Cellular mechanisms of cyclic nucleotide-induced vasorelaxation

Chronic hypoxia attenuates the vasodilator efficacy of protein kinase G in fetal and adult ovine cerebral arteries

Long-term hypoxia (LTH) attenuates nitric oxide-induced vasorelaxation in ovine middle cerebral arteries. Because cGMP-dependent protein kinase (PKG) is an important mediator of NO signaling in vascular smooth muscle, we tested the hypothesis that LTH diminishes the ability of PKG to interact with target proteins and cause vasorelaxation. Prominent among proteins that regulate vascular tone is the large-conductance Ca²⁺-sensitive K⁺ (BK) channel, which is a substrate for PKG and is responsive to phosphorylation on multiple serine/threonine residues. Given the influence of these proteins, we also examined whether LTH attenuates PKG and BK channel protein abundances and PKG activity. Middle cerebral arteries were harvested from normoxic and hypoxic (altitude of 3,820 m for 110 days) fetal and adult sheep. These arteries were denuded and equilibrated with 95% O₂-5% CO₂ in the presence of N-nitro-l-arginine methyl ester (l-NAME) to inhibit potential confounding influences of events upstream from PKG. Expression and activity of PKG-I were not significantly affected by chronic hypoxia in either fetal or adult arteries. Pretreatment with the BK inhibitor iberiotoxin attenuated vasorelaxation induced by 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate in normoxic but not LTH arteries. The spatial proximities of PKG with BK channel α- and β1-proteins were examined using confocal microscopy, which revealed a strong dissociation of PKG with these proteins after LTH. These results support our hypothesis that hypoxia reduces the ability of PKG to attenuate vasoconstriction in part through suppression of the ability of PKG to associate with and thereby activate BK channels in arterial smooth muscle.NEW & NOTEWORTHY Using measurements of contractility, protein abundance, kinase activity, and confocal colocalization in fetal and adult ovine cerebral arteries, the present study demonstrates that long-term hypoxia diminishes the ability of cGMP-dependent protein kinase (PKG) to cause vasorelaxation through suppression of its colocalization and interaction with large-conductance Ca²⁺-sensitive K⁺ (BK) channel proteins in cerebrovascular smooth muscle. These experiments are among the first to demonstrate hypoxic changes in BK subunit abundances in fetal cerebral arteries and also introduce the use of advanced methods of confocal colocalization to study interaction between PKG and its targets.

The principal pathways involved in the in vivo modulation of hypoxic pulmonary vasoconstriction, pulmonary arterial remodelling and pulmonary hypertension

Hypoxic pulmonary vasoconstriction (HPV) serves to optimize ventilation-perfusion matching in focal hypoxia and thereby enhances pulmonary gas exchange. During global hypoxia, however, HPV induces general pulmonary vasoconstriction, which may lead to pulmonary hypertension (PH), impaired exercise capacity, right-heart failure and pulmonary oedema at high altitude. In chronic hypoxia, generalized HPV together with hypoxic pulmonary arterial remodelling, contribute to the development of PH. The present article reviews the principal pathways in the in vivo modulation of HPV, hypoxic pulmonary arterial remodelling and PH with primary focus on the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways. In summary, endothelin-1 and thromboxane A₂ may enhance, whereas nitric oxide and prostacyclin may moderate, HPV as well as hypoxic pulmonary arterial remodelling and PH. The production of prostacyclin seems to be coupled primarily to cyclooxygenase-1 in acute hypoxia, but to cyclooxygenase-2 in chronic hypoxia. The potential role of adenine nucleotides in modulating HPV is unclear, but warrants further study. Additional modulators of the pulmonary vascular responses to hypoxia may include angiotensin II, histamine, serotonin/5-hydroxytryptamine, leukotrienes and epoxyeicosatrienoic acids. Drugs targeting these pathways may reduce acute and/or chronic hypoxic PH. Endothelin receptor antagonists and phosphodiesterase-5 inhibitors may additionally improve exercise capacity in hypoxia. Importantly, the modulation of the pulmonary vascular responses to hypoxia varies between species and individuals, with hypoxic duration and age. The review also define how drugs targeting the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways may improve pulmonary haemodynamics, but also impair pulmonary gas exchange by interference with HPV in chronic lung diseases.

Treatment with transducible phosphopeptide analogues of the small heat shock–related protein, HSP20, after experimental subarachnoid hemorrhage: prevention and reversal of delayed decreases in cerebral perfusion

Object

Delayed vasospasm is a significant cause of morbidity and mortality after subarachnoid hemorrhage (SAH). Proteomic therapeutics offers a new modality in which biologically active proteins or peptides are transduced into cells via covalent linkage to cell permeant peptides (CPPs). The hypothesis of this study was that either intrathecal or intravenous delivery of a phosphopeptide mimetic of the small heat shock–related protein, HSP20, linked to a CPP, would inhibit delayed decreases in cerebral perfusion after experimental SAH in a rat model.

Methods

This study was conducted in 3 parts: 1) prevention and 2) reversal of delayed decreases in cerebral perfusion via either intrathecal or intravenous administration of a CPP linked to phosphopeptide mimetics of HSP20 (AZX100) and 3) determining the effect of intravenous administration of AZX100 on blood pressure and heart rate. Subarachnoid hemorrhage was induced in rats by endovascular perforation. Subsequently, AZX100 was administered intrathecally via a cisternal catheter or intravenously. Cerebral perfusion was determined by laser Doppler monitoring. Blood pressure was monitored by telemetry in a separate group of naïve animals treated with AZX100 for 24 hours.

Results

The maximal decrease in cerebral perfusion occurred 3 days after SAH. Cisternal administration of AZX100 (0.14–0.57 mg/kg) 24 hours after hemorrhage prevented decreases in cerebral perfusion after SAH. Animals receiving lower doses of AZX100 (0.068 mg/kg) or a scrambled sequence of the active HSP20 peptide linked to CPP developed decreases in cerebral perfusion similar to those seen in control animals. Intravenous administration of AZX100 (1.22 mg/kg) 24 hours after hemorrhage prevented the decreases in cerebral perfusion seen in the controls. Intravenous administration (0.175 mg/kg and 1.22 mg/kg) of AZX100 on Days 2 and 3 after SAH reversed decreases in cerebral perfusion as early as Day 3. There was no impact of AZX100 on blood pressure or heart rate at doses up to 2.73 mg/kg.

Conclusions

Cisternal administration of AZX100 24 hours after hemorrhage prevented decreases in cerebral perfusion. Intravenous administration of AZX100 also prevented and reversed decreases in cerebral perfusion at doses that did not induce hypotension. Transduction of biologically active motifs of downstream regulators like HSP20 represents a potential novel treatment for SAH.

Effect of cardioplegic and organ preservation solutions and their components on coronary endothelium-derived relaxing factors

Cardioplegic (and organ preservation) solutions were initially designed to protect the myocardium (cardiac myocytes) during cardiac operation (and heart transplantation). Because of differences between cardiac myocytes and vascular (endothelial and smooth muscle) cells in structure and function, the solutions may have an adverse effect on coronary vascular cells. However, such effect is often complicated by many other factors such as ischemia-reperfusion injury, temperature, and perfusion pressure or duration. To evaluate the effect of a solution on the coronary endothelial function, a number of points should be taken into consideration. First, the overall effect on endothelium should be identified. Second, the effect of the solution on the individual endothelium-derived relaxing factors (nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor) must be distinguished. Third, the effect of each major component of the solution should be investigated. Lastly, the effect of a variety of new additives in the solution may be studied. Based on available literature these issues are reviewed to provide information for further development of cardioplegic or organ preservation solutions.

Sildenafil and an early stage of chronic hypoxia-induced pulmonary hypertension in newborn piglets

Devising therapies that might prevent the onset or progression of pulmonary hypertension in newborns has received little attention. Our major objective was to determine whether sildenafil, a selective phosphodiesterase inhibitor, prevents the development of an early stage of chronic hypoxia-induced pulmonary hypertension in newborn pigs. Another objective was to determine whether sildenafil causes pulmonary vasodilation without systemic vasodilation in piglets with chronic pulmonary hypertension. Piglets were raised in room air (control, n = 5) or 10-11% O(2) (hypoxic, n = 17) for 3 days. Some piglets (n = 4) received oral sildenafil, 12 mg/kg/day, throughout exposure to hypoxia. All piglets were anesthetized and catheterized, and pulmonary arterial pressure (Ppa), pulmonary wedge pressure (Pw), aortic pressure (Ao), and cardiac output (CO) were measured. Then for some piglets raised in hypoxia for 3 days, a single oral sildenafil dose (3 mg/kg, n = 6) or placebo (n = 5) was given, and hemodynamic measurements were repeated. For piglets raised in hypoxia for 3 days, mean Ppa and calculated PVR were elevated above respective values in control piglets. Mean Ppa and PVR did not differ between piglets that received sildenafil throughout exposure to hypoxia and those that did not. For piglets with chronic hypoxia-induced pulmonary hypertension that received a single oral dose of sildenafil, mean Ppa and PVR decreased, while mean Pw, CO, mean Ao, and systemic vascular resistance remained the same. All hemodynamic measurements were unchanged after placebo. Oral sildenafil did not influence the early stage of chronic hypoxia-induced pulmonary hypertension in newborn piglets. However, a single oral dose of sildenafil caused pulmonary vasodilation, without systemic vasodilation, in piglets with chronic hypoxia-induced pulmonary hypertension, which may have therapeutic implications.

The paradox of smooth muscle physiology

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (contraction) and the release of force (relaxation). The signaling events that activate contraction include Ca(2+)-dependent myosin light chain phosphorylation. The signaling events that mediate relaxation include the removal of a contractile agonist (passive relaxation) and activation of cyclic nucleotide-dependent signaling pathways in the continued presence of a contractile agonist (active relaxation). The major questions that remain in contractile physiology include (1) how is tonic force maintained when intracellular Ca(2+) levels and myosin light chain phosphorylation have returned to basal levels; and (2) what is the mechanism of cyclic nucleotide-dependent relaxation? This review focuses on these specific controversies surrounding the molecular mechanisms of contraction and relaxation of vascular smooth muscle.

Endothelial dysfunction in the pulmonary vascular bed

The pulmonary endothelium modulates vascular tone by the release of endothelium-derived constricting (EDCF) and relaxing (EDRF) factors, among them endothelin-1, nitric oxide, prostacyclin, and putative endothelium-derived hyperpolarizing factors. Abnormalities in EDCF and EDRF generation have been demonstrated in a number of cardiopulmonary disease states, such as primary and secondary pulmonary hypertension, chronic obstructive lung disease, cardiopulmonary bypass, and congestive heart failure. An imbalance between EDCF and EDRF, termed "pulmonary endothelial dysfunction," may contribute to the alteration in vascular tone characteristic of pulmonary disease. The following review summarizes the present knowledge of the role of EDCF and EDRF in such processes with major focus on pulmonary endothelial dysfunction in hypoxia-induced pulmonary hypertension.

The macromolecular associations of heat shock protein-27 in vascular smooth muscle

BACKGROUND

Behavioral stress is a risk factor for hypertension and atherosclerosis. Stress leads to increases in the expression and phosphorylation of heat shock proteins (HSPs) in vascular smooth muscle. Two small heat shock proteins, HSP27 and HSP20, have been implicated in the regulation of the contraction and relaxation of smooth muscle. We hypothesized that stress-induced alterations in the phosphorylation of HSP27 would effect the macromolecular associations of the small HSPs.

METHODS

Bovine carotid artery smooth muscle was treated with buffer alone or with the chemical stressor, arsenite. HSP27 phosphorylation was determined with isoelectric focusing immunoblotting. Macromolecular interactions were determined with subcellular fractionation, molecular sieving, and glutaraldehyde cross-linking and immunoblotting.

RESULTS

Arsenite treatment led to increases in the phosphorylation of HSP27, a redistribution of some HSP27 from a cytosolic to a particulate fraction and to the formation of larger macromolecular aggregates of HSP27. Glutaraldehyde cross-linking and immunoblotting demonstrated that HSP27 existed in monomeric and dimeric forms, which suggested that the large aggregates were not simply aggregates of HSP27 but contained other proteins.

CONCLUSIONS

Cellular stress leads to increases in the phosphorylation of HSP27 and to changes in the macromolecular associations of HSP27 in intact vascular smooth muscles. The functions of the small HSPs in the vascular smooth muscle may be dependent on both phosphorylation and macromolecular associations.

Functional expression of NOS 1 in vascular smooth muscle

Substances that increase intracellular calcium concentration ([Ca(2+)](i)), such as serotonin, are known to induce vascular smooth muscle (VSM) contraction. However, increases in [Ca(2+)](i) also activate Ca(2+)/calmodulin-dependent nitric oxide synthases (NOS), which leads to increases in cGMP and activation of cGMP-dependent protein kinase (PKG). One recently identified substrate protein of PKG is the small heat shock protein, HSP20. The purpose of this study was to determine if serotonin activates a Ca(2+)-dependent NOS in VSM. Strips of bovine carotid arterial smooth muscle denuded of endothelium were stimulated with serotonin in the presence and absence of the nonspecific NOS inhibitor N-monomethyl-L-arginine (L-NMMA). Activation of NOS was determined by increases in cGMP and in the phosphorylation of HSP20. Immunohistochemical and Western blotting techniques were performed to identify specific NOS isoforms in bovine carotid arterial smooth muscle preparations. Serotonin stimulation led to significant increases in cGMP and in the phosphorylation of HSP20, which were inhibited by pretreatment with L-NMMA. Antibodies against NOS 1 stained the media of bovine carotid and human renal arteries, whereas antibodies against NOS 3 stained only the endothelium. Additionally, the conversion of radiolabeled L-arginine to L-citrulline NOS activity demonstrated a consistent amount of activity present in the endothelium-denuded smooth muscle preparations that was reduced by 99% with an NOS 1 specific inhibitor. Finally, an NOS 1 specific inhibitor, 7-nitroindazole, augmented contractions induced by high extracellular KCl. This study demonstrates that NOS 1 is present in VSM and may effect physiological contractile responses.

Cellular stress inhibits vascular smooth muscle relaxation

PURPOSE

Cellular stress has been shown to induce a group of proteins called heat shock proteins (HSPs). Recent evidence suggests that a group of small HSPs may modulate vascular smooth muscle contraction (HSP27) and/or relaxation (HSP20). In this investigation, we hypothesized that cellular stress would alter contraction and/or relaxation of intact vascular smooth muscles and would lead to changes in the induction and/or phosphorylation of the small HSPs.

METHODS

Bovine carotid arteries were obtained from an abattoir, and physiologic contractile responses were determined in a muscle bath. Phosphorylation state-specific antibodies were produced and characterized against HSP27. Phosphorylation events were determined with phosphorylation state-specific antibodies or whole-cell phosphorylation and two-dimensional gel electrophoresis.

RESULTS

Cellular stress induced by arsenite or heat shock did not alter basal tone or the magnitude of contractions induced by serotonin or high extracellular potassium chloride. However, cellular stress led to inhibition of forskolin and sodium nitroprusside-induced vasorelaxation. This impaired vasorelaxation was associated with increases in the phosphorylation of HSP27 and decreases in forskolin-induced phosphorylation of HSP20.

CONCLUSION

Cellular stress, which leads to increases in the phosphorylation of HSP27, inhibits cyclic nucleotide-dependent vascular relaxation and cyclic nucleotide-dependent increases in the phosphorylation of HSP20.

Soluble guanylate cyclase gene expression and localization in rat lung after exposure to hypoxia

The nitric oxide (NO)-cGMP signal transduction pathway plays an important role in the regulation of pulmonary vascular tone and resistance in pulmonary hypertension. A number of studies have demonstrated that endothelial (e) and inducible nitric oxide synthases (NOS) are upregulated in hypoxia-exposed rat lung. These changes in NOS expression have been found to correlate with the process of pulmonary vascular remodeling in hypoxia-induced pulmonary hypertension, and remodeling is increased in the absence of eNOS. In this study, we examined the expression and localization of soluble guanylate cyclase (sGC), the primary receptor for NO, in hypoxia- and normoxia-treated rat lungs. Male Sprague-Dawley rats were exposed to hypoxia (10% O(2), normobaric) or normoxia for 1, 3, 5, and 21 days. The lungs were used for Western analysis of sGC protein, sGC enzyme activity, immunohistochemistry using antiserum against sGC alpha(1)- and beta(1)-subunits, and nonradioactive in situ hybridization (NRISH) using a digoxigenin-labeled sGC alpha(1)-subunit cRNA probe. Western blot analysis revealed a more than twofold increase of sGC protein alpha(1)-subunit in rat lungs exposed to 3, 5, and 21 days of hypoxia, correlating well with sGC enzyme activity. Immunohistochemistry and NRISH demonstrated increased expression of sGC in the smooth muscle cells of the pulmonary arteries and arterioles in the hypoxic rat lungs when compared with normoxic controls. Based on our results, the upregulation of sGC may play an important role in the regulation of smooth muscle tone and pressure in the pulmonary circulation during chronic hypoxia.

The small heat shock-related protein, HSP20, is phosphorylated on serine 16 during cyclic nucleotide-dependent relaxation

The small heat shock-related protein 20 (HSP20) is present in four isoforms in bovine carotid artery smooth muscles. Three of the isoforms are phosphorylated and one is not. Increases in the phosphorylation of two isoforms of HSP20 (isoform 3, pI 5.9; and 8, pI 5.7) are associated with cyclic nucleotide-dependent relaxation of bovine carotid artery smooth muscles. Increases in the phosphorylation of another isoform (isoform 4, pI 6.0) are associated with phorbol ester-induced contraction of bovine carotid artery smooth muscles. In this investigation we determined that isoforms 3 and 8 are phosphorylated on Ser16 of the HSP20 molecule during activation of cAMP-dependent signaling pathways. Phosphorylation state-specific antibodies produced against a peptide containing phosphorylated Ser16 recognized isoforms 3 and 8 but not isoform 4. In human vascular tissue, only isoform 3 is present. Incubation of transiently permeabilized strips of bovine carotid artery smooth muscle with synthetic peptides in which Ser16 is phosphorylated, inhibits contractile responses to high extracellular KCl and to serotonin. These data suggest that phosphorylation of HSP20 on Ser16 modulates cAMP-dependent vasorelaxation.

Cyclosporine-induced renal artery smooth muscle contraction is associated with increases in the phosphorylation of specific contractile regulatory proteins

Cyclosporine A (CSA) is a type 2B phosphatase inhibitor which can induce contraction of renal artery smooth muscle. In this investigation, we examined the phosphorylation events associated with CSA-induced contraction of bovine renal artery smooth muscle. Contractile responses were determined in a muscle bath and the corresponding phosphorylation events were determined with whole cell phosphorylation and two-dimensional gel electrophoresis. CSA-induced contractions were associated with increases in the phosphorylation of the 20 kDa myosin light chains (MLC20) and different isoforms of the small heat shock protein, HSP27. Cyclic nucleotide-dependent relaxation of CSA-induced contractions was associated with increases in the phosphorylation of another small heat shock protein, HSP20, and decreases in the phosphorylation of the MLC20, and some isoforms of HSP27. These data suggest that CSA-induced contraction and relaxation of vascular smooth muscle is associated with increases in the phosphorylation of specific contractile regulatory proteins.

The small heat shock-related protein-20 is an actin-associated protein

PURPOSE

The activation of cyclic nucleotide-dependent signaling pathways in vascular smooth muscle is important for the prevention of vein graft spasm and neointimal hyperplasia. Cyclic nucleotide-dependent relaxation is associated with an increase in the phosphorylation of a small heat shock-related protein (HSP20). In this investigation, we examined the mechanisms by which HSP20 may modulate relaxation.

METHODS

The relaxation responses of the bovine carotid artery smooth muscles were determined in a muscle bath. HSP20 phosphorylation was quantitated with isoelectric-focusing immunoblots. The association with actin was determined with coimmunoprecipitation and cosedimentation. Molecular sieving columns were used to examine the macromolecular associations of HSP20.

RESULTS

The activation of cyclic nucleotide signaling pathways leads to the complete relaxation of carotid smooth muscle. This relaxation response is associated with an increase in the phosphorylation of HSP20. Actin coimmunoprecipitated with HSP20, and the association of actin with recombinant HSP20 in vitro was phosphorylation-state dependent. Finally, HSP20 exists in large (>100 kDa) aggregates, which dissociate with the activation of cyclic nucleotide signaling pathways.

CONCLUSION

These data support a role of HSP20 phosphorylation in mediating smooth muscle relaxation, possibly via a direct interaction of large aggregates of HSP20 with the contractile elements.

Cyclic nucleotide-dependent vasorelaxation is associated with the phosphorylation of a small heat shock-related protein

Activation of cyclic nucleotide-dependent signaling pathways leads to the relaxation of various smooth muscles. One of the major phosphorylation events associated with cyclic nucleotide-dependent vasorelaxation in bovine trachealis and carotid artery smooth muscle is the phosphorylation of two 20-kDa phosphoproteins with pI values of 5.7 and 5.9 (previously designated pp8 and pp3, respectively). The present studies sought to determine the identities of pp3 and pp8 in vascular smooth muscle. The phosphopeptide maps for the pp8 and pp3 proteins were similar. Preparative two-dimensional gel electrophoresis and amino acid sequencing of a peptide fragment of the pp3 protein revealed a sequence identical to a 20-kDa heat shock-related protein (HSP20) previously purified from skeletal muscle. Western blot and immunoprecipitation analysis with anti-HSP20 antibodies demonstrated that the pp3 and pp8 proteins are phosphorylated forms of HSP20. In addition, HSP20 could be phosphorylated in vitro by both cAMP-dependent protein kinase and cGMP-dependent protein kinase. These data suggest that the phosphorylation of the heat shock-related protein HSP20 is associated with cyclic nucleotide-dependent relaxation of vascular smooth muscle.