Somatostatin potentiates cholinergic neurotransmission in ferret trachea

We studied the effect of somatostatin on contractile responses to electrical field stimulation (EFS) in isolated ferret tracheal segments. Somatostatin (up to 10(-5) M) did not change resting tension, but it potentiated the contractile response to EFS dose dependently, with a maximum effect at 10(-6) M. Thus, at a concentration of 10(-6) M, somatostatin significantly decreased the mean log of EFS frequency producing 50% of maximum contraction from a control value of 0.52 +/- 0.07 to 0.24 +/- 0.06 (SE) Hz (P less than 0.01). The potentiating effect of somatostatin (10(-6) M) was not inhibited by hexamethonium, indomethacin, BW755C, pyrilamine, methysergide, or D,Pro2,D,Trp7,9-SP, but it was inhibited by atropine or by the somatostatin antagonist cyclo[7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)]. In contrast to EFS-induced contraction, contractions produced by acetylcholine (10(-9) to 10(-3) M) were not affected by somatostatin at a concentration of 10(-6) M. These results suggest that somatostatin potentiates contractions produced by EFS via presynaptic cholinergic mechanisms and probably through a specific somatostatin receptor.     

Nonadrenergic inhibitory nerves attenuate neurally mediated contraction in cat bronchi

Effects of nonadrenergic and noncholinergic (NANC) inhibitory nerves on cholinergic neurotransmission were examined in isolated bronchial segments from cats in the presence of propranolol (10(-6) M) and indomethacin (10(-6) M) by use of electrical field stimulation (EFS) techniques. EFS caused contraction alone in tissues at the baseline tension and biphasic responses (contraction and relaxation) in tissues precontracted with 5-hydroxytryptamine. Contraction was abolished by atropine (10(-6) M), and relaxation was abolished by tetrodotoxin (10(-6) M). At the baseline tension, EFS at frequencies greater than 10 Hz inhibited the subsequent (4 min later) contraction induced by EFS at 1-5 Hz. EFS-induced inhibition was stimulus frequency dependent and reached maximum at 20 Hz. However, EFS at 20 Hz did not inhibit the subsequent contractile response to acetylcholine (10(-7) to 10(-3) M). Exogenously applied vasoactive intestinal peptide mimicked EFS-induced inhibitory effects, but substance P and calcitonin gene-related peptide did not. The inhibitory effect of EFS at 20 Hz was not altered by pyrilamine, cimetidine, naloxone, methysergide, phentolamine, BW755C, AF-DX 116, or removal of epithelium. These results imply that the NANC transmitter acts via presynaptic cholinergic receptors.     

Responses of human basilar arteries to vasoactive intestinal polypeptide

The responses to 9 X 10(-7) M vasoactive intestinal polypeptide (VIP) by isolated human basilar arteries of 30 individuals were studied to further elucidate the role the peptide might play in modifying cerebrovascular tone normally and in disease. In most experiments the artery was precontracted with prostaglandin F2 alpha (PGF2 alpha), either with 1 or 2 X 10(-6) M or with 10(-5) M PGF2 alpha. The course of action to VIP was observed for 15 min following its application to the contracted vessel. Some arteries failed to respond to VIP (13%), otherwise the arteries relaxed 44% when the contraction was induced by 10(-5) M PGF2 alpha and 67.6% after the lower concentrations of PGF2 alpha. There was no significant decrement in the vasorelaxant effect of VIP throughout the period of observation. A second and third application of VIP to the precontracted artery produced significantly less of an effect than the first, but no consistent progressive pattern of tachyphylaxis was evident. In additional experiments, indomethacin (10(-5) M) did not prevent the vasorelaxant effect of VIP, suggesting that prostanoid synthesis was not involved. Pretreatment of the artery with VIP did not prevent the contractions generated by 10, 30, 50 and 90 mM KCl while antithrombin III (1.2 X 10(-7) M) did, indicating fundamental differences between these two vasorelaxants. In conclusion, VIP will inhibit contraction of isolated human cerebral arteries for prolong periods and could be a significant factor regulating cerebral blood flow in humans.     

Inhibitory effects of botulinum toxin on pyloric and antral smooth muscle

Botulinum toxin injection into the pylorus is reported to improve gastric emptying in gastroparesis. Classically, botulinum toxin inhibits ACh release from cholinergic nerves in skeletal muscle. The aim of this study was to determine the effects of botulinum toxin on pyloric smooth muscle. Guinea pig pyloric muscle strips were studied in vitro. Botulinum toxin type A was added; electric field stimulation (EFS) was performed every 30 min for 6 h. ACh (100 microM)-induced contractile responses were determined before and after 6 h. Botulinum toxin caused a concentration-dependent decrease of pyloric contractions to EFS. At a low concentration (2 U/ml), botulinum toxin decreased pyloric contractions to EFS by 43 +/- 9% without affecting ACh-induced contractions. At higher concentrations (10 U/ml), botulinum toxin decreased pyloric contraction to EFS by 75 +/- 7% and decreased ACh-induced contraction by 79 +/- 9%. In conclusion, botulinum toxin inhibits pyloric smooth muscle contractility. At a low concentration, botulinum toxin decreases EFS-induced contractile responses without affecting ACh-induced contractions suggesting inhibition of ACh release from cholinergic nerves. At higher concentrations, botulinum toxin directly inhibits smooth muscle contractility as evidenced by the decreased contractile response to ACh.     

Inhibition of small conductance K+ -channels attenuated melatonin-induced relaxation of serotonin-contracted rat gastric fundus

The aim of this study was to investigate the effects of melatonin on rat gastric fundus smooth muscle. Melatonin (10(-4) to 10(-3) M) had no effect on the basal tone of gastric smooth muscle. After precontraction with carbachol (10(-6) M) or serotonin (10(-7) M), melatonin caused a concentration dependent inhibitory action. The half maximal effect on serotonin-induced contraction was found with 1.12 +/- 0.86 x 10(-5) M of melatonin. Increasing concentrations of melatonin (10(-5) to 10(-3) M) resulted in a right shift of the serotonin concentration response curve (10(-10) to 10(-5) M). This inhibitory effect of melatonin was partially blocked in the presence of apamin (10(-10) to 10(-7) M), a specific blocker of the small conductance calcium-dependent potassium channel, but not in the presence of other potassium channel blockers like charybdotoxin (10(-8) M), glibenclamide (l0(-5) M), or tetraethylammonium (ODQ, 10(-4) M). The inhibitory effect was not changed in the presence of the neuronal blocker tetrodotoxin (10(-6) M), the selective P2-receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (3 x 10(-5) M), the nitric-oxide synthase inhibitor N-nitro-L-arginine (3 x 10(-4) M), or the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (10(-4) M), suggesting that neither the purinergic, nitrergic, nor guanylate cyclase pathways were involved. We further investigated inhibitory responses to electrical field stimulation (EFS) at different frequencies under non-adrenergic, non-cholinergic (NANC) conditions on a serotonin-induced contraction in the presence of melatonin (10)-5 to 10(-4) M). Melatonin significantly reduced these inhibitory NANC responses in higher (8-32 Hz), but not lower (05-4 Hz), frequencies (16 Hz without melatonin, 103 +/- 6.3%; melatonin 10(-5) M, 80.4 +/- 7.5%; melatonin 10(-4) M, 39.1 +/- 17.1%). Melatonin had no effect on contractile responses induced by EFS under basal tone. These results demonstrate that the inhibitory effect of melatonin in rat gastric fundus smooth muscle is apamin sensitive, but is not affected by other potassium channel blockers. This suggests that melatonin may be another transmitter candidate for the apamin sensitive responses within the gastrointestinal tract.     

Neural control of relaxation in cat airways smooth muscle

Functional innervation of cat airways smooth muscle was examined in isolated segments of trachea and bronchi using electrical field stimulation (EFS) techniques. Field stimulation caused contraction in tissues at resting tone and biphasic responses (contraction followed by relaxation) in tissues precontracted with 5-hydroxytryptamine (5-HT). Contractions were abolished by 10(-6) M atropine. Inhibitory responses were dependent on impulse voltage, duration, and frequency. At low voltages (less than or equal to 10 V) and pulse durations (less than or equal to 0.3 ms), EFS induced relaxations were abolished by 3 X 10(-6) M tetrodotoxin (TTX). Greater stimulus parameters elicited TTX-resistant relaxations. Pretreatment of the tissues with 10(-6) M propranolol and 10(-5) M guanethidine caused rightward shifts in relaxation frequency-response curves. These findings indicate that cat airways are innervated by excitatory cholinergic, inhibitory adrenergic, and inhibitory nonadrenergic noncholinergic (NANC) nerves. Pretreatment of the tissues with hexamethonium, cimetidine, indomethacin, or nordihydroguaiaretic acid did not affect NANC relaxation responses. It is concluded that NANC inhibitory responses in cat airway smooth muscle are mediated through intrinsic postganglionic nerve fibers and occur independently of histamine H2-receptor activation and without involvement of cyclooxygenase or lipoxygenase products of arachidonic acid metabolism.     

The effect of leukotriene D4 on the isolated stomach and colon of the rat

The effect of synthetic leukotriene D4 (LTD4) was evaluated on isolated gastric longitudinal or circular smooth muscle and distal colon of the rat. The concentrations of LTD4, 2.5 X 10(-10)M to 5 X 10(-7)M, evoked minimal to maximal contractile responses. In addition, selected prostaglandins were used to identify the mediator of LTD4-induced contraction of gastric smooth muscle. FPL 55712 inhibited LTD4-induced contractions of gastric longitudinal or circular muscle. Indomethacin inhibited only LTD4-induced contractions of the longitudinal muscle. A combination of FPL 55712 and indomethacin produced greater inhibition of LTD4-induced contractions of longitudinal muscle than either agent alone. However, the same combination of inhibitors showed no greater effect than FPL 55712 alone on LTD4-induced contractions of circular smooth muscle. Unlike PGI2, PGF2, PGA2, or PGD2, PGE2 evoked contraction of the longitudinal muscle and relaxation of the circular muscle of the stomach. The dissimilar effect of PGE2 in the two smooth muscle layers of the rat stomach may signify that PGE2 is the prostaglandin released by LTD4 from the longitudinal and circular gastric muscle. However, the opposing pharmacologic effects following LTD4-induced release of prostaglandins in the circular muscle of the stomach would preclude the appearance of an inhibitory effect of indomethacin in this tissue. In contrast, PGE2 and other prostaglandins contract gastric longitudinal muscle in response to LTD4. Thus, these studies clearly suggest that LTD4 has both a direct and indirect effect on gastric smooth muscle of the rat. Unlike the stomach, LTD4-induced contraction of the distal colon was not inhibited by indomethacin while FPL 55712 antagonized contractions. Thus, these findings indicate a differential mechanism of stimulation of rat gastrointestinal tissue by LTD4.     

Enkephalinase inhibitor potentiates substance P- and electrically induced contraction in ferret trachea

To determine the role of endogenous enkephalinase (EC 3.4.24.11) in regulating peptide-induced contraction of airway smooth muscle, we studied the effect of the enkephalinase inhibitor, leucine-thiorphan (Leu-thiorphan), on responses of isolated ferret tracheal smooth muscle segments to substance P (SP) and to electrical field stimulation (EFS). Leu-thiorphan shifted the dose-response curve to SP to lower concentrations. Atropine or the SP antagonist [D-Pro2,D-Trp7,9]SP significantly inhibited SP-induced contractions in the presence of Leu-thiorphan. Leu-thiorphan increased the contractile responses to EFS dose dependently, an effect that was significantly inhibited by the SP antagonist [D-Pro2,D-Trp7,9]SP. SP, in a concentration that did not cause contraction, increased the contractile responses to EFS. This effect was augmented by Leu-thiorphan dose dependently and was not inhibited by hexamethonium or by phentolamine but was inhibited by atropine. Because contractile responses to acetylcholine were not significantly affected by SP or by Leu-thiorphan, the potentiating effects of SP were probably on presynaptic-postganglionic cholinergic neurotransmission. Captopril, bestatin, or leupeptin did not augment contractions, suggesting that enkephalinase was responsible for the effects. These results suggest that endogenous tachykinins modulate smooth muscle contraction and endogenous enkephalinase modulates contractions produced by endogenous or exogenous tachykinins and tachykinin-induced facilitation of cholinergic neurotransmission.     

Effect of endogenous prostaglandins on acetylcholine release from dog trachealis muscle

We used a radioenzymatic technique to measure effects of the prostaglandin synthesis inhibitor indomethacin and of exogenous prostaglandin E2 (PGE2) and prostaglandin I2 (PGI2) on acetylcholine (ACh) efflux from canine tracheal smooth muscle (TSM) during sustained electrical field stimulation (EFS; 2 Hz, 2 ms pulse duration, 50 V for 15 min). ACh efflux from indomethacin (INDO, 10(-6) M)-pretreated and control TSM increased with consecutive stimulations. However, efflux of ACh was greater in INDO-treated than control muscles. INDO increased the tension produced by TSM in response to EFS. Neither PGE2 (10(-8) M) nor PGI2 (10(-6) M) had any effect on ACh efflux from INDO-pretreated TSM during the first of three periods of EFS. However, PGI2 and PGE2 prevented the progressive increase in ACh efflux observed on subsequent stimulations. PGE2 but not PGI2 decreased contractions of TSM caused by EFS. Our results demonstrate that endogenous prostaglandins, probably PGE2, do inhibit EFS-evoked ACh release from canine TSM in vitro, but suggest that these prostaglandins modulate EFS-evoked contractions predominantly by postsynaptic mechanisms.     

Cholinergic neuromodulation by prostaglandin D2 in canine airway smooth muscle

To determine whether prostaglandin D2 (PGD2) modulates cholinergic neurotransmission in airway smooth muscle and, if so, what the mechanism of action is, we studied bronchial segments from dogs under isometric conditions in vitro. PGD2 (10(-8)-10(-5) M) elicited dose-dependent muscle contraction, which was reduced after blockade of muscarinic receptors, so that 50% effective dose (ED50) increased from 1.3 +/- 0.3 X 10(-6) to 3.9 +/- 1.0 X 10(-6) M by atropine (10(-6) M) (mean +/- SE, P less than 0.05). Physostigmine, at a concentration insufficient to alter base-line tension (10(-8) M), enhanced the PGD2-induced contraction and decreased ED50 to 6.4 +/- 0.5 X 10(-7) M (P less than 0.05). When added at the highest doses that did not cause spontaneous contraction (1.9 +/- 0.5 X 10(-7) M), PGD2 increased the contractile response to electrical field stimulation (1-50 Hz) by 21.9 +/- 6.6% (P less than 0.001). In contrast to this effect, the response to administered acetylcholine was not affected by PGD2. On the other hand, PGD2-induced augmentation of the response to electrical field stimulation (5 Hz) was further increased from 23.6 +/- 3.0 to 70.4 +/- 8.8% in the presence of physostigmine (10(-8) M) and was abolished by atropine but not affected by the alpha-adrenergic antagonist phentolamine or the histamine H1-blocker pyrilamine. These results suggest that the contraction of airway smooth muscle induced by PGD2 is in in part mediated by a cholinergic action and that PGD2 prejunctionally augments the parasympathetic contractile response, likely involving the accelerated release of acetylcholine at the neuromuscular junction.     

Inhibition of cholinergic neurotransmission in human airways by beta 2-adrenoceptors

The purpose of the study was to determine whether catecholamines modulate cholinergic neurotransmission in isolated human airway smooth muscle. Bronchial rings were suspended in organ baths for isometric measurement of tension, and contractions were induced by either electrical field stimulation (EFS) or exogenous acetylcholine (ACh). Isoproterenol, epinephrine, and norepinephrine in that order of potency produced concentration-dependent inhibition of comparable responses to EFS and ACh. However a potency difference of 100-fold for isoproterenol (IC50 = 4.80 X 10(-8) M for EFS and 3.70 X 10(-6) M for ACh) and 10-fold for both epinephrine and norepinephrine was observed for inhibition of responses to EFS compared with responses to ACh. The inhibitory effects of isoproterenol on responses to EFS were prevented by propranolol and ICI 118551 (a beta 2-antagonist) but not by betaxolol (a beta 1-antagonist). Tyramine had no effect on contractions elicited by EFS. These experiments demonstrate that beta-agonists inhibit cholinergic nerve-induced contractions of human bronchi more potently than contractions induced by exogenous ACh, suggesting modulation of cholinergic neurotransmission by prejunctional beta 2-receptors.     

Pathways of substance P stimulation of canine tracheal ciliary beat frequency.

Substance P (SP), an inflammatory neuropeptide, may be released by intraepithelial nerves in response to an irritant or inflammatory stimulus. To investigate the neural and humoral pathways mediating the response of tracheal ciliary beat frequency (CBF) to topically applied SP, CBF was measured on the ventral midtracheal surface of anesthetized beagles by using heterodyne-mode correlation analysis laser light scattering. In the first study, aerosolized SP, delivered to the lungs of eight beagle dogs, stimulated CBF in a dose-dependent manner from a baseline of 4.9 +/- 0.4 Hz to a maximum of 14.9 +/- 1.5 Hz at dose of 10(-7) M. In the second study, the tracheal lumen was isolated from the bronchial airways by inflating the cuff of an endotracheal tube near the carina. Intravenous hexamethonium bromide (2 mg/kg), ipratropium bromide (0.5 micrograms/kg), and indomethacin (2 mg/kg) were used as blocking agents to inhibit the nicotinic, muscarinic, and cyclooxygenase pathways, respectively. Aerosolized 10(-9), 10(-8), or 10(-7) M SP was delivered sequentially to the tracheal lumen for 3 min at 30-min intervals. SP caused two distinct CBF stimulatory episodes at 4 min (mean time of the maximal response) and at 18 min (mean time of the maximal response) after onset of delivery and returned to baseline after 25 min. SP stimulated CBF from the baseline of 5.1 +/- 0.4 Hz to a maximum of 14.2 +/- 2.5 Hz during the first episode (P less than 0.01) and to 10.4 +/- 0.6 Hz during the second episode (P less than 0.01) at dose of 10(-8) M. These responses were inhibited by all the blocking agents. These data suggest that SP stimulates CBF via a cyclooxygenase-dependent parasympathetic reflex.     

Studies of the mechanism of passive anaphylaxis in human airway smooth muscle

This investigation was carried out to study allergic contraction of passively sensitized human airway smooth muscle in response to specific antigen challenge. We attempted to determine the role played by histamine, slow reaction substances (SRSs), and cyclooxygenase products in the mediation of this response in tracheal smooth muscle. Tissues were passively sensitized with serum from ragweed-sensitive patients (15 h, 4 degrees C). Subsequent challenge with ragweed antigen produced a slowly developing contraction. The peak contraction to a dose producing a maximal response was 37 +/- 6% of the carbachol maximum. Mepyramine (5 X 10(-6) M) did not alter the contraction. Methylprednisolone (2 X 10(-5) M) attenuated the response to antigen but had no significant effect on the contractile response to arachidonic acid. Indomethacin (5.6-28 X 10(-6) M) enhanced the peak antigen-induced contractions by 25 +/- 11% whereas 5,8,11,14-eicosatetraynoic acid (6.4 X 10(-5) M) selectively attenuated the antigen-induced contraction by 86 +/- 12%. Nordihydroguarietic acid (6-12 X 10(-6) M) attenuated both the antigen plus arachidonate induced responses. FPL-55712 (1-2 X 10(-6) M) antagonized the contractions to antigen. Compound 48/80 and goat antihuman immunoglobulin E produced similar slowly developing contractions in sensitized and in some nonsensitized tissues. These responses, except for an early component of the response to 48/80, were independent of histamine and were reversed by FPL-55712. These findings suggest that arachidonic acid metabolites mediate (slow reacting substances) and modulate (prostaglandins) allergic contraction of human airway smooth muscle while any histamine released contributes little or nothing to the contraction in the larger airways.     

Pre-junctional inhibitory action of prostaglandin E2 on excitatory neuro-effector transmission in the human bronchus

The effects of prostaglandin E2 (PGE2) and indomethacin on excitatory neuro-effector transmission in the human bronchus were investigated by tension recording and microelectrode methods. PGE2 (10(-10)-10(-9)M) suppressed the amplitude of twitch contractions and excitatory junction potentials (e.j.ps) evoked by field stimulation at a steady level of basal tension obtained by the combined application of indomethacin (10(-5) M) and FPL55712 (10(-6) M). In doses over 10(-8)M, PGE2 reduced the muscle tone and dose-dependently suppressed the amplitude of twitch contractions. Indomethacin (10(-5) or 5 x 10(-5) M) reduced the muscle tone and enhanced the amplitude of twitch contractions and e.j.ps evoked by field stimulation in the presence of FPL55712. PGE2 (10(-9) M) had no effect on the post-junctional response of smooth muscle cells to exogenously applied acetylcholine (ACh) (4 x 10(-7) M). However, indomethacin (10(-5) M) significantly enhanced the ACh-induced contraction of the human bronchus. These results indicate that PGE2 in low concentrations has a pre-junctional action to inhibit excitatory neuro-effector transmission in addition to a post-junctional action, presumably by suppressing transmitter release from the vagus nerve terminals in the human bronchial tissues.     

Potentiation of the responses to transmural nerve stimulation by alpha-agonists: a possible role in vivo

This study was undertaken to assess the effects of exogenous alpha-agonists on the effector response to transmural nerve stimulation in canine saphenous vein rings. The response to a fixed train (5 s duration) of transmural nerve stimulation (8 Hz, 0.3 ms, 9 V) applied every 5 min was determined in the control state and in the presence of subthreshold (for contraction) concentrations of noradrenaline, adrenaline, clonidine, and methoxamine. The maximum potentiations achieved by the three drugs were 246.2 +/- 36.9, 220.5 +/- 38.8, 384.3 +/- 78.7, and 353.3 +/- 68.0%, respectively. The potentiation observed was significantly inhibited by indomethacin (10(-6) mol/L) and propranolol (5 X 10(-6) mol/L). Both indomethacin and propranolol potentiated the response to transmural nerve stimulation. The potentiation of the responses to transmural nerve stimulation by alpha-agonists suggests that, presynaptic alpha 2-inhibition by circulating catecholamines is likely to be of limited biological significance in modulating the effector responses in the canine saphenous vein.     

Nitrergic relaxation in rat gastric fundus: influence of mechanism of induced tone and possible role of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase

The aim of this study was to investigate the influence of the mechanism of induced tone and the role of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) in nitrergic relaxation of rat gastric fundus. Prostaglandin F(2alpha) (PGF(2alpha)), thapsigargin (TSG) and cyclopiazonic acid (CPA) were used in concentrations that induced a similar contraction (20 g force/g tissue). Nifedipine (3 x 10(-7) M) completely relaxed PGF(2alpha)-contracted tissues and relaxed tissues contracted by TSG and CPA by 20 +/- 6% and 56 +/- 12% respectively; contraction induced by the three contractile agents was fully reversed by a general Ca2+ entry blocker 1-[2-(4-methoxyphenyl)-2-[3-(4-metoxyphenyl)propoxy]ethyl-1H-imidazole HCl (SKF 96365; 10(-5) M). In the presence of nifedipine (3 x 10(-7) M) or verapamil (10(-5) M), PGF(2alpha) and CPA-induced contractions were still approximately 50% relaxed by SKF 96365. This suggests that contractions induced by PGF(2alpha) are related to Ca2+ entry through L-type voltage-operated Ca2+ channels and that contractions by TSG are mainly related to Ca2+ entry through store-operated Ca2+ channels. Relaxant responses to exogenous nitric oxide (NO), to endogenous NO released by electrical field stimulation, and to vasoactive intestinal polypeptide (VIP) were studied in tissues contracted by TSG and CPA and compared to responses in tissues contracted by PGF(2alpha). Responses to exogenous and endogenous NO were greatly attenuated in TSG-contracted tissues, but not in CPA-contracted tissues. When contraction was induced by CPA in the presence of nifedipine or verapamil, relaxations to exogenous and endogenous NO were also significantly reduced. Relaxation induced by VIP was reduced in tissues contracted by either TSG or CPA in the presence of nifedipine or verapamil. These results suggest that the ability of the nitrergic neurotransmitter to induce relaxation of rat gastric fundus is influenced by the mechanism used to induce tone and are indicative for a role for SERCA in nitrergic relaxation. However, activation of SERCA appears to not be unique for nitrergic relaxation, but might also be used by VIP, a co-transmitter of NO in this tissue.     

Distribution and dilatory effect of vasoactive intestinal polypeptide (VIP) in human cerebral arteries

Vasoactive intestinal polypeptide (VIP)-containing nerve fibers were demonstrated in human pial arteries by immunocytochemistry. Fine varicose fibers were located in the adventitia close to the media layer. Measurements by radioimmunoassay revealed concentrations of VIP between 0.7 and 2.7 pmol/g in the major arteries at the base of the brain, obtained at autopsy. Isolated human pial arteries, obtained in conjunction with neurosurgery, relaxed in a concentration-dependent manner upon administration of VIP. The relaxation of the vessels amounted to 57 +/- 9% of the contraction elicited by prostaglandin F2 alpha (2.5 microM) with an EC50 value of (8.5 +/- 1.2) X 10(-9) M.     

Parasympathetic involvement in PAF-induced contraction in canine trachealis in vivo

We studied the contractile response elicited by platelet-activating factor (PAF) administered intra-arterially into the tracheal circulation of 34 dogs in vivo. A method that avoided tachyphylaxis encountered in prior investigations was developed for isometric measurement of multiple dose-response effects. PAF was a very potent contractile agent; active tension was elicited with 10(-11) mol ia PAF. To determine the mechanism by which contraction was induced, dose-response curves were generated in groups of five animals each treated with either 0.5 mg/kg (approximately 1.5 X 10(-5) mol) iv + 10(-3) mg/kg (3 X 10(-8) mol) ia atropine, 5 mg/kg iv indomethacin (INDO), or 7.5 mg/kg iv hexamethonium (HEX). After pretreatment with atropine, contraction still was elicited with 10(-11) mol ia PAF. However, maximal contraction was only 16.2 +/- 2.74 g/cm (vs. 35.7 +/- 5.74 g/cm for untreated controls; P less than 0.02). The dose at which maximal contraction was elicited after atropine was 10(-7) mol ia (vs. 1.9 X 10(-9) mol for controls; P less than 0.001). Pretreatment with INDO caused minimal attenuation, and HEX had no effect on the response elicited by ia PAF. We demonstrate a method for assessing the effects of PAF in central airways that avoids tachyphylaxis and permits dose-response studies in the same animal. We also demonstrate that PAF is an extremely potent mediator that elicits tracheal smooth muscle contraction at least in part by postganglionic activation of parasympathetic nerves. A direct contractile effect of PAF which is not related to secretion of products of the cyclooxygenase pathway is also suggested.     

Nicotine potentiates the neurogenic contractile response of rabbit bladder tissue via nicotinic acetylcholine receptors: nitric oxide and prostaglandins have no role in this process

Nicotine, a nicotinic acetylcholine receptors (nAChRs) agonist, has a role in modulation of the neurotransmitter release following nerve stimulation in both the central and peripheral nervous systems. The aim of this study was to determine whether electrical field stimulation (EFS)-evoked contractions are altered in rabbit bladder in the presence of nicotine and, if an alteration occurs, to investigate the effects of nitric oxide and prostaglandins on nicotine-induced alternation in isolated rabbit bladder. EFS-evoked contractile responses from rabbit bladder obtained were recorded with isometric force displacement transducers. Nicotine was added to preparations at various concentrations. The effects of hexamethonium, cadmium (Cd(2+)), indomethacin and N-nitro-L-arginine methyl ester (L-NAME) were tested on the EFS-evoked contractions in the presence of nicotine. Nicotine led to a dose-dependent increase in the amplitude of the EFS-evoked contractile responses. Cd(2+) and hexamethonium inhibited the nicotine-induced increase in EFS-evoked responses, whereas indomethacin and L-NAME had no effect. In conclusion, nicotine increased the EFS-evoked contractile responses possibly by facilitating release of neurotransmitters from nerve terminals by a mechanism dependent on the influx of Ca(2+) from voltage-gated Ca(2+) channels (VGCCs) via activation of nAChRs in isolated rabbit bladder. Nitric oxide and prostaglandins do not have a physiological role in the regulation of neurotransmitter release.     

Substance P and gastrin releasing peptide in bovine mesenteric lymphatic vessels: chemical characterization and action

Alcoholic extracts of bovine mesenteric lymphatic vessels were assayed for the presence of SP, GRP, VIP, PHI, GIP and NT using specific radioimmunoassays. SP and GRP immunoreactivities were detected at concentrations of 190 +/- 20 and 1,000 +/- 130 pg.g-1, respectively. No significant levels of immunoreactivity were detected for any of the other peptides. SP and GRP immunoreactivities coeluted with their synthetic counterparts from both Sephadex G-50 and reversed phase HPLC columns. Synthetic SP (10(-9)-10(-7) M) and the naturally occurring analogue of GRP, bombesin (10(-9)-10(-7) M), increased spontaneous contraction rate in isolated vessel segments. This excitatory effect was not blocked by the alpha-adrenoceptor antagonist phentolamine (3 x 10(-6) M).