Autonomic nerve-smooth muscle transmission

Reply to Comment on 'New photoplethysmogram indicators for improving cuffless and continuous blood pressure estimation accuracy'

OBJECTIVE

This article was written by the invitation of the editorial board of Physiological Measurement. It is a Reply to the Comment regarding our recently published paper entitled 'New photoplethysmogram indicators for improving cuffless and continuous blood pressure estimation accuracy' (Lin et al 2018 Physiol. Meas. 29 025005).

APPROACH

We appreciate van Helmond and Joseph's (2018 Physiol. Meas. 098001) interests and comments on our previous paper. In the Comment, they discussed in detail the physiology underlying the pulse arrive time (PAT)-based methods for blood pressure (BP) measurement, and concluded that there are inherent physiological reasons precluding the development of an accurate continuous cuffless BP measurement using PAT-based methods. We could agree with the comments of van Helmond and Joseph about the physiology underlying PAT-based methods for BP measurement. It may be difficult to minimize the confounding effects of physiological factors such as pre-ejection period and smooth muscle tone, etc. However, in this Reply, we discuss some potential solutions to deal with these problems from an engineering point of view.

MAIN RESULTS

When heart rate, more photoplethysmogram (PPG) features, PAT, robust machine learning models, and other techniques were adopted for BP estimation, it is promising for improving the accuracy of BP estimation to an acceptable range that can meet professional standards (e.g. Advancement of Medical Instrumentation (AAMI) standard, British Hypertension Society (BHS) protocol).

SIGNIFICANCE

PAT- and/or PPG-based methods may be a promising technique for continuous and unobtrusive blood pressure measurement.

P2X receptors

Extracellular adenosine 5'-triphosphate (ATP) activates cell surface P2X and P2Y receptors. P2X receptors are membrane ion channels preferably permeable to sodium, potassium and calcium that open within milliseconds of the binding of ATP. In molecular architecture, they form a unique structural family. The receptor is a trimer, the binding of ATP between subunits causes them to flex together within the ectodomain and separate in the membrane-spanning region so as to open a central channel. P2X receptors have a widespread tissue distribution. On some smooth muscle cells, P2X receptors mediate the fast excitatory junction potential that leads to depolarization and contraction. In the central nervous system, activation of P2X receptors allows calcium to enter neurons and this can evoke slower neuromodulatory responses such as the trafficking of receptors for the neurotransmitter glutamate. In primary afferent nerves, P2X receptors are critical for the initiation of action potentials when they respond to ATP released from sensory cells such as taste buds, chemoreceptors or urothelium. In immune cells, activation of P2X receptors triggers the release of pro-inflammatory cytokines such as interleukin 1β. The development of selective blockers of different P2X receptors has led to clinical trials of their effectiveness in the management of cough, pain, inflammation and certain neurodegenerative diseases.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Purinergic signalling: pathophysiology and therapeutic potential

The article begins with a review of the main conceptual steps involved in the development of our understanding of purinergic signalling, including non-adrenergic, non-cholinergic (NANC) neurotransmission; identification of ATP as a NANC transmitter; purinergic cotransmission; recognition of two families of purinoceptors [P1 (adenosine) and P2 (ATP/ADP)]; and, later, cloning and characterisation of P1 (G protein-coupled), P2X (ion channel) and P2Y (G protein-coupled) receptor subtypes. Further studies have established the involvement of ATP in synaptic neurotransmission in both ganglia and in the central nervous system; long-term (trophic) purinergic signalling in cell proliferation, differentiation and death occurring in development and regeneration; and short-term purinergic signalling in neurotransmission, neuromodulation and secretion. ATP is released from most cell types in response to gentle mechanical stimulation and is rapidly degraded to adenosine by ecto-nucleotidases. This review then focuses on the pathophysiology of purinergic signalling in a wide variety of systems, including urinogenital, cardiovascular, airway, musculoskeletal and gastrointestinal. Consideration is also given to the involvement of purinoceptors in pain, cancer and diseases of the central nervous system. Purinergic therapeutic approaches for the treatment of some of these diseases are discussed.

Differential activation of an identified motor neuron and neuromodulation provide Aplysia's retractor muscle an additional function

To survive, animals must use the same peripheral structures to perform a variety of tasks. How does a nervous system employ one muscle to perform multiple functions? We addressed this question through work on the I3 jaw muscle of the marine mollusk Aplysia californica's feeding system. This muscle mediates retraction of Aplysia's food grasper in multiple feeding responses and is innervated by a pool of identified neurons that activate different muscle regions. One I3 motor neuron, B38, is active in the protraction phase, rather than the retraction phase, suggesting the muscle has an additional function. We used intracellular, extracellular, and muscle force recordings in several in vitro preparations as well as recordings of nerve and muscle activity from intact, behaving animals to characterize B38's activation of the muscle and its activity in different behavior types. We show that B38 specifically activates the anterior region of I3 and is specifically recruited during one behavior, swallowing. The function of this protraction-phase jaw muscle contraction is to hold food; thus the I3 muscle has an additional function beyond mediating retraction. We additionally show that B38's typical activity during in vivo swallowing is insufficient to generate force in an unmodulated muscle and that intrinsic and extrinsic modulation shift the force-frequency relationship to allow contraction. Using methods that traverse levels from individual neuron to muscle to intact animal, we show how regional muscle activation, differential motor neuron recruitment, and neuromodulation are key components in Aplysia's generation of multifunctionality.

Purinergic signalling in the urinary tract in health and disease

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

Purinergic signalling: from discovery to current developments

This lecture is about the history of the purinergic signalling concept. It begins with reference to the paper by Paton & Vane published in 1963, which identified non-cholinergic relaxation in response to vagal nerve stimulation in several species, although they suggested that it might be due to sympathetic adrenergic nerves in the vagal nerve trunk. Using the sucrose gap technique for simultaneous mechanical and electrical recordings in smooth muscle (developed while in Feldberg's department in the National Institute for Medical Research) of the guinea-pig taenia coli preparation (learned when working in Edith Bülbring's smooth muscle laboratory in Oxford Pharmacology), we showed that the hyperpolarizations recorded in the presence of antagonists to the classical autonomic neurotransmitters, acetylcholine and noradrenaline, were inhibitory junction potentials in response to non-adrenergic, non-cholinergic neurotransmission, mediated by intrinsic enteric nerves controlled by vagal and sacral parasympathetic nerves. We then showed that ATP satisfied the criteria needed to identify a neurotransmitter released by these nerves. Subsequently, it was shown that ATP is a cotransmitter in all nerves in the peripheral and central nervous systems. The receptors for purines and pyrimidines were cloned and characterized in the early 1990 s, and immunostaining showed that most non-neuronal cells as well as nerve cells expressed these receptors. The physiology and pathophysiology of purinergic signalling is discussed.

Cotransmission in the autonomic nervous system

After some early hints, cotransmission was proposed in 1976 and then "chemical coding" later established for sympathetic nerves (noradrenaline/norepinephrine, adenosine 5'-triphosphate (ATP), and neuropeptide Y), parasympathetic nerves (acetylcholine, ATP, and vasoactive intestinal polypeptide (VIP)), enteric nonadrenergic, noncholinergic inhibitory nerves (ATP, nitric oxide, and VIP), and sensory-motor nerves (calcitonin gene-related peptide, substance P, and ATP). ATP is a primitive signaling molecule that has been retained as a cotransmitter in most, if not all, nerve types in both the peripheral and central nervous systems. Neuropeptides coreleased with small molecule neurotransmitters in autonomic nerves do not usually act as cotransmitters but rather as prejunctional neuromodulators or trophic factors. Autonomic cotransmission offers subtle, local variation in physiological control mechanisms, rather than the dominance of inflexible central control mechanisms envisaged earlier. The variety of information imparted by a single neuron then greatly increases the sophistication and complexity of local control mechanisms. Cotransmitter composition shows considerable plasticity in development and aging, in pathophysiological conditions and following trauma or surgery. For example, ATP appears to become a more prominent cotransmitter in inflammatory and stress conditions.

Discovery of purinergic signalling, the initial resistance and current explosion of interest

There has been a remarkable growth of papers published about purinergic signalling via ATP since 1972. I am most grateful to the wonderful PhD students and postdoctoral fellows who have worked with me over the years to pursue the purinergic hypothesis despite early opposition and to the many outstanding scientists around the world who are currently extending the story. Recently, therapeutic approaches to pathological disorders include the development of selective P1 and P2 receptor subtype agonists and antagonists, as well as of inhibitors of extracellular ATP breakdown and of ATP transport enhancers and inhibitors. Medicinal chemists are starting to develop small molecule purinergic drugs that are orally bioavailable and stable in vivo.

Modulation of purinergic neuromuscular transmission by phorbol dibutyrate is independent of protein kinase C in murine urinary bladder

Parasympathetic control of murine urinary bladder consists of contractile components mediated by both muscarinic and purinergic receptors. Using intracellular recording techniques, the purinergic component of transmission was measured as both evoked excitatory junctional potentials (EJPs) in response to electrical field stimulation and spontaneous events [spontaneous EJPs (sEJPs)]. EJPs, but not sEJPs, were abolished by the application of the Na(+) channel blocker tetrodotoxin and the Ca(2+) channel blocker Cd(2+). Both EJPs and sEJPs were abolished by the application of the P2X(1) antagonist 8,8'-[carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino)]bis-1,3,5-naphthalenetrisulfonic acid hexasodium salt (NF279). Application of phorbol dibutyrate (PDBu) increased electrically evoked EJP amplitudes with no effect on mean sEJP amplitudes. Similar increases in EJP amplitudes were produced by PDBu in the presence of either the nonselective protein kinase inhibitor staurosporine or the specific protein kinase C (PKC) inhibitor 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X). These results suggest that PDBu increases the purinergic component of detrusor transmission through increasing neurogenic ATP release via a PKC-independent mechanism.

Current spread in the smooth muscle of the guinea-pig vas deferens

1. The membrane polarization in response to intracellular stimulation and external stimulation, the junction potentials evoked by nerve and field stimulation and the spontaneous junction potentials were studied in the guinea-pig vas deferens.2. The responses to intracellular stimulation differed from those to external stimulation applied through a large electrode in the following ways: short time constant of the electrotonic potential; linearity of current-voltage relation; all-or-none spike only in a small proportion of the cells; high critical firing level; short latency; weak tendency for repetitive activity during depolarization; and sharp spatial decay of the response.3. The difference between intracellular and external stimulation could be explained by differences in current distribution in the tissue, if many muscle fibres were aggregated in functional bundles, with three-dimensional cell-to-cell connexions, so that the membrane near an intracellular stimulating electrode was shunted by a large area of surrounding membrane.4. The time course of the junction potentials depended on the manner by which they were produced. The junction potential evoked by hypogastric nerve stimulation was recorded in every cell with almost the same amplitude. The spontaneous junction potential decayed very sharply with distance and the time course of the falling phase was about 10 times faster than that of the evoked junction potential.The difference between the time course of the junction potentials was also explained by the difference in current distribution in the tissue.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Do sympathetic nerves release noradrenaline in "quanta"?

The discovery of excitatory junction potentials (EJPs) in guinea-pig vas deferens by Burnstock and Holman (1960) showed for the first time that a sympathetic transmitter, now known to be ATP, is secreted in "quanta". As it was assumed at the time that EJPS are triggered by noradrenaline, this discovery led to attempts to use the fractional overflow of noradrenaline from sympathetically innervated tissues to assess, indirectly, the number of noradrenaline molecules in the average "quantum". The basic finding was that each pulse released 1/50000 of the tissue content of noradrenaline, when reuptake was blocked and prejunctional alpha(2)-adrenoceptors were intact. This provided the constraints, two extreme alternatives: (i) each pulse releases 0.2-3% of the content of a vesicle from all varicosities, or (ii) each pulse releases the whole content of a vesicle from 0.2 to 3% of the varicosities. New techniques have made it possible to address questions about the release probability in individual sites, or the "quantal" size, more directly. Results by optical (comparison of the labelling of SV2 and synaptotagmin, proteins in the membrane of transmitter vesicles), electrophysiological (excitatory junction currents, EJCs, at single visualized varicosities) and amperometric (the noradrenaline oxidation current at a carbon fibre electrode) methods reveal that transmitter exocytosis in varicosities is intermittent. The EJC and noradrenaline oxidation current responses (in rat arteries) to a train of single pulses were observed to be similar in intermittency and amplitude fluctuation. This suggests that they are caused by exocytosis of single or very few "quanta" of ATP and noradrenaline, respectively, equal to the contents of single vesicles, from a small population of release sites. These findings support, but do not conclusively prove the validity of the "intermittent" model of noradrenaline release. The question if noradrenaline is always secreted in packets of preset size ("quanta") and if the "quantum" is a subfraction or the whole content of single synaptic vesicles, still remains open.

CORRELATION OF FINE STRUCTURE AND PHYSIOLOGY OF THE INNERVATION OF SMOOTH MUSCLE IN THE GUINEA PIG VAS DEFERENS

An electron microscope study of the innervation of smooth muscle of the guinea pig vas deferens was undertaken in order to find a structural basis for recent electrophysiological observations. The external longitudinal muscle coat was examined in transverse section. Large areas of the surfaces of adjacent muscle cells were 500 to 800 A apart. Closer contacts were rare. A special type of close contact suggested cytoplasmic transfer between neighbouring cells. Groups of non-myelinated axons from ganglia at the distal end of the hypogastric nerve ramified throughout the muscle. Some small axon bundles and single axons lay in narrow fissures within closely packed muscle masses. Many axons contained "synaptic vesicles." About 25 per cent of the muscle fibres in the plane of section were within 0.25 µ of a partly naked axon; of these 15 per cent were within 500 A of the axon, and about 1 per cent made close contact (200 A) with a naked axon. It is unlikely that every muscle fibre is in close contact with an axon, and it is not possible for every fibre to have many such contacts. Muscle fibres are probably activated by both diffusion of transmitter from naked portions of axons a fraction of a micron distant, and electrotonic spread of activity from neighbouring cells.

Involvement of cholinergic nerves in excitatory junction potentials through prejunctional nicotinic receptors in the guinea-pig vas deferens

1. The effects of various drugs on excitatory junction potentials (EJPs) elicited by field stimulation of the longitudinal smooth muscle of the guinea-pig vas deferens were examined using the sucrose-gap method. 2. Field stimulation with a single pulse (1.0 ms, 20 Vcm-1) produced a depolarization of 3-7 mV. When a single pulse was repetitively applied, the depolarization showed summation, resulting in action potential firing. 3. Tetrodotoxin (1 microM) and guanethidine (1 microM) completely suppressed EJPs whilst propranolol (1 microM) or atropine (0.1 microM) did not affect them. Phentolamine (1 microM) slightly potentiated EJPs. 4. d-Tubocurarine (dTC) at lower concentrations (0.5-2 microM) suppressed EJPs, but at a higher concentration (20 microM) the drug little affected them. 5. Hexamethonium (C6) suppressed EJPs at low concentrations (0.5-5 microM), but potentiated them at a higher concentration (50 microM). 6. Either dTC or C6, each at a concentration of 0.5 microM, did not affect the depolarization induced by exogenously applied ATP or noradrenaline. 7. These results suggest that in this tissue cholinergic nerve activity can contribute to the magnitude of EJPs, and that this effect is mediated through nicotinic receptors.

Possible interaction of cholinergic nerves with two different (pre and post) sites of the neuromuscular junction in guinea-pig vas deferens

Effects of various drugs on the mechanical responses of the longitudinal smooth muscle of guinea-pig vas deferens evoked by ACh and field nerve stimulation were examined. ACh (28-280 microM) produced a contraction consisting of two phases. The first phase of the contraction was suppressed by guanethidine and by nicotinic antagonists. The second phase was suppressed only by atropine. Both phases were unaffected by TTX or prazosin. Field stimulation (0.1 msec, 40 Hz) evoked contractions which also consisted of two (early and late) phases. Guanethidine (0.1-1 microM) suppressed both phases whilst prazosin (1 microM) suppressed only the late phase. Atropine (0.1 microM) suppressed both phases whilst physostigmine (5 microM) potentiated both phases of field stimulation-evoked contractions. Pentolinium suppressed both phases of field stimulation-evoked contractions at low concentrations (2-10 microM), but potentiated them at a higher concentration (100 microM). dTC at a low concentration (0.5 microM) suppressed the early phase, but slightly enhanced the late phase of field stimulation responses. At a higher concentration (20 microM), dTC potentiated both phases of the response. Pentolinium and dTC did not affect the contractions induced by 90 mM-K ions, ATP or NA. These results suggest that cholinergic nerves possess an excitatory action not only directly at the smooth muscle but also at the noradrenergic nerve terminals. The role of each receptor is discussed further.

Dual excitatory actions of acetylcholine at the neuromuscular junction in the guinea-pig vas deferens

The action of acetylcholine (ACh) on the smooth muscle of guinea-pig vas deferens was studied using the sucrose-gap method. ACh, when applied at a concentration of 10(-6) M, evoked a depolarization of the smooth muscle membrane which was slow in time course (slow depolarization). When ACh was applied at higher concentrations, another depolarization which was fast in time course (fast depolarization) occurred, overlapping the early part of the slow depolarization. The magnitudes of both depolarizations were concentration-dependent on ACh. TTX and adrenergic receptor antagonists had little effect on either depolarizations, while guanethidine and nicotinic receptor antagonists mainly suppressed the fast depolarization. In contrast, atropine suppressed the slow depolarization. The membrane conductance observed by current application, was reduced during the slow depolarization, and the reversal potential of the depolarization was 18.3 mV negative to the resting membrane potential. Whereas, the reversal potential of the fast depolarization was 27.6 mV positive to the resting membrane potential. This reversal potential was quite similar to that of the adenosine triphosphate (ATP)-induced depolarization, previously observed in the same tissue. From these observations, it is suggested that in the guinea-pig vas deferens, ACh acts on nicotinic receptors at the sympathetic postganglionic nerve terminal, causing the release mostly of a non-adrenergic transmitter, probably ATP. In addition, ACh also acts on muscarinic receptors on the smooth muscle membrane, inducing membrane depolarization resulting from a reduction of the membrane conductance to potassium ions.

Review lecture. Neurotransmitters and trophic factors in the autonomic nervous system

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