Proximal cerebral arteries develop myogenic responsiveness in heart failure via tumor necrosis factor-α-dependent activation of sphingosine-1-phosphate signaling.

BACKGROUND: Heart failure is associated with neurological deficits, including cognitive dysfunction. However, the molecular mechanisms underlying reduced cerebral blood flow in the early stages of heart failure, particularly when blood pressure is minimally affected, are not known. METHODS AND RESULTS: Using a myocardial infarction model in mice, we demonstrate a tumor necrosis factor-α (TNFα)-dependent enhancement of posterior cerebral artery tone that reduces cerebral blood flow before any overt changes in brain structure and function. TNFα expression is increased in mouse posterior cerebral artery smooth muscle cells at 6 weeks after myocardial infarction. Coordinately, isolated posterior cerebral arteries display augmented myogenic tone, which can be fully reversed in vitro by the competitive TNFα antagonist etanercept. TNFα mediates its effect via a sphingosine-1-phosphate (S1P)-dependent mechanism, requiring sphingosine kinase 1 and the S1P(2) receptor. In vivo, sphingosine kinase 1 deletion prevents and etanercept (2-week treatment initiated 6 weeks after myocardial infarction) reverses the reduction of cerebral blood flow, without improving cardiac function. CONCLUSIONS: Cerebral artery vasoconstriction and decreased cerebral blood flow occur early in an animal model of heart failure; these perturbations are reversed by interrupting TNFα/S1P signaling. This signaling pathway may represent a potential therapeutic target to improve cognitive function in heart failure.

Sphingosine-1-phosphate-dependent activation of p38 MAPK maintains elevated peripheral resistance in heart failure through increased myogenic vasoconstriction.

RATIONALE: Mechanisms underlying vasomotor abnormalities and increased peripheral resistance exacerbating heart failure (HF) are poorly understood. OBJECTIVE: To explore the role and molecular basis of myogenic responses in HF. METHODS AND RESULTS: 10 weeks old C57Bl6 mice underwent experimental myocardial infarction (MI) or sham surgery. At 1 to 12 weeks postoperative, mice underwent hemodynamic studies, mesenteric, cerebral, and cremaster artery perfusion myography and Western blot. Organ weights and hemodynamics confirmed HF and increased peripheral resistance after MI. Myogenic responses, ie, pressure-induced vasoconstriction, were increased as early as 1 week after MI and remained elevated. Vasoconstrictor responses to phenylephrine were decreased 1 week after MI, but not at 2 to 6 weeks after MI, whereas those to endothelin (ET)-1 and sphingosine-1-phosphate (S1P) were increased at all time points after MI. An antagonist (JTE-013) for the most abundant S1P receptor detected in mesenteric arteries (S1P(2)R) abolished the enhanced myogenic responses of HF, with significantly less effect on controls. Mice with genetic absence of sphingosine-kinases or S1P(2)R (Sphk1(-/-); Sphk1(-/-)/Sphk2(+/-); S1P(2)R(-/-)) did not manifest enhanced myogenic responses after MI. Mesenteric arteries from HF mice exhibited increased phosphorylation of myosin light chain, with deactivation of its phosphatase (MLCP). Among known S1P-responsive regulators of MLCP, GTP-Rho levels were unexpectedly reduced in HF, whereas levels of activated p38 MAPK and ERK1/2 (extracellular signal-regulated kinase 1/2) were increased. Inhibiting p38 MAPK abolished the myogenic responses of animals with HF, with little effect on controls. CONCLUSIONS: Rho-independent p38 MAPK-mediated deactivation of MLCP underlies S1P/S1P(2)R-regulated increases in myogenic vasoconstriction observed in HF. Therapeutic targeting of these findings in HF models deserves study.

A microfluidic platform for probing small artery structure and function.

Although pathologic changes to the structure and function of small blood vessels are hallmarks of various cardiovascular diseases, limitations of conventional investigation methods (i.e. pressure myography) have prohibited a comprehensive understanding of the underlying mechanisms. We developed a microfluidic device to facilitate assessment of resistance artery structure and function under physiological conditions (37 degrees C, 45 mmHg transmural pressure). The platform allows for on-chip fixation, long-term culture and fully automated acquisition of up to ten dose-response sequences of intact mouse mesenteric artery segments (diameter approximately 250 micrometres and length approximately 1.5 mm) in a well-defined microenvironment. Even abluminal application of phenylephrine or acetylcholine (homogeneous condition) yielded dose-response relationships virtually identical to conventional myography. Unilateral application of phenylephrine (heterogeneous condition) limited constriction to the drug-exposed side, suggesting a lack of circumferential communication. The microfluidic platform allows us to address new fundamental biological questions, replaces a manually demanding procedure with a scalable approach and may enable organ-based screens to be routinely performed during drug development.

Tumor necrosis factor-α enhances microvascular tone and reduces blood flow in the cochlea via enhanced sphingosine-1-phosphate signaling.

BACKGROUND AND PURPOSE: We sought to demonstrate that tumor necrosis factor (TNF)-α, via sphingosine-1-phosphate signaling, has the potential to alter cochlear blood flow and thus, cause ischemic hearing loss. METHODS: We performed intravital fluorescence microscopy to measure blood flow and capillary diameter in anesthetized guinea pigs. To measure capillary diameter ex vivo, capillary beds from the gerbil spiral ligament were isolated from the cochlear lateral wall and maintained in an organ bath. Isolated gerbil spiral modiolar arteries, maintained and transfected in organ culture, were used to measure calcium sensitivity (calcium-tone relationship). In a clinical study, a total of 12 adult patients presenting with typical symptoms of sudden hearing loss who were not responsive or only partially responsive to prednisolone treatment were identified and selected for etanercept treatment. Etanercept (25 mg s.c.) was self-administered twice a week for 12 weeks. RESULTS: TNF-α induced a proconstrictive state throughout the cochlear microvasculature, which reduced capillary diameter and cochlear blood flow in vivo. In vitro isolated preparations of the spiral modiolar artery and spiral ligament capillaries confirmed these observations. Antagonizing sphingosine-1-phosphate receptor 2 subtype signaling (by 1 µmol/L JTE013) attenuated the effects of TNF-α in all models. TNF-α activated sphingosine kinase 1 (Sk1) and induced its translocation to the smooth muscle cell membrane. Expression of a dominant-negative Sk1 mutant (Sk1(G82D)) eliminated both baseline spiral modiolar artery calcium sensitivity and TNF-α effects, whereas a nonphosphorylatable Sk1 mutant (Sk1(S225A)) blocked the effects of TNF-α only. A small group of etanercept-treated, hearing loss patients recovered according to a 1-phase exponential decay (half-life=1.56 ± 0.20 weeks), which matched the kinetics predicted for a vascular origin. CONCLUSIONS: TNF-α indeed reduces cochlear blood flow via activation of vascular sphingosine-1-phosphate signaling. This integrates hearing loss into the family of ischemic microvascular pathologies, with implications for risk stratification, diagnosis, and treatment.

RGS4 regulates parasympathetic signaling and heart rate control in the sinoatrial node.

Heart rate is controlled by the opposing activities of sympathetic and parasympathetic inputs to pacemaker myocytes in the sinoatrial node (SAN). Parasympathetic activity on nodal myocytes is mediated by acetylcholine-dependent stimulation of M(2) muscarinic receptors and activation of Galpha(i/o) signaling. Although regulators of G protein signaling (RGS) proteins are potent inhibitors of Galpha(i/o) signaling in many tissues, the RGS protein(s) that regulate parasympathetic tone in the SAN are unknown. Our results demonstrate that RGS4 mRNA levels are higher in the SAN compared to right atrium. Conscious freely moving RGS4-null mice showed increased bradycardic responses to parasympathetic agonists compared to wild-type animals. Moreover, anesthetized RGS4-null mice had lower baseline heart rates and greater heart rate increases following atropine administration. Retrograde-perfused hearts from RGS4-null mice showed enhanced negative chronotropic responses to carbachol, whereas SAN myocytes showed greater sensitivity to carbachol-mediated reduction in the action potential firing rate. Finally, RGS4-null SAN cells showed decreased levels of G protein-coupled inward rectifying potassium (GIRK) channel desensitization and altered modulation of acetylcholine-sensitive potassium current (I(KACh)) kinetics following carbachol stimulation. Taken together, our studies establish that RGS4 plays an important role in regulating sinus rhythm by inhibiting parasympathetic signaling and I(KACh) activity.

Sphingosine-1-Phosphate Signaling Regulates Myogenic Responsiveness in Human Resistance Arteries.

We recently identified sphingosine-1-phosphate (S1P) signaling and the cystic fibrosis transmembrane conductance regulator (CFTR) as prominent regulators of myogenic responsiveness in rodent resistance arteries. However, since rodent models frequently exhibit limitations with respect to human applicability, translation is necessary to validate the relevance of this signaling network for clinical application. We therefore investigated the significance of these regulatory elements in human mesenteric and skeletal muscle resistance arteries. Mesenteric and skeletal muscle resistance arteries were isolated from patient tissue specimens collected during colonic or cardiac bypass surgery. Pressure myography assessments confirmed endothelial integrity, as well as stable phenylephrine and myogenic responses. Both human mesenteric and skeletal muscle resistance arteries (i) express critical S1P signaling elements, (ii) constrict in response to S1P and (iii) lose myogenic responsiveness following S1P receptor antagonism (JTE013). However, while human mesenteric arteries express CFTR, human skeletal muscle resistance arteries do not express detectable levels of CFTR protein. Consequently, modulating CFTR activity enhances myogenic responsiveness only in human mesenteric resistance arteries. We conclude that human mesenteric and skeletal muscle resistance arteries are a reliable and consistent model for translational studies. We demonstrate that the core elements of an S1P-dependent signaling network translate to human mesenteric resistance arteries. Clear species and vascular bed variations are evident, reinforcing the critical need for further translational study.

Metoprolol impairs resistance artery function in mice.

Acute ß-blockade with metoprolol has been associated with increased mortality by undefined mechanisms. Since metoprolol is a relatively high affinity blocker of ß(2)-adrenoreceptors, we hypothesized that some of the increased mortality associated with its use may be due to its abrogation of ß(2)-adrenoreceptor-mediated vasodilation of microvessels in different vascular beds. Cardiac output (CO; pressure volume loops), mean arterial pressure (MAP), relative cerebral blood flow (rCBF; laser Doppler), and microvascular brain tissue Po(2) (G2 oxyphor) were measured in anesthetized mice before and after acute treatment with metoprolol (3 mg/kg iv). The vasodilatory dose responses to ß-adrenergic agonists (isoproterenol and clenbuterol), and the myogenic response, were assessed in isolated mesenteric resistance arteries (MRAs; ∼200-µm diameter) and posterior cerebral arteries (PCAs ∼150-µm diameter). Data are presented as means ± SE with statistical significance applied at P < 0.05. Metoprolol treatment did not effect MAP but reduced heart rate and stroke volume, CO, rCBF, and brain microvascular Po(2), while concurrently increasing systemic vascular resistance (P < 0.05 for all). In isolated MRAs, metoprolol did not affect basal artery tone or the myogenic response, but it did cause a dose-dependent impairment of isoproterenol- and clenbuterol-induced vasodilation. In isolated PCAs, metoprolol (50 µM) impaired maximal vasodilation in response to isoproterenol. These data support the hypothesis that acute administration of metoprolol can reduce tissue oxygen delivery by impairing the vasodilatory response to ß(2)-adrenergic agonists. This mechanism may contribute to the observed increase in mortality associated with acute administration of metoprolol in perioperative patients.