin, but not to TRP-14, was inhibited by hirudin. Acetylation of TRP-14 or omission of NH-2-terminus serine from the heptapeptide caused loss of vasodilator activity. The results suggest that proteolytic cleavage of the thrombin receptor's NH-2-terminus and exposure of a tethered ligand sequence cause receptor autoactivation, thereby initiating a pulmonary vasodilation response in the neonate.