Hypoxic regulation of nitric oxide signaling in vascular smooth muscle

Abstract

THE CURRENT ARTICLE IN FOCUS by Mingone and colleagues (Ref. 16, see p. L296 in this issue) describes a novel permissive effect of hypoxia on cGMP-independent relaxation to nitric oxide (NO) in bovine pulmonary and coronary arteries. Interestingly, whereas NO-mediated vasorelaxation appeared to be largely a function of cGMP at ambient PO 2 , a cGMP-independent mechanism involving activation of the sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA) was revealed during hypoxic exposure. This study emphasizes the emerging complexity of NO signaling in vascular smooth muscle and raises important new questions regarding mechanisms by which PO 2 regulates cGMP-independent actions of NO.Both cGMP-dependent and -independent mechanisms of NO signaling have been implicated in vascular smooth muscle (1, 2, 4-8, 10, 13, 15, 16, 19, 20, 31, 32). It is well established that soluble guanylyl cyclase is an important target of NO in both the pulmonary and systemic circulations (8,10,13,15,19). Activation of soluble guanylyl cyclase by NO leads to increased cGMP synthesis and stimulation of protein kinase G (PKG) (8,15,19). PKG, in turn, elicits relaxation of vascular smooth muscle through a myriad of signaling pathways, leading to a decrease in the intracellular free Ca 2ϩ concentration ([Ca 2ϩ ] i ) and desensitization of the contractile apparatus to Ca 2ϩ (8,15). Those pathways involving a decrease in [Ca 2ϩ ] i are, for the most part, poorly understood. However, evidence exists for PKG-dependent activation of large-conductance Ca 2ϩ -activated K ϩ (BK) channels and associated membrane hyperpolarization, inhibition of L-type voltage-gated Ca 2ϩ channels, stimulation of Ca 2ϩ -ATPases on both the plasma membrane and sarcoplasmic reticulum, inhibition of inositol trisphosphate receptors, and decreased inositol trisphosphate synthesis (8,15). Those mechanisms involving a decrease in sensitivity of the contractile apparatus to Ca 2ϩ in response to PKG activation are even less well defined but appear to be predominantly mediated by regulation of myosin light chain phosphorylation subsequent to activation of myosin light chain phosphatase (8,15,19,21,22).In addition, NO signals through cGMP-independent mechanisms in many tissues by nitrosylation of cysteine thiol groups or transition metals to posttranslationally modify enzymatic activity (3,11,12,18,(23)(24)(25)29). Many cGMP-independent influences of NO in vascular smooth muscle appear to involve an increase in sarcolemmal K ϩ permeability. For example, NO has been reported to activate BK channels either directly (7) or indirectly via inhibition of 20-hydroxyeicosatetraenoic (20-HETE) production (4, 5, 31) or increased calcium spark activity (20). Additional evidence supports a role for cGMP-independent activation of delayed rectifier K ϩ channels in pulmonary arterial smooth muscle cells (32). Other direct effects of NO include stimulation of SERCA in rabbit aortic smooth muscle, leading to increased Ca 2ϩ reuptake by intracellular stores, a fall in [Ca 2ϩ ] i , and ...

Vascular nitric oxide resistance in type 2 diabetes

“…3 . Signal transduction of NO• in VSM involves two major pathways: (1) the indirect pathway of NO•-sGC-cGMP-PKG [ 128 , 150 , 151 ] and (2) the direct pathway of protein S -nitrosylation [ 128 , 150 , 151 ], also called cGMP-dependent and -independent pathways, respectively [ 152 ] ( Fig. 3 ) .…”

“…3 ) . Although both pathways are found in the VSM, NO•-mediated vasorelaxation is mostly cGMP-dependent at a normal oxygen pressure [ 152 ]. The NO• concentrations eliciting a physiological response in VSM have been estimated to fall from 100 pM to 5 nM [ 153 ].…”

“…cGMP exerts its effects in VSMCs by cGMP-dependent protein kinase (PKG, or cGK) and by affecting cGMP-binding phosphodiesterase [ 154 ]. PKG, the principal mediator of cGMP-induced VSM relaxation, acts via Ca 2+ -dependent mechanisms, i.e., decreasing [Ca 2+ ] i [ 152 , 154 , 155 ] and Ca 2+ desensitization [ 152 , 154 ], as well as Ca 2+ -independent mechanisms [ 156 ].…”

Routes for Formation of S-Nitrosothiols in Blood

“…However, when • NO complexes with thiol groups to form RSNO, it increases the life span by protecting the • NO from inactivation by metalloproteins (especially hemoglobin) and free radicals (especially superoxide and lipid peroxyl radical). RSNO can elicit their biological activity by releasing • NO, resulting in the activation of the classical guanylyl cyclase-/cGMP-dependent pathway or the cGMP-independent pathway, which includes post-translational modification of proteins [3–7]. RSNO are potent vasodilators [8, 9], strong inhibitors of platelet aggregation [10, 11] and leukocyte binding to the vascular endothelium [12–14].…”