Hypoxia selectively induces proliferation in a specific subpopulation of smooth muscle cells in the bovine neonatal pulmonary arterial media.

Medial thickening of the pulmonary arterial wall, secondary to smooth muscle cell (SMC) hyperplasia, is commonly observed in neonatal hypoxic pulmonary hypertension. Because recent studies have demonstrated the existence of multiple phenotypically distinct SMC populations within the arterial media, we hypothesized that these SMC subpopulations would differ in their proliferative responses to hypoxic pulmonary hypertension and thus contribute in selective ways to the vascular remodeling process. Expression of meta-vinculin, a muscle-specific cytoskeletal protein, has been shown to reliably distinguish two unique SMC subpopulations within the bovine pulmonary arterial media. Therefore, to assess the proliferative responses of phenotypically distinct SMC subpopulations in the setting of neonatal pulmonary hypertension, we performed double immunofluorescence staining on pulmonary artery cryosections from control and hypertensive calves with antibodies against meta-vinculin and the proliferation-associated nuclear antigen, Ki-67. We found that, although neonatal pulmonary hypertension caused significant increases in overall cell replication, proliferation occurred almost exclusively in one, the meta-vinculin-negative SMC population, but not the other SMC population expressing meta-vinculin. We also examined fetal pulmonary arteries, where proliferative rates were high and meta-vinculin expression again reliably distinguished two SMC subpopulations. In contrast to the hypertensive neonate, we found in the fetus that the relative proliferative rates of both SMC subpopulations were equal, thus suggesting the existence of different mechanisms controlling proliferation and expression of cytoskeletal proteins in the fetus and neonate. We conclude that phenotypically distinct SMC populations in the bovine arterial media exhibit specific and selective proliferative responses to neonatal pulmonary hypertension. Distinct SMC subpopulations may, thus, contribute in unique ways to vascular homeostasis under both normal and pathologic conditions.

Pilot dose-finding and safety study of bivalirudin in infants <6 months of age with thrombosis.

BACKGROUND: Thrombosis is not uncommon in children with serious medical conditions. Unfractionated heparin, the most commonly used anticoagulant in the acute management of thrombosis, has significant pharmacologic limitations, especially in infants. Newer anticoagulants have improved properties relative to heparin, and this may enhance the outcome in children. OBJECTIVE: To determine dosing, and to assess the safety and efficacy of bivairudin for infants with thrombosis. METHODS: Infants <6 months old were chosen for this pilot study as they may most benefit from a direct thrombin inhibitor because of their physiologically low antithrombin levels. This was an open label, dose-finding and safety study. Patients received one of three bolus doses and one of two initial infusion doses with subsequent dosing adjusted utilizing the activated partial thromboplastin time. Safety was assessed by specific bleeding endpoints. Efficacy was determined by reassessing the initial imaging study at 48-72 h and by measurement of molecular markers of thrombin generation. RESULTS: Sixteen patients completed the study. All three bolus doses resulted in therapeutic anticoagulation, as did both initial infusion doses. A dose-response effect was noted for the continuous infusion but not the bolus dosing. Two patients met the study criteria for major bleeding, both with gross hematuria, which resolved with a reduction in the bivalirudin infusion rate. In terms of efficacy, 37.5% of patients had complete or partial resolution of their thrombosis by 48-72 h. There was a significant decrease in all three molecular markers of thrombin generation. CONCLUSION: This study demonstrates the potential utility of bivalirudin in the pediatric population.

Expression and localization of tropoelastin mRNA in the developing bovine pulmonary artery is dependent on vascular cell phenotype.

During vascular development, the expression of tropoelastin (TE) messenger ribonucleic acid (mRNA) has been shown to be time dependent and to form complex patterns along the longitudinal and radial arterial axes. The factors contributing to these patterns of TE expression are not known, but it has been suggested that they reflect phenotypic changes in developing smooth muscle cells (SMC). In order to examine a possible correlation between the developmental state of the SMC and TE expression during lung vascular development, we localized and assessed relative TE mRNA expression in the developing bovine main pulmonary artery (PA), and correlated the observed patterns of TE expression to changes in SMC phenotype as determined by the expression of various developmentally related SMC proteins. Further, because TE expression can be modulated by physical forces such as pressure, fetal PA TE expression was evaluated with regard to changes in fetal arterial pressure. We found that expression of TE mRNA exhibited a biphasic pattern during fetal development. In early gestation, expression was noted throughout the entire PA wall; at midgestation, expression was markedly decreased in the outer wall but maintained in the inner vascular media; at late gestation, reexpression was observed throughout the entire PA wall, albeit in a heterogeneous pattern. Immunohistochemical studies showed that the decrease in SMC TE expression during midgestation coincided with the acquisition of SMC-specific proteins such as smooth muscle myosin heavy chains and desmin. The reexpression of TE late in gestation occurred in these "differentiated" SMC and was temporally associated with a large increase in arterial pressure shown to occur in late gestation. In addition, we identified an SMC population defined by its immunoreactivity to the muscle-specific cytoskeletal protein meta-vinculin that did not express TE mRNA either during fetal PA development or postnatally when PA hypertension was induced. We conclude that both the developmental state of the SMC and hemodynamic forces correlate with the pattern of PA TE mRNA expression during pulmonary vascular development. Further, a subpopulation of SMC defined by meta-vinculin expression exists in the fetal and neonatal bovine vascular wall and does not express detectable levels of TE mRNA regardless of vascular pressure.