Pathogenesis of hypoxic pulmonary hypertension is initiated by oxidative injury to the pulmonary vascular wall. Because nitric oxide (NO) can contribute to oxidative stress and because the inducible isoform of NO synthase (iNOS) is often upregulated in association with tissue injury, we hypothesized that iNOS-derived NO participates in the pulmonary vascular wall injury at the onset of hypoxic pulmonary hypertension. An effective and selective dose of an iNOS inhibitor, L-N 6 -(1-iminoethyl)lysine (L-NIL), for chronic peroral treatment was first determined (8 mg/l in drinking water) by measuring exhaled NO concentration and systemic arterial pressure after LPS injection under ketamineϩxylazine anesthesia. A separate batch of rats was then exposed to hypoxia (10% O2) and given L-NIL or a nonselective inhibitor of all NO synthases, N G -nitro-L-arginine methyl ester (L-NAME, 500 mg/l), in drinking water. Both inhibitors, applied just before and during 1-wk hypoxia, equally reduced pulmonary arterial pressure (PAP) measured under ketamineϩxylazine anesthesia. If hypoxia continued for 2 more wk after L-NIL treatment was discontinued, PAP was still lower than in untreated hypoxic controls. Immunostaining of lung vessels showed negligible iNOS presence in control rats, striking iNOS expression after 4 days of hypoxia, and return of iNOS immunostaining toward normally low levels after 20 days of hypoxia. Lung NO production, measured as NO concentration in exhaled air, was markedly elevated as early as on the first day of hypoxia. We conclude that transient iNOS induction in the pulmonary vascular wall at the beginning of chronic hypoxia participates in the pathogenesis of pulmonary hypertension. pulmonary circulation; nitric oxide; rat; inducible nitric oxide synthase SINCE THE DISCOVERY THAT NITRIC OXIDE (NO) is formed in mammalian cells as an endogenous mediator, many attempts were made to define its possible role in the pathogenesis of pulmonary hypertension (reviewed in Ref. 23). Although the capacity of lung vessels to produce NO can be reduced in terminal phases of severe pulmonary hypertension (15), possibly due to the progressive endothelial damage, less advanced stages (at least in adults) are associated with increased expression of NO synthase (NOS) and augmented NO production (reviewed in Ref. 23). This is particularly well documented in the frequently used and clinically relevant model of pulmonary hypertension elicited by chronic hypoxia.In principle, as the actions of NO in the body are multifaceted, two main functional consequences of the elevated lung NO synthesis in chronic hypoxic pulmonary hypertension are possible. On one hand, the vasodilator and antiproliferative effects of NO may limit the extent of pulmonary vascular resistance elevation. This possibility is supported by numerous reports that acute administration of NOS blockers, such as N G -nitro-L-arginine methyl ester (L-NAME), increases perfusion pressure in lungs isolated from chronically hypoxic animals more than in normoxic controls (rev...

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The anorexic agent fenfluramine considerably increases the risk of primary pulmonary hypertension. The mechanism of this effect is unknown. The appetite-reducing action of fenfluramine is mediated by its interaction with the metabolism of serotonin [5-hydroxytryptamine (5-HT)] in the brain. We tested the hypothesis that the pulmonary vasoconstrictive action of fenfluramine is at least in part mediated by 5-HT receptor activation. In addition, we sought to determine whether pharmacological reduction of voltage-gated potassium (K(V)) channel activity would potentiate the pulmonary vascular reactivity to fenfluramine. Using isolated rat lungs perfused with Krebs-albumin solution, we compared the inhibitory effect of ritanserin, an antagonist of 5-HT(2) receptors, on fenfluramine- and 5-HT-induced vasoconstriction. Both 5-HT (10(-5) mol/l) and fenfluramine (5 x 10(-4) mol/l) caused significant increases in perfusion pressure. Ritanserin at a dose (10(-7) mol/l) sufficient to inhibit >80% of the response to 5-HT reduced the response to fenfluramine by approximately 50%. A higher ritanserin dose (10(-5) mol/l) completely abolished the responses to 5-HT but had no more inhibitory effect on the responses to fenfluramine. A pharmacological blockade of K(V) channels by 4-aminopyridine (3 x 10(-3) mol/l) markedly potentiated the pulmonary vasoconstrictor response to fenfluramine but was without effect on the reactivity to 5-HT. These data indicate that the pulmonary vasoconstrictor response to fenfluramine is partly mediated by 5-HT receptors. Furthermore, the pulmonary vasoconstrictor potency of fenfluramine is elevated when the K(V)-channel activity is low. This finding suggests that preexisting K(V)-channel insufficiency may predispose some patients to the development of pulmonary hypertension during fenfluramine treatment.

Exposure to hypoxia, leading to hypoxic pulmonary hypertension (HPH), is associated with activation of alveolar macrophages (AM). However, it remains unclear how AM participate in this process. There are studies which imply that the AM product monocyte chemoattractant protein-1 (MCP-1) plays an important role. Thus we tested: 1. if the selective elimination of AM attenuates HPH in rats, 2. the correlation of MCP-1 plasmatic concentrations with the presence and absence of AM during exposure to hypoxia, 3. the direct influence of hypoxia on MCP-1 production in isolated AM. We found that experimental depletion of AM attenuated the chronic hypoxia-induced increase in mean pulmonary arterial pressure, but did not affect the serum MCP-1 concentrations. Furthermore, the MCP-1 production by AM in vitro was unaffected by hypoxia. Thus we conclude that AM play a significant role in the mechanism of HPH, but MCP-1 release from these cells is most likely not involved in this process. The increase of MCP-1 accompanying the development of HPH probably originates from other sources than AM.

Background: Chronic hypoxia induces lung vascular remodeling, which results in pulmonary hypertension. Vascular remodeling is associated with collagenolysis and activation of matrix metalloproteinases (MMPs). One of the possible sources of MMPs in hypoxic lung are mast cells. Objective: The role of lung mast cell collagenolytic activity in hypoxic pulmonary hypertension was tested by the inhibitor of mast cell degranulation disodium cromoglycate (DSCG). Methods: Rats were treated with DSCG in an early or later phase of isobaric hypoxia. Control groups were exposed to hypoxia only or to normoxia. Lung hemodynamics, muscularization and collagen metabolism in the walls of peripheral pulmonary vessels in the lungs were measured. Results: DSCG applied at an early phase of exposure to hypoxia reduced the development of pulmonary hypertension, inhibited muscularization in peripheral pulmonary arteries and decreased the amount of collagen cleavage fragments in prealveolar vessels. Conclusions: Mast cell degranulation plays a role in the initiation of hypoxic pulmonary vascular remodeling.