The effect of omega conotoxin GVIA, a peptide modulator of the N-type voltage sensitive calcium channels, on motor responses produced by activation of efferent and sensory nerves in mammalian smooth muscle

ATP as a cotransmitter in sympathetic and parasympathetic nerves - another Burnstock legacy

Geoff Burnstock created an outstanding scientific legacy that includes identification of adenosine 5'-triphosphate (ATP) as an inhibitory neurotransmitter in the gut, the discovery and characterisation of a large family of purine and uridine nucleotide-sensitive ionotropic P2X and metabotropic P2Y receptors and the demonstration that ATP is as an excitatory cotransmitter in autonomic nerves. The evidence for cotransmission includes that: 1) ATP is costored with noradrenaline in synaptic vesicles in postganglionic sympathetic nerves innervating smooth muscle tissues, including the vas deferens and most arteries. 2) When coreleased with noradrenaline, ATP acts at postjunctional P2X1 receptors to elicit depolarisation, Ca²⁺ influx, Ca²⁺ sensitisation and contraction. 3) ATP is also coreleased with acetylcholine from postganglionic parasympathetic nerves innervating the urinary bladder, where it stimulates postjunctional P2X1 receptors, and a second, as yet unidentified site to evoke contraction of detrusor smooth muscle. In both systems membrane-bound ecto-enzymes and soluble nucleotidases released from postganglionic nerves dephosphorylate ATP and so terminate its neurotransmitter actions. Currently, the most promising potential area of therapeutic application relating to cotransmission is treatment of dysfunctional urinary bladder. This family of disorders is associated with the appearance of a purinergic component of neurogenic contractions. This component is an attractive target for drug development and targeting it may be a rewarding area of research.

Sympathetic and Sensory-Motor Nerves in Peripheral Small Arteries

Small arteries, which play important roles in controlling blood flow, blood pressure, and capillary pressure, are under nervous influence. Their innervation is predominantly sympathetic and sensory motor in nature, and while some arteries are densely innervated, others are only sparsely so. Innervation of small arteries is a key mechanism in regulating vascular resistance. In the second half of the previous century, the physiology and pharmacology of this innervation were very actively investigated. In the past 10-20 yr, the activity in this field was more limited. With this review we highlight what has been learned during recent years with respect to development of small arteries and their innervation, some aspects of excitation-release coupling, interaction between sympathetic and sensory-motor nerves, cross talk between endothelium and vascular nerves, and some aspects of their role in vascular inflammation and hypertension. We also highlight what remains to be investigated to further increase our understanding of this fundamental aspect of vascular physiology.

Bioactive Mimetics of Conotoxins and other Venom Peptides

Ziconotide (Prialt^®), a synthetic version of the peptide ω-conotoxin MVIIA found in the venom of a fish-hunting marine cone snail Conus magnus, is one of very few drugs effective in the treatment of intractable chronic pain. However, its intrathecal mode of delivery and narrow therapeutic window cause complications for patients. This review will summarize progress in the development of small molecule, non-peptidic mimics of Conotoxins and a small number of other venom peptides. This will include a description of how some of the initially designed mimics have been modified to improve their drug-like properties.

ATP as a cotransmitter in the autonomic nervous system

The role of adenosine 5'-triphosphate (ATP) as a major intracellular energy source is well-established. In addition, ATP and related nucleotides have widespread extracellular actions via the ionotropic P2X (ligand-gated cation channels) and metabotropic P2Y (G protein-coupled) receptors. Numerous experimental techniques, including myography, electrophysiology and biochemical measurement of neurotransmitter release, have been used to show that ATP has several major roles as a neurotransmitter in peripheral nerves. When released from enteric nerves of the gastrointestinal tract it acts as an inhibitory neurotransmitter, mediating descending muscle relaxation during peristalsis. ATP is also an excitatory cotransmitter in autonomic nerves; 1) It is costored with noradrenaline in synaptic vesicles in postganglionic sympathetic nerves innervating smooth muscle preparations, such as the vas deferens and most arteries. When coreleased with noradrenaline, ATP acts at postjunctional P2X1 receptors to evoke depolarisation, Ca(2+) influx, Ca(2+) sensitisation and contraction. 2) ATP is also coreleased with acetylcholine from postganglionic parasympathetic nerves innervating the urinary bladder and again acts at postjunctional P2X1 receptors, and possibly also a P2X1+4 heteromer, to elicit smooth muscle contraction. In both cases the neurotransmitter actions of ATP are terminated by dephosphorylation by extracellular, membrane-bound enzymes and soluble nucleotidases released from postganglionic nerves. There are indications of an increased contribution of ATP to control of blood pressure in hypertension, but further research is needed to clarify this possibility. More promising is the upregulation of P2X receptors in dysfunctional bladder, including interstitial cystitis, idiopathic detrusor instability and overactive bladder syndrome. Consequently, these roles of ATP are of great therapeutic interest and are increasingly being targeted by pharmaceutical companies.

Challenging the catechism of therapeutics for chronic neuropathic pain: Targeting CaV2.2 interactions with CRMP2 peptides

Chronic neuropathic pain management is a worldwide concern. Pharmaceutical companies globally have historically targeted ion channels as the therapeutic catechism with many blockbuster successes. Remarkably, no new pain therapeutic has been approved by European or American regulatory agencies over the last decade. This article will provide an overview of an alternative approach to ion channel drug discovery: targeting regulators of ion channels, specifically focusing on voltage-gated calcium channels. We will highlight the discovery of an anti-nociceptive peptide derived from a novel calcium channel interacting partner - the collapsin response mediator protein 2 (CRMP2). In vivo administration of this peptide reduces pain behavior in a number of models of neuropathic pain without affecting sympathetic-associated cardiovascular activity, memory retrieval, sensorimotor function, or depression. A CRMP2-derived peptide analgesic, with restricted access to the CNS, represents a completely novel approach to the treatment of severe pain with an improved safety profile. As peptides now represent one of the fastest growing classes of new drugs, it is expected that peptide targeting of protein interactions within the calcium channel complex may be a paradigm shift in ion channel drug discovery.

Calcium channel subtypes for cholinergic and nonadrenergic noncholinergic neurotransmission in isolated guinea pig trachea

The Ca(2+) channel subtypes in the neurotransmission of isolated guinea pig trachea were elucidated by monitoring the effects of specific Ca(2+) channel blockers on cholinergic contractions and nonadrenergic noncholinergic (NANC) relaxation elicited by electrical field stimulation (EFS). In isolated guinea pig trachea, cholinergic contractile responses to low- and high-frequency EFS were inhibited by the selective N-type calcium channel blocker, ω-conotoxin MVIIA. ω-Agatoxin IVA (a selective P-type blocker), ω-conotoxin MVIIC (a nonselective N-, Q-, and P-type blocker), and nifedipine (a selective L-type blocker) were ineffective, whereas Ni(2+) (a T- and R-type blocker) facilitated cholinergic contractions and produced a late contracture when its concentration exceeded 30 μM. The more the concentration of Ni(2+) increased, the greater the number of incidences and the late contracture areas which occurred. Late contracture did not seem to be due to the effects of acetylcholine, tachykinins, or other polypeptides, but disappeared in the absence of indomethacin. The NANC relaxant responses elicited by the low- and high-frequency EFS were inhibited by ω-conotoxin MVIIA or Ni(2+), but unaffected by ω-Agatoxin IVA, ω-conotoxin MVIIC, and nifedipine. In the absence of indomethacin, Ni(2+) did not alter the ω-conotoxin MVIIA (100 nM)-resistant component of cholinergic contraction, but significantly further inhibited that of NANC relaxation. These results suggest that in isolated guinea pig trachea, cholinergic contraction is regulated by N-type calcium channels which may mask T- and R-type calcium channels and may be co-modulated by both, while NANC relaxation is mainly and independently controlled by N-, T-, and R-type calcium channels.

Spontaneous release of acetylcholine from autonomic nerves in the bladder

BACKGROUND AND PURPOSE

Bladder contractility is regulated by intrinsic myogenic mechanisms interacting with autonomic nerves. In this study, we have investigated the physiological role of spontaneous release of acetylcholine in guinea pig and rat bladders.

EXPERIMENTAL APPROACH

Conventional isotonic or pressure transducers were used to record contractile activity of guinea pig and rat bladders.

KEY RESULTS

Hyoscine (3 micromol x L(-1)), but not tetrodotoxin (TTX, 1 micromol x L(-1)), reduced basal tension, distension-evoked contractile activity and physostigmine (1 micromol x L(-1))-evoked contractions of the whole guinea pig bladder and muscle strips in vitro. omega-Conotoxin GVIA (0.3 micromol x L(-1)) did not affect physostigmine-induced contractions when given either alone or in combination with omega-agatoxin IVA (0.1 micromol x L(-1)) and SNX 482 (0.3 micromol x L(-1)). After 5 days in organotypic culture, when extrinsic nerves had significantly degenerated, the ability of physostigmine to induce contractions was reduced in the dorso-medial strips, but not in lateral strips (which have around 15 times more intramural neurones). Most muscle strips from adult rats lacked intramural neurones. After 5 days in culture, physostigmine-induced or electrical field stimulation-induced contractions of the rat bladder strips were greatly reduced. In anaesthetized rats, topical application of physostigmine (5-500 nmol) on the bladder produced a TTX-resistant tonic contraction that was abolished by atropine (4.4 micromol x kg(-1) i.v.).

CONCLUSIONS AND IMPLICATIONS

The data indicate that there is spontaneous TTX-resistant release of acetylcholine from autonomic cholinergic extrinsic and intrinsic nerves, which significantly affects bladder contractility. This release is resistant to blockade of N, P/Q and R type Ca(2+) channels.

Glucagon-like peptide-2 relaxes mouse stomach through vasoactive intestinal peptide release

Glucagon-like peptide-2 (GLP-2) influences different aspects of the gastrointestinal function, including epithelial growth, digestion, absorption, motility, and blood flow. Intraluminal pressure from isolated mouse stomach was recorded to investigate whether GLP-2 affects gastric tone and to analyze its mechanism of action. Regional differences between diverse parts of the stomach were also examined using circular muscular strips from fundus and antrum. In the whole stomach, GLP-2 (0.3-100 nM) produced concentration-dependent relaxation with a maximum that was about 75% of relaxation to 1 microM isoproterenol (IC50=2.5 nM). This effect was virtually abolished by desensitization of GLP-2 receptors or by alpha-chymotrypsin. The relaxant response to GLP-2 was not affected by tetrodotoxin, a blocker of neuronal voltage-dependent Na+ channels, but it was significantly reduced by omega-conotoxin GVIA, a blocker of neuronal N-type voltage-operated Ca2+ channels. Nomega-nitro-L-arginine methyl ester, a blocker of nitric oxide synthase, or apamin, a blocker of Ca2+-dependent potassium channels, failed to affect the gastric response to the peptide. However, the relaxation was significantly antagonized by [Lys1,Pro2,5,Arg3,4,Tyr6]VIP7-28, a vasoactive intestinal peptide (VIP) receptor antagonist (GLP-2 maximum effect=45% of relaxation to 1 microM isoproterenol), and virtually abolished by desensitization of the VIP receptors. GLP-2 induced concentration-dependent relaxation in carbachol-precontracted fundic strips but not in antral strips. These results provide the first experimental evidence that GLP-2 is able to induce gastric relaxation acting peripherally on the mouse stomach. The effect appears to be mediated by prejunctional neural release of VIP and confined to fundic region.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Involvement of nitric oxide and tachykinins in the effects induced by protease-activated receptors in rat colon longitudinal muscle

(1) The aim of the present study was to verify a possible involvement of nitric oxide (NO) and of tachykinins in the contractile and relaxant effects caused by the activation of protease-activated receptor (PAR)-1 and PAR-2 in the longitudinal muscle of rat colon. (2) Mechanical responses to the PAR-1 activating peptides, SFLLRN-NH(2) (10 nM-10 micro M) and TFLLR-NH(2) (10 nM-10 micro M), and to the PAR-2-activating peptide, SLIGRL-NH(2) (10 nM-10 micro M), were examined in vitro in the absence and in the presence of different antagonists. (3) The relaxation induced by SFLLRN-NH(2), TFLLR-NH(2) and SLIGRL-NH(2) was antagonised by the inhibitor of NO synthase L-N(omega)-nitroarginine methyl ester (300 micro M), or by the inhibitor of the guanylyl cyclase, 1-H-oxodiazol-[1,2,4]-[4,3-a]quinoxaline-1-one (10 micro M). (4) The contractile responses to PAR-1 and PAR-2 activation were concentration-dependently attenuated by SR140333 (0.1-1 micro M), NK(1) receptor antagonist, or by SR48968 (0.1-1 micro M), NK(2) receptor antagonist. The combined pretreatment with SR140333 (1 micro M) and SR48968 (1 micro M) produced additive suppressive effects on the contractile responses to PAR activation. Pretreatment of the preparation with capsaicin (10 micro M) markedly reduced the contractions evoked by SFLLRN-NH(2), TFLLR-NH(2) and SLIGRL-NH(2), while omega-conotoxin GVIA (0.2 micro M) had no effect. (5) The present results suggest that in rat colonic longitudinal muscle, PAR-1 and PAR-2 activation can evoke (i) relaxation through the production of NO or (ii) contraction through the release of tachykinins, likely, from sensory nerves. These actions may contribute to motility disturbances during intestinal trauma and inflammation.

Neuroeffector mechanisms involved in the regulation of dog splenic arterial tone

It has been recognized that sympathetic neurons release several transmitters but mainly adenosine 5'-triphosphate (ATP), noradrenaline, and neuropeptide Y (NPY). Recently, we reported that periarterial nerve electrical stimulation (PNS) produced biphasic vasoconstrictions consisting of an initial transient, predominantly P2X-purinoceptor-mediated constriction followed by a prolonged, alpha(1)-adrenoceptor-mediated one in canine isolated splenic arteries. In this article, we tried to analyze the effects of several selective key drugs that influence the PNS-induced responses, and we functionally showed sympathetic transmitter releasing mechanisms by pharmacological analysis using purinergic, adrenergic, and NPYergic agonists and antagonists.

Role of voltage-gated cation channels and axon reflexes in the release of sensory neuropeptides by capsaicin from isolated rat trachea

In order to reveal the role of axon reflexes and sensory receptors in sensory neuropeptide release in response to capsaicin, liberation of substance P, calcitonin gene-related peptide and somatostatin from isolated rat tracheae was investigated in the presence of voltage-sensitive Na(+) and Ca(2+) channel blocking agents. Neuropeptide release induced by capsaicin (10 nM) remained unchanged in the presence of 25 mM lidocaine, 1 microM tetrodotoxin or the N-type Ca(2+) channel inhibitor, omega-conotoxin GVIA (100-300 nM). Peptide release by 100 pulses of 2 Hz field stimulation was prevented by lidocaine or tetrodotoxin. Omega-agatoxin TK (250 nM) significantly inhibited and Cd(2+) (200 microM) prevented capsaicin-induced neuropeptide release. These results suggest that chemical stimulation-induced neuropeptide release does not involve activation of fast Na(+) channels or N- and P-type voltage-dependent Ca(2+) channels, but contribution of Q-type Ca(2+) channels is possible. Sensory neuropeptides are released by capsaicin from sensory receptors without axon reflexes.

Involvement of tachykinin receptors in Clostridium perfringens beta-toxin-induced plasma extravasation

1 Clostridium perfringens beta-toxin causes dermonecrosis and oedema in the dorsal skin of animals. In the present study, we investigated the mechanisms of oedema induced by the toxin. 2 The toxin induced plasma extravasation in the dorsal skin of Balb/c mice. 3 The extravasation was significantly inhibited by diphenhydramine, a histamine 1 receptor antagonist. However, the toxin did not cause the release of histamine from mouse mastocytoma cells. 4 Tachykinin NK(1) receptor antagonists, [D-Pro(2), D-Trp(7,9)]-SP, [D-Pro(4), D-Trp(7,9)]-SP and spantide, inhibited the toxin-induced leakage in a dose-dependent manner. Furthermore, the non-peptide tachykinin NK(1) receptor antagonist, SR140333, markedly inhibited the toxin-induced leakage. 5 The leakage induced by the toxin was markedly reduced in capsaicin-pretreated mouse skin but the leakage was not affected by systemic pretreatment with a calcitonin gene-related peptide receptor antagonist (CGRP(8-37)). 6 The toxin-induced leakage was significantly inhibited by the N-type Ca(2+) channel blocker, omega-conotoxin MVIIA, and the bradykinin B(2) receptor antagonist, HOE140 (D-Arg-[Hyp(3), Thi(5), D-Tic(7), Oic(8)]-bradykinin), but was not affected by the selective L-type Ca(2+) channel blocker, verapamil, the P-type Ca(2+) channel blocker, omega-agatoxin IVA, tetrodotoxin (TTX), the TTX-resistant Na(+) channel blocker, carbamazepine, or the sensory nerve conduction blocker, lignocaine. 7 These results suggest that plasma extravasation induced by beta-toxin in mouse skin is mediated via a mechanism involving tachykinin NK(1) receptors.

Endothelial nitric oxide modulates perivascular sensory neurotransmission in the rat isolated mesenteric arterial bed

1. A possible role of nitric oxide (NO) as a modulator of capsaicin-sensitive sensory neurotransmission in blood vessels was investigated in the rat isolated mesenteric arterial bed. 2. Electrical field stimulation (EFS) of methoxamine-preconstricted mesenteric beds elicited frequency-dependent vasorelaxation mediated by capsaicin-sensitive sensory nerves. N(G)-nitro-L-arginine methyl ester (L-NAME, 10 and 300 microM) and 7-nitroindazole (7-NI, 100 microM), inhibitors of nitric oxide synthase (NOS), augmented sensory neurogenic vasorelaxation. D-NAME (300 microM), 6-aminoindazole (100 microM) and N(omega)-propyl-L-arginine (50 nM), a selective inhibitor of neuronal NOS, were without effect. The effect of 10 microM L-NAME was reversed by L-arginine (1 mM), the substrate for NOS. 3. L-NAME (300 microM) and 7-NI (100 microM) had no significant effect on vasorelaxations to calcitonin gene-related peptide (CGRP), the principal motor neurotransmitter of capsaicin-sensitive sensory nerves in rat mesenteric arteries, or to capsaicin, indicating a prejunctional action. The inhibitors of NOS had no effect on vasorelaxation to forskolin, but augmented vasorelaxation to sodium nitroprusside (SNP). 4. Removal of the endothelium augmented sensory neurogenic vasorelaxation, but did not affect vasorelaxation to CGRP, indicating a prejunctional action of endothelial NO. 5. In the absence of endothelium, L-NAME (300 microM) inhibited, and 7-NI (100 microM) caused no further augmentation of sensory neurotransmission. 6. SNP (100 nM), a nitric oxide donor, attenuated sensory neurogenic relaxations to EFS. 7. In rat isolated thoracic aortic rings, L-NAME (100 microM) and 7-NI (100 microM) attenuated concentration-dependent relaxations to acetylcholine. 8. These data show that NO modulates sensory neurotransmission evoked by EFS of the rat isolated mesenteric arterial bed, and that when NO synthesis is blocked sensory neurogenic relaxation is augmented. The source of NO is the vascular endothelium.

Differential involvement of conotoxin-sensitive mechanisms in neurogenic vasodilatation responses: effects of age

During aging there is a decline in sensory nerve function that is associated with reduced neurogenic inflammation and poor wound repair. The cellular mechanism(s) responsible for this decline in function with age is not well understood. We previously reported that sensory nerves in aged rats release sensory neuropeptides preferentially in response to low-frequency (5 Hz) as compared with higher-frequency (15 Hz) antidromic electrical stimulation, and that low-frequency transcutaneous electrical nerve stimulation accelerates wound healing. The present study investigates possible mechanisms for this preferential response. Using laser Doppler techniques, we have measured changes in blood flow in the base of vacuum-induced blisters induced in the rat hind footpad of young and old animals in response to low-frequency (5 Hz) or high-frequency (15 Hz) electrical stimulation (20 V, 2 ms for 1 minute) of the sciatic nerve. The relative contributions of the sensory neuropeptides, substance P and calcitonin gene-related peptide (CGRP), and of N-type voltage-gated calcium channels to the vascular responses were assessed by using the specific receptor antagonists RP67580, which is 2-(1-imino-2-(2 methoxy phyenyl) ethyl)-7,7 diphenyl-4 perhydroisoindolone-(3aR, 7aR); CGRP(8-37); and omega-conotoxin GVIA (Conus geographus), respectively. The results showed a greater involvement of substance P at high-frequency electrical stimulation and of CGRP at low-frequency stimulation. Our finding that omega-conotoxin-sensitive N-type calcium channel function was preserved with age and was only involved in the vascular response to low-frequency electrical stimulation could explain our previous report demonstrating beneficial effects of low-frequency transcutaneous electrical nerve stimulation to wound repair in aged animals. The current results have important practical implications for improving tissue repair in the aged.

Novel omega-conotoxins from Conus catus discriminate among neuronal calcium channel subtypes

omega-Conotoxins selective for N-type calcium channels are useful in the management of severe pain. In an attempt to expand the therapeutic potential of this class, four new omega-conotoxins (CVIA-D) have been discovered in the venom of the piscivorous cone snail, Conus catus, using assay-guided fractionation and gene cloning. Compared with other omega-conotoxins, CVID has a novel loop 4 sequence and the highest selectivity for N-type over P/Q-type calcium channels in radioligand binding assays. CVIA-D also inhibited contractions of electrically stimulated rat vas deferens. In electrophysiological studies, omega-conotoxins CVID and MVIIA had similar potencies to inhibit current through central (alpha(1B-d)) and peripheral (alpha(1B-b)) splice variants of the rat N-type calcium channels when coexpressed with rat beta(3) in Xenopus oocytes. However, the potency of CVID and MVIIA increased when alpha(1B-d) and alpha(1B-b) were expressed in the absence of rat beta(3), an effect most pronounced for CVID at alpha(1B-d) (up to 540-fold) and least pronounced for MVIIA at alpha(1B-d) (3-fold). The novel selectivity of CVID may have therapeutic implications. (1)H NMR studies reveal that CVID possesses a combination of unique structural features, including two hydrogen bonds that stabilize loop 2 and place loop 2 proximal to loop 4, creating a globular surface that is rigid and well defined.

Involvement of vanilloid receptors and purinoceptors in the Phoneutria nigriventer spider venom-induced plasma extravasation in rat skin

Phoneutria nigriventer venom causes stimulation of capsaicin-sensitive primary afferent neurons in the rat dorsal skin, leading to neurogenic plasma protein extravasation due to the release of tachykinin NK(1) receptor agonist. In this study we further investigated the mechanisms involved in the venom-induced activation of capsaicin-sensitive primary afferent neurons. The plasma extravasation in response to venom intradermally injected was measured in Wistar rats as the local accumulation of i.v. injected 125I-labelled human serum albumin into skin sites. The tachykinin NK(1) receptor agonist, D-Ala-[L-Pro(9),Me-Leu(8)]substance P-(7-11) (GR73632; 10-100 pmol/site), induced a significant plasma leakage that was abolished by the selective tachykinin NK(1) receptor antagonist, (S)-1-[2-[3-(3,4-dichlorphenyl)-1 (3-isopropoxyphenylacetyl) piperidin-3-yl] ethyl]-4-phenyl-1 azaniabicyclo [2.2.2]octane chloride (SR140333; 1 nmol/site), whereas the leakage after venom (1-10 microgram/site) was significantly inhibited (but not abolished) by SR140333. The calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP-(8-37), failed to further reduce the residual plasma extravasation induced by venom plus SR140333. The mu-opioid receptor agonist, [D-Ala(2), Me-Phe(4),Gly-ol(5)]enkephalin (DAMGO), and the local anaesthetic, lignocaine, had no effect on the venom-induced plasma extravasation. Similarly, the L-, N- and P/Q-type voltage-sensitive Ca(2+) channel blockers (verapamil, omega-conotoxin MVIIA and MVIIC, respectively) as well as the Na(+) channel blockers, tetrodotoxin and carbamazepine, had no effect on the venom-induced effect. Neither the systemic treatment nor the local injection of ruthenium red prevented the venom-induced plasma extravasation. However, the vanilloid receptor antagonist, N-[2-(4-chlorophenyl) ethyl]-1,3,4, 5-tetrahydro-7,8-dihydroxy-2H-2-benzazepine-2-carbothioamide (capsazepine; 120 micromol/kg, i.v.), reduced by 48% (P<0.05) the venom (10 microgram/site)-induced plasma extravasation. A significant inhibitory effect was also observed with the P(2) purinoceptor agonists, adenosine 5'-triphosphate (ATP; 10 and 30 nmol/site) and adenosine 5'-diphosphate (ADP; 10 nmol/site). The involvement of histamine and/or 5-hydroxytryptamine (5-HT) in the venom-induced plasma extravasation was ruled out since neither histamine and 5-HT receptor antagonists nor depletion of mast cells by compound 48/80 affected the venom response. This was further supported by the failure of venom to degranulate in vitro peritoneal mast cells. In conclusion, only vanilloid receptors and P(2) prejunctional purinoceptors had an inhibitory effect on the neurogenic plasma extravasation evoked by P. nigriventer venom in rat dorsal skin.

Effects of omega-conotoxin GVIA and diltiazem on double peaked vasoconstrictor responses to periarterial electric nerve stimulation in isolated canine splenic artery

The actions of omega-conotoxin (omega-CTX) and diltiazem on adrenergic and purinergic components of double peaked vasoconstrictor responses to periarterial nerve stimulation have been investigated in the isolated, perfused canine splenic arterial preparation. Double peaked vasoconstrictions (biphases of vasoconstrictors) were consistently observed in the conditions of 30 s trains of pulses at 1 - 10 Hz frequencies. omega-CTX (1 - 30 nM) produced similar inhibitory effects on the first phase and second phase responses in a dose-related manner. Thirty nM omega-CTX almost completely inhibited the biphasic vasoconstrictions at any used frequencies but did not affect the vasoconstrictor responses to exogenous applied ATP (0.01 - 1 micromol) and noradrenaline (0.03 - 3 nmol). Intraluminal application of a large dose of diltiazem (3 - 10 microM) also produced a dose-dependent inhibitory effect on biphasic vasoconstrictions at any used frequencies. Three microM diltiazem exerted rather a larger inhibitory effect on the second phase than the first phase response at low frequencies (1 - 3 Hz), but a similar inhibition on first and second phasic responses at high frequencies (6 - 10 Hz). An extremely high dose of diltiazem (10 microM) almost completely inhibited the biphasic vasoconstrictor responses to nerve stimulation, and slightly inhibited the contractile responses to exogenous applied ATP (0.01 - 1 micromol) and noradrenaline (0.03 - 3 nmol). The present results indicate that omega-CTX selectively acts prejunctionally to inhibit the release of transmitters from sympathetic nerve terminals, and omega-CTX-sensitive calcium channels may produce a parallel controlling of purinergic and adrenergic components of sympathetic cotransmission. A large dose of diltiazem has inhibitory effects on both prejunctional and postjunctional sympathetic co-transmission. British Journal of Pharmacology (2000) 129, 47 - 52

Multiple calcium channels regulate neurotransmitter release from vagus nerve terminals in the cat bronchiole

1. Twitch-like contractions and non-adrenergic non-cholinergic (NANC) relaxations evoked by electrical field stimulation (EFS) of the cat bronchiole were used to examine the voltage-activated calcium channels involved in excitatory and inhibitory neurotransmission in the cat bronchiole. 2. Nifedipine (50 microM), the L-type calcium channel antagonist, did not affect the twitch-like contraction and NANC relaxations. However, low concentrations of the N-type calcium channel blocker omega-conotoxin GVIA (omega-CgTX GVIA) (0.1 microM) irreversibly abolished twitch-like contractions evoked by trains of EFS </=10 stimuli at 20 Hz. 3. After the prolonged treatment with 0.1 microM omega-CgTX GVIA, EFS evoked initial fast and later slow NANC relaxations in the presence of 5-HT (10 microM), atropine and guanethidine (1 microM each). However increased concentration of omega-CgTX GVIA (1 microM) completely suppressed the slow NANC relaxation without affecting the initial fast component. 4. omega-agatoxin IVA (100 nM), the P- and Q-type calcium channel inhibitor, and nimodipine (10 microM), the L- and T-type calcium channel blocker, did not affect the amplitude of the initial fast NANC relaxation in the absence or presence of omega-CgTX GVIA (1 microM). 5. The contraction or relaxation induced by exogenous acetylcholine (ACh) (0.5 microM) or the nitric oxide donor, s-nitroso-N-acetyl penicillamine (SNAP) (1 microM) were not affected by omega-CgTX GVIA (1 microM). 6. Taken together, these results suggest that generation of twitch-like contraction and later slow NANC relaxation are regulated by N-type calcium channels, whereas generation of the initial fast NANC relaxation possibly involves R-type calcium channel.

Regulation of neuropeptide Y release by neuropeptide Y receptor ligands and calcium channel antagonists in hypothalamic slices

Neuropeptide Y (NPY) is an important regulator of energy balance in mammals through its orexigenic, antithermogenic, and insulin secretagogue actions. We investigated the regulation of endogenous NPY release from rat hypothalamic slices by NPY receptor ligands and calcium channel antagonists. High-potassium stimulation (60 mM) of the slices produced a calcium-dependent threefold increase in NPY release above basal release. The Y2 receptor agonists NPY(13-36) and N-acetyl[Leu28,Leu31]NPY(24-36), the Y4 agonist rat pancreatic polypeptide (rPP), and the Y4/Y5 agonist human pancreatic polypeptide (hPP) significantly reduced both basal and stimulated NPY release. NPY(13-36)-induced reduction of NPY release could be partially prevented in the presence of the weak Y2 antagonist T4-[NPY(33-36)]4, whereas the hPP- and rPP-induced inhibition of release was not affected by the Y5 antagonist CGP71683A or the Y1 antagonist BIBP3226. The selective Y1, Y2, and Y5 antagonists had no effect on either basal or potassium-stimulated release when administered alone. The calcium channel inhibitors omega-conotoxin GVIA (N-type), omega-agatoxin TK (P/Q-type), and omega-conotoxin MVIIC (Q-type) all significantly inhibited potassium-stimulated NPY release, without any effect on basal release, whereas nifedipine had no effect on either basal or stimulated release. Addition of both omega-conotoxin GVIA and omega-agatoxin TK together completely inhibited the potassium-stimulated release. In conclusion, we have demonstrated that NPY release from hypothalamic slices is calcium-dependent, involving N-, P-, and Q-type calcium channels. NPY release is also inhibited by Y2 agonists and rPP/hPP, suggesting that Y2 and Y4 receptors may act as autoreceptors on NPY-containing nerve terminals.

Role of N-type calcium channels in autonomic neurotransmission in guinea-pig isolated left atria

1. Calcium entry via neuronal calcium channels is essential for the process of neurotransmission. We investigated the calcium channel subtypes involved in the operation of cardiac autonomic neurotransmission by examining the effects of selective calcium channel blockers on the inotropic responses to electrical field stimulation (EFS) of driven (4 Hz) guinea-pig isolated left atria. In this tissue, a previous report (Hong & Chang, 1995) found no evidence for N-type channels involved in the vagal negative inotropic response and only weak involvement in sympathetic responses. 2. The effects of cumulative concentrations of the selective N-type calcium channel blocker, omega-conotoxin GVIA (GVIA; 0.1-10 nM) and the non-selective N-, P/Q-type calcium channel blocker, omega-conotoxin MVIIC (MVIIC; 0.01-10 nM) were examined on the positive (with atropine, 1 microM present) and negative (with propranolol, 1 microM and clonidine, 1 microM present) inotropic responses to EFS (eight trains, each train four pulses per punctate stimulus). 3. GVIA caused complete inhibition of both cardiac vagal and sympathetic inotropic responses to EFS. GVIA was equipotent at inhibiting positive (pIC50 9.29+/-0.08) and negative (pIC50 9.13+/-0.17) inotropic responses. MVIIC also mediated complete inhibition of inotropic responses to EFS and was 160 and 85 fold less potent than GVIA at inhibiting positive (pIC50 7.08+/-0.10) and negative (pIC50 7.20+/-0.14) inotropic responses, respectively. MVIIC was also equipotent at inhibiting both sympathetic and vagal responses. 4. Our data demonstrates that N-type calcium channels account for all the calcium current required for cardiac autonomic neurotransmission in the guinea-pig isolated left atrium.

Effects of Ca2+ concentration and Ca2+ channel blockers on noradrenaline release and purinergic neuroeffector transmission in rat tail artery

1. The effects of Ca2+ concentration and Ca2+ channel blockers on noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves have been investigated in rat tail arteries in vitro. Intracellularly recorded excitatory junction potentials (e.j.ps) were used as a measure of ATP release and continuous amperometry was used to measure NA release. 2. Varying the extracellular Ca2+ concentration similarly affected the amplitudes of e.j.ps and NA-induced oxidation currents evoked by trains of ten stimuli at 1 Hz. 3. The N-type Ca2+ blocker, omega-conotoxin GVIA (omega-CTX GVIA, 0.1 microM) reduced the amplitudes of both e.j.ps (evoked by trains of ten stimuli at 1 Hz) and NA-induced oxidation currents (evoked by trains of ten stimuli at 1 Hz and 50 stimuli at 10 Hz) by about 90%. 4. The omega-CTX GVIA resistant e.j.ps and NA-induced oxidation currents evoked by trains of 50 stimuli at 10 Hz were abolished by the non-selective Ca2+ channel blocker, Cd2+ (0.1 mM), and were reduced by omega-conotoxin MVIIC (0.5 microM) and omega-agatoxin IVA (40 nM). 5. Nifedipine (10 microm) had no inhibitory effect on omega-CTX GVIA resistant e.j.ps and NA-induced oxidation currents. 6. Thus both varying Ca2+ concentration and applying Ca2+ channel blockers results in similar effects on NA and ATP release from postganglionic sympathetic nerves. These findings are consistent with the hypothesis that NA and ATP are co-released together from the sympathetic nerve terminals.

Sympathetic modulation of sensory nerve activity with age: human and rodent skin models

1. Sensory nerves serve an afferent role and mediate neurogenic components of inflammation and tissue repair via an axon reflex release of sensory peptides at sites of injury. Dysfunction of these nerves with age could contribute to delayed tissue healing. 2. Complementary animal and human skin models were used in the present studies to investigate changes in the modulation of sensory nerve function by sympathetic efferents during ageing. Laser Doppler flowmetry was used to monitor neurogenic skin vascular responses. 3. The animal model used skin of the hind footpad of anaesthetized rats combined with electrical stimulation of the sciatic nerve, while the human model comprised capsaicin electrophoresis to the volar surface of the forearm. Sympathetic modulation was effected by systemic phentolamine pretreatment in animals and local application in the human model. 4. The results obtained from the human model confirmed the reported decline in sensory nerve function and showed no change in sympathetic modulation with age. The results from the animal model confirm and expand results obtained from the human model. 5. The use of low (5 Hz) and high (15 Hz) frequency electrical stimulation (20 V, 2 ms for 1 min) revealed a preferential response of aged sensory nerves to low-frequency electrical stimulation parameters with differential sympathetic modulation that is dependent on the frequency of stimulation.

Absence of histamine H3-receptors in the rabbit colon: species difference

1. The involvement of histamine H3-receptors in the control of colonic motility was investigated in the rabbit. 2. In vitro spontaneous and electrically evoked contractions were evaluated in preparations of proximal and distal colon, whereas in vivo spike activity was monitored in conscious rabbits, fitted long-term with bipolar electrodes along the distal colon. 3. In vitro, selective histamine H3-receptor agonists, R(alpha)-methylhistamine and immepip (1 nM to 10 microM), and antagonists, thioperamide and clobenpropit (1 nM to 1 microM), did not modify either spontaneous motility of neurogenic contractions elicited by electrical field stimulation. In the same conditions, Neurogenic contractions were partially prevented by agonists acting at alpha2-adrenoceptors and adenosine A1- and opioid kappa-receptors. 4. In vivo, R(alpha)-methylhistamine, immepip (at both 100 and 300 microgram kg-1 i.v.) and clobenpropit (1 mg kg-1 i.v.) did not significantly affect spontaneous spike activity in the rabbit distal colon. 5. These data suggest that histamine H3-receptors are not primarily involved in the control of colonic motility in the rabbit.

Histamine H3 receptors do not modulate reflex-evoked peristaltic motility in the isolated guinea-pig ileum

We investigated the role played by histamine H3 receptors in the control of intestinal peristalsis, using two different in vitro preparations of guinea-pig ileum. (a) Ileal segments were perfused from the oral end, inducing peristaltic movements (emptying waves), due to the activation of intramural reflexes. Such peristaltic motility was measured as changes in the perfusion pressure during the emptying phase and the threshold pressure for triggering the emptying wave was determined. (b) Ileal segments were mounted horizontally and circular muscle contraction evoked by the ascending peristaltic reflex was triggered by caudal distension of the intestinal wall. In perfused ileal segments, specific agonists acting at histamine H3 receptors, ((R)-alpha-methylhistamine and immepip, 1 nmol-10 micromol/l), did not cause any change in the threshold pressure for triggering the peristaltic wave, or in the rise of the perfusion pressure during the emptying phase. Similarly, circular muscle contractions evoked by caudal distension of the wall were not affected by these histamine H3 receptor agonists up to 10 micromol/l. In the same conditions, a complete inhibition of peristaltic movements was elicited by agonists acting at alpha2-adrenoceptors or adenosine A1 receptors (compound UK 14,304 and N6-cyclopentyladenosine, respectively), their effects being prevented by the respective receptor antagonists, idazoxan and 8-cyclopentyl-1,3-dimethyl-xanthine. These data demonstrate that, contrary to alpha2-adrenoceptors and adenosine A1 receptors, histamine H3 receptors are not primarily involved in the modulation of intramural reflexes that modulate the peristaltic motility of the isolated guinea-pig ileum.

The role of nitric oxide in inhibitory neurotransmission in the middle cerebral artery of the sheep

1. The involvement of nitric oxide (NO) as a mediator of inhibitory neurotransmission and its potential release mechanism in sheep isolated middle cerebral artery rings was investigated using NO synthase inhibitors, haemolysate, superoxide dismutase (SOD) and omega-conotoxin GVIA. In the presence of guanethidine (5 microM) and atropine (2 microM), transmural nerve stimulation of precontracted artery rings elicited an endothelium-independent vasodilator response that could be abolished by tetrodotoxin. 2. The magnitude of the vasodilator response was virtually abolished by NG-nitro-L-arginine-p-nitroanilide (L-NAPNA; 100-500 microM) and significantly reduced by NG-nitro-L-arginine (50 microM) or haemolysate (1 microliter ml-1). NG-nitro-D-arginine (50 microM) had no effect. In the presence of the NO synthase inhibitors, addition of L-arginine (300 microM) produced either no effect or a partial, transient restoration of inhibitor responses following electrical field stimulation (EFS). L-NAPNA (100 microM) did not affect the relaxant response to the NO donor SIN-1. These results suggest that NO is involved in the relaxation elicited by transmural nerve stimulation. 3. Superoxide dismutase (SOD; 150 Uml-1) did not produce any significant changes in the magnitude of the EFS-induced vasodilation. Thus, superoxide anions appear not to be a limiting factor for NO-mediated neurogenic vasodilation in sheep MCA. 4. omega-Conotoxin GVIA (100 nM) caused an almost immediate abolition of the EFS-induced vasoconstrictor response at resting tension, but had no effect on the vasodilator response at all frequencies of stimulation (0.5-8 Hz) tested. Thus, the neurotransmission process mediating this vasodilator response does not appear to involve Ca2+ entry via N-type Ca2+ channels.

Multiple calcium channels control neurotransmitter release from rat postganglionic sympathetic nerve terminals

1. Intracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals of the rat isolated anococcygeus muscle. 2. Low concentrations of the N-type calcium channel blocker omega-conotoxin GVIA (omega-CgTX GVIA) irreversibly abolished excitatory junction potentials (EJPs) evoked by trains of < or = five stimuli at 10 Hz. When the frequency of stimulation was increased (10-50 Hz) trains of stimuli evoked EJPs even in the presence of 1 microM omega-CgTX GVIA. We have termed this omega-CgTX GVIA-resistant release 'residual release'. EJP amplitude in the presence of omega-CgTX GVIA depended on both the frequency and number of stimuli in a train. 3. Residual release was inhibited by the P-type calcium channel blocker omega-agatoxin IVA (100 nM). However, even in the presence of both toxins, longer trains of stimuli could still evoke neurotransmitter release. 4. Residual release was abolished by omega-conotoxin MVIIC and by the non-specific calcium channel antagonist omega-grammotoxin SIA. Therefore, it would appear that a heterogeneous population of calcium channels is involved in mediating neurotransmitter release from these sympathetic nerve terminals.

omega-Conotoxins block neurotransmission in the rat vas deferens by binding to different presynaptic sites on the N-type Ca2+ channel

Electrically-induced twitch responses of the prostatic segment of vas deferens (0.1 Hz, 65 V, 1 ms) are mainly due to the transient presynaptic release of ATP, which acts postsynaptically on non-adrenergic receptors to contract smooth muscle cells. These responses were fully blocked by nanomolar concentrations of the omega-conotoxins GVIA, MVIIA, and MVIIC, most likely by inhibiting Ca2+ entry through presynaptic N-type Ca2+ channels controlling the release of ATP. Repeated washout of the toxins allowed the recovery of contractions, except for omega-conotoxin GVIA, whose inhibitory effects remained unchanged for at least 60 min. In addition, micromolar concentrations of omega-conotoxin MVIIC were unable to protect against the irreversible inhibition of twitch contractions induced by nanomolar concentrations of omega-conotoxin GVIA. At low extracellular Ca2+ concentrations (1.5 mM), 20 nM of omega-conotoxin GVIA or MVIIA inhibited completely the twitch contractions in about 10 min. In 5 mM Ca2+ the blockade of twitch contractions after 10 min was 70% for both toxins. In 1.5 mM Ca2+ omega-conotoxin MVIIC (1 microM) inhibited completely the twitch contraction after 10 min. In 5 mM Ca2+ blockade developed very slowly and was very poor after 30 min, omega-conotoxin MVIIC depressed the response by only 20%. These results are compatible with the idea that the three omega-conotoxins block the purinergic neurotransmission of the vas deferens by acting on presynaptic N-type voltage-dependent Ca2+ channels. However, omega-conotoxin MVIIC seems to bind to sites different from those recognised by omega-conotoxin GVIA and MVIIA, which are markedly differentiated by their Ca2+ requirements for binding to their receptors.

Role of N-, P- and Q-type voltage-gated calcium channels in transmitter release from sympathetic neurones in the mouse isolated vas deferens

1. N-type voltage-gated calcium channels are known to play an important role in transmitter release from autonomic neurones, and recent studies have demonstrated that non-N-type calcium channels are also involved. The calcium channels coupled to transmitter release from sympathetic neurones in the mouse isolated vas deferens were investigated in the present study. 2. Contractions of the mouse vas deferens were evoked by electrical stimulation at 1-50 Hz. The contractions were entirely nerve-mediated, since they were abolished by tetrodotoxin, and were used as an indirect measure of transmitter release. 3. The N-type calcium channel blocker, omega-conotoxin GVIA, inhibited contractions in a concentration-dependent manner, with a maximal effect at 30 nM. Contractions evoked by stimulation frequencies less than 10 Hz were abolished, and those evoked by 20 and by 50 Hz stimulation were decreased in amplitude by 51.3 +/- 13.9% and 9.3 +/- 2.6%, respectively. 4. The N-, P- and Q-type channel blocker, omega-conotoxin MVIIC, inhibited contractions in a concentration-dependent manner and caused greater maximum inhibition than omega-conotoxin GVIA, suggesting an action on P- and/or Q-type channels, in addition to N-type. 5. The P-type channel blocker, omega-agatoxin IVA, alone did not have a significant effect at concentrations up to 300 nM, but inhibited contractions in the presence of omega-conotoxin GVIA. Subsequent addition of omega-conotoxin MVIIC abolished the remaining contractions. Identical results were obtained when the three toxins were tested cumulatively on the purinergic and noradrenergic components of the contraction in the presence of (1.3 microM prazosin and following desensitization to 10 microM alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-NeATP), respectively. 6. The results suggest that N-, P- and Q-type channels are involved in the release of noradrenaline and ATP from sympathetic neurones in the mouse vas deferens.

Effect of different calcium channel blockers on inhibitory junction potentials and slow waves in porcine ileum

The effect of several calcium channel blockers was evaluated: (i) on spontaneous electrical and mechanical activities and (ii) on the response to electrical field stimulation. The study was carried out on whole-thickness preparation of porcine ileum. Glass microelectrodes were used to record membrane potential from smooth muscle cells. Resting membrane potential was -60 +/- 2mV (n = 18) and preparations generated spontaneous slow waves. Electrical field stimulation (EFS) was applied using different parameters. The amplitude and duration of inhibitory junction potentials (IJPs) increased with EFS strength. IJPs were abolished by tetrodotoxin (1 microM). Nifedipine (1 microM) did not modify the amplitude or duration of IJPs. The frequency of slow waves was not modified, however a slight but significant (p < 0.001) reduction in slow wave duration was observed. Mechanical activity was abolished in presence of nifedipine within approximately 6 min. omega-agatoxin IVA (50 nM) or omega-conotoxin MVIIC (100 nM), respectively a P-type and a Q-type calcium channel blockers, did not modify slow wave and IJP characteristics. In contrast, in presence of omega-conotoxin GVIA (100 nM), a N-type calcium channel blocker, or omega-conotoxin MVIIC (1 microM), IJPs were completely abolished. These data suggest that, in porcine ileum, N-type but not P-,Q- or L-type calcium channels are involved in the release of the non-adrenergic non-cholinergic neurotransmitters mediating IJPs. L-type calcium channels underlie electrical mechanical coupling but are not involved in slow wave generation.

Multiple subtypes of voltage-gated calcium channel mediate transmitter release from parasympathetic neurons in the mouse bladder

Multiple subtypes of voltage-gated calcium channels are coupled to transmitter release from central neurons; however, only N-type channels have been shown to play a role in autonomic neurons. The aim of the present study was to investigate potential roles for other channel subtypes in transmitter release from parasympathetic neurons in the mouse bladder using calcium channel toxins alone and in combination. Transmitter release was measured indirectly by recording the contraction of bladder dome strips in response to electrical stimulation of the neurons by single pulses or trains of 20 pulses at 1-50 Hz. omega-Conotoxin-GVIA (GVIA) and omega-conotoxin-MVIIC (MVIIC) inhibited contractions in a concentration-dependent manner, with IC50 values of approximately 30 and 200 nM, respectively, at low stimulation frequencies. omega-Agatoxin-IVA (agatoxin) alone did not have any significant effect up to 300 nM. Cumulative addition of the toxins demonstrated that 300 nM agatoxin had a significant effect after N-type channels were blocked with 100 nM GVIA. MVIIC (3 microM) reduced the contraction amplitude further. Testing the toxins on the cholinergic or purinergic component of the contraction separately showed that acetylcholine release depends primarily on N-type channels and, to a lesser extent, on P- and Q-type channels, whereas ATP release involves predominantly P- and Q-type channels. In conclusion, parasympathetic neurons in the mouse bladder, like central neurons, use multiple calcium channel subtypes. Furthermore, the release of the two main transmitters in these neurons has differing dependencies on the calcium channel subtypes.

Electrical properties of smooth muscle in the guinea-pig urinary bladder

1. The effects of transmural nerve stimulation were examined on preparations of detrusor smooth muscle from guinea-pig urinary bladder using intracellular recording techniques. Most recordings were made from preparations in which spontaneous and evoked action potentials had been inhibited by nifedipine (10 microM), a dihydropyridine that blocks L-type Ca2+ channels. 2. Supramaximal stimuli evoked excitatory junction potentials (EJPs) which could be divided into three basic types. Type 1 EJPs had short latencies (< 30 ms) and fast rise times (< 60 ms). Type 2 EJPs consisted of two components: a small depolarization that was followed by a second depolarization with a faster rise time. In a third type of cell, at high strengths of stimulation, EJPs resembled type 1 EJPs but at lower strengths of stimulation were similar in time course to type 2 EJPs. 3. All EJPs were abolished by tetrodotoxin (1 microM) and reduced by omega-conotoxin (0.1 microM), but were unaffected by hexamethonium (0.1 mM), suggesting that they result from the release of transmitter from post-ganglionic nerve fibres. All responses persisted in the presence of atropine (1 microM) but were abolished following the desensitization of P2-purinoceptors with alpha, beta-methylene ATP (m-ATP; 10 microM). 4. Spontaneous excitatory junction potentials (SEJPs) were also recorded from most cells. SEJPs were similar in appearance to fast single-component EJPs; however, in general they had a briefer time course. SEJPs persisted in the presence of tetrodotoxin (1 microM). 5. The electrical properties of urinary bladder smooth muscle were also examined. Voltage changes induced by point current injection into cells had fast rates of rise and decay (time constant, 5-20 ms). The input resistance of cells ranged between 12 and 108 M omega. When recordings were taken from cells near the point of current injection, resultant electrotonic potentials could be detected in only a small proportions of cells. 6. The results are discussed in relation to the idea that transmural nerve stimulation in the guinea-pig urinary bladder causes the activation of at least two different membrane conductances. Cells appear to be electrically coupled with one another. However, it is likely that coupling exists within discrete bundles of the smooth muscle.

Regulation by phosphodiesterase isoenzymes of non-adrenergic non-cholinergic contraction in guinea-pig isolated main bronchus

1. We have investigated the role of phosphodiesterase isoenzymes in modulating electric field stimulation (EFS), substance P and capsaicin-induced contraction of the guinea-pig isolated main bronchus. 2. Non-adrenergic non-cholinergic contractile responses were elicited by EFS (3 Hz, 20 s) in the guinea-pig isolated main bronchus in the presence of the non-selective muscarinic antagonist, atropine (0.1 microM), the non-selective beta-adrenoceptor antagonist, propranolol (1 microM), the neutral endopeptidase inhibitor, thiorphan (10 microM) and the cyclo-oxygenase inhibitor, indomethacin (5 microM). The type III, type III/IV, type IV and type V phosphodiesterase isoenzyme inhibitor, SKF 94836, benzafentrine, Ro-20-1724 and zaprinast respectively, significantly attenuated the contractile response to EFS. The IC50 (95% confidence limits) value for SKF 94836, benzafentrine, Ro-20-1724 and zaprinast was 8.3 microM (0.89-78); 0.7 microM (0.1-4.5); 0.5 microM (0.2-1.2) and 13 microM (2-87) respectively. 3. The phosphodiesterase isoenzyme inhibitors, SKF 94836, Ro-20-1724 and zaprinast, partially attenuated the contractile response to substance P (10 nM). Benzafentrine significantly inhibited the contractile response to substance P, yielding an IC50 value of 1.9 microM (0.9-3.8). 4. The phosphodiesterase isoenzyme inhibitor, Ro-20-1724 (0.1-100 microM) failed to reduce significantly the contractile potency of capsaicin (P > 0.05). In contrast, SKF 94836 (1 microM), benzafentrine (10 microM) and zaprinast (100 microM) significantly reduced the contractile potency of capsaicin (P < 0.05). 5 The selective phosphodiesterase isoenzyme inhibitors, SKF 94836, benzafentrine, Ro-20-1724 andzaprinast (0.01-100 microM) reversed in a concentration-dependent manner the contractile response toexogenously administered capsaicin (EC50) yielding ICm values of 3.91 microM (0.68-22); 3.37 microM (1.86-6.11); 0.366 microM (0.201-0.564) and 50.1 microM (18.6- 135) respectively.6 In conclusion, phosphodiesterase isoenzymes appear to regulate the contractile response to electricalfield stimulation and our results provide circumstantial evidence for a regulatory role ofphosphodiesterase type IV isoenzyme on sensory nerve function in vitro.

A role for Q type Ca2+ channels in neurotransmission in the rat urinary bladder

1. In isolated bladder strips of the rat, a substantial component (46%) of the Ca(2+)-dependent contractile response to electrical field stimulation (5 Hz) was resistant to combined block of both N and P type Ca2+ channels by omega-conotoxin-GVIA (300 nM) and omega-agatoxin-IVA (100 nM) respectively. 2. The resistant portion (non-N, non-P) was sensitive to omega-conotoxin-MVIIC (3 microM), which in addition to N and P also blocks Q type channels at this concentration. omega-Conotoxin-MVIIC administered alone, inhibited the neurogenic response to the same degree as that observed in the combined presence of omega-agatoxin-IVA, omega-conotoxin-GVIA and omega-conotoxin-MVIIC. 3. omega-Agatoxin-IVA (100 nM), a concentration that fully inhibits P type channels, had a negligible effect on the neurogenic response. Following blockade of N type Ca2+ channels with omega-conotoxin-GVIA (300 nM), omega-agatoxin-IVA (3 microM) (a concentration well above that used to block P channels, inhibits Q type channels, but spares N type channels), inhibited the residual response to the same degree as omega-conotoxin-MVIIC alone. 4. Results suggest that neurotransmission in rat urinary bladder is supported by both N and Q type Ca2+ channels.

Effects of omega-toxins on noradrenergic neurotransmission in beating guinea pig atria

The effects of four omega-toxins, known to block various subtypes of neuronal voltage-activated Ca2+ channels, on the beating guinea pig left atrium have been analyzed. Atria were suspended in oxygenated Krebs-bicarbonate solution at 32 degrees C and driven with electrical pulses delivered by a stimulator at 1 Hz, 1 ms, 4 V. A 10-fold increase of voltage caused a potent and rapid enhancement of the size of contractions (about 3- to 4-fold above basal), which reflects the release of endogenous noradrenaline from sympathetic nerve terminals. omega-Conotoxin MVIIC, omega-conotoxin MVIIA and omega-conotoxin GVIA inhibited the inotropic responses to 10 x V stimulation with IC50 values of 191, 44 and 20.4 nM, respectively. omega-Agatoxin IVA did not affect the contractile responses. The inotropic responses to exogenous noradrenaline were unaffected by the toxins. The potent blocking effects of omega-conotoxin GVIA were present even in conditions in which the release of noradrenaline was strongly facilitated by presynaptic alpha 2-adrenoceptor blockade by phenoxybenzamine. These effects were not reversed upon repeated washing of the tissue with toxin-free medium. In contrast, the blockade induced by omega-conotoxin MVIIC and omega-conotoxin MVIIA were fully reversed, with t1/2 of 13.5 and 31.2 min, respectively. omega-Conotoxin MVIIC (1 microM) protected against the irreversibility of the blockade induced by omega-conotoxin GVIA (100 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of omega-agatoxin-IVA on autonomic neurotransmission

omega-Agatoxin-IVA, a peptide from the venom of the funnel-web spider Agelenopsis aperta and a P type Ca2+ channel inhibitor, was examined for effects on responses to nerve stimulation in isolated autonomic neuroeffector preparations from the rabbit, guinea-pig and rat. Ca(2+)-dependent, tetrodotoxin sensitive, noradrenergic excitatory responses of rabbit pulmonary artery, rat vas deferens, and anococcygeus muscles, and cholinergic guinea-pig myenteric plexus preparations (all highly sensitive to the N type Ca2+ channel inhibitor omega-conotoxin-GVIA) were unaffected by omega-agatoxin-IVA (100 nM). Similarly, the neurogenic response of rat bladder, which has cholinergic, and non-adrenergic non-cholinergic (NANC) excitatory components, and the NANC inhibitory response of rat jejunum (atropine 0.5 microM- and guanethidine 5.0 microM-treated), which are partially sensitive and insensitive to omega-conotoxin-GVIA, respectively, were unaffected by omega-agatoxin-IVA (100 nM). Neurogenic NANC inhibitory responses of the guinea-pig taenia caecum, and rat anococcygeus muscles (atropine- and guanethidine-treated, and tone raised with prostaglandin F2 alpha), were also insensitive to omega-agatoxin-IVA. These results suggest that P type Ca2+ channels, if present, play an insignificant role in supplying the Ca2+ necessary for neurotransmitter release in the peripheral autonomic nervous system.

Electrophysiological evidence for different release mechanism of ATP and NO as inhibitory NANC transmitters in guinea-pig colon

1. The effect of the P2-purinoceptor antagonist, suramin, the specific N-type voltage-dependent calcium channel blocker, omega-conotoxin GVIA (omega-CgTx) and the delta-opioid receptor agonist [D-Pen2,D-Pen5] enkephalin (DPDPE) on the apamin-sensitive and apamin-resistant inhibitory junction potentials (i.j.ps) produced by electrical field stimulation (EFS) were investigated by means of a sucrose-gap technique in the circular muscle of the guinea-pig colon. 2. After incubation of muscle strips in either atropine (1 microM), guanethidine (3 microM) and NG-nitro-L-arginine (L-NOARG, 30 microM) or atropine, guanethidine and apamin (0.3 microM), the addition of the NK1 receptor antagonist, SR 140,333 (1 microM) abolished the non-adrenergic, non-cholinergic (NANC) excitatory junction potential (e.j.p.) and unmasked a pure apamin-sensitive i.j.p. (in the presence of L-NOARG) or a pure apamin-resistant i.j.p. (in the presence of apamin). Both types of i.j.p. were abolished by tetrodotoxin. 3. Suramin (30-300 microM) concentration-dependently inhibited the apamin-sensitive i.j.p., while the apamin-resistant i.j.p. was not significantly affected by suramin (up to 300 microM). L-NOARG (30 microM) markedly reduced the apamin-resistant i.j.p. 4. The delta-opioid receptor agonist, DPDPE (0.03-3 microM) concentration-dependently reduced the apamin-sensitive i.j.p., while leaving the apamin-resistant i.j.p. unaffected. Naloxone (1 microM) prevented the i.j.p. inhibition evoked by DPDPE (0.3 microM). 5. omega-CgTx (0.3 microM) markedly reduced the apamin-sensitive but not the apamin-resistant i.j.p. The application of DPDPE (3 MicroM), after development of a steady state inhibitory effect by omega-CgTx, evoked further inhibition of the apamin-sensitive ij.p., similar to the effect produced by DPDPE alone. The L-type calcium channel blocker, nifedipine (1 MicroM) did not significantly affect either the apamin-sensitive or the apamin-resistant ij.ps.6. These findings support the purinergic origin of the fast, apamin-sensitive ij.p. produced by EFS in the circular muscle of the guinea-pig colon and strongly suggest that the apamin-sensitive and the apamin-resistant components of the evoked ij.p. utilize different mechanisms for the secretion of theNANC transmitters, ATP and NO, respectively.

Release of secretoneurin and noradrenaline from hypothalamic slices and its differential inhibition by calcium channel blockers

Secretoneurin is a newly discovered peptide found in high concentrations in brain. We have studied the release of secretoneurin and noradrenaline from superfused hypothalamic slices from rat brain. Both electrical stimulation and potassium induced depolarisation released secretoneurin and noradrenaline from these slices in a calcium-dependent manner. Electrical stimulation caused a preferential release of noradrenaline when compared to the secretion elicited by high potassium. The time course of secretoneurin release was more protracted than that of noradrenaline. The calcium channel blocker omega-conotoxin inhibited only the electrically induced release of noradrenaline, whereas nifedipine inhibited only that of secretoneurin. These results establish that secretoneurin is secreted from neurons. Inhibition of this release by nifedipine is consistent with the concept that secretion from large dense core vesicles occurs at sites different from that of small vesicles and depends on calcium influx via L-type calcium channels.

Localized L-type calcium channels control exocytosis in cat chromaffin cells

Depolarizing 1-s pulses to 0 mV from a holding potential of -70 mV, induced whole-cell currents through Ca2+ channels (ICa) in patch-clamped cat adrenal medulla chromaffin cells. The dihydropyridine (DHP) furnidipine (3 microM) reduced the peak current by 47% and the late current by 80%. omega-Conotoxin GVIA (CgTx, 1 microM) reduced the peak ICa by 42% and the late ICa by 55%. Pulses (10 s duration) with 70 mM K+/2.5 mM Ca2+ solution (70 K+/2.5 Ca2+), applied to single fura-2-loaded cat chromaffin cells increased the cytosolic Ca2+ concentration ([Ca2+]i) from 0.1 to 2.21 microM; this increase was reduced by 43.7% by furnidipine and by 42.5% by CgTx. In the perfused cat adrenal gland, secretion evoked by 10-s pulses of 70 K+/2.5 Ca2+ was reduced by 25% by CgTx and by 96% by furnidipine. Similar results were obtained when secretion from superfused isolated cat adrenal chromaffin cells was studied and when using a tenfold lower [Ca2+]o. The results are compatible with the existence of DHP-sensitive (L-type) as well as CgTx-sensitive (N-type) voltage-dependent Ca2+ channels in cat chromaffin cells. It seems, however, that though extracellular Ca2+ entry through both channel types leads to similar increments of averaged [Ca2+]i, the control of catecholamine release is dominated only by Ca2+ entering through L-type Ca2+ channels. This supports the idea of a preferential segregation of L-type Ca2+ channels to localized "hot spots" in the plasmalemma of chromaffin cells where exocytosis occurs.

N- and P-type Ca2+ channels are involved in acetylcholine release at a neuroneuronal synapse: only the N-type channel is the target of neuromodulators

Cholinergic transmission in an identified neuro-neuronal synapse of the Aplysia buccal ganglion was depressed by application of a partially purified extract of the funnel-web-spider venom (FTx) or of its synthetic analog (sFTx). This specific blocker of voltage-dependent P-type Ca2+ channels did not interfere with the effect of the N-type Ca2+ channel blocker omega-conotoxin, which could further decrease synaptic transmission after a previous application of FTx. Similar results were obtained when the reversal order of application of these two Ca2+ channel blockers was used. Both P- and N-type Ca2+ currents trigger acetylcholine release in the presynaptic neuron. The neuromodulatory effects of FMRF-amide, histamine, and buccalin on transmitter release disappeared after the blockade of the N-type Ca2+ channels but remained still effective in the presence of FTx. These results indicate that only N-type Ca2+ channels appear to be sensitive to the neuromodulators we have identified.

Effect of longitudinal muscle-myenteric plexus removal and indomethacin on the response to tachykinin NK-2 and NK-3 receptor agonists in the circular muscle of the guinea-pig ileum

1. The effect of removal of the longitudinal muscle-myenteric plexus (LM-MP) and/or indomethacin (10 microM) on the response to the tachykinin NK-2 receptor selective agonist, [beta Ala8]NKA(4-10), or to the NK-3 receptor selective agonist, senktide, was investigated by measuring mechanical activity (isotonic recording) of circular muscle (ring preparation) of the guinea-pig ileum. 2. Indomethacin (10 microM) increased the percentage of ileal rings displaying spontaneous activity, either intact or LM-MP-free. The response to senktide (10 nM and 1 microM) was lower in LM-MP-free than in intact ileal rings, either in the absence or presence of indomethacin. The response to a low concentration (10 nM) of [beta Ala8] NKA (4-10) was enhanced in LM-MP-free rings and by indomethacin. 3. In intact ileal rings, the response to senktide was unaffected by atropine (3 microM) alone or by the tachykinin NK-2 receptor antagonist MEN 10,376 (10 microM) alone while it was reduced by the combined administration of the two antagonists. The response to senktide was greatly reduced by tetrodotoxin (TTX, 1 microM). Senktide-induced contractions (10 nM) were also reduced by the blocker of N-type voltage-sensitive calcium channels, omega-contoxin (CTX, 0.1 microM). 4. In about 30% of preparations tested, an inhibitory response (decrease in spontaneous activity) to 10 nM senktide, was disclosed in CTX-treated intact ileal rings. This inhibitory effect was TTX-sensitive. 5. In LM-MP-free ileal rings, the response to senktide was abolished or reduced by atropine and MEN 10,376, alone or in combination, and was also reduced or abolished by TTX and CTX. 6. The response to [beta Ala8]NKA (4-10) was inhibited by MEN 10,376, in both intact and LM-MP-free ileal rings while it was unaffected by atropine, TTX or CTX. 7. These results indicate that indomethacin pretreatment induces a regular background activity for studying the motor response to tachykinins in the circular muscle of the ileum, probably by blocking the formation of relaxant prostanoids. A further increase in sensitivity to direct smooth muscle stimulation (NK-2 receptor agonist) can be obtained by removal of the LM-MP. The response to NK-3 receptor stimulation is diminished but not abolished by removal of the LM-MP, suggesting that NK-3 receptors are located on neuronal bodies of myenteric neurons, but possibly also at other sites (possibly, nerve terminals).(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+ dependency of the release of nitric oxide from non-adrenergic non-cholinergic nerves

1. The role of Ca2+ in nitrergic neurotransmission was studied in the canine ileocolonic junction. 2. The specific N-type voltage-sensitive Ca2+ channel blocker omega-conotoxin GVIA (CTX, 10-100 nM) significantly reduced the electrically-evoked (2-16 Hz, 1-2 ms pulse width) non-adrenergic non-cholinergic (NANC) relaxations, preferentially affecting those to low frequency stimulation, in circular muscle strips of the ileocolonic junction. In contrast, the nerve-mediated NANC-relaxations in response to acetylcholine (30 microM), gamma-aminobutyric acid (100 microM) and adenosine 5'-triphosphate (100 microM), as well as the relaxations to nitric oxide (NO) (3-10 microM) and nitroglycerin (1 microM), remained unaffected. 3. A NO-related substance (NO-R), released from the ileocolonic junction in response to NANC nerve stimulation (4 and 16 Hz, 2 ms pulse width), was assayed with a superfusion bioassay cascade. CTX (50 nM) reduced the release of NO-R induced by electrical impulses (4 Hz: from 18 +/- 4% to 6 +/- 4%; 16 Hz: from 33 +/- 2% to 14 +/- 4%, n = 5), but not that in response to the nicotinic receptor agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 0.3 mM). In Ca(2+)-free medium, the release of NO-R evoked by electrical impulses or DMPP was inhibited. The L-type Ca2+ channel blockers verapamil (1-3 microM) and nifedipine (1 microM) had no effect. 4. From these results we conclude that the release of NO-R in response to NANC nerve stimulation is Ca(2+)-dependent. The electrically-evoked release of NO-R results from Ca2+ entry through CTX-sensitive N-type voltage-sensitive Ca2+ channels, whereas that induced by nicotinic receptor activation involves CTX-insensitive Ca2+ channels, different from the L- or N-type.

Multiple calcium channel types control glutamatergic synaptic transmission in the hippocampus

N-type calcium channels play a dominant role in controlling synaptic transmission in many peripheral neurons. Transmitter release from mammalian central nerve terminals, however, is relatively resistant to the N channel antagonist omega-conotoxin GVIA. We studied the sensitivity of glutamatergic synaptic transmission in rat hippocampal slices to omega-conotoxin and to omega-Aga-IVA, a P channel antagonist. Both toxins reduced the amplitude of excitatory postsynaptic potentials in CA1 pyramidal neurons, but omega-Aga-IVA was the more rapid and efficacious. These results were corroborated by biochemical studies measuring subsecond, calcium-dependent [3H]glutamate release from hippocampal synaptosomes. Thus, at least two calcium channel types trigger glutamate release from hippocampal neurons, but P-type plays a more prominent role. Eliminating synaptic transmission in the CNS, therefore, may require inhibiting more than a single calcium channel type.

Calcium-channel blockers and gastrointestinal motility: basic and clinical aspects

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.

Evidence for sympathetic neurotransmission through presynaptic N-type calcium channels in human saphenous vein

1. The specific type(s) of voltage-sensitive calcium channels (VSCCs) involved in sympathetic neurotransmission have not yet been characterized in human vascular tissues. We therefore examined the functional role of the N- and L-type VSCCs in human saphenous veins. 2. Contractile response curves for transmural nerve stimulation (TNS) and for exogenously administered noradrenaline (NA) were obtained in superfused saphenous vein rings. The contractions induced by TNS, but not by NA, were inhibited by 1 microM tetrodotoxin and by 10 microM guanethidine. Both responses were substantially reduced by 1 microM phentolamine, indicating that the contractions evoked by TNS were mediated by endogenous NA released from noradrenergic nerves. 3. In the presence of 2 microM omega-conotoxin GVIA (omega Conus Geographus toxin, fraction VI A; omega-CgTx), a polypeptide with specific inhibitory activity on N- and L-type calcium channels, the neurally evoked contractions were almost completely abolished. In contrast, the responses induced by exogenous NA were not affected by the neurotoxin, thus providing evidence of the exclusive presynaptic action of omega-CgTx. 4. In the presence of the calcium antagonist verapamil (10 microM), which selectively blocks L-type VSCCs, the contractions induced by both TNS and NA were diminished to the same extent, suggesting that the organic calcium blocker is active only at the postjunctional level. 5. It is concluded that N-type calcium channels are the main pathway of calcium entry controlling the functional responses induced by activating sympathetic nerves; the role of L-type channels appears to be limited to the postjunctional level, modulating smooth muscle contractions.