Chronic hypoxia attenuates cGMP-dependent pulmonary vasodilation

Abstract

Jernigan, Nikki L., and Thomas C. Resta. Chronic hypoxia attenuates cGMP-dependent pulmonary vasodilation. Am J Physiol Lung Cell Mol Physiol 282: L1366-L1375, 2002. First published January 18, 2002 10.1152/ajplung.00273. 2001.-Chronic hypoxia (CH) augments endothelium-derived nitric oxide (NO)-dependent pulmonary vasodilation; however, responses to exogenous NO are reduced following CH in female rats. We hypothesized that CH-induced attenuation of NO-dependent pulmonary vasodilation is mediated by downregulation of vascular smooth muscle (VSM) soluble guanylyl cyclase (sGC) expression and/or activity, increased cGMP degradation by phosphodiesterase type 5 (PDE5), or decreased VSM sensitivity to cGMP. Experiments demonstrated attenuated vasodilatory responsiveness to the NO donors S-nitroso-N-acetylpenicillamine and spermine NONOate and to arterial boluses of dissolved NO solutions in isolated, saline-perfused lungs from CH vs. normoxic female rats. In additional experiments, the sGC inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, blocked vasodilation to NO donors in lungs from each group. However, CH was not associated with decreased pulmonary sGC expression or activity as assessed by Western blotting and cGMP radioimmunoassay, respectively. Consistent with our hypothesis, the selective PDE5 inhibitors dipyridamole and T-1032 augmented NO-dependent reactivity in lungs from CH rats, while having little effect in lungs from normoxic rats. However, the attenuated vasodilatory response to NO in CH lungs persisted after PDE5 inhibition. Furthermore, CH similarly inhibited vasodilatory responses to 8-bromoguanosine 3Ј5Ј-cyclic monophosphate. We conclude that attenuated NO-dependent pulmonary vasodilation after CH is not likely mediated by decreased sGC expression, but rather by increased cGMP degradation by PDE5 and decreased pulmonary VSM reactivity to cGMP. isolated rat lung; nitric oxide; soluble guanylate cyclase; phosphodiesterase; pulmonary hypertension CHRONIC EXPOSURE TO HYPOXIA results in structural as well as functional alterations in the pulmonary vasculature, leading to the development of pulmonary hypertension and right ventricular hypertrophy. The principal mediators of chronic hypoxia (CH)-induced pulmonary hypertension are polycythemia, pulmonary arterial remodeling, and arterial constriction. Among the corresponding functional alterations is responsiveness to vasoactive factors. Studies from our laboratory (23,24,26) and others (12,20,22,28,29) have shown that CH augments pulmonary vasodilatory responses to endothelium-derived nitric oxide (EDNO)-dependent vasodilators. EDNO is synthesized in the vasculature by endothelial nitric oxide synthase (eNOS), and several studies have demonstrated that CH is associated with increased pulmonary eNOS levels, gene expression, and activity (12,15,20,23,24,32,39). Consistent with elevated eNOS levels, nitric oxide (NO) synthesis appears to be greater in lungs isolated from CH rats compared with normoxic controls (12,20,30,36). Despite enhanced reactivit...

Pulmonary PKG-1 is upregulated following chronic hypoxia

“…The reason for this discrepancy is unclear but may be accounted for by altered activity and/or expression of cellular targets of cGMP. Consistent with this possibility are findings from our laboratory that responses to the membrane-permeable cGMP analog 8-bromoguanosine 3',5'-cyclic monophosphate are similarly impaired following CH (19). Therefore, we hypothesized that attenuated cGMP-dependent pulmonary vasodilation following CH is mediated by downregulation of VSM PKG-1 expression and/or activity.…”

“…Consistent with elevated eNOS levels, nitric oxide (NO) synthesis appears to be greater in lungs isolated from CH rats compared with controls (18,26,44). In contrast to enhanced reactivity to EDNO-dependent vasodilators, our laboratory (19,34) and others (35) have demonstrated impaired responsiveness to exogenous NO following CH. However, the mechanism(s) responsible for this diminished reactivity is not fully understood.…”

“…Right ventricular (RV) hypertrophy was assessed as an index of CH-induced pulmonary hypertension, as previously described (19,33,34). In brief, after isolation of the heart, the atria and major vessels were removed from the ventricles.…”

“…Rats from each group were anesthetized with pentobarbital sodium (52 mg ip), and the lungs were isolated for recirculating perfusion with a physiological saline solution (PSS) as previously described (19,33,34). After the trachea was cannulated with a 17-gauge needle stub, the lungs were ventilated with a Harvard positive-pressure rodent ventilator (model 683) at a frequency of 55 breaths/min and a tidal volume of 2.5 ml with a warmed and humidified gas mixture (6% CO 2 in room air).…”

“…The subsequent elevation in cGMP activates protein kinase G-1 (PKG-1), causing relaxation through a variety of mechanisms involving decreased VSM intracellular calcium and desensitization of the contractile apparatus to calcium (24). Interestingly, CH appears to elevate pulmonary sGC activity, protein, and mRNA expression (22), despite evidence for attenuated NO-mediated vasodilation (19). The reason for this discrepancy is unclear but may be accounted for by altered activity and/or expression of cellular targets of cGMP.…”