Stavros Katsoulis

Pituitary adenylate cyclase activating peptide, a novel VIP-like gut-brain peptide, relaxes the guinea-pig taenia caeci via apamin-sensitive potassium channels.

SummaryEffects of pituitary adenylate cyclase activating peptide (PACAP-(1-27)) and vasoactive intestinal polypeptide (VIP) on the guinea-pig taenia caeci were studied in the presence of guanethidine and scopolamine. Both peptides (1 nmol/1-1 μmol/1) concentration-dependently relaxed the smooth muscle of the taenia. PACAP-(1-27) and VIP were nearly equipotent. Apamin (30 nmol/1), a selective blocker of calcium-activated potassium channels, abolished the relaxation induced by PACAP-(1–27) whereas the effect of VIP remained unaffected. PACAP-(1–27) may be a candidate for the noncholinergic, non-adrenergic inhibitory neurotransmitter which induces apamin-sensitive relaxation in the intestinal tract.

Pituitary Adenylate Cyclase-Activating Peptide is a Potent Modulator of Human Colonic Motility

Pituitary adenylate cyclase-activating peptide (PACAP) represents a novel brain-gut peptide with high sequence homology to vasoactive intestinal polypeptide (VIP). Since PACAP has been identified in the human gut, the effect of the two molecular forms PACAP-(1-38) and PACAP-(1-27), the hybrid PACAP-(1-23)VIP-(24-28), and VIP on the contractility of the longitudinal muscle of human sigmoid colon was tested in vitro. All peptides inhibited the spontaneous phasic contractions and relaxed concentration-dependently carbachol-precontracted preparations. The effects of the peptides remained unaffected by tetrodotoxin, by inhibition of phosphodiesterase activity, and by inhibition of nitric oxide synthesis. Apamin reduced only the effects of the PACAP peptides, whereas tetraethylammonium blocked only the effect of VIP. In conclusion, PACAP peptides and VIP mediate their relaxant effects via activation of specific PACAP and VIP receptors coupled to different potassium channels.

Calcitonin gene-related peptides relax guinea pig and rat gastric smooth muscle.

Rat and human alpha- and beta-calcitonin gene-related peptide (CGRP), in the concentration range 1-100 nM, produced sustained relaxations of longitudinal muscle from the rat fundus and guinea pig gastric corpus. The peptides were equipotent and equally effective. Tetrodotoxin, adrenoceptor and purine receptor antagonists, somatostatin, apamin and Tyr-rat alpha-CGRP-(28-37) peptide did not modify the action of the CGRP peptides. The CGRP-induced responses were inhibited by verapamil and potentiated by Bay K-8644. Incubation of the tissues with indomethacin markedly reduced the magnitude of the CGRP- and adrenaline-induced relaxations, but their responsiveness was restored by addition of prostaglandins E1, E2 and F2 alpha in concentrations that alone did not affect the motility of the indomethacin-treated strips. It is suggested that an inhibitory receptor for CGRP on gastric smooth muscle cells is linked to calcium channels and may be activated or sensitized by endogenous prostaglandins.

Pituitary adenylate cyclase activating polypeptide (PACAP) is a potent relaxant of the rat ileum

The effect and mode of action of pituitary adenylate cyclase activating polypeptide (PACAP) were studied in rat ileal strips. PACAP relaxed, concentration dependently, rat ileum and was 50 times more potent than the structurally related vasoactive intestinal polypeptide (VIP). The inhibitory action of PACAP was not modified by TTX, omega-conotoxin, adrenergic, or ganglionic blockade, antagonists of adrenoreceptors and muscarinic receptors, indicating a direct myogenic effect probably through specific PACAP receptors. The lack of cross-tachyphylaxis between PACAP and VIP suggests that both peptides act by activation of distinct receptors. Structure-function analysis revealed that the N-terminal region of the PACAP molecule is crucial for biological activity.

Effects of galanin, its analogues and fragments on rat isolated fundus strips.

1 Rat and porcine galanin (rGal and pGal) produced dose‐dependent contraction of rat fundus strips in a concentration range of 6 nm–100 nm. 2 The stimulatory effect of rGal on rat fundus strips was not modified in the presence of somatostatin (250 nm), naloxone (1 μm), guanethidine (10 μm), a mixture of propranolol (3 μm) and phentolamine (3 μm), tetrodotoxin (1 μm), indomethacin (10 μm), atropine (1 μm), a mixture of methysergide (2.5 μm) and ketanserin (2.5 μm), a mixture of mepyramine (10 μm) and cimetidine (10 μm), and saralasin (10 μm) or when strips were desensitized to substance P and neurotensin. 3 These results suggest the localization of specific Gal receptors on the surface of smooth muscle cells of rat fundus. 4 The galanin analogues [d‐Trp2]‐rGal, [NLe4]‐rGal, [d‐Ala7]‐rGal, [d‐Trp2‐NLe4‐d‐Ala7]‐rGal and fragments [Cys23]‐Gal (1–23), Gal (1–18) were fully active. In contrast, rGal (3–29) was completely inactive and showed no antagonistic properties to the contractile effect of intact galanin. 5 The order of potency of the galanin peptides, analogues and fragments to contract rat fundus strips was: pGal > rGal > [NLe4]‐rGal > [Cys23]‐Gal (1–23) > Gal (1–18) > [d‐Ala7]‐rGal > [Trp2]‐rGal > [d‐Trp2‐NLe4‐d‐Ala7]‐rGal. 6 The data originating from our structure‐activity study suggest that the C‐terminal portion of Gal contributes mainly to the affinity of Gal receptors whereas the N‐terminal portion of Gal is responsible for the full activation of Gal receptors in this tissue. In particular the amino acids in position 1 and 2 of Gal (Gly‐Trp) appear to be essential for binding and intrinsic activity.

[Arg3]substance P and neurokinin A from chicken small intestine

Using antisera specific for NH2-terminal and COOH-terminal regions of substance P and for the COOH-terminal region of neurokinin A, peptides with tachykinin-like immunoreactivity were isolated from extracts of chicken small intestine. The peptide Arg-Pro-Arg-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2 differs from human substance P by substitution of the lysyl residue by an arginyl residue at position 3. Synthetic [Arg3]substance P showed identical chromatographic and immunochemical properties to chicken substance P and was equipotent with substance P in contracting the guinea pig ileum. A second peptide His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH2 isolated from the extracts is identical to human neurokinin A. A third peptide was immunoreactive towards the COOH-terminally directed anti-serum to substance P only but was not characterized structurally in this study.

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.

Calcitonin Gene-Related Peptide (CGRP) modulates cholinergic neurotransmission in the small intestine of man, pig and guinea-pig via presynaptic CGRP receptors

The effect of calcitonin gene-related peptide (CGRP) on the cholinergically mediated twitch contraction in longitudinal muscle strips of the small intestine (duodenum, jejunum, ileum) of guinea-pig, pig and man was investigated. Independently of the anatomical region, CGRP inhibited the twitch response in the different specimens of all three species by about 40% with similar IC50 values (1.5-2.4 nmol/l). Only in the guinea-pig small intestine CGRP induced a contraction of the smooth muscle which was sensitive to scopolamine and tetrodotoxin. The electrically evoked [3H]acetylcholine release from jejunal longitudinal muscle strips with myenteric plexus attached of the guinea-pig, which were incubated with [3H]choline, was concentration-dependently inhibited by CGRP. A direct relaxant effect of CGRP on smooth muscle tone of carbachol precontracted preparations was only observed in specimens of the guinea-pig. In conclusion, presynaptic inhibitory CGRP receptors on cholinergic neurones modulate the release of acetylcholine in different parts of the small intestine.

Effects of neurotensin-related peptides on the motility of the guinea pig oesophagus

Abstract Neurotensin, neuromedin N and xenopsin induced a monophasic and concentration-dependent contraction of the intact guinea pig oesophagus but kinetensin was without effect. The responses were completely abolished by tetrodoxin and atropine but were unaffected by hexamethonium. The maximum response induced by neurotensin was reduced (20–30%) by somatostatin and by dynorphin-(1–13) in a naloxone-reversible manner. Neurotensin did not contract the isolated muscularis mucosae. The effects of neurotensin-related peptides on the motility of the oesophagus are mediated exclusively through the release of acetylcholine.

Effects of guinea pig neurotensin ([Ser7]neurotensin) on gastrointestinal smooth muscle

Guinea pig neurotensin differs from other mammalian neurotensin by the substitution of proline by serine at position 7. [Ser7]neurotensin produced a concentration-dependent contraction of the guinea pig oesophagus and rat fundus, a biphasic effect (initial relaxation followed by more sustained contraction) on the guinea pig stomach corpus, duodenum and longitudinal muscle of the ileum and a relaxation of the guinea pig colon and rat ileum. The potencies (EC50) and % maximum contraction compared with the maximum effect produced by either histamine or acetylcholine of [Ser7]neurotensin and neurotensin were not significantly different in any tissue studied.