Szurszewski, Joseph H.

Significance Mechanical forces are important for normal gastrointestinal tract function. The enterochromaffin cells in the gastrointestinal epithelium have been proposed, but not previously shown, to be specialized sensors that convert forces into serotonin release, and serotonin released from these cells is important for normal gastrointestinal secretion and motility. The findings in this study show that some enterochromaffin cells are indeed mechanosensitive, and that they use mechanosensitive Piezo2 channels to generate an ionic current that is critical for the intracellular Ca 2+ increase, serotonin release, and epithelial fluid secretion.

Abstract Background Gastric emptying is a complex physiological process regulating the division of a meal into smaller partitions for the small intestine. Disrupted gastric emptying contributes to digestive disease, yet current measures may not reflect different mechanisms by which the process can be altered. Methods We have developed high temporal resolution solid and liquid gastric emptying breath tests in mice using [ 13 C]‐octanoic acid and off axis‐ integrated cavity output spectroscopy ( OA ‐ ICOS ). Stretched gamma variate and 2‐component stretched gamma variate models fit measured breath excretion data. Key Results These assays detect acceleration and delay using pharmacological (7.5 mg/kg atropine) or physiological (nutrients, cold exposure stress, diabetes) manipulations and remain stable over time. High temporal resolution resolved complex excretion curves with 2 components, which was more prevalent in mice with delayed gastric emptying following streptozotocin‐induced diabetes. There were differences in the gastric emptying of Balb/c vs C57Bl6 mice, with slower gastric emptying and a greater occurrence of two‐phase gastric emptying curves in the latter strain. Gastric emptying of C57Bl6 could be accelerated by halving the meal size, but with no effect on the occurrence of two‐phase gastric emptying curves. A greater proportion of two‐phase gastric emptying was induced in Balb/c mice with the administration of PYY (8‐80 nmol) 60 min following meal ingestion. Conclusions and Inferences Collectively, these results demonstrate the utility of high temporal resolution gastric emptying assays. Two‐phase gastric emptying is more prevalent than previously reported, likely involves intestinal feedback, but contributes little to the overall rate of gastric emptying.

Methods developed previously for studying the effect of cerebroventricular injection or ventriculocisternal perfusion of test substances are unsatisfactory because the test substance is not confined to the central compartment. Most likely the test substance enters the peripheral circulation via the arachnoid villi. The purpose of this paper is to describe a method for perfusing the cerebroventricular system of conscious dogs without passage of test substances to the peripheral circulation. With the method described, the mean (+/- SE) cerebroventricular pressure in conscious dogs was 7.4 +/- 0.8 cmH2O (n = 16), and the mean (+/- SE) production of cerebrospinal fluid (CSF) was 25 +/- 0.3 microliter/min (n = 16). Endogenously occurring migrating myoelectric complexes (MMCs) of the small intestine were recorded in dogs before catheters were implanted in the left and right lateral ventricles and the fourth ventricle and after catheter implantation during cerebroventricular perfusion with artificial CSF alone or with CSF containing sulfated (S-CCK-OP) or nonsulfated cholecystokinin octapeptide (NS-CCK-OP). Only cerebroventricular perfusion with S-CCK-OP (1.2 pmol.kg-1.min-1; n = 20) replaced spontaneously occurring MMCs with a fed-like pattern of myoelectric activity. The results suggest that replacement of the fasting pattern of myoelectric activity with a fed-like pattern in the fasted dog was mediated by CCK-A receptors located in one or more brain nuclei surrounding the third ventricle.

In mammalian peripheral sympathetic ganglia GABA acts presynaptically to facilitate cholinergic transmission and postsynaptically to depolarize membrane potential. The GABA effect on parasympathetic pancreatic ganglia is unknown. We aimed to determine the effect of locally applied GABA on cat pancreatic ganglion neurons. Ganglia with attached nerve trunks were isolated from cat pancreata. Conventional intracellular recording techniques were used to record electrical responses from ganglion neurons. GABA pressure microejection depolarized membrane potential with an amplitude of 17.4 ± 0.7 mV. Electrically evoked fast excitatory postsynaptic potentials were significantly inhibited (5.4 ± 0.3 to 2.9 ± 0.2 mV) after GABA application. GABA-evoked depolarizations were mimicked by the GABAAreceptor agonist muscimol and abolished by the GABAAreceptor antagonist bicuculline and the Cl−channel blocker picrotoxin. GABA was taken up and stored in ganglia during preincubation with 1 mM GABA; β-aminobutyric acid application after GABA loading significantly ( P < 0.05) increased depolarizing response to GABA (15.6 ± 1.0 vs. 7.8 ± 0.8 mV without GABA preincubation). Immunolabeling with antibodies to GABA, glial cell fibrillary acidic protein, protein gene product 9.5, and glutamic acid decarboxylase (GAD) immunoreactivity showed that GABA was present in glial cells, but not in neurons, and that glial cells did not contain GAD, whereas islet cells did. The data suggest that endogenous GABA released from ganglionic glial cells acts on pancreatic ganglion neurons through GABAAreceptors.

Intracellular recordings were made from the circular smooth muscle cells of the canine jejunum to study the effect of exogenous ATP and to compare the ATP response to the nonadrenergic, noncholinergic (NANC) inhibitory junction potential (IJP) evoked by electrical field stimulation (EFS). Under NANC conditions, exogenous ATP evoked a transient hyperpolarization (6.5 ± 0.6 mV) and EFS evoked a NANC IJP (17 ± 0.4 mV). ω-Conotoxin GVIA (100 nM) and a low-Ca2+, high-Mg2+solution abolished the NANC IJP but had no effect on the ATP-evoked hyperpolarization. The ATP-evoked hyperpolarization and the NANC IJP were abolished by apamin (1 μM) and NG-nitro-l-arginine (100 μM). Oxyhemoglobin (5 μM) partially (38.8 ± 5.5%) reduced the amplitude of the NANC IJP but had no effect on the ATP-evoked hyperpolarization. Neither the NANC IJP nor the ATP-evoked hyperpolarization was affected by P2 receptor antagonists or agonists, including suramin, reactive blue 2, 1-( N, O-bis-[5-isoquinolinesulfonyl]- N-methyl-l-tyrosyl)-4-phenylpiperazine, pyridoxal phosphate-6-azophenyl-2′,4′-disulfonic acid, α,β-methylene ATP, 2-methylthioadenosine 5′-triphosphate tetrasodium salt, and adenosine 5′- O-2-thiodiphosphate. The data suggest that ATP evoked an apamin-sensitive hyperpolarization in circular smooth muscle cells of the canine jejunum via local production of NO in a postsynaptic target cell.

Effects of nitric oxide (NO) generated from sodium nitroprusside (SNP) on neurons of mouse superior mesenteric ganglion (SMG) were studied in vitro using intracellular recording techniques. SNP solutions caused a membrane hyperpolarization in the majority (64%) of the neurons tested or a hyperpolarization followed by a depolarization in 8% of the neurons tested. The SNP-induced hyperpolarization persisted in a low-Ca2+ high-Mg2+ solution, indicating a direct effect of NO on the postsynaptic membrane. The hyperpolarizing effect of SNP was reduced or abolished by oxyhemoglobin. Electrical stimulation of the colonic nerves evoked a late slow excitatory postsynaptic potential (late sEPSP) in a population of neurons in normal Krebs solution. The amplitude of the late sEPSP was significantly enhanced in the presence of NG-nitroL-arginine, a NO synthase inhibitor. The results, particularly those observed with the NO synthase inhibitor, suggest that endogenous NO was released in the mouse SMG by repetitive nerve stimulation and that it modulated slow synaptic transmission, presumably by a direct action on ganglionic neurons.

Previous studies have demonstrated the presence of cholecystokinin (CCK)-like peptides in nerve terminals surrounding ganglion neurons of the cat pancreas. The present study was undertaken to determine the effect of cholecystokinin octapeptide (CCK-8) on ganglionic transmission. Recordings were made intracellularly in vitro from ganglion neurons in isolated pieces of the pancreas. Sulfated CCK-8 (S-CCK-8) and nonsulfated CCK-8 initiated or increased ongoing fast excitatory postsynaptic potential (fEPSP) activity, an effect antagonized by hexamethonium. Superfusion of S-CCK-8 in concentrations ranging from 10(-11) to 10(-8) M significantly augmented the amplitude of nerve-evoked subthreshold fEPSPs without a significant change in either membrane potential or membrane input resistance. S-CCK-8 (10(-8)M) also increased the quantal content and quantal size of nerve-evoked fEPSPs and increased the response to exogenously applied acetylcholine (ACh). Concentrations of S-CCK-8 higher than 10(-8)M caused depolarization and an increase in membrane input resistance, an effect unaltered by a low-Ca+, high-Mg2+ solution. It was concluded that S-CCK-8 potentiated nicotinic transmission by facilitating release of ACh from preganglionic nerve terminals and by increasing the postsynaptic membrane sensitivity to ACh.

The neurotransmitter(s) that generates the fast component of the inhibitory junction potential (IJP-F) in human jejunal circular smooth muscle is not known. The aim of this study was to determine the role of ATP and purinergic receptors in the generation of the IJP-F in human jejunal circular smooth muscle strips. The P2-receptor antagonist suramin (100 μM) reduced the IJP-F by 28%. Apamin (1 μM) reduced the IJP-F by 25%. Desensitization of muscle strips with the putative P2x-receptor agonist α,β-methylene ATP (α,β-MeATP, 100 μM) decreased the IJP-F by 44%, and desensitization with the putative P2y-receptor agonist adenosine 5′- O-2-thiodiphosphate (ADPβS) completely abolished the IJP-F. Desensitization with the putative P2y-receptor agonist 2-methylthioATP had no effect on the IJP-F. Exogenous ATP evoked a hyperpolarization with a time course that matched the IJP-F. The ATP-evoked hyperpolarization was reduced by apamin and suramin, reduced by desensitization with α,β-MeATP (69% decrease), and abolished by desensitization with ADPβS. These data suggest that the IJP-F in human jejunal circular smooth muscle is mediated in part by ATP through an ADPβS-sensitive P2receptor.

Carbon monoxide (CO) has been postulated to be a messenger in the gastrointestinal tract. The aims of this study were to determine the distribution of heme oxygenase (HO), the source for endogenous CO in the canine jejunum, and to determine the effects of CO on jejunal circular smooth muscle cells. HO-2 isoform was present in a population of myenteric and submucosal neuronal cell bodies, in nerve fibers innervating the muscle layers, and in smooth muscle cells. HO-1 isozyme was not detected in the canine jejunum. Exogenous CO increased whole cell current by 285 ± 86%, hyperpolarized the membrane potential by 8.5 ± 2.9 mV, and increased guanosine 3′,5′-cyclic monophosphate (cGMP) levels in smooth muscle cells. 8-Bromo- cGMP also increased the whole cell current. The data suggest that endogenous activity of HO-2 may be a source of CO in the canine jejunum and that exogenously applied CO can modulate intestinal smooth muscle electrical activity. It is therefore reasonable to suggest a role for endogenously produced CO as a messenger in the canine jejunum.

This contribution to the centennial commemorative issue of the American Journal of Physiology: Gastrointestinal and Liver Physiology identifies some of the important studies of spontaneous electrical and motor activity in the gastrointestinal tract published in the Journal between 1898 and 1996. Emphasis is given to the contributions made by Walter B. Cannon, Walter C. Alvarez, Emil Bozler, C. Ladd Prosser, and James Christensen.