Spontaneous release of large packets of noradrenaline from sympathetic nerve terminals in rat mesenteric arteries in vitro

Respiratory modulated sympathetic activity: a putative mechanism for developing vascular resistance?

KEY POINTS

Sympathetic activity exhibits respiratory modulation that is amplified in hypertensive rats. Respiratory modulated sympathetic activity produces greater changes in vascular resistance than tonic stimulation of the same stimulus magnitude in normotensive but not hypertensive rats. Mathematical modelling demonstrates that respiratory modulated sympathetic activity may fail to produce greater vascular resistance changes in hypertensive rats because the system is saturated as a consequence of a dysfunctional noradrenaline reuptake mechanism. Respiratory modulated sympathetic activity is an efficient mechanism to raise vascular resistance promptly, corroborating its involvement in the ontogenesis of hypertension.

ABSTRACT

Sympathetic nerve activity (SNA) exhibits respiratory modulation. This component of SNA is important - being recruited under cardiorespiratory reflex conditions and elevated in the spontaneously hypertensive (SH) rat - and yet the exact influence of this modulation on vascular tone is not understood, even in normotensive conditions. We constructed a mathematical model of the sympathetic innervation of an arteriole, and used it to test the hypothesis that respiratory modulation of SNA preferentially increases vasoconstriction compared to a frequency-matched tonic pattern. Simulations supported the hypothesis, where respiratory modulated increases in vasoconstriction were mediated by a noradrenergic mechanism. These predictions were tested in vivo in adult Wistar rats. Stimulation of the sympathetic chain (L3) with respiratory modulated bursting patterns, revealed that bursting increases vascular resistance (VR) more than tonic stimulation (57.8 ± 3.3% vs. 44.8 ± 4.2%; P < 0.001; n = 8). The onset of the VR response was also quicker for bursting stimulation (rise time constant = 1.98 ± 0.09 s vs. 2.35 ± 0.20 s; P < 0.01). In adult SH rats (n = 8), the VR response to bursting (44.6 ± 3.9%) was not different to tonic (37.4 ± 3.5%; P = 0.57). Using both mathematical modelling and in vivo techniques, we have shown that VR depends critically on respiratory modulation and revealed that this pattern dependency in Wistar rats is due to a noradrenergic mechanism. This respiratory component may therefore contribute to the ontogenesis of hypertension in the pre-hypertensive SH rat - raising VR and driving vascular remodelling. Why adult SH rats do not exhibit a pattern-dependent response is not known, but further modelling revealed that this may be due to dysfunctional noradrenaline reuptake.

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.

Boron-doped diamond nano/microelectrodes for biosensing and in vitro measurements

Since the fabrication of the first diamond electrode in the mid 1980s, repid progress has been made on the development and application of this new type of electrode material. Boron-doped diamond (BDD) electrodes exhibit outstanding properties compared to oxygen-containing sp2 carbon electrodes. These properties make BDD electrodes an ideal choice for use in complex samples. In recent years, BDD microelectrodes have been applied to in vitro measurements of biological molecules in tissues and cells. This review will summarize recent progress in the development and applications of BDD electrodes in bio-sensing and in vitro measurements of biomolecules. In the first section, the methods for BDD diamond film deposition and BDD microelectrodes preparation are described. This is followed by a description and discussion of several approaches for characterization of the BDD electrode surface structure, morphology, and electrochemical activity. Further, application of BDD microelectrodes for use in the in vitro analysis of norepinephrine (NE), serotonin (5-HT), nitric oxide (NO), histamine, and adenosine from tissues are summarized and finally some of the remaining challenges are discussed.

Alterations in sympathetic neuroeffector transmission to mesenteric arteries but not veins in DOCA-salt hypertension

We studied hypertension-associated changes in prejunctional alpha(2) adrenergic receptor (alpha(2)-AR) function using amperometry to monitor in vitro norepinephrine (NE) measured as oxidation currents. Vasoconstriction was measured using video imaging. NE release was induced by electrical stimulation of sympathetic nerves associated with mesenteric arteries (MA) and veins (MV) of sham and DOCA-salt hypertensive rats. NE oxidation currents were larger in DOCA-salt compared to sham MA; there were no differences between currents in sham and DOCA-salt MV. Increases in NE oxidation currents followed a multi-exponential time course in sham MA. In DOCA-salt MA and sham and DOCA-salt MV, the time course was mono-exponential. Yohimbine (alpha(2)-AR antagonist, 1 microM), caused a mono-exponential increase in NE oxidation currents in sham and DOCA-salt MA. Yohimbine increased NE oxidation currents and constrictions more in sham compared to DOCA-salt MA and compared to MV. UK 14,304 (alpha(2)-AR agonist, 1.0 microM), reduced currents less in DOCA-salt MA and sham and DOCA-salt MV compared to sham MA. Prazosin (alpha(1)-AR antagonist, 0.1 microM) did not alter NE oxidation currents. Prazosin inhibited constrictions more in DOCA-salt compared to sham MA and almost completely blocked constrictions in sham and DOCA-salt MV. Prazosin-resistant constrictions in MA were blocked by the P2 receptor antagonist, PPADS (10 microM). Prejunctional alpha(2)-ARs modify NE concentrations near neuroeffector junctions in MA and MV. alpha(2)-AR function is most prominent in MA and is impaired in DOCA-salt MA but not MV. Purinergic transmission predominates in sham MA. NE is the dominant vasoconstrictor in DOCA-salt MA and sham and DOCA-salt MV.

Differences in sympathetic neuroeffector transmission to rat mesenteric arteries and veins as probed by in vitro continuous amperometry and video imaging

As arteries are resistance blood vessels while veins perform a capacitance function, it might be expected that sympathetic neural control of arteries and veins would differ. The function of sympathetic nerves supplying mesenteric arteries (MA) and veins (MV) in rats was investigated using in vitro continuous amperometry with a carbon fibre microelectrode and video imaging. We simultaneously measured noradrenaline (NA) overflow at the blood vessel adventitial surface and vasoconstriction evoked by electrical stimulation of perivascular sympathetic nerves. Sympathetic nerve arrangement was studied using glyoxylic acid-induced fluorescence of NA. We found that: (i) there were significant differences between MA and MV in the arrangement of sympathetic nerves; (ii) frequency-response curves for NA overflow and vasoconstriction for MV were left-shifted compared to MA; (iii) the P2X receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microm), reduced constrictions in MA but not in MV while the alpha(1)-adrenergic receptor antagonist, prazosin (0.1 microm), blocked constrictions in MV but not in MA; (iv) NA overflow for MA was enhanced by the alpha(2)-adrenergic receptor antagonist, yohimbine (1.0 microm), and attenuated by the alpha(2)-adrenergic receptor agonist, UK 14,304 (1.0 microm), while yohimbine and UK 14,304 had little effect in MV; (v) cocaine (10 microm) produced larger increases in NA overflow in MA than in MV; (vi) UK 14,304 constricted MV but not MA while yohimbine reduced constrictions in MV but not MA. We conclude that there are fundamental differences in sympathetic neuroeffector mechanisms in MA and MV, which are likely to contribute to their different haemodynamic functions.

In vitro continuous amperometry with a diamond microelectrode coupled with video microscopy for simultaneously monitoring endogenous norepinephrine and its effect on the contractile response of a rat mesenteric artery

Continuous amperometry with a diamond microelectrode and video microscopy were used to record (in vitro) endogenous norepinephrine release simultaneously with the evoked contractile response of a mesenteric artery from a healthy Sprague Dawley rat. Norepinephrine (NE) is a vasoconstricting neurotransmitter released from sympathetic nerves that innervate the smooth muscle cell layers surrounding arteries and veins. Using these two techniques along with several drugs, the NE released at sympathetic neuroeffector junctions nearby the microelectrode was measured as an oxidation current. Key to the amperometric measurement was the use of a diamond microelectrode because of the response sensitivity, reproducibility, and stability it provided. NE release was elicited by electrical stimulation at frequencies between 1 and 60 Hz, with a maximum response seen at 20 Hz. Confirmation that the oxidation current was, in fact, associated with endogenous NE came from the results of several drugs. Tetrodotoxin (TTX, 0.3 microM), a voltage-dependent sodium channel antagonist that blocks nerve conduction, abolished both the oxidation current and the arterial constriction. The alpha(2)-adrenergic autoreceptor antagonist, yohimbine (1.0 microM), caused an increase in the oxidation current and the corresponding constriction. The addition of cocaine (10 microM), an antagonist that inhibits neuronal NE reuptake, caused both the oxidation current and the contractile response to increase. These results, combined with the fact that the hydrodynamic voltammetric E(1/2) for endogenous NE was identical to that for a standard solution, confirmed that the oxidation current was due to NE and that this compound caused, at least in part, the contractile response. The results demonstrate that continuous amperometric monitoring of NE with a diamond microelectrode and video imaging of vascular tone allow real time local measurement of the temporal relationship between nerve-stimulated NE release and arterial constriction.

Sympathetically evoked Ca2+ signaling in arterial smooth muscle

The sympathetic nervous system plays an essential role in the control of total peripheral vascular resistance and blood flow, by controlling the contraction of small arteries. Perivascular sympathetic nerves release ATP, norepinephrine (NE) and neuropeptide Y. This review summarizes our knowledge of the intracellular Ca2+ signals that are activated by ATP and NE, acting respectively on P2X1 and alpha1-adrenoceptors in arterial smooth muscle. Each neurotransmitter produces a unique type of post-synaptic Ca2+ signal and associated contraction. The neural release of ATP and NE is thought to vary markedly with the pattern of nerve activity, probably reflecting both pre- and post-synaptic mechanisms. Finally, we show that Ca2+ signaling during neurogenic contractions activated by trains of sympathetic nerve fiber action potentials are in fact significantly different from that elicited by simple bath application of exogenous neurotransmitters to isolated arteries (a common experimental technique), and end by identifying important questions remaining in our understanding of sympathetic neurotransmission and the physiological regulation of contraction of small arteries.

Selective release of ATP from sympathetic nerves of rat vas deferens by the toxin TsTX-I from Brazilian scorpion Tityus serrulatus

1. The effects of the main component of the Tityus serrulatus scorpion venom, toxin TsTX-I, were studied on the contractility and release of neurotransmitters in the rat vas deferens. Since TsTX-I is known to act on sodium channels, we used veratridine, another sodium channel agent, for comparison. 2. Toxin TsTX-I induced concentration-dependent contractions with an EC(50) value of 47.8+/-0.1 nM and a maximum effect of 84.4+/-10.4% of that for BaCl(2). 3. Contractions by TsTX-I were abolished by denervation or tetrodotoxin (0.1 microM), showing that the toxin effects depend on the integrity of sympathetic nerve terminals. 4. To check for the presence of a noradrenergic component, experiments were conducted after removal of adrenergic stores in nerve terminals by reserpinization (10 mg kg(-1), 24 h prior to experiments) or blockade of alpha(1) adrenoceptors by prazosin (30 microM), showing that these procedures did not modify the response to TsTX-I, and therefore that adrenoceptors were not involved in contractions. 5. To check for the presence of a purinergic component, experiments were carried out after blockade of P(2X) receptors by suramin (0.1 mM) or desensitization by alpha,beta-methylene-ATP (30 microM). These agents greatly abolished the contractile response to TsTX-I (about 83% by desensitization and 96% by suramin), showing the involvement of purinergic receptors. 6. The release of noradrenaline and purinergic agents (ATP, ADP, AMP and adenosine) was detected by HPLC. Together, the total release of purines in the presence of TsTX-I was about 42 times higher than in the control group. In contrast, TsTX-I did not modify the overflow of noradrenaline, showing that the release was selective for purines. 7. The release of purinergic agents was reduced by the N-type calcium channel blocker omega-conotoxin GVIA (1 microM) and by the P/Q-type blocker omega-conotoxin MVIIC (1 microM), showing that the effects of TsTX-I are calcium-dependent. 8. The results show that TsTX-I produced a selective release of purines from postganglionic sympathetic nerves in the rat vas deferens.

Differential sensitivity to calcium and osmotic pressure of fast and slow ATP currents at sympathetic varicosities in mouse vas deferens

Secretion of noradrenaline from large dense-core vesicles in chromaffin cells involves both rapid and slow components of exocytosis which are differentially sensitive to changes in external calcium, osmotic pressure and interruption of the interacting SNARE proteins. Electrical signs of secretion of ATP from sympathetic nerve terminals of mouse vas deferens, the excitatory junctional currents (EJCs), also indicate both rapid and slow mechanisms of exocytosis, which might also show such differential sensitivity. We report here that the large and fast EJCs are highly sensitive to changes in extracellular calcium ions whereas the small and slow EJCs are not. Furthermore, the frequency of fast EJCs is accelerated by hypotonic solutions whereas the slow EJCs are accelerated by hypertonic solution. Fast EJCs, but not slow EJCs, are blocked by peptide fragments of alpha-SNAP and syntaxin whereas slow EJCs are not. These observations point to two classes of exocytosis from sympathetic nerve terminals that parallel those of exocytosis from chromaffin cells.

Botulinum neurotoxin A attenuates release of norepinephrine but not NPY from vasoconstrictor neurons

We examined effects of botulinum neurotoxin A (BoNTA) on sympathetic constrictions of the vena cava and uterine artery from guinea pigs to test the role of soluble NSF attachment protein receptor (SNARE) proteins in release of the cotransmitters norepinephrine (NE) and neuropeptide Y (NPY). Protein extracts of venae cavae and uterine arteries showed partial cleavage of synaptosomal associated protein of 25 kDa (SNAP-25) after treatment in vitro with BoNTA (50-100 nM). The rising phase of isometric contractions of isolated venae cavae to field stimulation at 20 Hz, mediated by NE acting on alpha-adrenoceptors, was reduced significantly by 100 nM BoNTA. However, sustained sympathetic contractions mediated by NPY were not affected by BoNTA. In uterine arteries, noradrenergic contractions to 1-Hz stimulation were almost abolished by BoNTA, and contractions at 10 Hz were reduced by 50-60%. We conclude that SNARE proteins are involved in exocytosis of NE from synaptic vesicles at low frequencies of stimulation but may not be essential for exocytosis of NPY and NE from large vesicles at high stimulation frequencies.

ATP as a cotransmitter in sympathetic nerves and its inactivation by releasable enzymes

ATP and norepinephrine (NE) are cotransmitters released from many postganglionic sympathetic nerves. In this article, we review the evidence for ATP and NE cotransmission in the rodent vas deferens with special attention to the mechanisms involved in removing the cotransmitters from the neuroeffector junction. Although the clearance of NE is well understood (e.g., the primary mechanism being reuptake into the nerves), the clearance of ATP is just beginning to be explained. The general belief has been that ATP is metabolized by cell-fixed ecto-nucleotidases. It now seems, however, that when ATP is released from nerves as a transmitter there is a concomitant release of nucleotidases that rapidly degrade ATP sequentially to ADP, AMP, and adenosine, thereby terminating the action of ATP. In the guinea pig vas deferens, there appear to be at least two enzymes, one that converts ATP to ADP and ADP to AMP (an ATPDase) and a second enzyme that converts AMP to adenosine (an AMPase). An important feature of this process is that the transmitter-metabolizing nucleotidases are released into the synaptic space as opposed to being fixed to cell membranes. A preliminary characterization of these enzymes suggests that the releasable ATPDase exhibits some similarities to known ectonucleoside triphosphate/diphosphohydrolases, whereas the releasable AMPase exhibits some similarities to ecto-5'-nucleotidases.

An in vivo probe based on mechanically strong but structurally small carbon electrodes with an appreciable surface area

Physically small carbon electrodes were fabricated by pyrolyzing acetylene in a nitrogen atmosphere using pulled quartz capillaries as the supporting substrate. A carbon disk geometry was obtained when a parallel flow of acetylene (50 kPa) and nitrogen (10 mL min(-1)) was introduced into the system. Further, carbon was found to deposit at the tip and on the shank of the quartz capillaries when the nitrogen flow rate was increased (80 mL min(-1)), yielding an approximately cylindrical geometry. A series of electrochemical and spectroscopic analyses was carried out to examine the type of carbon surface obtained by pyrolysis of acetylene. The results suggested that a surface consisting of an almost defect-free highly oriented pyrolytic graphite type structure was formed by the pyrolyzed acetylene. However, this contradicts the kinetically reversible electron transfer observed for dopamine oxidation at these electrodes. Meanwhile, the nonpolar and relatively oxygen-free characteristics indicate that these electrodes also behave similarly to a hydrogenated carbon surface. The formation of a hydrogenated carbon-type surface may be plausible as a result of the attack on the carbon surface by a surplus of hydrogen produced by the pyrolysis of acetylene to form graphitic carbon. These characteristics are expected to aid in reducing electrode fouling, which is often encountered in electrochemical detection of neurotransmitters in vivo. In conjunction with a miniature physical dimension, their appreciable surface area and enhanced mechanical strength make these carbon electrodes well suited to the detection of neurotransmitters in vivo.