Fading and tachyphylaxis to the contractile effects of substance P in the guinea-pig ileum

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.

Ranakinin, a naturally occurring tachykinin, stimulates phospholipase C activity in the frog adrenal gland

We have previously shown that the frog adrenal gland is innervated by a dense network of fibers containing ranakinin, one of the endogenous tachykinins in the amphibian Rana ridibunda, and we have found that ranakinin stimulates in vitro corticosteroid secretion by frog adrenal tissue. To elucidate the mechanism of action of ranakinin on the frog adrenal gland, we investigated the effect of ranakinin on cAMP formation and polyphosphoinositide metabolism. Incubation of frog adrenal explants with various tachykinins, including ranakinin, substance P, neurokinin A, or neurokinin B, did not produce any significant modification of cAMP concentrations. In contrast, ranakinin induced a time- and dose-dependent stimulation of inositol phosphate formation with a concomitant decrease in membrane polyphosphoinositides. Pretreatment of the tissue slices with the phospholipase C inhibitor U-73122 or with pertussis toxin completely abolished the stimulatory effect of ranakinin on inositol phosphate formation. Prolonged administration of U-73122 to perifused frog adrenal explants markedly attenuated the ranakinin-evoked stimulation of corticosterone and aldosterone secretion. Taken together, these data indicate that in the frog adrenal gland, ranakinin has no effect on the adenylyl cyclase system, but enhances polyphosphoinositide hydrolysis. The stimulatory action of ranakinin on inositol phosphate formation and corticosteroid secretion is mediated through activation of a phospholipase C positively coupled to a pertussis toxin-sensitive G protein.

Quadruple colocalization of calretinin, calcitonin gene-related peptide, vasoactive intestinal peptide, and substance P in fibers within the villi of the rat intestine

Double-labeling immunofluorescent histochemistry demonstrates that calretinin, a calcium-binding protein, coexists with calcitonin gene-related peptide, vasoactive intestinal peptide, and substance P in the fibers innervating the lamina propria of the rat intestinal villi. An acetylcholinesterase histochemical stain revealed that the majority of calretinin-containing cells in the myenteric ganglia were cholinergic and that about one half of the submucosal calretinin-containing cells colocalized with acetylcholinesterase. In situ hybridization studies confirmed the presence of calretinin mRNA in the dorsal root ganglia, and a ribonuclease protection assay verified the presence of calretinin message in the intestine. The coexistence of calretinin in calcitonin-gene-related-peptide-containing cells that also contained substance P and vasoactive intestinal polypeptide in the dorsal root ganglia suggest that these ganglia are the source of the quadruple colocalization within the sensory fibers of the villi. Although the function of calretinin in these nerves is unknown, it is hypothesized that the coexistence of three potent vasodilatory peptides influences the uptake of metabolized food products within the vasculature of the villi.

Effect of substance P and capsaicin on stomach fundus and ileum of streptozotocin-diabetic rats

The in vitro responses of longitudinal preparations of rat stomach fundus and ileum to capsaicin at 1, 8, 4, 16 and 26 weeks and to substance P at 1 and 8 weeks from diabetes induction were studied. The results were compared with those obtained in age-matched control rats. The contractile responses to exogenous substance P and capsaicin were not affected in the stomach fundus from diabetic rats. Atropine (1 microM) did not antagonize the substance P-induced response whereas it inhibited about 90% of the capsaicin-induced response in controls and about 60% of the response in diabetic rats. At the resting tone, capsaicin induced a relaxation followed by a contraction in stomach fundus of control rats. Only a contraction was evoked in diabetic rats. In carbachol (0.05-0.1 microM) pre-stimulated strips, a complete restoration of the biphasic response was obtained in the diabetic state. The contractile response elicited by exogenous substance P was not significantly increased in the ileum preparations from diabetic rats; nevertheless the EC50 value for substance P was reduced 8 weeks after the onset of diabetes. The response elicited by capsaicin in the ileum of control rats was also biphasic. The capsaicin-induced contraction was greater in tissue from diabetic rats as compared with controls and relaxation was not evident. An age-related decrease of the contraction was also evident in both groups. Atropine (1 microM) partially antagonized the responses to substance P and capsaicin. The inhibition of the responses with atropine was more evident in control than in diabetic rats. These results suggest that the myogenic actions of several agonists in these two tissues are differently modified in experimental diabetes.

Tachykinin receptors: a radioligand binding perspective

The last decade has witnessed major breakthroughs in the study of tachykinin receptors. The currently described NK-1, NK-2, and NK-3 receptors have been sequenced and cloned from various mammalian sources. A far greater variety of tachykinin analogues are now available for use as selective agonists and antagonists. Importantly, potent nonpeptide antagonists highly selective for the NK-1 and NK-2 receptors have been developed recently. These improved tools for tachykinin receptor characterization have enabled us to describe at least three distinct receptor types. Furthermore, novel antagonists have yielded radioligand binding and functional data strongly favoring the existence of putative subtypes of NK-1 and especially NK-2 receptors. Whether these subtypes are species variants or true within-species subtypes awaits further evidence. As yet undiscovered mammalian tachykinins, or bioactive fragments, may have superior potency at a specific receptor class. The common C terminus of tachykinins permits varying degrees of interaction at essentially all tachykinin receptors. Although the exact physiological significance of this inherent capacity for receptor "cross talk" remains unknown, one implication is for multiple endogenous ligands at a single receptor. For example, NP gamma and NPK appear to be the preferred agonists and binding competitors at some NK-2 receptors, previously thought of as exclusively "NKA-preferring." Current evidence suggests that tachykinin coexistence and expression of multiple receptors may also occur with postulated NK-2 and NK-1 receptor subtypes. Other "tachykinin" receptors may recognize preprotachykinins and the N terminus of SP. In light of these recent developments, the convenient working hypothesis of three endogenous ligands (SP, NKA, and NKB) for three basic receptor types (NK-1, NK-2, and NK-3) may be too simplistic and in need of amendment as future developments occur (Burcher et al., 1991b). In retrospect, the 1980s contributed greatly to our understanding of the structure, function, and regulation of tachykinins and their various receptors. The development of improved, receptor subtype-selective antagonists and radioligands, in addition to recent advances in molecular biological techniques, may lead to a more conclusive pharmacological and biochemical characterization of tachykinin receptors. The 1990s may prove to be the decade of application, where a better understanding of the roles played by endogenous tachykinins (at various receptor subtypes) under pathophysiological conditions will no doubt hasten the realization of clinically useful therapeutic agents.

Vasoactive intestinal polypeptide induces neurogenic contraction of guinea-pig ileum. Involvement of acetylcholine and substance P

The effect and mode of action of vasoactive intestinal polypeptide (VIP), a peptidergic neuromodulator in the gastrointestinal nervous system, were investigated in isolated muscle strips of the guinea-pig ileum. VIP induced concentration-dependent (20 nM-1 microM) contractions of longitudinal ileal strips. TTX (1 microM), a mixture of atropine (3 microM) and spantide (30 microM), a mixture of atropine (3 microM) and omega-conotoxin GVIA (100 nM), somatostatin (60 nM) and dynorphin (100 nM) abolished the effect of VIP. In most cases a small relaxation became evident. Desensitization to substance P in the presence of atropine prevented VIP-induced contraction. A partial inhibition was observed in the presence of atropine (3 microM), spantide (30 microM), omega-conotoxin GVIA (100 nM), beta-endorphin (265 nM), met-enkephalin (1100 nM) and a mixture of spantide (30 microM) and omega-conotoxin GVIA (100 nM). The action of VIP was not significantly modified by guanethidine (3 microM) or hexamethonium (150 microM). In circular ileal strips VIP (10-300 nM) caused concentration-dependent relaxations through a direct myogenic effect. These results indicate that the VIP produced contractions of the guinea-pig ileum are exclusively neurally mediated and involve a cholinergic as well as a noncholinergic-nonadrenergic (NANC) pathway. It is concluded that besides acetylcholine (Ach) VIP releases the peptidergic transmitter substance P from postganglionic nerve fibers of myenteric plexus. Opioid peptides and somatostatin modulate the activity of cholinergic and peptidegic nerves in the guinea-pig ileum. The release of substance P appears to depend completely on N-type voltage sensitive calcium channels.

Effects of some autonomic drugs and neuropeptides on the mechanical activity of longitudinal and circular muscle strips isolated from the carp intestinal bulb (Cyprinus carpio)

1. The mechanical responses to some autonomic drugs and neuropeptides of longitudinal muscle (LM) and circular muscle (CM) strips isolated from the carp intestinal bulb were investigated in vitro. 2. Acetylcholine and carbamylcholine caused concentration-dependent transient contraction of both LM and CM strips. Tetrodotoxin had no effect, but atropine selectively decreased the contractile responses to acetylcholine and carbamylcholine. 3. Excitatory alpha-2 and inhibitory beta adrenoceptors were present in both LM and CM strips. 4. 5-Hydroxytryptamine (5-HT) caused concentration-dependent contraction of both LM and CM strips. Tetrodotoxin, atropine and methysergide decreased the contractile responses to 5-HT. 5. Some neuropeptides (angiotensin I, angiotensin II, bombesin, bradykinin, neurotensin, somatostatin and vasoactive intestinal polypeptide) did not cause any mechanical response (contraction or relaxation) in either smooth muscle strip. 6. Substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) caused contraction of both LM and CM strips. However, the time course of the contraction in LM was different from that in CM. The order of potency was NKA greater than SP greater than NKB in LM strips and NKA greater than SP much greater than NKB in CM strips. In LM strips, the contractile responses to tachykinins were unaffected by spantide and methysergide, but partly decreased by tetrodotoxin and atropine. On the other hand, the contractile responses of CM strips were unaffected by tetrodotoxin, atropine, methysergide and spantide. 7. Dynorphin (1-13) (DYN), leucine-enkephalin (L-Enk) and methionine-enkephalin (M-Enk) caused concentration-dependent contraction of both LM and CM strips. The order of potency was DYN greater than M-Enk greater than L-Enk. Naloxone selectively decreased the responses to opiate peptides. 8. The present results indicate that acetylcholine, carbamylcholine, catecholamines, 5-HT, tachykinins (SP, NKA and NKB) and opiate peptides (DYN, L-Enk and M-Enk) affect the mechanical activity of LM and CM strips isolated from the carp intestinal bulb through their specific receptors.

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)

Non-cholinergic ascending excitatory response in the cat small intestine: possible involvement of substance P

Using an electromyographic technique, an ascending excitatory response was recorded "in vitro" in the presence of atropine in the cat small intestine up to 70 mm orally with respect to the site of repetitive transmural nerve stimulation. This non-cholinergic ascending excitatory response was characterized by an increase in the slow wave amplitude and spiking activity. This response was reversibly abolished by Tetrodotoxin (3,1 X 10(-6) M) but remained unchanged after exposure of the intestine to: Hexamethonium (4,9 X 10(-6) M) plus Tubocurarine (1,4 X 10(-5) M), Guanethidine (5 X 10(-7) to 5 X 10(-5) M), Domperidone (2,3 X 10(-7) to 2,3 X 10(-5) M), Naloxone (3 X 10(-7) to 3 X 10(-5) M), Methysergide (2,8 X 10(-7) to 2,8 X 10(-5) M), Metergoline (2,4 X 10(-5) M), Methiotepin (2,1 X 10(-5) M) and Mepyramine (2,3 X 10(-5) M). This response was unaffected by the substance P analogues, D-Pro2, D-Phe7, D-Trp9-Substance P (10(-5) M) or D-Pro2, D-Trp7-9-Substance P (10(-5) M) but was reversibly abolished after exposure of the intestine to substance P (10(-6) M). Moreover substance P still effectively abolished this response in the presence of any two of the above analogues. The results of the present study show that the non-cholinergic excitatory response elicited in the cat small intestine due to the activity of long ascending pathways probably involved substance P. The functional significance of this response is discussed.

Desensitization to intrathecal substance P in mice: possible involvement of opioids

Intrathecal (i.t.) injection of substance P (SP) results in a behavioral syndrome characterized by caudally directed biting and scratching. The present results indicate that repeated injections of 10 ng of SP, administered at 60 sec intervals, result in the rapid development of tachyphylaxis to this SP-induced behavioral phenomenon. Three injections of SP produced a greatly attenuated behavioral response compared to that of a single injection of SP. Mice injected with Freund's adjuvant in the hind foot pads to produce an arthritic-like condition exhibited significantly less desensitization to SP after repeated i.t. injection compared to that in saline-injected control mice. While large doses of naloxone (5 mg/kg s.c. or 1 microgram i.t.) combined with each injection of SP completely blocked the desensitization to the effect of SP in naive mice, a smaller dose of naloxone (0.5 mg/kg administered s.c.) failed to alter the apparent tachyphylaxis to SP similar to that seen in saline-treated control mice. Pretreatment with 0.5 micrograms i.t. of beta-funaltrexamine (beta-FNA) also failed to alter the degree of apparent tachyphylaxis to SP. Co-administration of 1 microgram of fluoxetine, a serotonin reuptake inhibitor, or 1 microgram of phentolamine, an alpha-adrenergic antagonist, completely blocked the development of tachyphylaxis to i.t. SP. These results indicate that tachyphylaxis to SP appears to involve activation of the endogenous opioid system. Desensitization also appears to involve activation and inhibition of the descending adrenergic and serotonergic systems respectively.

Contractile response to substance P in isolated smooth muscle strips from the intestinal bulb of the carp (Cyprinus carpio)

1. The effect of substance P on the mechanical activity of carp intestinal bulb smooth muscle was investigated in vitro. 2. Bath-applied substance P (1 nM-1 microM) caused concentration-dependent contraction of the smooth muscle. The EC50 value was 20 +/- 3 nM (N = 13). 3. Pretreatment with tetrodotoxin (780 nM) or atropine (500 nM) partially decreased the contractile response to substance P, while methysergide (3 microM) did not decrease the response. 4. The contractile response to substance P was not decreased by [D-Pro2, D-Trp7.9]-substance P or [D-Pro4, D-Trp7.9]-substance P (4-11) pretreatment (10 microM for 5 min). 5. Exposure of the intestinal bulb to substance P (100 nM and 1 microM for 15 min) decreased the response to subsequent application of substance P, physalaemin and eledoisin in a concentration dependent manner, while the contractile response to acetylcholine or methionine-enkephalin was not affected. 6. Exposure of the intestinal bulb to physalaemin and eledoisin (100 nM for 15 min) decreased the response to subsequent application of substance P. 7. The above results indicate that substance P causes the contraction of the carp intestinal bulb smooth muscle through its direct action on the smooth muscle and its indirect action through enteric cholinergic nerves. Long-term exposure to substance P causes desensitization of the preparation to substance P, physalaemin and eledoisin at the receptor level.

Somatostatin: peripheral venoconstrictive activity and interaction with monoamines in man

The mechanism of somatostatin venoconstriction and tachyphylaxis in the human hand vein in vivo has been investigated. No cross-tachyphylaxis was observed between somatostatin and 5-hydroxytryptamine, noradrenaline, adrenaline, dopamine or tyramine-induced venoconstriction. Somatostatin potentiates the venoconstrictive activity of noradrenaline, adrenaline and dopamine, but not that of 5-hydroxytryptamine and tyramine. Phentolamine antagonizes the somatostatin-induced venoconstriction, whereas methysergide, haloperidol and morphine do not. It is suggested that somatostatin could act on specific receptors in the hand vein, but the mechanism of somatostatin venoconstriction and interaction with vasoactive monoamines remains to be defined.

Involvement of substance P in the excitatory action of GABAA agonists on cholinergic neurons in the guinea-pig ileum

The possible involvement of substance P (SP) in cholinergic contractions induced by GABAA agonists in the guinea-pig ileum was further investigated. Responses evoked by 3-aminopropane sulphonic acid (3-APS) or muscimol consisted of a rapid phasic contraction followed in 70% of preparations by a tonic contraction, usually smaller in amplitude but considerably longer in duration. Phasic and tonic components were sensitive to bicuculline, neurogenic (cholinergic) in nature and susceptible to desensitization. Capsaicin (0.2 microM) pretreatment and SP receptor desensitization caused by 3 different priming SP concentrations (10 nM, 30 nM, 100 nM), depressed both components of the 3-APS-induced response, the magnitude of antagonism being greater for tonic contractions. Similar findings were obtained by using 10 microM (D-Pro4,D-Trp7.9)SP-(4-11), even though the degree of antagonism caused by this SP antagonist was consistently lower. These results indicate that depression of SP receptor function achieved by three different procedures decreases cholinergic contractile responses to GABAA agonists in the guinea-pig ileum. This provides further support for the hypothesis that GABAA receptor activation evokes both direct and indirect stimulation of enteric cholinergic neurons and that SP and/or a related peptide play an important role in mediating the indirect component of the cholinergic response.

Cholecystokinin octapeptide activates an opioid mechanism in the guinea-pig ileum: a possible role for substance P

To test the hypothesis that excitatory peptides release endogenous opioids from the myenteric plexus longitudinal muscle (MPLM) preparation of the guinea-pig ileum (GPI), the effect of cholecystokinin (CCK8) was studied in the absence and presence of the opioid antagonist naloxone. The maximum height of the contracture induced by CCK8 was not altered by the presence of naloxone in the incubation medium, however, the subsequent sustained excitation was clearly increased. This effect is interpreted as being a result of the release of endogenous opioids during the first moments of the CCK8-evoked excitation of the plexus. CCK8 still induced neurogenic contractures in the presence of atropine; these contractures were probably mediated by the release of substance P. Naloxone was used to evidence the opioid control of the CCK8-induced release of substance P. Desensitization to the effect of substance P reduced the action of CCK8 and also abolished the non-cholinergic contractures evoked by CCK8 and the subsequent effect of naloxone. These facts suggest the release of endogenous opioids within the plexus in response to the neurally mediated excitatory action of CCK8.

Enkephalinase inhibitors potentiate substance P-induced secretion of 35SO4-macromolecules from ferret trachea

To study the roles of substance P (SP) and endogenous peptidases in regulating mucus secretion from ferret trachea, we measured the SP-induced release of 35SO4-labeled macromolecules after incubating segments of trachea in Ussing chambers in the presence and absence of selected inhibitors of proteolytic enzymes. Our strategy was based on the idea that if endogenous peptidases degrade SP, then inhibitors of these enzymes should potentiate SP-induced secretion. We found that tracheas of ferrets contained SP-like immunoreactivity, and that SP stimulated the release of bound 35SO4 with rapid onset and offset. Eighty-five percent of the total macromolecular radioactivity released was contained in fractions of molecular weights greater than 10(6). The response to SP was concentration-dependent and reproducible. Thiorphan potentiated the secretory response to SP in a concentration-dependent fashion and phosphoramidon potentiated SP-induced secretion, whereas other inhibitors of proteinases and peptidases were without effects. These results suggest that substance P may regulate mucus secretion in ferrets, and that enkephalinase (dipeptidyl carboxypeptidase II, EC 3.4.24.11) in the airway degrades SP in a physiologically significant fashion, and thereby regulates peptide-induced secretion.

A pharmacological and immunohistochemical study of the splanchnic innervation of ileal longitudinal muscle in the toad Bufo marinus

A study was made of the innervation of the longitudinal muscle of the toad ileum with particular emphasis on the splanchnic innervation by non-adrenergic, non-cholinergic (NANC) nerves. Nerve fibres containing substance P-like immunoreactivity (SP-LI) were observed in the gut wall and in the ileal wall after degenerative section of the splanchnic nerves. Incubation overnight in a high concentration of capsaicin (3 X 10(-4) M) caused degeneration of SP-LI fibres. No evidence was obtained for enteric neurons containing SP-LI. Substance P caused a contraction of the longitudinal muscle similar to that produced by nerve stimulation. The response to nerve stimulation was decreased by about 60% by treatment with alpha-chymotrypsin. Capsaicin normally evoked a contraction of the longitudinal muscle, but did not do so after degenerative section of the splanchnic nerves. Prolonged treatments with high concentrations of capsaicin (5 X 10(-5) M) abolished the excitatory response to nerve stimulation. The results suggest that substance P is the transmitter mediating the NANC contraction. The fibres releasing the transmitter are possibly antidromically activated, sensory afferents. Both transmural stimulation and capsaicin caused a NANC inhibition of longitudinal muscle. Stimulation of perivascular nerves after splanchnic nerve section caused a NANC excitation, as did transmural stimulation even after nerve section or capsaicin treatment.

Antagonist discrimination between subtypes of tachykinin receptors in the guinea-pig ileum

The effects of substance P and eledoisin on spontaneous and electrically-evoked release of [3H]acetylcholine, and on smooth muscle were studied in the guinea-pig myenteric plexus-longitudinal muscle preparation preloaded with [3H]choline. Substance P and eledoisin caused transient increases in spontaneous release of [3H]-acetylcholine and in longitudinal muscle tone. Both tachykinins were equipotent in contracting the muscle, but eledoisin was more potent than substance P in eliciting [3H]acetylcholine release. The release caused by substance P was enhanced in the presence of naloxone and scopolamine which suggests that the release is modulated through opioid and muscarinic receptors. Substance P and eledoisin inhibited the release of [3H]acetylcholine evoked by electrical stimulation at 0.1 Hz. The inhibition was not due to an activation of alpha-adrenoceptors, histamine or opioid receptors. The substance P antagonists (D-Pro2, D-Trp7,9)SP (10 and 30 microM) and (Arg5, D-Trp7,9, Nle11)SP5-11 (1 and 10 microM) competitively antagonized both the contractile effects of substance P and eledoisin, and the inhibition by the tachykinins of the electrically-evoked release of [3H]acetylcholine. The increase in spontaneous [3H]acetylcholine release elicited by substance P and eledoisin was not prevented by the substance P antagonists. The results suggest that the neuronal receptor whose activation causes inhibition of acetylcholine release and the smooth muscle receptor correspond to the SP-P type, whereas the neuronal receptor mediating an increase in spontaneous acetylcholine release is of the SP-E type. The two antagonists, (D-Pro2, D-Trp7,9)SP and (Arg5, D-Trp7,9, Nle11)SP5-11, selectively block only the SP-P receptor.

Desensitization of substance P-induced K+ release in rat parotid

Challenge of rat parotid slices with substance P or its analogs, at concentrations which cause less than maximal response resulted in the transient release of K+ into the medium. Reuptake of the released K+ into the cell was accompanied by a parallel decrease in the biologically active concentration of the peptide in the medium, indicating that at low concentrations inactivation of the peptide is a mechanism for termination of the substance P response. At concentrations of substance P and its analogs which are higher than needed for a maximal response, a second mechanism for the termination of the response enters into play, resulting in desensitization of the response to substance P. Desensitization was specific for substance P and was not influenced by activation of the cholinergic or alpha-adrenoceptors. Inactivation of the peptide by proteolytic breakdown does not take part in the development of desensitization to substance P.

Blockade of substance P receptors inhibits non-nicotinic, non-adrenergic colonic contractions induced by stimulation of the lumbar sympathetic nerves to the feline large intestine

Experiments were performed on chloralosed cats, pretreated with guanethidine and hexamethonium and with ligated adrenals. The motility of an arterially isolated colonic segment was monitored by a volumetric method. Colonic contractions were induced either by electric stimulation of the centrally cut lumbar splanchnic nerves, or by close i.a. injections of acetylcholine (ACh). Close i.a. injections of substance P (SP) were performed in order to test the degree of SP-receptor blockade. Blockade of colonic SP-receptors, either by close i.a. administration of large amounts of SP itself or by Spantide, a competitive SP-receptor antagonist, markedly reduced or abolished the neurally induced colonic contractions, while responses to ACh were unchanged. These results indicate a role for SP, or a closely related peptide, as an intermediate neurotransmitter in the neurally induced colonic responses. Thus, the present study gives support to the hypothesis that non-nicotinic, non-adrenergic colonic contractions, in response to electric stimulation of sympathetic nerves, are due to an antidromic activation of sensory neurons with peripheral excitatory collaterals which release SP, in turn activating the colonic smooth muscle, mainly via a muscarinic mechanism.

Effect of substance P on somatostatin release from the isolated perfused rat stomach

The effect of substance P (SP) on the release of gastric somatostatinlike immunoreactivity (SLI) was studied. Substance P inhibited both basal SLI release and SLI release stimulated by gastric inhibitory polypeptide or 1-isoproterenol. This inhibitory action of SP was not blocked by atropine or hexamethonium, suggesting that a cholinergic mechanism was not involved. The SP-suppressed SLI release was also not reversed by the administration of naloxone, which indicated that enkephalinergic involvement was absent. However, the SP antagonist [D-Pro2, D-Trp7,9]-SP abolished the inhibition of SLI release by SP. It is concluded that SP might be involved in the control of gastric SLI release and that the inhibitory action of SP was probably mediated directly or indirectly on the gastric D cells through SP receptors.

Neurotensin receptors on the ileum of the guinea-pig: evidence for the coexistence of inhibitory and excitatory receptors

Neurotensin caused a complex muscular response of the longitudinal muscle-myenteric plexus preparation of guinea-pig ileum: picomoles of neurotensin produced inhibition while larger concentrations caused an inhibitory effect followed by a delayed dose-dependent contraction. The inhibitory phase of the neurotensin-induced muscular activity was not modified by tetrodotoxin but was potently antagonized in a non-competitive manner by apamin, a bee venom toxin. The contractile component was blocked by tetrodotoxin but not by apamin. These toxins were used to dissect the neurotensin muscular response into an inhibitory phase and an excitatory component. It was possible to further characterize the two neurotensin muscular components by their kinetics of desensitization. The inhibitory neurotensin response showed a fast rate of desensitization and presented a relatively low spare receptor capacity. In contrast, desensitization to the excitatory action of neurotensin was much slower, the excitatory receptors apparently having a larger spare receptor capacity. Desensitization to the action of neurotensin was selective for the neuropeptide not altering the contractile activity of substance P, angiotensin II, bradykinin, histamine or acetylcholine. These results strongly suggest the presence of two subsets of neurotensin receptors in the ileum: the inhibitory set probably localized at the postsynaptic effector level and excitatory neurotensin receptors probably of neuronal origin whose function is probably to modulate the release of neurotransmitters. The physiological implications of these two subtypes of neurotensin receptors in the control of gastrointestinal motility are discussed.

Specific and non-specific desensitization of guinea-pig ileal smooth muscle

The effects of cis-2-methyl-4-dimethylaminomethyl-1-3-dioxolane methiodide (CD), a muscarinic agonist, histamine, substance P and K+-stimulation on the mechanical responses, Ca2+-dependence and desensitization in guinea-pig ileal longitudinal smooth muscle have been studied. The mechanical responses to all four stimulants are highly dependent upon extracellular Ca2+(Ca2+EXT) and are blocked by the Ca2+ channel antagonist nicardipine. The tonic (slow) components of response are more dependent on Ca2+EXT and are more sensitive to nicardipine (IC50 values 5.0 X 10(-8) - 2.5 X 10(-9)M) than are the phasic (fast) components of response. Tissue exposure to CD (5 X 10(-7)M, 10 min) or histamine (3 X 10(-6)M and 3 X 10(-4)M, 10 min) produces short term nonspecific desensitization but substance P (5 X 10(-8)M, 10 min) produces only specific desensitization. K+-induced responses neither desensitize nor are desensitized. Desensitization is concentration- and time-dependent for both specific and nonspecific processes. Nonspecific desensitization is protected by elevation of K+ concentration (5.36mM) in the incubating medium, by dithiothreitol and by inhibitors (mepacrine,p-bromophenacyl bromide and phenylgloxal) of phospholipase A2 and is potentiated by mellitin, an activator of phospholipase A2. Desensitization produced by the muscarinic agonist CS is protected by Gpp(NH)p (10(-4)M), but histamine-induced desensitization is unaffected. There is no loss of muscarinic receptors, measured by [3H]QNB binding following tissue exposure to low concentrations of CD (5.0 X 10(-7)M) for up to 72 h. However, an apparent loss of receptors (20-30%) is measured following 10-90 min exposure of tissue to 10(-3)M CD. It is suggested that contractions of guinea-pig ileal longitudinal smooth muscle elicited by CD, histamine, substance P or K+ mobilize a common pool of Ca2+ through a Ca2+ channel antagonist (nicardipine) sensitive pathway. However, the existence of short term nonspecific desensitization (CD and histamine), specific desensitization (substance P) or no desensitization (K+ stimulation) indicates that significant differences exist in the pathways linking initial stimulus to mechanical response. The ability of elevated K+ to protect against nonspecific desensitization suggest that post stimulus membrane hyperpolarization may represent one contributing component to nonspecific desensitization. Products of phospholipid degradation may also contribute to desensitization since inhibitors or activators of phospholipase A2 prevented or potentiated respectively, nonspecific desensitization.

Post-tetanic potentiation at the nerve-muscle junction in the longitudinal muscle of the guinea-pig ileum. Possible role of substance P

The effect of short tetanic stimulation (30 Hz for 25 s) on the following twitch responses of the myenteric plexus-longitudinal muscle preparation of guinea-pig ileum to electric stimulation (0.1 Hz) was investigated in the presence of naloxone and indomethacin. Post-tetanic potentiation (PTP) of the twitches observed in control experiments was abolished in preparations desensitized by substance P but it was not affected in preparations desensitized by serotonin or pretreated with methysergide. Immediately after 5 min tetanic stimulation a decreased sensitivity to substance P but unchanged sensitivity to serotonin were observed. Electromyogram (EMG) of the longitudinal muscle layer was picked up 4 and 10 mm aborally from the stimulation site in response to 1 to 16 impulse trains delivered at 100 Hz. In control conditions only the longer trains triggered this neurogenic response at the distal recording site. In the presence of substance P but not serotonin facilitation occurred so that the distal site was frequently recruited to respond with an EMG even to single impulses. A substance P-like compound rather than serotonin may be a candidate for the neuromodulator or neurotransmitter substance involved in PTP and changes in the response topography of muscarinic transmission.

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.

Contractile responses to substance P and related peptides of the isolated muscularis mucosae of the guinea-pig oesophagus

The site of action of substance P and related tachykinins with respect to isotonic contractions was examined on the isolated muscularis mucosae attached to the submucous plexus of the guinea-pig oesophagus. Substance P (greater than 30 nM) produced a concentration-dependent contraction of the muscularis mucosae (EC50 1.9 +/- 0.5 microM, n = 10). The contractions were rapid in onset (2 min or less), sustained, reversible by washing and the preparation did not show tachyphylaxis. Eledoisin and physalaemin produced similar sustained contraction of the muscularis mucosae. The order of sensitivity was eledoisin greater than substance P greater than physalaemin. Contractions induced by 1 microM of each tachykinin were not significantly modified by incubation of the tissue with substance P or eledoisin (10 microM for 30 min). The contractile responses to tachykinins were unaffected by tetrodotoxin (0.3 microM), atropine (0.3 microM), phentolamine (1 microM), chlorpheniramine (1 microM), methysergide (1 microM), baclofen (100 microM) and verapamil (10 microM), but were abolished by the incubation of the tissue with calcium-free, EGTA (0.1 mM)-containing Tyrode solution. A substance P antagonist, [D-Pro2, D-Trp7,9]-substance P (greater than 0.1 microM), produced a transient contraction of the muscularis mucosae and the smooth muscle regained its original tone within 6 to 10 min. Contractions to the tachykinins were now inhibited by the antagonist (0.1-10 microM) in a concentration-dependent manner, the order of sensitivity being physalaemin greater than substance P = eledoisin. The cholinergically mediated electrically (0.1 Hz, 0.5 ms, supramaximal voltage)-induced twitch contractions of the muscularis mucosae were not significantly modified by substance P (0.01-0.3 microM). 7 The present results indicate that substance P and related tachykinins contract the isolated muscularis mucosae of the guinea-pig oesophagus by a direct action on the smooth muscle, probably by stimulating SP-E receptors.

Pharmacological characterization of the inhibitory effects of neurotensin on the rabbit ileum myenteric plexus preparation

Neurotensin in picomolar concentrations caused a concentration-related inhibition of the spontaneous contractile activity of the longitudinal muscle from the rabbit isolated ileum. Neurotensin was approximately 100 times more potent than adrenaline and about 10000 times as active as adenosine triphosphate (ATP) in producing similar relaxations. The neurotensin-induced inhibitory effect did not follow activation of adrenoceptors or P1-purinoceptors since the effect of the neuropeptide was not antagonized by a combination of phentolamine plus (-)-propranolol, nor by pretreatment with theophylline. Tetrodotoxin did not reduce the potency of neurotensin in relaxing the rabbit ileum, suggesting that the neurotensin-induced inhibition is not neuronally mediated. The inhibitory responses of neurotensin were blocked non-competitively by apamin. The inhibitory effect of neurotensin was short-lived with the subsequent development of tachyphylaxis, which was not crossed to the inhibitory action of adrenaline or ATP. Similarly, when tachyphylaxis to adrenaline or to ATP was established, the inhibitory action of neurotensin was unaffected. Desensitization was characterized by a gradual shift of the neuropeptide concentration-response curve to the right and downwards. Preincubation of rabbit ileum strips with 10 nM dynorphin (1-13) significantly increased the inhibitory action of neurotensin.

Effect of substance P on non-cholinergic fast and slow post-stimulus depolarization in the guinea-pig ileum

Membrane potentials were recorded with intracellular electrodes from the circular muscle cells of the guinea-pig ileum, in vitro, in the presence of atropine (1.4 X 10(-6) M) at 30 degrees C. Under such conditions, the preparations did not show any spontaneous activity. Exposure of the tissue to substance P (10(-7)-10(-6)M) for a short period of time (less than 5 min) caused depolarization and action potentials. At this time the membrane resistance appeared to be decreased and after transmural nerve stimulation the fast post-stimulus depolarization, which followed the inhibitory junction potential (IJP), triggered action potentials. After prolonged exposure of the tissue to substance P (greater than 10 min), the contractile activity of the preparation subsided, the membrane potential was decreased by 6.4 +/- 1.1 mV and the membrane resistance was increased; following transmural nerve stimulation, the amplitude of the IJP was increased, the fast post-stimulus depolarization was abolished and the later slow post-stimulus depolarization was enhanced and in some preparations reached threshold for action potential generation. In the latter preparations in the absence of transmural nerve stimulation, the membrane potential occasionally showed regular oscillations of approximately 10 mV in amplitude every 5-8 s. When the tissue was hyperpolarized by anodal current, after prolonged exposure to substance P, the fast post-stimulus depolarization did not return indicating that its abolition was not a secondary effect due to the depolarization of the tissue by substance P.

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

The contractile effect of substance P on the longitudinal muscle of the isolated guinea-pig small intestine and the desensitization of the muscle which occurs on prolonged exposure to the peptide have been investigated. All experiments were performed in the presence of atropine. The response to a substance P concentration which produced a nearly maximal effect was not sustained but faded rapidly. It was found that not elimination of substance P from the bath, but desensitization of the muscle to substance P was the main cause for the fading of contraction. Desensitization of the muscle to substance P only developed if the muscle was exposed to the peptide for a certain time. The degree of and the time needed for recovery from desensitization were directly related to concentration of substance P and contact time. Tetraethylammonium (3 mM), which reduces the membrane conductance for K+, enhanced the potency of substance P in contracting the muscle and reduced the fading of contraction. Noradrenaline (295 nM), which increases the K+ conductance, produced opposite effects. Lowering the extracellular Ca2+ concentration to one-tenth decreased the potency of substance P in contracting the muscle, accelerated the fading of contraction, and reduced the ability of the muscle to respond to a second addition of substance P after the response to the first addition had faded away. Concentrations of substance P (22 nM) and tetraethylammonium (30 mM), which produced nearly maximal contractions, slightly enhanced the efflux of 86Rb from pre-loaded muscle strips. Both substances, however, caused a sustained reduction of 86Rb efflux from strips depolarized by high [K+], the effect of substance P being smaller than that of tetraethylammonium. The effect of substance P and tetraethylammonium on 86Rb efflux appeared independent of the extracellular [Ca2+]. On exposure of the muscle to substance P (22 nM) for 8 min the intracellular uptake of 45Ca was first decreased and then increased while the 45Ca influx was instantly enhanced by tetraethylammonium (30 mM) or K+ (108 mM). The delayed increase in 45Ca influx caused by substance P was also observed in muscle strips depolarized with high [K+].(ABSTRACT TRUNCATED AT 400 WORDS)

[D-Met2, Pro5]enkephalinamide and dynorphin-(1-13) inhibit the cholinergic contraction induced in the guinea-pig ileum by substance P

In the isolated guinea-pig ileum, the sustained phase of the longitudinal contractile response to substance P is, unlike the initial peak response, mediated by stimulation of cholinergic neurons. This cholinergically mediated response to substance P was inhibited by the specific substance P antagonist, [D-Pro2,D-Trp7,9]substance P, which suggests that substance P stimulates enteric cholinergic neurons through a specific site of action. [D-Met2,Pro5]enkephalinamide and dynorphin-(1-13) also decreased whereas naloxone increased the sustained response to substance P. The results indicate that the enteric cholinergic neurons, which are stimulated by substance P, are also under the control of enkephalin and/or dynorphin neurons.

Hydrolysis of substance P and bradykinin by black widow spider venom gland extract

Black widow spider venom gland extract was found to contain significant peptidase activity. Aliquots of the venom gland extract incubated at 37 degrees inactivated substance P (SP) and bradykinin but not angiotensin II or the enkephalins. The peptide inactivation was proportional to the duration of the incubation and the amount of extract used. Analysis of the peptides on high pressure liquid chromatography demonstrated that the loss in biological activity of SP and bradykinin in the longitudinal muscle of the guinea pig ileum was correlated with cleavage of the peptides into several fragments. Kinetic studies revealed that SP was initially split into two fragments but that these products underwent further degradation into smaller peptides. The optimal pH for the peptidase activity was 6.5. At 0 degree the enzymatic activity was undetectable, and it was irreversibly destroyed by incubation at 100 degrees for 5 min or by pretreatment of the extract with 100 microM diisopropyl fluorophosphate. In addition, the gland extract preparation hydrolyzed artificial substrates designed to detect trypsin or chymotrypsin-like activity.