Antioxidant treatment restores prejunctional regulation of purinergic transmission in mesenteric arteries of deoxycorticosterone acetate-salt hypertensive rats

Spinal cord injury alters purinergic neurotransmission to mesenteric arteries in rats

Complications associated with spinal cord injury (SCI) result from unregulated reflexes below the lesion level. Understanding neurotransmission distal to the SCI could improve quality of life by mitigating complications. The long-term impact of SCI on neurovascular transmission is poorly understood, but reduced sympathetic activity below the site of SCI enhances arterial neurotransmission (1). We studied sympathetic neurovascular transmission using a rat model of long-term paraplegia (T2-3) and tetraplegia (C6-7). Sixteen weeks after SCI, T2-3 and C6-7 rats had lower blood pressure (BP) than sham rats (103  ±  2 and 97  ±  4 vs. 117  ±  6 mmHg, P < 0.05). T2-3 rats had tachycardia (410  ±  6 beats/min), and C6-7 rats had bradycardia (299  ±  10 beats/min) compared with intact rats (321  ±  4 beats/min, P < 0.05). Purinergic excitatory junction potentials (EJPs) were measured in mesenteric arteries (MA) using microlectrodes, and norepinephrine (NE) release was measured using amperometry. NE release was similar in all groups, while EJP frequency-response curves from T2-3 and C6-7 rats were left-shifted vs. sham rats. EJPs in T2-3 and C6-7 rats showed facilitation followed by run-down during stimulation trains (10 Hz, 50 stimuli). MA reactivity to exogenous NE and ATP was similar in all rats. In T2-3 and C6-7 rats, NE content was increased in left cardiac ventricles compared with intact rats, but was not changed in MA, kidney, or spleen. Our data indicate that peripheral purinergic, but not adrenergic, neurotransmission increases following SCI via enhanced ATP release from periarterial nerves. Sympathetic BP support is reduced after SCI, but improving neurotransmitter release might maintain cardiovascular stability in individuals living with SCI.NEW & NOTEWORTHY This study revealed increased purinergic, but not noradrenergic, neurotransmission to mesenteric arteries in rats with spinal cord injury (SCI). An increased releasable pool of ATP in periarterial sympathetic nerves may contribute to autonomic dysreflexia following SCI, suggesting that purinergic neurotransmission may be a therapeutic target for maintaining stable blood pressure in individuals living with SCI. The selective increase in ATP release suggests that ATP and norepinephrine may be stored in separate synaptic vesicles in periarterial sympathetic varicosities.

VNUT and VMAT2 segregate within sympathetic varicosities and localize near preferred Cav2 isoforms in the rat tail artery

ATP and norepinephrine (NE) are coreleased from peripheral sympathetic nerve terminals. Whether they are stored in the same vesicles has been debated for decades. Preferential dependence of NE or ATP release on Ca²⁺ influx through specific voltage-gated Ca²⁺ channel (Cav2) isoforms suggests that NE and ATP are stored in separate vesicle pools, but simultaneous imaging of NE and ATP containing vesicles within single varicosities has not been reported. We conducted an immunohistochemical study of vesicular monoamine transporter 2 (VMAT2/SLC18A2) and vesicular nucleotide translocase (VNUT/SLC17A9) as markers of vesicles containing NE and ATP in sympathetic nerves of the rat tail artery. A large fraction of varicosities exhibited neighboring, rather than overlapping, VNUT and VMAT2 fluorescent puncta. VMAT2, but not VNUT, colocalized with synaptotagmin 1. Cav2.1, Cav2.2, and Cav2.3 are expressed in nerves in the tunica adventitia. VMAT2 preferentially localized adjacent to Cav2.2 and Cav2.3 rather than Cav2.1. VNUT preferentially localized adjacent to Cav2.3 > Cav2.2 >> Cav2.1. With the use of wire myography, inhibition of field-stimulated vasoconstriction with the Cav2.3 blocker SNX-482 (0.25 µM) mimicked the effects of the P2X inhibitor suramin (100 µM) rather than the α-adrenergic inhibitor phentolamine (10 µM). Variable sensitivity to SNX-482 and suramin between animals closely correlated with Cav2.3 staining. We concluded that a majority of ATP and NE stores localize to separate vesicle pools that use different synaptotagmin isoforms and that localize near different Cav2 isoforms to mediate vesicle release. Cav2.3 appears to play a previously unrecognized role in mediating ATP release in the rat tail artery. NEW & NOTEWORTHY Immunofluorescence imaging of vesicular nucleotide translocase and vesicular monoamine transporter 2 in rat tail arteries revealed that ATP and norepinephrine, classical cotransmitters, localize to well-segregated vesicle pools. Furthermore, vesicular nucleotide translocase and vesicular monoamine transporter 2 exhibit preferential localization with specific Cav2 isoforms. These novel observations address long-standing debates regarding the mechanism(s) of sympathetic neurotransmitter corelease.

Macrophage-dependent impairment of α2-adrenergic autoreceptor inhibition of Ca2+ channels in sympathetic neurons from DOCA-salt but not high-fat diet-induced hypertensive rats

DOCA-salt and obesity-related hypertension are associated with inflammation and sympathetic nervous system hyperactivity. Prejunctional α₂-adrenergic receptors (α₂ARs) provide negative feedback to norepinephrine release from sympathetic nerves through inhibition of N-type Ca²⁺ channels. Increased neuronal norepinephrine release in DOCA-salt and obesity-related hypertension occurs through impaired α₂AR signaling; however, the mechanisms involved are unclear. Mesenteric arteries are resistance arteries that receive sympathetic innervation from the superior mesenteric and celiac ganglia (SMCG). We tested the hypothesis that macrophages impair α₂AR-mediated inhibition of Ca²⁺ channels in SMCG neurons from DOCA-salt and high-fat diet (HFD)-induced hypertensive rats. Whole cell patch-clamp methods were used to record Ca²⁺ currents from SMCG neurons maintained in primary culture. We found that DOCA-salt, but not HFD-induced, hypertension caused macrophage accumulation in mesenteric arteries, increased SMCG mRNA levels of monocyte chemoattractant protein-1 and tumor necrosis factor-α, and impaired α₂AR-mediated inhibition of Ca²⁺ currents in SMCG neurons. α₂AR dysfunction did not involve changes in α₂AR expression, desensitization, or downstream signaling factors. Oxidative stress impaired α₂AR-mediated inhibition of Ca²⁺ currents in SMCG neurons and resulted in receptor internalization in human embryonic kidney-293T cells. Systemic clodronate-induced macrophage depletion preserved α₂AR function and lowered blood pressure in DOCA-salt rats. HFD caused hypertension without obesity in Sprague-Dawley rats and hypertension with obesity in Dahl salt-sensitive rats. HFD-induced hypertension was not associated with inflammation in SMCG and mesenteric arteries or α₂AR dysfunction in SMCG neurons. These results suggest that macrophage-mediated α₂AR dysfunction in the mesenteric circulation may only be relevant to mineralocorticoid-salt excess. NEW & NOTEWORTHY Here, we identify a contribution of macrophages to hypertension development through impaired α₂-adrenergic receptor (α₂AR)-mediated inhibition of sympathetic nerve terminal Ca²⁺ channels in DOCA-salt hypertensive rats. Impaired α₂AR function may involve oxidative stress-induced receptor internalization. α₂AR dysfunction may be unique to mineralocorticoid-salt excess, as it does not occur in obesity-related hypertension.

Macrophage depletion lowers blood pressure and restores sympathetic nerve α2-adrenergic receptor function in mesenteric arteries of DOCA-salt hypertensive rats

We tested the hypothesis that vascular macrophage infiltration and O2 (-) release impairs sympathetic nerve α2-adrenergic autoreceptor (α2AR) function in mesenteric arteries (MAs) of DOCA-salt hypertensive rats. Male rats were uninephrectomized or sham operated (sham). DOCA pellets were implanted subcutaneously in uninephrectomized rats who were provided high-salt drinking water or high-salt water with apocynin. Sham rats received tap water. Blood pressure was measured using radiotelemetry. Treatment of sham and DOCA-salt rats with liposome-encapsulated clodronate was used to deplete macrophages. After 3-5, 10-13, and 18-21 days of DOCA-salt treatment, MAs and peritoneal fluid were harvested from euthanized rats. Norepinephrine (NE) release from periarterial sympathetic nerves was measured in vitro using amperometry with microelectrodes. Macrophage infiltration into MAs as well as TNF-α and p22(phox) were measured using immunohistochemistry. Peritoneal macrophage activation was measured by flow cytometry. O2 (-) was measured using dihydroethidium staining. Hypertension developed over 28 days, and apocynin reduced blood pressure on days 18-21. O2 (-) and macrophage infiltration were greater in DOCA-salt MAs compared with sham MAs after day 10. Peritoneal macrophage activation occurred after day 10 in DOCA-salt rats. Macrophages expressing TNF-α and p22(phox) were localized near sympathetic nerves. Impaired α2AR function and increased NE release from sympathetic nerves occurred in MAs from DOCA-salt rats after day 18. Macrophage depletion reduced blood pressure and vascular O2 (-) while restoring α2AR function in DOCA-salt rats. Macrophage infiltration into the vascular adventitia contributes to increased blood pressure in DOCA-salt rats by releasing O2 (-), which disrupts α2AR function, causing enhanced NE release from sympathetic nerves.

Aging-associated oxidative stress leads to decrease in IAS tone via RhoA/ROCK downregulation

Internal anal sphincter (IAS) tone plays an important role in rectoanal incontinence (RI). IAS tone may be compromised during aging, leading to RI in certain patients. We examined the influence of oxidative stress in the aging-associated decrease in IAS tone (AADI). Using adult (4-6 mo old) and aging (24-30 mo old) rats, we determined the effect of oxidative stress on IAS tone and the regulatory RhoA/ROCK signal transduction cascade. We determined the effect of the oxidative stress inducer LY83583, which produces superoxide anions (O2 (·-)), on basal and stimulated IAS tone before and after treatment of intact smooth muscle strips and smooth muscle cells with the O2 (·-) scavenger SOD. Our data showed that AADI was associated with a decrease in RhoA/ROCK expression at the transcriptional and translational levels. Oxidative stress with a LY83583-mediated decrease in IAS tone and relaxation of IAS smooth muscle cells was associated with a decrease in RhoA/ROCK signal transduction, which was reversible by SOD. In addition, LY83583 caused a significant decrease in IAS contraction produced by the RhoA activator and a known RhoA/ROCK agonist, U46619, that was also reversible by SOD. The inhibitory effects of LY83583 and the ROCK inhibitor Y27632 on the U46619-induced increase in IAS tone were similar. We conclude that an increase in oxidative stress plays an important role in AADI in the elderly and may be one of the underlying mechanisms of RI in certain aging patients.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Increased catecholamine secretion from single adrenal chromaffin cells in DOCA-salt hypertension is associated with potassium channel dysfunction

The mechanism of catecholamine release from single adrenal chromaffin cells isolated from normotensive and DOCA-salt hypertensive rats was investigated. These cells were used as a model for sympathetic nerves to better understand how exocytotic release of catecholamines is altered in this model of hypertension. Catecholamine secretion was evoked by local application of acetylcholine (1 mM) or high K+ (70 mM), and continuous amperometry was used to monitor catecholamine secretion as an oxidative current. The total number of catecholamine molecules secreted from a vesicle, the total number of vesicles fusing and secreting, and the duration of secretion in response to a stimulus were all significantly greater for chromaffin cells from hypertensive rats as compared to normotensive controls. The greater catecholamine secretion from DOCA-salt cells results, at least in part, from functionally impaired large conductance, Ca2+-activated (BK) and ATP-sensitive K+ channels. This work reveals that there is altered vesicular release of catecholamines from these cells (and possibly from perivascular sympathetic nerves) and this may contribute to increased vasomotor tone in DOCA-salt hypertension.

Influence of pressure on adenosine triphosphate function as a sympathetic neurotransmitter in small mesenteric arteries from the spontaneously hypertensive rat

OBJECTIVES

Enhanced sympathetic neurotransmission contributes to hypertension in the spontaneously hypertensive rat (SHR). We recently reported a method for studying sympathetic neurotransmission in pressurized small arteries, demonstrating a major role of adenosine triphosphate (ATP) as a sympathetic neurotransmitter under these physiological conditions. We have now used this methodology to assess the role of ATP as a sympathetic neurotransmitter in small mesenteric arteries isolated from SHRs.

METHODS

Small arteries were mounted in a suction electrode, cannulated and pressurized to either 30 or 90 mmHg. Nerve-evoked alterations in membrane potential were assessed using sharp microelectrodes. Neurally evoked vasoconstrictor responses were measured in the absence and presence of the α1-adrenoceptor antagonist, tamsulosin (0.1 μmol/l), or the P2 purinoceptor antagonist suramin (0.1 mmol/l).

RESULTS

At 30 mmHg the P2X-receptor-mediated excitatory junctional potential (EJP) was larger in arteries from SHRs (7.9 ± 0.9 mV) than Wistar-Kyoto (WKY) rats (3.2 ± 0.4 mV, P < 0.05). Increasing pressure increased the amplitude of the EJP, which again, was larger in SHRs. At 90 mmHg, activation of the perivascular nerves produced a larger vasoconstriction in arteries isolated from SHRs compared with WKY rats. The vasoconstrictor response in SHRs was abolished by either suramin or tamsulosin.

CONCLUSION

These data provide electrophysiological evidence for enhanced purinergic function in the SHR and show that ATP is fundamentally important in contributing to the vasoconstriction produced after activation of the perivascular nerves in pressurized arteries from the SHR. This involves a synergistic interaction with noradrenaline to causes enhanced mesenteric arterial vasoconstriction, which may contribute to the hypertension in this model.

Oxidative stress disrupts purinergic neuromuscular transmission in the inflamed colon

Colitis, induced by trinitrobenzene sulfonic acid (TNBS) in guinea pig, leads to decreased purinergic neuromuscular transmission resulting in a reduction in inhibitory junction potentials (IJPs) in colonic circular muscle. We explored possible mechanisms responsible for this inflammation-induced neurotransmitter plasticity. Previous studies have suggested that the deficit in inflamed tissue involves decreased ATP release. We therefore hypothesized that decreased purinergic transmission results from inflammation-induced free radical damage to mitochondria, leading to decreased purine synthesis and release. Stimulus-induced release of purines was measured using high-performance liquid chromatography, and quantities of all purines measured were significantly reduced in the inflamed colons as compared to controls. To test whether decreased mitochondrial function affects the IJP, colonic muscularis preparations were treated with the mitochondrial ATP synthase inhibitors oligomycin or dicyclohexylcarbodiimide, which resulted in a significant reduction of IJP amplitude. Induction of oxidative stress in vitro, by addition of H2O2 to the preparation, also significantly reduced IJP amplitude. Purinergic neuromuscular transmission was significantly restored in TNBS-inflamed guinea pigs, and in dextran sodium sulfate-inflamed mice, treated with a free radical scavenger. Furthermore, propulsive motility in the distal colons of guinea pigs with TNBS colitis was improved by in vivo treatment with the free radical scavenger. We conclude that oxidative stress contributes to the reduction in purinergic neuromuscular transmission measured in animal models of colitis, and that these changes can be prevented by treatment with a free radical scavenger, resulting in improved motility.

Impaired function of prejunctional adenosine A1 receptors expressed by perivascular sympathetic nerves in DOCA-salt hypertensive rats

Increased sympathetic nervous system activity contributes to deoxycorticosterone acetate (DOCA)-salt hypertension in rats. ATP and norepinephrine (NE) are coreleased from perivascular sympathetic nerves. NE acts at prejunctional α2-adrenergic receptors (α2ARs) to inhibit NE release, and α2AR function is impaired in DOCA-salt rats. Adenosine, an enzymatic ATP degradation product, acts at prejunctional A1 adenosine receptors (A1Rs) to inhibit NE release. We tested the hypothesis that prejunctional A1R function is impaired in sympathetic nerves supplying mesenteric arteries (MAs) and veins (MVs) of DOCA-salt rats. Electrically evoked NE release and constrictions of blood vessels were studied in vitro with use of amperometry to measure NE oxidation currents and video microscopy, respectively. Immunohistochemical methods were used to localize tyrosine hydroxylase (TH) and A1Rs in perivascular sympathetic nerves. TH and A1Rs colocalized to perivascular sympathetic nerves. Adenosine and N(6)-cyclopentyl-adenosine (CPA, A1R agonist) constricted MVs but not MAs. Adenosine and CPA (0.001-10 µM) inhibited neurogenic constrictions and NE release in MAs and MVs. DOCA-salt arteries were resistant to adenosine and CPA-mediated inhibition of NE release and constriction. The A2A adenosine receptor agonist CGS21680 (C23H29N7O6.HCl.xH2O) (0.001-0.1 μM) did not alter NE oxidation currents. We conclude that there are prejunctional A1Rs in arteries and both pre- and postjunctional A1Rs in veins; thus, adenosine selectively constricts the veins. Prejunctional A1R function is impaired in arteries, but not veins, from DOCA-salt rats. Sympathetic autoreceptor dysfunction is not specific to α2ARs, but there is a more general disruption of prejunctional mechanisms controlling sympathetic neurotransmitter release in DOCA-salt hypertension.

The roles of purinergic signaling during gastrointestinal inflammation

Extracellular purines play important roles as neurotransmitters and paracrine mediators in the gastrointestinal (GI) tract. Inflammation of the GI tract causes marked changes in the release and extracellular catabolism of purines, and can modulate purinoceptor expression and/or signaling. The functional consequences of this include suppression of the purinergic component of inhibitory neuromuscular and neurovascular transmission, increased release of purines from immune and epithelial cells, loss of enteric neurons to damage through P2X(7) purinoceptors, and enhanced activation of pain fibres. The purinergic system represents an important target for drug therapies that may improve GI inflammation and its consequences.

Splanchnic sympathetic nerves in the development of mild DOCA-salt hypertension

We previously reported that mild deoxycorticosterone acetate (DOCA)-salt hypertension develops in the absence of generalized sympathoexcitation. However, sympathetic nervous system activity (SNA) is regionally heterogeneous, so we began to investigate the role of sympathetic nerves to specific regions. Our first study on that possibility revealed no contribution of renal nerves to hypertension development. The splanchnic sympathetic nerves are implicated in blood pressure (BP) regulation because splanchnic denervation effectively lowers BP in human hypertension. Here we tested the hypothesis that splanchnic SNA contributes to the development of mild DOCA-salt hypertension. Splanchnic denervation was achieved by celiac ganglionectomy (CGX) in one group of rats while another group underwent sham surgery (SHAM-GX). After DOCA treatment (50 mg/kg) in rats with both kidneys intact, CGX rats exhibited a significantly attenuated increase in BP compared with SHAM-GX rats (15.6 ± 2.2 vs. 25.6 ± 2.2 mmHg, day 28 after DOCA treatment). In other rats, whole body norepinephrine (NE) spillover, measured to determine if CGX attenuated hypertension development by reducing global SNA, was not found to be different between SHAM-GX and CGX rats. In a third group, nonhepatic splanchnic NE spillover was measured as an index of splanchnic SNA, but this was not different between SHAM (non-DOCA-treated) and DOCA rats during hypertension development. In a final group, CGX effectively abolished nonhepatic splanchnic NE spillover. These data suggest that an intact splanchnic innervation is necessary for mild DOCA-salt hypertension development but not increased splanchnic SNA or NE release. Increased splanchnic vascular reactivity to NE during DOCA-salt treatment is one possible explanation.

Vascular-dependent effects of elevated glucose on postganglionic sympathetic neurons

Perivascular sympathetic nerves are important determinants of vascular function that are likely to contribute to vascular complications associated with hyperglycemia and diabetes. The present study tested the hypothesis that glucose modulates perivascular sympathetic nerves by studying the effects of 7 days of hyperglycemia on norepinephrine (NE) synthesis [tyrosine hydroxylase (TH)], release, and uptake. Direct and vascular-dependent effects were studied in vitro in neuronal and neurovascular cultures. Effects were also studied in vivo in rats made hyperglycemic (blood glucose >296 mg/dl) with streptozotocin (50 mg/kg). In neuronal cultures, TH and NE uptake measured in neurons grown in high glucose (HG; 25 mM) were less than that in neurons grown in low glucose (LG; 5 mM) (P < 0.05; n = 4 and 6, respectively). In neurovascular cultures, elevated glucose did not affect TH or NE uptake, but it increased NE release. Release from neurovascular cultures grown in HG (1.8 ± 0.2%; n = 5) was greater than that from cultures grown in LG (0.37 ± 0.28%; n = 5; P < 0.05; unpaired t-test). In vivo, elevated glucose did not affect TH or NE uptake, but it increased NE release. Release in hyperglycemic animals (9.4 + 1.1%; n = 6) was greater than that in control animals (5.39 + 1.1%; n = 6; P < 0.05; unpaired t-test). These data identify a novel vascular-dependent effect of elevated glucose on postganglionic sympathetic neurons that is likely to affect the function of perivascular sympathetic nerves and thereby affect vascular function.