In vivo assessment of precursor induced prostaglandin release within the rat gastric lumen.

Gastroprotection associated with the intragastric administration of prostaglandin (PG) precursor fatty acids such as linoleic (LA), gamma-linolenic (GLA), and arachidonic acid (AA) has been reported to be mediated via their conversion to PGs. This report examines the relationship between gastroprotection and the extent/rate of PG-release in rats intragastrically administered PG biosynthetic precursors: LA, AA, dihomo-gamma-linolenic acid (DHGL) or oleic acid (OA, a nonprecursor fatty acid). At various times following intragastric administration of a fatty acid, gastric fluid was collected, extracted, chromatographed, and assayed for PGE1 or PGE2 by specific radioimmunoassay. AA and DHGL dose dependently elevated gastric PGE2 and PGE1 levels, respectively. Maximal PGE elevation, 200-400 ng/stomach, was over 400-fold above basal values, and observed within 5-10 minutes of administration. Conversely, OA and LA elicited only a minor (2-10 fold) stimulation of PGE release. In contrast to effects on PG release, all four fatty acids protected the gastric mucosa against macroscopic damage induced by ethanol. The apparent rank order of potency was AA greater than DHGL = LA greater than OA (the difference in potency between DHGL or LA and OA was not significant). Since LA and OA (a nonprecursor) only marginally elevated lumenal PGs relative to DHGL or AA, yet were equally efficacious in the gastroprotection assay, it is likely that other fatty acid-related mechanisms play an important role in protecting the stomach against ethanol-induced injury.

Gastrointestinal motility stimulating drugs and 5-HT receptors on myenteric neurons.

5-HT3 receptor antagonists may have both antiemetic and gastric and intestinal motility stimulating properties, but they differ in their relative potencies and efficacies for these two activities. Since the 5-HT3 receptor is present on enteric neurons, intracellular recordings of myenteric neuronal transmembrane potential were used to assess the actions of four proposed motility stimulating drugs, metoclopramide, BRL 24924, ICS 205-930 and cisapride. BRL 24924 (10(-6) M), ICS 205-930 (10(-7) M) and cisapride (5 x 10(-6) M) each antagonized the 5-HT3-mediated fast depolarization of myenteric neurons. Metoclopramide (10(-5) M) was less consistent in its ability to antagonize this response, and the response often returned in the continued presence of metoclopramide. In the present study, BRL 24924 (10(-6) M) and, as previously shown, cisapride (5 x 10(-6) M) antagonized the slow depolarization of myenteric neurons induced by 5-HT. Metoclopramide (10(-5) M), BRL 24924 (10(-6) M) and cisapride (5 x 10(-6) M), but not ICS 205-930 (10(-7) M) depolarized myenteric neurons within the first 2 min of contact with myenteric neurons. These data support the view that there are separate receptors that may be responsible for the prokinetic actions of these drugs and a series of 5-HT3-mediated actions which include antiemesis.

Effects of forskolin on electrical behaviour of myenteric neurones in guinea-pig small intestine.

The actions of forskolin on electrical behaviour of myenteric neurones were investigated with intracellular recording methods in guinea-pig small intestine. The actions of forskolin were: membrane depolarization, increased input resistance, suppression of post-spike hyperpolarizing potentials and repetitive spike discharge. These effects occurred always in AH/Type 2 myenteric neurones and never in the cells classified as S/Type 1. Reversal potentials for the depolarizing effects were near the estimated potassium equilibrium potential. Analyses based on the 'constant field equation' indicated that the permeability ratios of K+ to other permeant ionic species were reduced by forskolin. Pretreatment of the neurones with a phosphodiesterase inhibitor potentiated the effects of forskolin. The results suggest that activation of adenylate cyclase by forskolin and subsequent elevation of intraneuronal adenosine 3',5'-phosphate (cyclic AMP) mimic slow synaptic excitation in AH/Type 2 myenteric neurones. They support the possibility that cyclic AMP functions as a second messenger in signal transduction which appears to involve closure of calcium-dependent K+ channels and other membrane changes that lead to depolarization and a dramatic increase in the excitability of the neurones.

Cyclic 3',5'-adenosine monophosphate mimics slow synaptic excitation in myenteric plexus neurons.

Iontophoretic injection of cyclic adenosine monophosphate (cAMP) into the somas of ganglion cells in the myenteric plexus of guinea pig small intestine mimicked the slow excitatory postsynaptic potentials that occur in these neurons. These effects were: (1) membrane depolarization. (2) decreased membrane conductance, (3) augmented excitability with increased probability of spontaneous spike discharge and increased spike discharge during depolarizing current pulses, (4) anodal break excitation, (5) suppression of hyperpolarizing afterpotentials. The same effects were produced by application of membrane-permeable analogs of cAMP in the bathing medium. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, potentiated the effects of the analogs. The results suggest that the biochemical basis for long-lasting excitatory neuromodulation in myenteric neurons is activation of adenylate cyclase and elevation of intraneuronal cAMP.

Effects of cholecystokinin, caerulein and pentagastrin on electrical behavior of myenteric neurons.

Intracellular methods were used to record the electrical behavior of myenteric neurons from guinea-pig ileum in vitro. Cholecystokinin-octapeptide (CCK-8) at 1 to 100 microM in the ejection pipettes were applied to the neurons by pressure microejection. CCK-8 (0.01 to 1 microM) and caerulein (0.01 to 0.1 microM) were applied also in the superfusion solution. CCK-8, applied by either method, evoked a long-lasting depolarization of the cell membranes that was associated with an increase in the input resistance, suppression of hyperpolarizing afterpotentials and enhanced excitability. The enhanced excitability was reflected by increased probability of action potential discharge during membrane depolarization by intracellularly injected current. Caerulein evoked the same excitatory responses as CCK-8. The actions of both agents simulated slow synaptic excitation in the neurons. In another group of AH/type 2 neurons, CCK-8 evoked a long-lasting membrane hyperpolarization and decreased input resistance that mimicked stimulus-evoked slow inhibitory postsynaptic potentials. Hyperpolarizing responses were never evoked by caerulein. About 25% of tested neurons failed to respond to CCK-8 and 70% did not respond to caerulein. Pentagastrin (1 microM in the superfusion solution) did not affect the electrical behavior of the neurons. The results are consistent with a neurotransmitter role for cholecystokinin that could be either excitatory of inhibitory in the enteric nervous system.

Bombesin, gastrin releasing peptide and vasoactive intestinal peptide excite myenteric neurons.

Intracellular recording methods were used to study the actions of bombesin, gastrin releasing peptide and vasoactive intestinal peptide on electrical behavior of AH/Type 2 myenteric neurons in guinea-pig ileum in vitro. Each peptide evoked membrane depolarization associated with increased input resistance, enhanced excitability and suppression of hyperpolarizing after-potentials. The effects of the peptides simulated slow synaptic excitation in the myenteric plexus and are consistent with a neurotransmitter or neuromodulatory function.

Nerve-mediated action of forskolin on guinea pig ileal mucosa.

The effects of forskolin on myenteric neuronal activity and mucosal function were examined in guinea pig ileum. Forskolin increased the excitability of myenteric neurons, and increased mucosal chloride secretion by stimulating enteric neurons as well as by acting directly on enterocytes.

Functional and structural changes in intestinal smooth muscle after jejunoileal bypass in rats.

Active stress and cross-sectional area of intestinal muscle were assessed in tissues taken from unoperated rats, from rats that had undergone bypass of 70% of the small bowel, and from rats that had undergone transection and anastomosis of the bowel. Thirty-five days after operation, muscle from the intestine of transected and bypassed animals elicited active stresses that were equal to or greater than those developed by muscle taken from unoperated animals. The total cross-sectional areas of the in-continuity segment and the area and thickness of the muscle layers of both the in-continuity and bypassed segments were greater when compared with unoperated animals. Significant differences also existed among tissues taken from bypassed and transected animals. Additionally, transection induced increases in active stress, area of muscle in the distal intestine, and circular muscle thickness in the mid- and distal intestine when compared with tissues from unoperated animals. These findings support the hypothesis that intestinal bypass induces increases in functioning smooth muscle tissue.

Effects of methylene blue on electrical behavior of myenteric neurons.

Intracellular recording methods were used to investigate the action of methylene blue on electrical behavior of myenteric neurons in guinea pig small intestine. The neurophysiological studies were done in parallel with studies on contractile activity of the intestinal musculature. Methylene blue depolarized the membranes, increased the input resistance, augmented excitability and reduced postspike hyperpolarizing potentials in AH/Type 2 myenteric neurons. These effects, with the exception of suppression of postspike hyperpolarization, were reversed by exposure to elevated calcium. The mechanism of action of methylene blue appeared to be suppression of calcium-dependent potassium conductance in the neuronal membranes. The neuronal action of methylene blue was manifest as a release of excitatory neurontransmitter substances which evoked contraction of the small intestinal longitudinal muscle.

Interactions between serotonin and cisapride on myenteric neurons.

Intracellular recording methods were used to investigate the interactions between serotonin (5-HT) and cisapride on myenteric neurons of guinea-pig small intestine. Serotonin had three actions on the neurons. One was a slowly rising depolarization associated with increased input resistance and discharge of spikes that lasted six or more times longer than the duration of the 5-HT application. The second action was a transient depolarization associated with decreased input resistance and brief discharge of spikes. This response desensitized quickly and could be evoked only at intervals of 2 to 3 min. The third action of 5-HT was presynaptic inhibition of acetylcholine release at nicotinic synapses. Cisapride reduced or abolished both the prolonged and transient responses to 5-HT. The threshold concentration for reduction of the responses was 0.1 microM and the responses were abolished at 1.0 to 10 microM. Cisapride suppressed stimulus-evoked slow excitatory postsynaptic potentials (EPSPs) in the same cells for which cisapride blocked the prolonged responses to 5-HT. There were no effects of cisapride on resting electrical behavior or spike generation. Cisapride reduced the amplitude of fast cholinergic EPSPs, suggesting that it behaved as an agonist at the presynaptic serotonergic receptors.

Intracellular study of effects of histamine on electrical behaviour of myenteric neurones in guinea-pig small intestine.

The actions of histamine on myenteric neurones were investigated with intracellular recording methods in guinea-pig small intestine. The actions of histamine at the ganglion cell soma were: membrane depolarization, increased input resistance, suppression of post-spike hyperpolarizing potentials, augmented excitability and repetitive spike discharge. Excitability was enhanced also at spike initiation sites remote from the cell body. Both H1, and H2, receptors were involved in the response to histamine. Dimaprit mimicked the responses to histamine in 80% and 2-methylhistamine in 50% of the trials. Cimetidine was an antagonist for histamine in 82% and for dimaprit in all of the trials. Pyrilamine blocked the actions of histamine in 59% of the cells and always blocked the action of 2-methylhistamine. Histamine mimicked slow synaptic excitation in the neurones, but was ruled out as a neurotransmitter for the slow excitatory post-synaptic potential (e.p.s.p.). Histamine either did not affect the responses to 5-hydroxytryptamine, substance P and acetylcholine or it potentiated the responses to these putative neurotransmitters for slow synaptic excitation. The results support the possibility that histamine released from mast cells by circulating peptidergic messengers, by neurotransmitters or during anaphylaxis could influence enteric nervous function.

Histamine action on guinea pig ileal mucosa.

Nerve-mediated and direct actions of histamine on mucosal transport function in the guinea pig ileum were investigated. Addition of histamine to the serosal side of flat sheet preparations in Ussing chambers evoked a transient increase in base-line short-circuit current that was due primarily to an increase in active chloride secretion. The mucosal response to histamine was mimicked by the H1-receptor agonist 2-methylhistamine, but not by the H2-receptor agonist dimaprit. The histamine-evoked response was prevented by the H1-receptor blocker pyrilamine, but not by the H2-receptor antagonist cimetidine. Thirty percent of the mucosal response to histamine was inhibited by tetrodotoxin. Intracellular electrical recording showed that histamine activated AH/type 2 myenteric neurons, and this response was abolished in the presence of pyrilamine. Local anesthetic action of pyrilamine was ruled out by direct electrical recording from myenteric neurons in the presence and absence of pyrilamine. Electrical field stimulation evoked a biphasic increase in short-circuit current. Histamine and 2-methylhistamine did not alter the sustained phase of the short-circuit current response to electrical field stimulation, although pyrilamine reduced the electrically evoked response by 22%. Muscarinic blockade with atropine reduced the stimulus-evoked response by 55%. When muscarinic receptors were blocked and electrical field stimulation applied, histamine increased the stimulus-evoked mucosal response by 22.3%. These results suggest that histamine increases short-circuit current and chloride secretion by acting at H1-receptor sites on both the enteric innervation of the mucosa and on the enterocytes.

Adaptation of intestinal muscle in bypassed loops after jejunoileal bypass in rat.

The function and structure of intestinal smooth muscle in the bypassed intestine of rats with 70% intestinal bypass were compared with the function and structure of muscle from equivalent areas of intestine from transected and nontransected controls. Muscle function was assessed by evaluating changes in intestinal transit. At all times studied after operation, transit in transected controls was identical to that seen in nontransected controls. In the bypassed intestine at 3 days after operation, transit in fasted animals was significantly slower than intestinal transit in either control group. Over a period of 14-35 days, transit in the bypassed intestine of fasted animals returned toward control values. In fed animals, on the other hand, transit was delayed when measured at both 3 and 35 days after bypass operation. These findings demonstrate a persistent change in muscle function in bypassed intestine in response to the ingestion of a meal. No changes in intestinal structure were found when bypassed intestine was compared with tissue from transected controls. Thus there were no indications of mucosal or muscular atrophy in the bypassed intestine. The weights of the combined submucosal, muscular, and serosal layers in the study segments were increased at 35 days after either bypass or transection compared with tissue from nontransected controls. These changes appear to be a nonspecific result of operation.

Adaptation of intestinal muscle in continuity after jejunoileal bypass in the rat.

Functional and structural adaptations of the intestine that remained in continuity after jejunoileal bypass [the in-continuity (IC) segment] were characterized in the rat. Three days after bypass, transit was rapid in fed rats, but, by 35 days, transit had slowed to mimic that seen in the intact intestine of control animals. In fasted rats, transit was as rapid as in the fed animals at 3 days after bypass; however, in fasted rats, transit did not slow when tested up to 35 days after operation. Transit in animals at 3, 14, and 35 days after sham operation was not different from control. Mucosal weights in the proximal and distal portions of the IC segment were increased at 3, 14, and 35 days after bypass. The wet weights and protein contents of the muscle and serosal layers of both portions of the IC segment were increased at 35 days. These findings support the hypothesis that the adaptation seen in the IC segment after bypass involves changes in function and structure of the intestinal smooth muscle as well as the intestinal mucosa.