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Acute hypoxic pulmonary vasoconstriction (HPV) and the development of chronic hypoxic pulmonary hypertension (PHTN) are cardinal features of the pulmonary circulation that differentiate this vascular bed from the systemic circulation. Nitric oxide (NO) produced by pulmonary vascular endothelium is thought to modulate pulmonary vascular responses to a variety of vasoconstrictor stimuli, including hypoxia (1-8). However, despite intensive investigation over the past decade, the role of endotheliumderived nitric oxide (EDNO) in modulating tone and structural remodeling of the chronically hypoxic pulmonary circulation remains controversial (9-13).Simultaneous pharmacologic inhibition of all three isoforms of nitric oxide synthase (NOS) results in acutely increased pulmonary vascular resistance and augmented HPV (1,(14)(15). However, chronic NOS inhibition does not result in PHTN and does not augment the development of hypoxic PHTN (16). These results present a paradox: If EDNO modulates acute HPV, why is pharmacologic inhibition of NOS not associated with either normoxic PHTN or accentuated chronic hypoxic PHTN? One possibility is that HPV is redundantly modulated, and the loss of endothelial nitric oxide synthase (eNOS)-derived NO alone is not sufficient to produce PHTN. Alternatively, the experimental approaches taken in the past to test this question may have been inadequate because of nonspecific effects of pharmacologic inhibitors of NOS, difficulty maintaining NOS inhibition, or confounding effects of simultaneous inhibition of all three isoforms of NOS.Mice with targeted disruption of eNOS (eNOS -/-mice) have recently been constructed and the vascular phenotype explored (17,18). Systemic hypertension and augmented structural remodeling after vascular injury have been reported (19,20). Steudel et al. (21) investigated the pulmonary vascular phenotype of eNOS -/-mice, finding increased pulmonary vascular resistance, but only minimal PHTN, and no evidence of pulmonary vascular remodeling. More recently, this group also found enhanced chronic hypoxic PHTN in eNOS-null mice (22). These studies were limited, however, to mice exposed to severe hypoxia (FiO 2 = 11%) and studied under general anesthesia with hyperoxic (FiO 2 = 80%) mechanical ven- Acute hypoxic vasoconstriction and development of hypoxic pulmonary hypertension (PHTN) are unique properties of the pulmonary circulation. The pulmonary endothelium produces vasoactive factors, including nitric oxide (NO), that modify these phenomena. We tested the hypothesis that NO produced by endothelial nitric oxide synthase (eNOS) modulates pulmonary vascular responses to hypoxia using mice with targeted disruption of the eNOS gene (eNOS -/-). Marked PHTN was found in eNOS -/-mice raised in mild hypoxia when compared with either controls or eNOS -/-mice raised in conditions simulating sea level. We found an approximate twofold increase in partially and fully muscularized distal pulmonary arteries in eNOS -/-mice compared with controls. Consistent with vasoconstriction...

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Abstract-Bone morphogenetic peptides (BMPs), a family of cytokines critical to normal development, were recently implicated in the pathogenesis of familial pulmonary arterial hypertension. The type-II receptor (BMPRII) is required for recognition of all BMPs, and targeted deletion of BMPRII in mice results in fetal lethality before gastrulation. To overcome this limitation and study the role of BMP signaling in postnatal vascular disease, we constructed a smooth muscle-specific transgenic mouse expressing a dominant-negative BMPRII under control of the tetracycline gene switch (SM22-tet-BMPRII delx4ϩ mice). When the mutation was activated after birth, mice developed increased pulmonary artery pressure, RV/LVϩS ratio, and pulmonary arterial muscularization with no increase in systemic arterial pressure. Studies with SM22-tet-BMPRII delx4ϩ mice support the hypothesis that loss of BMPRII signaling in smooth muscle is sufficient to produce the pulmonary hypertensive phenotype. Key Words: artery Ⅲ bone morphogenetic peptide Ⅲ hypertension Ⅲ smooth muscle Ⅲ vascular A ssociation between abnormal bone morphogenetic peptide (BMP) signaling and pulmonary vascular disease was suggested by genetic studies of familial pulmonary arterial hypertension (PAH), a disorder characterized by the pathological development of increased pressure and structural remodeling in the pulmonary circulation later in life. 1,2 Fifteen to 25% of cases of PAH occur in families, with autosomal-dominant inheritance, and in the year 2000, two groups independently identified the presence of mutations in the BMRII gene in familial PAH. 3,4 Subsequently, approximately 25% of sporadic cases of PAH were also found to be associated with mutations in BMPRII. 5 Dissecting the function of BMPII using transgenic mice has identified a critical role for BMP signaling in development. 6 Although there is redundancy at the level of ligands, type I receptors, intra-and extracellular inhibitors and downstream signaling via SMADs, functional BMPRII is an absolute requirement for BMP signaling. Consequently, BM-PRII Ϫ/Ϫ mice die early in development, before gastrulation, whereas BMPRII ϩ/Ϫ mice develop normally and have no apparent phenotype. 7 To overcome the developmental lethality identified by traditional transgenic approaches, we constructed a conditional, tissue-specific BMPRII transgenic mouse, using a smooth muscle cell-specific promotor and a dominant-negative BMPRII identified in a family with PAH. 8 Using these mice, we asked two questions: (1) is expression of a strong loss-of-function mutation in BMPRII sufficient to produce pulmonary hypertension, and (2) is loss of BMPRII function in smooth muscle alone sufficient to produce pulmonary hypertension? Materials and Methods Construction of SM22-tet-BMPRII delx4؉ Transgenic MiceThe mice were generated at the University of Cincinnati Transgenic Mouse Science from plasmids we provided. The murine SM22 promotor (488 bp starting 340 bp before the start site and extending 148 bp into the 5Ј UTR) was used to drive...