On the mechanism of contraction and desensitization induced by substance P in the intestinal muscle of the guinea-pig

Replacement substance P reduces cardiac fibrosis in monkeys with type 2 diabetes

BACKGROUND

Type 2 diabetes mellitus (T2DM)-associated cardiac fibrosis contributes to heart failure. We previously showed that diabetic mice with cardiomyopathy, including cardiac fibrosis, exhibit low levels of the neuropeptide substance P; exogenous replacement of substance P reversed cardiac fibrosis, independent of body weight, blood glucose and blood pressure. We sought to elucidate the effectiveness and safety of replacement substance P to ameliorate or reverse cardiac fibrosis in type 2 diabetic monkeys.

METHODS

Four female T2DM African Green monkeys receive substance P (0.5 mg/Kg/day S.Q. injection) for 8 weeks. We obtained cardiac magnetic resonance imaging and blood samples to assess left ventricular function and fibrosis by T1 map-derived extracellular volume as well as circulating procollagen type I C-terminal propeptide. Hematological parameters for toxicities were also assessed in these monkeys and compared with three female T2DM monkeys receiving saline S.Q. as a safety comparison group.

RESULTS

Diabetic monkeys receiving replacement substance P exhibited a ∼20% decrease in extracellular volume (p = 0.01), concomitant with ∼25% decrease procollagen type I C-terminal propeptide levels (p = 0.008). Left ventricular ejection fraction was unchanged with substance P (p = 0.42); however, circumferential strain was improved (p < 0.01). Complete blood counts, glycosylated hemoglobin A1c, lipids, liver and pancreatic enzymes, and inflammation markers were unchanged (p > 0.05).

CONCLUSIONS

Replacement substance P reversed cardiac fibrosis in a large preclinical model of type 2 diabetes, independent of glycemic control. No hematological or organ-related toxicity was associated with replacement substance P. These results strongly support a potential application for replacement substance P as safe therapy for diabetic cardiac fibrosis.

Regional differences of tachykinin effects on smooth muscle and pacemaker potentials of the stomach, duodenum, ileum and colon of an emetic model, the house musk shrews

BACKGROUND AND AIMS

The contractile effects of tachykinins on the gastrointestinal tract are well-known, but how they modulate slow-waves, particularly in species capable of emesis, remains largely unknown. We aimed to elucidate the effects of tachykinins on myoelectric and contractile activity of isolated gastrointestinal tissues of the Suncus murinus.

METHODS

The effects of substance P (SP), neurokinin (NK)A, NKB and selective NK₁ (CP122,721, CP99,994), NK₂ (SR48,968, GR159,897) and NK₃ (SB218,795, SB222,200) receptor antagonists on isolated stomach, duodenum, ileum and colon segments were studied. Mechanical contractile activity was recorded using isometric force displacement transducers. Electrical pacemaker activity was recorded using a microelectrode array.

RESULTS

Compared with NKA, SP induced larger contractions in stomach tissue and smaller contractions in intestinal segments, where oscillation magnitudes increased in intestinal segments, but not the stomach. CP122,721 and GR159,897 inhibited electrical field stimulation-induced contractions of the stomach, ileum and colon. NKB and NK₃ had minor effects on contractile activity. The inhibitory potencies of SP and NKA on the peristaltic frequency of the colon and ileum, respectively, were correlated with those on electrical pacemaker frequency. SP, NKA and NKB inhibited pacemaker activity of the duodenum and ileum, but increased that of the stomach and colon. SP elicited a dose-dependent contradictive pacemaker frequency response in the colon.

CONCLUSION

This study revealed distinct effects of tachykinins on the mechanical and electrical properties of the stomach and colon vs. the proximal intestine, providing a unique aspect on neuromuscular correlation in terms of the effects of tachykinin on peristaltic and pacemaker activity in gastrointestinal-related symptoms.

Contractile effect of tachykinins on Suncus murinus (house musk shrew) isolated ileum

Recent studies used Suncus murinus to investigate the anti-emetic potential of NK(1) tachykinin receptor antagonists. However, the pharmacology of tachykinin receptors in this species has not been fully characterized. In the present studies, therefore, we examined a range of tachykinin receptor agonists for a capacity to induce contractions of the isolated ileum. The tachykinin NK1 receptor preferring agonists substance P, septide and [Sar9Met(O2)11] substance P, and the tachykinin NK2 preferring agonists neurokinin A and GR 64349 (Lys-Asp-Ser-Phe-Val-Gly-R-gamma-lactam-Leu-Met-NH2) caused concentration dependent contractions with EC50 values in the nanomolar range. However, the tachykinin NK3 preferring agonists neurokinin B and senktide (1nM-1microM) induced only weak contractions. The action of senktide, but not [Sar9Met(O2)11] substance P, septide, or GR 64349, was antagonized significantly by atropine (P<0.05); tetrodotoxin and hexamethonium were inactive. The tachykinin NK1 receptor antagonist CP-99,994 ((+)-[(2S,3S)-3-(2-methoxy-benzyl-amino)-2-phenylpiperidine]) (10-100nM) inhibited substance P- and septide-induced contractions non-competitively. The pA2 value estimated for CP-99,994 against septide was 7.3+/-0.1. It also non-competitively antagonized the contractile responses induced by [Sar9Met(O2)11] substance P with a pA2 of 7.4+/-0.1. CP-99,994 also had a slight inhibitory action on neurokinin A-induced contractions, but did not modify the action of GR 64349. Conversely, the tachykinin NK2 receptor antagonist, saredutant, competitively antagonized GR 64349-induced contractions with a pA2 of 7.34+/-0.02. On the other hand, the presence of both CP-99,994 and saredutant competitively antagonized substance P-induced contraction. The present studies indicate that tachykininNK1 and NK2 receptors exist in the ileum of S. murinus and are involved in mediating contractions directly on smooth muscle, whereas tachykinin NK3 receptors may play a minor role involving a release of acetylcholine.

Sensory neuron and substance P involvement in symptoms of a zymosan-induced rat model of acute bowel inflammation

Intestinal inflammation is a painful syndrome with multiple symptoms, including chronic pain. This study examined the possible role of sensory neurons and substance P in symptoms of an animal model of acute intestinal inflammation. The model was induced by injecting ethanol and zymosan into the colon of anesthetized male rats. Three hours later, sections of the colon were stained with hematoxylin and eosin. To determine the role of substance P, 5 mg/kg of the neurokinin-1 receptor (NK-1r) antagonist, CP-96,345, or 300 microg/kg of an antisense oligonucleotide targeted at NK-1r mRNA was administered. Spinal cord sections were examined for internalization of NK-1r, as an indicator of substance P release. Sections of colon revealed infiltration of inflammatory cells following ethanol and zymosan treatment. Plasma extravasation in rats given ethanol and zymosan was significantly greater than in controls given saline only (P<0.0001) or saline and ethanol (P<0.001). In ethanol- and zymosan-treated rats given CP-96,345, plasma extravasation was significantly less than in rats given ethanol and zymosan without the antagonist (P<0.0001). Administration of the antisense oligonucleotide also resulted in lower levels of plasma extravasation compared with controls (P<0.01). Internalization of the NK-1r was observed in neurons of lamina I in the T13-L2 and L6-S2 regions of the spinal cord, as well as in sympathetic preganglionic neurons at the L1 level. This internalization was observed in the absence of any other stimulus besides the inflammation itself. This study implicates substance P and its receptor, the NK-1r, in acute inflammation of the colon.

Tachykinins mediate non-adrenergic, non-cholinergic excitatory neurotransmission to the hamster ileum via NK1 and NK2 receptors

The present study was designed to investigate Substance P (SP) and a related tachykinin, Neurokinin A (NKA), contributions to the excitatory neurotransmission to the circular smooth muscle of the hamster ileum. In the presence of atropine (0.5 microM), guanethidine (3 microM) and NG-nitro-L-arginine methyl ester (L-NAME) (200 microM), electrical field stimulation (EFS) evoked a non-adrenergic, non-cholinergic (NANC) excitatory junction potential (EJP) and contraction of circular smooth muscle. Applications of SP and NKA produced depolarizing and contractile responses in a concentration-dependent fashion. The EJP and contraction were almost abolished by the non-specific tachykininergic antagonist, spantide (3 microM). Application of SP antagonist, L-732,138, (1 microM) markedly inhibited EJP (82.5%) and contraction (68.9%) and completely blocked excitatory responses produced by exogenous application of SP. While application of NKA antagonist, SR48968 (1 microM) completely blocked the depolarising and contractile responses to NKA, it only slightly inhibited those to EFS (17.2% and 31.4% respectively). These results provide evidence that, in the circular muscle of hamster ileum, endogenous tachykinins are the main NANC excitatory neurotransmitters and their action is mediated by both NK1 and NK2 receptors.

The cellular localization of the neuropeptides substance P, neurokinin A, calcitonin gene-related peptide and neuropeptide Y in guinea-pig vestibular sensory organs: a high-resolution confocal microscopy study

Four neuropeptides, substance P, neurokinin A, calcitonin gene-related peptide and neuropeptide Y, were detected by radioimmunoassay in guinea-pig vestibular end-organs. High-resolution confocal microscopy visualization of immunofluorescence staining was used to determine the cellular localization of these peptides. Substance P- and neurokinin A-like immunoreactivities were found to co-exist in afferent fibers innervating the peripheral regions of both the utricular and ampullar sensory organs. The immunoreactivity was more concentrated in the distal ends of the calyceal-shaped nerve endings that innervate type I sensory cells. While in the guinea-pig, nerve calyces and type I cells are distributed in both the central and peripheral regions of the sensory epithelia, immunoreactive calyces were found only in the peripheral regions. Calcitonin gene-related peptide-like immunoreactivity was localized in small bouton endings situated at the level of the base of the hair cells. These boutons were in a position to make axosomatic contacts with type II sensory cells and axodendritic contacts with afferent nerve endings. Calcitonin gene-related peptide immunoreactivity co-existed with choline acetyltransferase immunoreactivity. The localization and shape of these boutons identified them as the axonal endings of efferent vestibular fibers. Neuropeptide Y-like immunoreactivity was not observed in the actual sensory epithelium but in the underlying connective tissue, where it was located in varicose fibers along blood vessels. The synaptic position of the tachykinins is clearly distinct from that of calcitonin gene-related peptide. This segregation distinguishes the vestibular end-organs from most peripheral tissues where these peptides are co-localized. The tachykinin-immunoreactive afferent fibers are postsynaptic to the hair cells. If, as in somatic sensory endings, these fibers can be triggered to release the neuropeptides by an axon reflex type of activation, then the tachykinins could interfere directly with the function of type I and type II vestibular hair cells. Calcitonin gene-related peptide co-exists with acetylcholine in the efferent axonal endings that are presynaptic to type II hair cells and to afferent fibers. Calcitonin gene-related peptide can thus interfere by direct synaptic action with type II hair cells only. It may also regulate the activity of the tachykinin-containing afferents.

Subtypes and excitation-contraction coupling mechanisms for neurokinin receptors in smooth muscle of the guinea-pig Taenia caeci

This study investigated the subtype and coupling mechanisms mediating the direct contractile response to tachykinins in the guinea-pig Taenia caeci preparation in vitro. Coupling of neurokinin receptors was compared throughout with coupling of muscarinic receptors. The smooth muscle neurokinin receptors seem to be predominantly of the NK-1 subtype. Thus, the relative activities of the common naturally-occurring tachykinins fell within one order of magnitude, and the selective NK-1 receptor agonist substance P methyl ester was high in activity (0.38 relative to substance P). Some contribution from NK-3 receptors is, however, possible in view of the appreciable activity of the selective NK-3 agonist succ-[Asp6, N-MePhe8]-SP(6-11) (senktide; activity 0.004 relative to substance P), and NK-2 or NK-3 receptors in view of the higher activity of the D-isomer of [Glp6, *Pro9]-SP(6-11) as compared to its NK-1 selective L-isomer (D/L-activity ratio 1.53). Contractile actions of tachykinins were compared with carbachol for reliance on membrane-potential dependent (electromechanical) and membrane-potential independent (pharmacomechanical) coupling mechanisms. Log concentration-response curves to carbachol and substance P in normal Krebs' medium were compared with curves obtained in a high-K+ solution where processes dependent on changes in membrane potential could play no part in excitation. In the high-K+ depolarizing solution, a concentration-related relationship was maintained, though with some diminution in the maximal additional tension generated: the maximum tension with carbachol was under both conditions greater than that with substance P. The relative effects of several tachykinins and carbachol in producing receptor-mediated changes in membrane permeability through presumed receptor-operated ion channel opening, was estimated in terms of the ability to increase 86Rb-efflux, as a marker for K+, in a high-K+ depolarizing solution. Carbachol (10 microM) consistently increased 86Rb-efflux. In contrast, no permeability increase could be detected with any tachykinin tested (substance P, eledoisin, substance P methyl ester, neurokinin A, neurokinin B, 1 or 10 microM). Tachykinins and carbachol were compared in terms of ability to increase phosphatidylinositol hydrolysis. Both substance P and carbachol showed a concentration-related increase in accumulation of total inositol phosphates; though the maximal response to carbachol was considerably greater than that to any tachykinin (substance P, eledoisin, substance P methyl ester, senktide, neurokinin A, neurokinin B), or combination of two tachykinins (substance P and eledoisin, senktide and substance P methyl ester).(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of action of substance P in guinea-pig ileum longitudinal smooth muscle: a re-evaluation

1. A proposed mechanism of contractile action of substance P in guinea-pig ileum longitudinal smooth muscle involving a decrease in membrane K+ permeability (PK) has been re-examined. 2. Potentiation of responses to substance P by the K+ channel blocker tetraethylammonium (TEA) was originally proposed as evidence for a mechanism of action of substance P involving a decrease in PK. Potentiation was confirmed; however this was found not to be specific to substance P since a similar potentiation of responses was seen with agonists not thought to act via a decrease in PK. 3. Antagonism of contractile responses to substance P by noradrenaline was similarly confirmed. However, this antagonism was found to represent a non-specific functional interaction through the inhibitory actions of beta-adrenoceptors rather than the proposed specific interaction with an increase in PK by noradrenaline which is normally alpha 1-adrenoceptor mediated. 4. Experiments were made measuring 86Rb efflux, in depolarized guinea-pig ileum longitudinal smooth muscle, to estimate PK. These studies confirmed a reported decrease in PK with TEA, but failed to detect the previously reported decrease with substance P. 5. These results, although not disproving a suggested mechanism of direct contractile action of substance P in guinea-pig ileum longitudinal smooth muscle involving a decrease in PK, do throw doubt on either the evidence, or its interpretation, as proposed by the original authors in support of such a mechanism.

The diacylglycerol kinase inhibitor, R 59022, suppresses contractility of intestinal smooth muscle

R 59022, a compound which causes the accumulation of diacylglycerol by inhibition of the enzyme diacylglycerol kinase, was tested for its effect on stimulated motor activity of the longitudinal muscle from the guinea-pig ileum. Motor responses to histamine were very potently inhibited by R 59022 (greater than or equal to 0.03 microM), which confirms its known activity as a histamine receptor antagonist. Contractions elicited by acetylcholine, substance P, or K+ depolarization were also concentration dependently depressed by R 59022 (1-30 microM); further analysis showed that substance P was antagonized in a non-competitive manner. R 59022 was significantly more active to block the tonic than the phasic component of the contractile response to K+ depolarization. The Ca2+-induced activation of the contractile apparatus in chemically skinned muscle strips was depressed by similar concentrations of R 59022 (3-30 microM). These data indicate that R 59022 suppresses the contractile activity of intestinal smooth muscle at an intracellular site of action but it is not yet clear whether this action can be accounted for by the accumulation of diacylglycerol and subsequent stimulation of protein kinase C.

Protein kinase C may regulate the tonic component of intestinal smooth muscle contraction in response to substance P

(1) The study investigated a possible involvement of protein kinase C (PKC) in the substance P-induced contraction of the longitudinal muscle of the guinea-pig isolated ileum. (2) The predominant effect of the PKC activator, phorbol-12,13-dibutyrate (PDB), was to change the time course of the response to substance P. While the initial peak contraction was hardly influenced by PDB, the fading of the contraction was accelerated to an extent that any tonic contraction which normally followed the initial peak response was prevented. This inhibitory effect of PDB on the tonic contraction was immediate in onset and related to its concentration (20-200 nM); responses to half-maximally (2-7 nM) or maximally effective (0.74 microM) concentrations of substance P were affected in the same manner. Tetrodotoxin (0.6 microM) did not alter the effect of PDB. Phorbol-13-monoacetate (2 microM), a phorbol ester which does not stimulate PKC, failed to change the time course of the substance P-induced contraction. (3) The tonic component of half-maximal contractile responses to histamine (0.2-0.4 microM) was also depressed by PDB (0.2 microM) whereas the tonic component of maximal responses to histamine (9 microM) was enhanced. (4) PDB (0.2 microM) reduced desensitization to substance P as judged by the reduction of the peak response to substance P (2-7 nM) following a 10-min exposure to a high concentration of the peptide (0.74 microM). (5) The PKC inhibitor, polymyxin B (0.1-0.3 mM), reduced the peak contractile response to substance P, slowed the fading of the contraction, and antagonized the inhibitory effect of PDB on the tonic contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for substance P as a functional neurotransmitter in guinea pig small intestinal mucosa

We showed previously that electrical transmural stimulation (TS) of guinea pig jejunal mucosa in vitro released neurotransmitters from submucosal plexus neurons which caused alterations in ion transport. The present studies were performed to obtain information regarding the identity of the neurotransmitters. The addition of exogenous substance P (SP) to the serosal side of the tissue caused a transient increase in short-circuit current (Isc) which closely mimicked the response to TS. Both TS and SP caused net secretion of Cl- ions by stimulating movement toward the luminal side. Tetrodotoxin abolished the response to TS, inhibited approximately 70% of the response to SP but did not affect the response to urecholine, a cholinergic muscarinic agonist. In the presence of the muscarinic antagonist, atropine, Isc responses to both TS and SP were reduced suggesting that a portion of both responses was due to action on enteric nerves causing release of acetylcholine. Following desensitization of the tissue with supramaximal doses of SP the response to TS was significantly reduced but the response to urecholine was unchanged. In the presence of atropine, SP desensitization reduced the nerve-stimulated response by approximately 65%; treatment of tissue with SP antibodies reduced the response by approximately 55%. Under the same conditions Isc responses to histamine were unaltered. Our results suggest that both SP (or a structurally analogous neurotransmitter) and acetylcholine as well as additional unidentified neurotransmitter(s) are functionally important in the regulation of intestinal ion transport in guinea pig jejunum.

Effects of substance P on inositol triphosphate accumulation, on contractile responses and on arachidonic acid release and prostaglandin biosynthesis in rabbit iris sphincter muscle

Addition of substance P (10(-7) to 10(-6) M) to rabbit iris sphincter muscle induced: (a) a rapid phosphodiesteratic breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) into 1,2-diacylglycerol (DG), measured as phosphatidic acid, and inositol triphosphate (IP3), measured by anion-exchange chromatography; (b) a rapid and strong contractile response, and (c) a rapid release of prostaglandin E2 (PGE2), measured by radioimmunoassay, and rapid release of 14C-labeled arachidonic acid, measured by radiochromatography. These substance P actions are concentration and time-dependent, and are blocked by substance P antagonist, [D-Pro2, D-Trp7,9]SP. The effects of substance P on arachidonic acid release and PG synthesis are not mediated through the cyclo-oxygenase and lipoxygenase pathways. Substance P exerted little effect on PGE2 release by the iris dilator muscle. We conclude that substance P, which is liberated from the sensory nerves that innervate the sphincter region of the iris and plays a role in miosis, may function as a Ca2+-mobilizing agonist in this tissue. Thus, a substance P-induced release of IP3 and formation of DG, a source for arachidonic acid in PG synthesis, followed by Ca2+ mobilization could underlie the mechanism for the biological actions, such as muscle contraction, of the neuropeptide reported in the eye. However, the precise relationship remains to be established.

Multiple neuropeptides exert a direct effect on the same isolated single smooth muscle cell

The contractile effect of various neuropeptides was examined by pressure ejecting these agents from a pipette onto single smooth muscle cells freshly dissociated from the stomach of Bufo marinus. Substance P, cholecystokinin-octapeptide, and bombesin caused contraction, whereas vasoactive intestinal peptide, secretin, and dopamine inhibited acetylcholine-induced contractions. Acetylcholine and the three peptides which produced contraction were found in some instances to act on the same cell, suggesting that receptors for these agents exist on one and the same cell.

Pharmacological characterization of the receptor mediating the adrenergic inhibition of responses to substance P in the cingulate cortex

The excitatory responses of neurones in the anterior cingulate cortex of the rat to iontophoretically applied substance P (SP) are reduced by noradrenaline (NA) applied iontophoretically or released from noradrenergic pathways. In order to determine the receptor involved in this inhibitory effect we have studied the effects of a number of receptor-specific adrenergic agonists and antagonists on responses of cingulate neurones to SP in rats anaesthetized with chloral hydrate. Low iontophoretic currents (0-15 nA) of NA, adrenaline and the beta-agonist, clenbuterol, all strongly reduced responses to SP. Isoprenaline was also effective but less consistently so, although problems were experienced with its iontophoretic release from micropipettes. The alpha 1-agonists, phenylephrine and methoxamine were also able to reduce responses to SP. However, this reduction required higher iontophoretic currents (15-60 nA) and was associated with depressant effects on baseline firing rate. The alpha 2-agonist clonidine was only weakly active at high currents and this too was associated with depression of baseline firing. Similar weak effects were noted with dopamine. The inhibitory effects of NA on SP responses were convincingly blocked or reversed by the beta-antagonist, practolol, but not by the alpha 1-antagonist, prazosin. The reduction of SP responses by phenylephrine was also blocked by practolol but unaffected by prazosin. Finally, reduction of SP excitations by activation of the coeruleocortical pathway was also blocked by practolol applied iontophoretically to the cortical cells. These results are consistent with the hypothesis that the effect of NA on SP responsiveness in the cingulate cortex is mediated by beta-adrenoreceptors.

Actions of polypeptides at the neuromuscular junction

The effects of several polypeptides, e.g. angiotensin II, substance P, oxytocin and vasopressin, on the isolated frog gastrocnemius, chick biventer cervicis and rat hemodiaphragm preparations were studied using electrophysiological and neurochemical techniques. The effects of angiotensin II, substance P, oxytocin and vasopressin on neuromuscular transmission and muscle contraction were investigated by studying the following parameters: the directly and indirectly-elicited twitch and tetanic contractions, nerve compound action potential, uptake of 3H-methylcholine into nerve-muscle preparations, the contractures produced by depolarizing drugs, e.g. ACh or TEA. The results showed that angiotensin II (10(-10)-10(-6) M) and substance P (10(-7)-10(-6) M) enhanced neuromuscular transmission and muscle contraction by increasing the amplitudes of the indirectly-elicited twitch and tetanic contractions. Oxytocin and vasopressin (1-100 mU/ml-1) both depressed neuromuscular transmission by reducing the contractile and electrical response in the frog, chick and rat skeletal muscle. It was concluded that, like their effects on ganglionic transmission, the peptides can modify neuromuscular transmission. The mechanism by which these peptides produce their effects may be dependent on external calcium concentration. These peptides may affect both pre- and postjunctional mechanisms; prejunctionally by increasing/decreasing the release of ACh, and postjunctionally by affecting the sensitivity of the postjunctional membrane to depolarizing drugs and/or producing a contracture in the skeletal muscle.

Immunochemical evidence for substance P-like peptide in tissues of the earthworm Lumbricus terrestris: action on intestinal contraction

The presence of a substance P-like peptide in intestinal and body wall tissues, ventral nerve fiber and seminal vesicles of the earthworm Lumbricus terrestris has been demonstrated by means of a radioimmunoassay technique. The greatest substance P-like immunoreactivity was measured in intestinal tissues where it stimulates the rate of spontaneous contraction. This effect is inhibited by the substance P antagonist (D-pro2, D-trp7,9)-SP suggesting a possible involvement of receptor mechanisms. Dual localization of substance P-like immunoreactivity in earthworm intestinal and nerve tissues follows the pattern observed of peptidal hormones in vertebrates which are common to both endocrine and non-endocrine tissues.

Neuromuscular blocking agents inhibit receptor-mediated increases in the potassium permeability of intestinal smooth muscle

The neuromuscular blocking agents tubocurarine, atracurium and pancuronium have been tested for their ability to inhibit receptor-mediated increases in the K+ permeability of intestinal smooth muscle. All three agents, as well as the bee venom peptide apamin, reduced both the resting efflux of 86Rb and the increase in efflux caused by the application of either bradykinin (1 microM) or an alpha 1-adrenoceptor agonist, amidephrine (20 microM), to depolarized strips of guinea-pig taenia caeci. This suggested that like apamin, the neuromuscular blocking agents inhibit the Ca2+-dependent K+ permeability (PK(Ca] mechanism which in this tissue is activated by a variety of membrane receptors. The concentrations (IC50S) of atracurium, pancuronium and (+)-tubocurarine which reduced the effect of amidephrine on 86Rb efflux by 50% were 12, 37 and 67 microM respectively. Also in keeping with an ability to block PK(Ca), the neuromuscular blockers and apamin reduced the inhibition by amidephrine and bradykinin of physalaemin-mediated contractions of the taenia caeci. The IC50 values were 15, 31 and 120 microM for atracurium, tubocurarine and pancuronium respectively, and 2.3 nM for apamin. Each of the neuromuscular blockers, and apamin, increased the spontaneous contractions of the rabbit duodenum and blocked the inhibitory effect of amidephrine thereon. It is concluded that the PK(Ca) mechanism in the longitudinal smooth muscle of the intestine It is concluded that the PK(Ca) mechanism in the longitudinal smooth muscle of the intestine resembles that of hepatocytes and sympathetic ganglion cells in its susceptibility to inhibition by neuromuscular blocking agents, as well as by apamin.

Actions of polypeptides at the neuromuscular junction

The effects of several polypeptides, e.g. angiotensin II, substance P, oxytocin and vasopressin, on the isolated frog gastrocnemius, chick biventer cervicis and rat hemodiaphragm preparations were studied using electrophysiological and neurochemical techniques. The effects of angiotensin II, substance P, oxytocin and vasopressin on neuromuscular transmission and muscle contraction were investigated by studying the following parameters: the directly and indirectly-elicited twitch and tetanic contractions, nerve compound action potential, uptake of 3H-methylcholine into nerve-muscle preparations, the contractures produced by depolarizing drugs, e.g. ACh or TEA. The results showed that angiotensin II (10(-10)-10(-6) M) and substance P (10(-7)-10(-6) M) enhanced neuromuscular transmission and muscle contraction by increasing the amplitudes of the indirectly-elicited twitch and tetanic contractions. Oxytocin and vasopressin (1-100 mU/ml-1) both depressed neuromuscular transmission by reducing the contractile and electrical response in the frog, chick and rat skeletal muscle. It was concluded that, like their effects on ganglionic transmission, the peptides can modify neuromuscular transmission. The mechanism by which these peptides produce their effects may be dependent on external calcium concentration. These peptides may affect both pre- and postjunctional mechanisms; prejunctionally by increasing/decreasing the release of ACh, and postjunctionally by affecting the sensitivity of the postjunctional membrane to depolarizing drugs and/or producing a contracture in the skeletal muscle.

In vivo and in vitro actions of mammalian tachykinins

Recently, two kassinin-like tachykinins have been isolated from mammalian nervous tissue. The potencies of these peptides, substance K (neurokinin alpha) and neuromedin K (neurokinin beta) were compared with those of substance P, eledoisin, and kassinin in various pharmacological systems in vivo and in vitro. In contracting the isolated guinea-pig ileum and rabbit jejunum the potencies of eledoisin, kassinin, substance K, and neuromedin K were 13-80% that of substance P. In the rat vas deferens substance K and neuromedin K potentiated the electrically induced contractions with potencies similar to those of eledoisin and kassinin; they were 46-236 times as potent as substance P. In stimulating salivation in the rat after intravenous injection, eledoisin, kassinin, and substance K were respectively 2.3, 1.3 and 0.33 times as potent as substance P. In contrast, neuromedin K exhibited negligible activity. Each peptide tested led to a short fall in blood pressure after intravenous injection in the rabbit, substance P being 12-250 times as potent as the other peptides. Substance P was 20 times as potent as substance K or neuromedin K in inducing vasodilatation in the rat hind paw in vivo. Of the peptides tested, only substance P (10 nmol/min) significantly increased the release of histamine from the rat isolated hindquarter preparation. The results are discussed with respect to several theories of tachykinin receptor heterogeneity.

Possible involvement of substance P in the contraction produced by periarterial nerve stimulation in the rat ileum

The effect of substance P (SP) on the contractile responses produced by periarterial (mesenteric) nerve stimulation was studied in the rat isolated ileum. Periarterial nerve stimulation at 1-50 Hz, with 10 V (maximum) and 0.2 msec pulse duration, for 15-20 sec, produced frequency-dependent contractions in the rat ileum. In the presence of guanethidine (10 microM) and 6-hydroxydopamine (1 microM), to block noradrenergic responses, periarterial nerve stimulation at 1-20 Hz still produced small contractions which were reduced by atropine (1 microM) and morphine (1 microM). In the presence of atropine, morphine, guanethidine and 6-hydroxydopamine, the contraction produced by periarterial nerve stimulation was readily abolished by tetrodotoxin (1 microM), capsaicin (3.3 microM) and an SP-antagonist (SPA1, 10 microM). SP in low concentrations (0.01-1.0 microM) potentiated the contractions produced by periarterial nerve stimulation at 1-2 Hz by 20-30%. High concentrations of SP (1.0-10.0 microM) reduced the contractile response by 40-50%. Indomethacin (2.8 microM) amd mepyramine (1 microM) had no effect on these responses. When the mesenteric nerve supply to the gut was cut, periarterial nerve stimulation produced no contraction in the rat ileum. However, SP in low concentrations, still produced small contractions which were abolished by an SP-antagonist but not by tetrodotoxin. SP in low concentrations, slightly increased the contractions produced by ACh (0.5-50 microM) or TEA (2.4-12 mM). High concentrations of SP significantly reduced the ACh and TEA-induced contractions in the rat ileum.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P action on phosphoinositides in guinea-pig intestinal muscle: a possible transduction mechanism?

The effect of substance P on the metabolism of membrane phosphoinositides and the possible role of this effect in the contractile response to substance P was investigated in longitudinal muscle strips obtained from the guinea-pig small intestine and prelabelled with [3H] inositol. Substance P (2.2 microM) failed to change significantly the tissue content of phosphoinositides but caused an accumulation of their water-soluble hydrolysis products, inositol bisphosphate (InsP2) and inositol monophosphate (InsP). These experiments were carried out in the presence of Li+ (12 mM), since only under these conditions could an accumulation of InsP2 be observed. The rate at which InsP2 and InsP accumulated was highest during the first 0.5 min of exposure to substance P (2.2 microM) and then decreased rapidly. Thus, the rate of inositol phosphate accumulation paralleled the time course of the contractile response to substance P. The magnitude of inositol phosphate accumulation was related to the concentration of substance P (22 nM-22 microM). The substance P-induced accumulation of InsP2 and InsP was not reduced when muscle strips had been incubated in Ca2+-free medium, for a time period sufficient to deplete the intracellular Ca2+ store which can be released by substance P, or in Ca2+-free medium containing high [K+]. These findings are compatible with the concept that hydrolysis of membrane phosphoinositides is a mechanism that links substance P receptor activation to contraction but further work is needed to establish a causal relationship.

Substance P can contract the longitudinal muscle of the guinea-pig small intestine by releasing intracellular calcium

The contraction of the longitudinal muscle of the guinea-pig isolated ileum in response to substance P (SP) in high [K+] medium and in Ca2+-free solution which contained EGTA has been investigated in order to examine whether excitation-contraction coupling involves the release of Ca2+ from an intracellular store. In tissues contracted by K+, high concentrations of SP (greater than or equal to 0.1 microM) were still capable of causing a slight, transient contraction. Bathing the ileum in a Ca2+-free medium for 2.5 min greatly diminished the potency and efficacy of SP in contracting the longitudinal muscle but concentrations of 2.2-22 microM SP were still able to produce a response 30-40% of maximal. The responsiveness to SP was completely lost within 10 min of bathing in Ca2+-free medium but was partially restored by a brief exposure (0.5-2 min) to high concentrations of Ca2+ (9-72 mM). The restorative effect of Ca2+ depended on the concentration of Ca2+ and on the time for which the tissue was exposed to this Ca2+ concentration. The fact that high concentrations of SP were able to elicit a contraction in media containing high [K+] or no Ca2+, suggested that they may do so by releasing Ca2+ from an intracellular store which is not as sensitive to removal of extracellular Ca2+ or as easily accessible to EGTA as the extracellular space of the muscle. The location of this store is not known; it may be associated with the internal side of the cell membrane. There is apparently an overlap between the Ca2+ pool associated with the action of SP and the Ca2+ pools utilized by acetylcholine, histamine or tetraethylammonium, which accounts for the cross-desensitization observed between these agonists. It was not possible to determine whether autodesensitization to SP also results from depletion of an intracellular Ca2+ store.

Pardaxin produces postjunctional muscle contraction in guinea-pig intestinal smooth muscle

The action of pardaxin (PX), a toxin isolated from the secretion of the Red Sea flatfish, Pardachirus marmoratus, was studied on longitudinal muscle of guinea-pig ileum. Pardaxin contracted the ileum and subsequently abolished muscle contraction to 5-hydroxytryptamine (5-HT), but did not affect the responses to acetylcholine (ACh) and substance P(SP). Pardaxin-induced contraction was only partially suppressed by atropine and not affected by tetrodotoxin or morphine. Preparations desensitized to 5-HT or SP responded normally to pardaxin. Pardaxin-induced contractions were normal in K+-depolarizing Krebs Ringer solution and not affected by black widow spider venom. It is concluded that the pardaxin-induced muscle contractions are not mediated through the release of neurotransmitters and do not involve 5-HT, SP or ACh receptors, but are due to a direct action on the muscle contractile mechanism.