Defining comparative physiology: results from an online survey and systematic review

August Krogh's 1929 principle is referenced as the cornerstone of comparative physiology (CP). However, there are diverse views as to what type of research falls under the CP approach. This study had three aims: 1) determine how CP is defined through an online survey (OS) of physiologists and a systematic review (SR), 2) put forth an updated definition of CP by summarizing OS and SR results, and 3) outline the numerous CP research approaches. Professional physiology societies (n = 54) were invited to share the OS with their members, and a SR was conducted, which yielded 197 and 70 definitions, respectively. The three most common words in descending order in the OS definitions were "different," "animals," and "species" and in the SR definitions, "animals," "species," and "organisms." The three most prevalent themes from the OS and SR definitions were comparing/differences/diversity across species (78% and 51%, respectively), response to the environment/ecology (28% and 43%, respectively), and included evolution or adaptation (24% and 60%, respectively). Ten research approaches were identified, which include broad comparison (i.e., many species generalization), specific comparison (e.g., 2 species; for traits that are different, exaggerated, extreme, missing, or not induced), or comparison while considering evolution (i.e., evolutionary physiology), ecology (i.e., ecophysiology), or human physiology/medicine. Only 5% and 33% of OS and SR definitions described or mentioned Krogh's principle. In conclusion, CP can best be defined as a compilation of research approaches that utilize different types of comparisons to elucidate physiological mechanisms and not simply comparing physiologies as the name implies.

Hibernation induces glutathione redox imbalance in ground squirrel intestine

Glutathione (GSH) is the major thiol-disulfide redox buffer in cells and is a critical component of antioxidant defense. Here we examined GSH redox balance in the intestinal mucosa during the annual cycle of 13-lined ground squirrels (Spermophilus tridecemlineatus). The ratio of reduced GSH to its oxidized form (glutathione disulfide, GSSG), which is an index of oxidative stress, was five-fold lower in hibernating compared with summer-active squirrels, an effect due primarily to elevated GSSG concentration in hibernators. During hibernation the total pool of GSH equivalents was lowest in squirrels undergoing arousal and highest in squirrels during interbout arousals. Hibernation decreased intestinal GSSG reductase activity by approximately 50%, but had no effect on activities of glutathione peroxidase or glucose-6-phosphate dehydrogenase. Within the hibernation season, expression of the stress protein HSP70 in intestinal mucosa was highest in squirrels entering torpor and early in a torpor bout, and lowest in squirrels arousing from torpor and during interbout euthermia. The results suggest that hibernation in ground squirrels is associated with a shift in intestinal GSH redox balance to a more oxidized state. Higher levels of HSP70 during the early phases of torpor may reflect induction of the stress response due to aberrations in protein folding or may be a mechanism to increase enterocyte tolerance to subsequent stress imposed by extended torpor or the arousal process.

D-glucose releases 5-hydroxytryptamine from human BON cells as a model of enterochromaffin cells

BACKGROUND & AIMS

5-Hydroxytryptamine (5-HT) is released from enterochromaffin cells and activates neural reflex programs regulating motility and secretion. Although sugars are reported to release 5-HT in vivo, it is unclear whether they act directly on enterochromaffin cells or indirectly through an intermediary messenger. The aim was to determine if D-glucose is a stimulus for 5-HT release.

METHODS

Human BON cells, derived from enterochromaffin cells, were treated with D-glucose, galactose, and the nonmetabolizable methyl alpha-D-glucopyranoside, or with fructose.

RESULTS

Reverse-transcription polymerase chain reaction together with Western blot analysis revealed an SGLT-like protein. D-glucose caused a concentration-dependent increase in 5-HT release, which was mimicked by methyl alpha-D-glucopyranoside and galactose but not fructose. D-glucose-stimulated 5-HT release was significantly reduced by phloridzin. Concentrations of mannitol below 75 mmol/L were ineffective in releasing 5-HT. Brefeldin A abolished forskolin-stimulated 5-HT release without affecting basal or constitutive release.

CONCLUSIONS

The results show that high concentrations of metabolizable and nonmetabolizable hexoses activate signal transduction pathways, leading to release of 5-HT. These findings imply a role for enterochromaffin cells as "glucose sensors" during ingestion of a meal.

Reactive nitrogen species inhibit alveolar epithelial fluid transport after hemorrhagic shock in rats

Our recent experimental work demonstrated that a neutrophil-dependent inflammatory response in the lung prevented the normal up-regulation of alveolar fluid clearance by catecholamines following hemorrhagic shock. In this study, we tested the hypothesis that the release of NO within the airspaces of the lung was responsible for the shock-mediated failure of the alveolar epithelium to respond to catecholamines in rats. Hemorrhagic shock was associated with an inducible NO synthase (iNOS)-dependent increase in the lung production of NO and a failure of the alveolar epithelium to up-regulate vectorial fluid transport in response to beta-adrenergic agonists. Inhibition of iNOS restored the normal catecholamine-mediated up-regulation of alveolar liquid clearance. Airspace instillation of dibutyryl cAMP, a stable analog of cAMP, restored the normal fluid transport capacity of the alveolar epithelium after prolonged hemorrhagic shock, whereas direct stimulation of adenyl cyclase by forskolin had no effect. Pretreatment with pyrrolidine dithiocarbamate or sulfasalazine attenuated the iNOS-dependent production of NO in the lung and restored the normal up-regulation of alveolar fluid clearance by catecholamines after prolonged hemorrhagic shock. Based on in vitro studies with an alveolar epithelial cell line, A549 cells, the effect of sulfasalazine appeared to be mediated in part by inhibition of NF-kappaB activation, and the protective effect was mediated by the inhibition of IkappaBalpha protein degradation. In summary, these results provide the first in vivo evidence that NO, released within the airspaces of the lung probably secondary to the NF-kappaB-dependent activation of iNOS, is a major proximal inflammatory mediator that limits the rate of alveolar epithelial transport after prolonged hemorrhagic shock by directly impairing the function of membrane proteins involved in the beta-adrenergic receptor-cAMP signaling pathway in alveolar epithelium.

Intestinal transport during fasting and malnutrition

Fasting or malnutrition (FM) has dramatic effects on small intestinal mucosal structure and transport function. Intestinal secretion of ions and fluid is increased by FM both under basal conditions and in response to secretory agonists. Intestinal permeability to ions and macromolecules may also be elevated by FM, which increases the potential for fluid and electrolyte losses and for anaphylactic responses to luminal antigens. Mucosal atrophy induced by FM reduces total intestinal absorption of nutrients, but nutrient absorption normalized to mucosal mass may actually be enhanced by a variety of mechanisms, including increased transporter gene expression, electrochemical gradients, and ratio of mature to immature cells. These observations underscore the value of enteral feeding during health and disease.

Involvement of PI 3-kinase in IGF-I stimulation of jejunal Na+-K+-ATPase activity and nutrient absorption

Mechanisms responsible for increased jejunal transport rates observed in tissues treated with orally administered insulin-like growth factor-I (IGF-I) were studied in 5-day-old colostrum-deprived piglets. Human recombinant IGF-I (3.5 mg. kg(-1). day(-1)) or control vehicle was given orogastrically for 4 days. Disaccharidase activity, fructose uptake, and Na+-glucose cotransporter SGLT-1 protein abundance were similar between groups. Oral IGF-I produced greater rates of enterocyte Na+-K+-ATPase activity with no significant differences in Na+-K+-ATPase abundance. Cellular mechanisms responsible for transport changes were studied in Ussing chambers. In control tissues, the presence of IGF-I in mucosal solutions increased basal short-circuit current (I(sc)), potential difference, D-glucose-stimulated I(sc), and Na+-K+-ATPase activity; these changes were abolished by preincubation of tissues with wortmannin, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor. The results suggest that the effect of IGF-I on jejunal ion and nutrient transport involves activation of PI 3-kinase and stimulation of Na+-K+-ATPase activity in enterocytes.

Hibernation induces oxidative stress and activation of NK-kappaB in ground squirrel intestine

Dramatic changes in blood flow occur during torpor-arousal cycles in mammalian hibernators that could increase the risk of oxidative stress to sensitive tissues. We used 13-lined ground squirrels (Spermophilus tridecemlineatus) to determine the effect of hibernation on lipid peroxidation and expression of stress-activated signaling pathways in the intestine, a tissue highly susceptible to ischemia-reperfusion injury. Compared with summer-active squirrels, levels of the mitochondrial stress protein GRP75 were consistently higher in intestinal mucosa of hibernators in each of five hibernation states (entrance, short-bout torpid, long-bout torpid, arousal and interbout euthermia). The redox-sensitive transcription factor, nuclear factor-kappaB (NF-kappaB), was strongly activated in each hibernation state compared with summer squirrels except for squirrels during an arousal from torpor. In contrast, NF-kappaB activation in brown adipose tissue (BAT) was low in active and hibernating squirrels regardless of season. Levels of conjugated dienes (products of lipid peroxidation) were higher in intestine of hibernators entering torpor and early in a torpor bout compared with summer squirrels. Conjugated diene levels were also higher in short-bout torpid vs arousing squirrels. The results suggest that the intestinal mucosa is vulnerable to oxidative stress during the hibernation season and in response may activate cellular defense pathways that help minimize severe oxidative damage induced by torpor-arousal cycles.

Enterotrophic effect of insulin-like growth factor-I but not growth hormone and localized expression of insulin-like growth factor-I, insulin-like growth factor binding protein-3 and -5 mRNAs in jejunum of parenterally fed rats

BACKGROUND

Administration of insulin-like growth factor (IGF)-I, but not growth hormone (GH), stimulates mucosal hyperplasia in surgically stressed rats with intestinal atrophy induced by hypocaloric total parenteral nutrition (TPN). Our aim was to characterize the basis for this disparity in enterotrophic action by assessing the relationships between stimulation of intestinal growth, nutritional adequacy, and localization of expression of IGF-I, insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5 mRNAs in jejunum.

METHODS

Rats were maintained with TPN for 8 days and treated with IGF-I or GH and adequate nutrition for 5 days after recovery from surgery. Jejunal mass, morphology, and sucrase activity were assessed. Localization of expression of IGF-I, IGFBP-3, and IGFBP-5 mRNAs in jejunum was accomplished by in situ hybridization.

RESULTS

Serum IGF-I and body weight gain were significantly increased by IGF-I or GH. Jejunal mucosal dry mass, morphology, and sucrase activity were improved with IGF-I but not GH. There were no differences in IGF-I mRNA. IGFBP-3 mRNA was localized in the lamina propria of the villi. IGF-I or GH stimulated IGFBP-3 expression. IGF-I strongly stimulated IGFBP-5 expression in the lamina propria and the muscularis and induced a twofold increase in IGFBP-5 mRNA based on RNase protection assay of intact jejunum total RNA. GH induced a modest increase in IGFBP-5 expression in the muscularis with no effect on intact jejunum total RNA.

CONCLUSIONS

The GH resistance observed in the jejunal mucosa of TPN rats cannot be fully explained by inadequate nutrition. The expression of IGFBP-5 in the lamina propria suggests it may modulate the enterotrophic action of exogeneous IGF-I.

Neural control of intestinal ion transport and paracellular permeability is altered by nutritional status

This study examined the effect of fasting on the neural control of ion transport and paracellular permeability in piglet jejunum. Muscle-stripped tissues from fed or 48-h fasted piglets were mounted in Ussing chambers. Neural blockade with tetrodotoxin (TTX) or antagonists of muscarinic or nicotinic receptors caused reductions in basal short-circuit current that were approximately threefold greater in fasted piglets. The TTX-induced reduction in short-circuit current in fasted piglets was due to a decrease in residual ion flux and was abolished in the absence of HCO(-)(3). Intestinal paracellular permeability, as indicated by tissue conductance (G(t)) and fluxes of inulin and mannitol, was significantly increased by fasting. TTX increased inulin flux and G(t) in fed but not fasted piglets. In fasted piglets, carbachol reduced G(t) by 29% and mannitol flux by 27% but had no effect on these parameters in the fed state. We conclude that fasting enhances enteric neural control of basal ion transport and increases paracellular permeability in piglet jejunum. Tonic release of enteric neurotransmitters regulates paracellular permeability in the fed state, and cholinergic stimulation restores fasting-induced elevations in paracellular permeability to fed levels.

Differential jejunal and colonic adaptation due to resection and IGF-I in parenterally fed rats

Patients with severe short-bowel syndrome (SBS) often require long-term total parenteral nutrition (TPN) to maintain their nutritional status because of limited intestinal adaptation. Growth factors, including insulin-like growth factor I (IGF-I), are under investigation to promote intestinal adaptation and tolerance to oral feeding. We investigated structural and functional adaptation of the jejunum and colon in four groups of rats maintained with TPN for 7 days after a 60% jejunoileal resection and cecectomy or sham surgery and treatment with IGF-I or vehicle. Resection alone did not stimulate jejunal growth. IGF-I significantly increased jejunal mucosal mass, enterocyte proliferation, and migration rates. IGF-I decreased jejunal sucrase specific activity and reduced active ion transport and ionic permeability; resection alone had no effect. In contrast, resection significantly increased colonic mass and crypt depth but had no effect on active ion transport or ionic permeability. IGF-I had minimal effects on colonic structure. IGF-I but not resection stimulates jejunal adaptation, whereas resection but not IGF-I stimulates colonic growth in rats subjected to a model for human SBS. IGF-I treatment may improve intestinal adaptation in humans with SBS.

Oral IGF-I enhances nutrient and electrolyte absorption in neonatal piglet intestine

The effect of orally administered insulin-like growth factor-I (IGF-I) on small intestinal structure and function was studied in 5-day-old colostrum-deprived piglets. Human recombinant IGF-I (3.5 mg. kg(-1). day(-1)) or control vehicle was given orogastrically for 4 days. Body weights, jejunal and ileal mucosa wet and dry weights, and serum IGF-I levels were similar in the two groups. Small intestinal villus height and crypt depth and jejunal enterocyte microvillar dimensions were also similar between groups. Oral IGF-I produced higher rates of jejunal ion transport because of increased basal Na+ absorption. Short-circuit current responses to mucosal addition of D-glucose and L-alanine and net transepithelial absorption of 3-O-methylglucose were increased by IGF-I. Carrier-mediated uptake of D-glucose per milligram in everted jejunal sleeves was greater in IGF-I-treated piglets because of a significantly greater maximal rate of uptake. We conclude that rates of net Na+ and Na+-dependent nutrient absorption are enhanced in piglets treated with oral IGF-I, and this effect is independent of changes in mucosal mass or surface area.

Psyllium improves fecal consistency and prevents enhanced secretory responses in jejunal tissues of piglets infected with ETEC

Infection with enterotoxigenic E. coli (ETEC) induces secretory diarrhea by stimulating net secretion of fluid and electrolytes. We tested the hypothesis that ETEC potentiates jejunal ion secretion induced by other agonists and also examined whether the soluble fiber psyllium ameliorates effects of ETEC-induced pathophysiology. Noninfected or ETEC-infected piglets were given oral electrolyte solution twice daily or electrolyte solution supplemented with psyllium for 48 hr. Jejunal tissues were mounted in flux chambers and basal and stimulated ion transport responses, as reflected by short-circuit current (I(SC)) were measured. The severity of ETEC-induced diarrhea was reduced by psyllium. I(SC) responses to carbachol and 5-hydroxytryptamine were greater in tissues from infected piglets compared with noninfected controls or infected piglets given psyllium. These results suggest that psyllium ameliorates ETEC-induced diarrhea and prevents the enhanced secretory responses to calcium-mediated agonists that occur in ETEC-infected piglet jejunum.

Hibernation induces expression of moesin in intestinal epithelial cells

Identification of proteins that are differentially expressed in mammals that hibernate can provide insight into mechanisms that preserve cellular function at low temperatures. A candidate protein was identified in intestinal brush border membranes of 13-lined ground squirrels. Intestinal brush border membrane proteins were separated using SDS-PAGE and gels were stained with Coomassie blue. We observed a approximately 75-kDa band that was specifically increased in brush border membranes isolated from torpid squirrels compared with summer active squirrels. The 75-kDa band was cut from one-dimensional gels and sequenced. A 17 amino acid sequence was identified of which amino acids 2-17 matched exactly a portion of moesin, a membrane-cytoskeletal linking protein and member of the ERM (ezrin/radixin/moesin) family. The sequence results were confirmed using anti-moesin antibodies that detected strong bands at approximately 75 kDa on Western blots of brush border membranes in torpid squirrels (Tb approximately 7 degreesC) and only faint signals in summer squirrels (Tb approximately 37 degrees C) or aroused hibernators (Tb approximately 37 degrees C). In contrast, signals obtained using anti-ezrin antibodies were uniformly strong in all squirrels, regardless of activity state. Intestinal brush borders of mice and rats expressed ezrin but not moesin. These results provide evidence for the physiological induction of an ERM protein in intestinal epithelial cells of torpid hibernators and support the idea that hibernation involves differential expression of gene products that may facilitate viability of cells at low temperatures.

Reduced susceptibility of mice overexpressing transforming growth factor alpha to dextran sodium sulphate induced colitis

BACKGROUND

Transforming growth factor alpha (TGF-alpha) knockout mice have increased susceptibility to dextran sodium sulphate (DSS) induced colitis.

AIM

To substantiate the findings that TGF-alpha is a key mediator of colonic mucosal protection and/or repair mechanisms by evaluating the susceptibility of mice overexpressing TGF-alpha to DSS induced colitis.

METHODS

TGF-alpha overexpression was induced in transgenic mice by ZnSO4 administration in drinking water (TG+). Three groups were used as controls: one transgenic group without ZnSO4 administration (TG-), and two non-transgenic littermate groups receiving ZnSO4 (Non-TG+) or only water (Non-TG-). Acute colitis was induced in all groups by administration of DSS (5%, w/v) in drinking water for six days and libitum.

RESULTS

About 35-39% of the entire colonic mucosa was destroyed in Non-TG-, Non-TG+, and TG- animals compared with 9% in TG+ mice. the crypt damage score was 18.7 (0.9), 18.2 (1.0), 18.9 (0.8), and 6.8 (1.5) (means (SEM)) in Non-TG-, Non-TG+, TG-, and TG+ mice respectively. Mucin and bromodeoxyuridine staining were markedly enhanced in colons of TG+ mice compared with controls, indicating increased mucosal protection and regeneration.

CONCLUSIONS

The significantly reduced susceptibility of mice overexpressing TGF-alpha to DSS further substantiates that endogenous TGF-alpha is a pivotal mediator of protection and/or healing mechanisms in the colon.

Muscarinic inhibition of substance P induced ion secretion in piglet jejunum

We examined the effects of the muscarinic agonist carbachol on ion secretion induced by substance P (SP) in piglet jejunal tissues mounted in Ussing chambers. Tetrodotoxin was present in all solutions to inhibit neural activity. Carbachol added 10 min prior to 0.75 microM SP dose dependently inhibited subsequent SP responses, with 90% inhibition at 10 microM carbachol. Addition of an equipotent dose of SP (7.5 microM) had no effect on subsequent carbachol-induced secretion. Carbachol's inhibition of SP-induced secretion was evident for at least 45 min and was abolished by prior addition of the M3 receptor antagonist 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP), but remained intact in the presence of the M2 antagonist gallamine or the nicotinic antagonist mecamylamine. Atropine added 10 min after carbachol restored subsequent SP responses toward control levels. Carbachol also reduced secretory responses to histamine and, to a lesser extent, prostaglandin E2 (PGE2). SP-induced secretion was not affected by prior addition of histamine and was reduced by PGE2 only at the highest PGE2 concentration. The results suggest that activation of the epithelial M3 receptor by carbachol inhibits subsequent secretory responses to the calcium-mediated agonists SP and histamine in piglet jejunum. This may reflect muscarinic activation of a negative messenger in epithelial cells that limits Cl- secretion.

Stimulation of intestinal growth is associated with increased insulin-like growth factor-binding protein 5 mRNA in the jejunal mucosa of insulin-like growth factor-I-treated parenterally fed rats

Surgically stressed rats maintained with total parenteral nutrition (TPN) exhibit jejunal atrophy, which can be attenuated by insulin-like growth factor-I (IGF-I) but not by growth hormone (GH) treatment. In order to understand the basis for the selective action of IGF-I, the levels of mRNAs encoding IGF-I, IGF-binding proteins (IGFBPs), IGF-I receptor, and GH receptor/binding protein (GHR/GHBP) were determined in rats given TPN and treated with GH, IGF-I, or GH + IGF-I. GH treatment significantly stimulated hepatic IGF-I mRNA. IGF-I treatment did not alter liver IGF-I mRNA, nor was there any evidence for interaction between GH and IGF-I. Jejunal mucosa IGF-I mRNA was extremely low and was not altered by TPN or by any of the hormonal treatments. The inability of GH to stimulate jejunal growth was not associated with a deficiency in GHR/GHBP mRNA. In jejunal mucosa, IGF-I and GH treatment independently and synergistically stimulated IGFBP-3 mRNA. IGF-I stimulated jejunal IGFBP-5 mRNA, but GH had no effect on IGFBP-5 mRNA. The levels of IGF-I receptor and IGFBP-1, 2, 4, and 6 mRNAs were extremely low and/or were not altered by any of the treatments. These results suggest that the ability of exogenous IGF-I, but not GH, to induce IGFBP-5 mRNA in jejunal mucosa may lead to the selective growth-promoting effect of IGF-I. Jejunal mucosa IGFBP-3 mRNA levels were not correlated with altered growth. We postulate that IGFBP-5 positively modulates the anabolic effects induced by exogenous IGF-I in the jejunum.

GH elevates serum IGF-I levels but does not alter mucosal atrophy in parenterally fed rats

Growth hormone (GH) action is primarily mediated by insulin-like growth factor I (IGF-I), although both growth factors show tissue-selective effects. We investigated the effects of GH, IGF-I, and GH plus IGF-I on jejunal growth and function in rats maintained with total parenteral nutrition (TPN) and given recombinant human GH (rhGH) (400 micrograms/day sc, twice daily) and/or rhIGF-I (800 micrograms/day in TPN solution) for 5 days. Administration of GH or IGF-I alone produced similar increases in serum IGF-I levels and body weight; GH plus IGF-I further increased these parameters. TPN reduced mucosal mass, protein and DNA content, villus height, crypt depth, and enterocyte migration rate. IGF-I or GH plus IGF-I produced equivalent increases in all intestinal growth parameters; GH alone had no effect. GH, IGF-I, or GH plus IGF-I reduced TPN-induced increases in sucrase-specific activity. IGF-I, but not GH, attenuated TPN-induced increases in tissue conductance and carbachol-stimulated ion secretion. In contrast to IGF-I, GH does not stimulate intestinal growth during TPN and has less effect on normalizing TPN-induced changes in epithelial function.

Beneficial effects of insulin-like growth factor I on epithelial structure and function in parenterally fed rat jejunum

BACKGROUND & AIMS

The functional significance of intestinal hyperplasia stimulated by insulin-like growth factor (IGF)-I is unclear and has not been studied in a model of mucosal atrophy induced by total parenteral nutrition (TPN). The aim of this study was to determine how IGF-I affects intestinal structure and epithelial function in the absence of luminal nutrition caused by TPN.

METHODS

Rats were maintained with TPN with or without IGF-I (800 micrograms/day), and jejunal histology and epithelial ion transport were measured after 5 days. In a third TPN group without IGF-I, a short-term dose of IGF-I was added during in vitro flux chamber experiments.

RESULTS

Rats given TPN with IGF-I had greater jejunal mucosal weight, greater protein and DNA content, and increased villus height and crypt depth compared with rats given TPN only. TPN increased ionic permeability and ion transport responses to secretory and absorptive agents. IGF-I in vivo reversed most of these changes; IGF-I in vitro enhanced sodium-dependent glucose absorption but had no other effects.

CONCLUSIONS

Coinfusion of recombinant human IGF-I with TPN solution stimulates intestinal hyperplasia and attenuates transport changes induced by TPN. The latter effect seems to be primarily associated with the growth state of the epithelium.

Preservation of intestinal gene expression during hibernation

The role of luminal nutrients in regulating enterocyte gene expression was studied in a natural model for long-term fasting, the hibernating ground squirrel. Squirrels were studied during the active season and during the hibernation season when they had not eaten for at least 12 wk. The specific activities of sucrase, isomaltase, and intestinal alkaline phosphatase in jejunal brush-border membranes were similar in hibernating and active squirrels, whereas amino-oligopeptidase was reduced in hibernators. Na(+)-K(+)-adenosinetriphosphatase activity in jejunal mucosa was unchanged by hibernation. Densitometric analysis of Western blots showed that abundance of sucrase-isomaltase (SI), amino-oligopeptidase, and the Na(+)-glucose cotransporter SGLT1 was similar in the two activity states. Preservation of SI abundance in hibernation was confirmed by immunocytochemistry. Slot-blot analysis revealed no differences in mRNA levels for these proteins between hibernating and active squirrels. Enterocyte proliferation and migration rates were greatly suppressed in torpid squirrels but increased immediately upon rewarming during arousals. These results demonstrate the striking constancy of enterocyte gene expression despite long-term fasting in a hibernating mammal.

Cellular localization of cystic fibrosis transmembrane regulator protein in piglet and mouse intestine

Antibodies raised against the cystic fibrosis transmembrane regulator protein (CFTR) were used to localize CFTR in intestinal tissues of piglets and mice. Positive staining for CFTR was detected in goblet cells of both species. A second population of epithelial cells of unknown phenotype was also labeled by anti-CFTR antibodies. The labeling pattern was abolished by preincubation of anti-CFTR antibodies with the immunogen or when non-immune IgG was used in place of anti-CFTR antibodies. These results support other studies that suggest that alterations in goblet cell function may be involved in the intestinal abnormalities associated with cystic fibrosis.

Role of prostaglandins and enteric nerves in Escherichia coli heat-stable enterotoxin (STa)-induced intestinal secretion in pigs

OBJECTIVE

To examine the role of prostaglandins and enteric nerves in mediating intestinal secretion induced by enterotoxigenic Escherichia coli heat-stable enterotoxin (STa) in pig jejunum and distal portion of the colon.

ANIMALS

Two- to 3-week-old suckling crossbred pigs were studied.

DESIGN

Changes in ion transport in response to mucosal addition of E coli STa to jejunal and colonic tissues were studied in the presence and absence of inhibitors.

PROCEDURE

Flat sheets of muscle-stripped proximal portions of the jejunum and distal portions of the colon were mounted in Ussing chambers equipped to measure changes in short-circuit current (Isc), a measure of active ion transport. Tissues were exposed to 200 ng of STa/ml administered to mucosal solutions, and subsequent changes in Isc were recorded.

RESULTS

In control tissues, changes in Isc induced by STa in the distal colon were significantly greater (21.4 +/- 4 muA/cm2) than those observed in the jejunum (14.0 +/- 2 muA/cm2). When either segment was exposed to the neurotoxin, tetrodotoxin, or to the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine-methyl ester, Isc responses to STa were unchanged, suggesting no involvement of submucosal nerves in mediating STa-induced secretion. When tissues from the distal portion of the colon and jejunum were pretreated with piroxicam, a prostaglandin synthesis inhibitor, the STa-induced Isc response was significantly reduced by 52 and 57%, respectively.

CONCLUSIONS

These results indicate that the pig jejunum and distal portion of the colon are sensitive to the secretory actions of enterotoxigenic E coli STa, and that the responses are mediated, in part, by release of prostaglandins.

Hibernation enhances D-glucose uptake by intestinal brush border membrane vesicles in ground squirrels

The ability to actively transport nutrients is maintained in intestinal tissues of hibernating ground squirrels compared with their active counterparts, and shows apparent upregulation in hibernators when transport rates are normalized to tissue mass. To identify the mechanisms responsible for the preservation of transport function during the extended fast of hibernation, we studied D-glucose uptake into jejunal brush border membrane vesicles prepared from active and hibernating 13-lined ground squirrels. Hibernators were without food and showing regular bouts of torpor for at least 6 weeks before sacrifice. Electron micrographs indicated similar microvillus heights of jejunal enterocytes in the two activity states, whereas microvillus density was slightly greater in the hibernators. Glucose uptake into brush border membrane vesicles was inversely related to medium osmolarity, indicating negligible binding of substrate to brush border membrane vesicles surfaces, and intravesicular spaces were similar in hibernating and active squirrels. Glucose uptake showed strong Na+ dependency in both groups, with equivalent overshoot values in the presence of Na+. Kinetic analysis revealed a significant increase in the maximal velocity of transport (Jmax) in hibernators (55.9 +/- 5.6 nmol.min-1. mg-1) compared with active squirrels (36.7 +/- 5.1 nmol.min-1. mg-1, P < 0.05), with no change in K(m). Thus, the structure and absorptive capacity of the intestinal brush border persists in fasted hibernators, and the increase in Jmax for glucose uptake during hibernation likely contributes to the enhanced Na(+)-dependent glucose absorption previously observed at the tissue level.

Gut Feelings About Hibernation

The winter fast characteristic of mammalian hibernation presents an interesting challenge to the intestine. The absence of feeding reduces energy expenditure and causes atrophy of the mucosa and the potential to compromise absorptive capacity after spring arousal. The preservation of mucosal function in hibernators and ways this might occur despite the dramatic reduction in dietary intake are discussed.

Developmental changes in neurally mediated ion transport in piglet distal colon

To better understand the role of enteric nerves in the regulation of colonic ion transport in neonates, we examined the effects of endogenous and exogenous neurotransmitters on ion transport across distal colonic tissues of piglets. Tissues were obtained from full-term fetuses; newborns; suckling piglets killed 1 d, 5 d, and 14 d after birth; and 21-d-old piglets that had been weaned for 2 d. Colonic tissues were stripped of external muscle layers and mounted in Ussing flux chambers. Short-circuit current (Isc), a measure of active ion transport, and transmural potential difference were lowest in fetal colons and increased during postnatal development. Tissue conductance remained constant throughout development until d 14 and then rose sharply after weaning. Blockade of enteric neural transmission with tetrodotoxin reduced basal Isc compared with control tissues in fetal, newborn, and 1-d-old piglets but had no effect in older animals. The Na(+)-channel blocker amiloride had no effect on basal Isc in fetal tissues but significantly reduced Isc in all other groups, with the effect increasing with age. Isc responses to electrical field stimulation of enteric neurons were similar in fetal through 14-d-old piglets and then increased after weaning. Increases in Isc after serosal additions of carbachol (10 microM), serotonin (10 microM), or norepinephrine (10 microM) in fetal and newborn piglets were as great or greater than in the older piglets. For serotonin and norepinephrine, Isc responses rose sharply immediately after weaning. In 1-d-old piglets, Isc responses to all stimuli were reduced significantly by removal of Cl- ions from the bathing solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

Fasting alters basal and stimulated ion transport in piglet jejunum

Three-week-old piglets were used to study the effects of short-term fasting on jejunal ion transport. A 48-h fast significantly reduced mucosal weight, villus height, and crypt depth. Fasting increased basal short-circuit current (Isc), which reflects active ion transport, and total tissue conductance (Gt) of muscle-stripped jejunal sheets mounted in Ussing chambers. Increases in Isc evoked by carbachol, serotonin, histamine, prostaglandin E2, or Escherichia coli heat-stable enterotoxin were significantly greater in the fasted piglets. Isc responses to mucosal D-glucose were also enhanced by the fast. Under basal conditions, unidirectional and net fluxes of Na+ and Cl-, as well as serosal-to-mucosal inulin fluxes, were significantly increased in fasted piglets. In fed piglets, carbachol increased net Cl- secretion by stimulating serosal-to-mucosal Cl- flux; Gt was not affected. In fasted piglets, carbachol increased net Cl- secretion by inhibiting mucosal-to-serosal fluxes with no effect on serosal-to-mucosal fluxes. In addition, carbachol significantly inhibited mucosal-to-serosal Na+ fluxes and reduced Gt in this group. Thus a 48-h fast increased unidirectional and net ion fluxes in piglet jejunum and enhanced ion transport responses to secretory agonists. The mechanism by which carbachol stimulated net Cl- secretion was also altered by the fast. These results suggest that the absence of luminal nutrition changes the ion transport characteristics of the jejunal epithelium.

Localization of amiloride-sensitive Na+ channels in intestinal epithelia

Polyclonal antibodies raised against purified bovine renal papillary amiloride-sensitive Na+ channels were used to localize Na(+)-channel proteins in mouse and piglet small intestine. Immunostaining using the avidin-biotin-peroxidase technique revealed epithelial Na(+)-channel epitopes localized to apical regions of villus enterocytes in jejunal tissues of both species. Anti-Na(+)-channel antibodies also stained apical borders of villus enterocytes in piglet ileum and apical borders of surface cells in the piglet distal colon. On immunoblots of jejunal, colonic, and renal tissues the anti-Na(+)-channel antibodies recognized one to three polypeptides of apparent molecular masses similar to those found in bovine renal epithelial Na(+)-channel protein (the 55-65, 110-116, and 150-kDa subunits). The antibodies also recognized a polypeptide in the 40- to 45-kDa range in mouse intestine, which is comparable to the 35- to 40-kDa subunit of a renal Na+ channel. The results demonstrate that epitopes comparable to those present in renal amiloride-sensitive Na+ channels are found in apical regions of absorptive epithelial cells in the mammalian small and large intestine.

Effects of fasting and hibernation on ion secretion in ground squirrel intestine

Ground squirrels were used to study the effects of fasting and hibernation on small intestinal secretory function. Muscle-stripped sheets of jejunum set up in flux chambers were challenged with a variety of secretory agonists, and changes in short-circuit current (Isc) were recorded. Mucosal wet weights per centimeter and crypt dimensions were not affected by a 3-day fast in active squirrels, but villus height and area were significantly reduced in hibernators that had not eaten for over 6 wk. Tissue conductance was significantly greater in fasted and hibernating squirrels compared with fed animals. Maximal changes in Isc evoked by electrical stimulation of submucosal neurons, normalized to serosal surface area, were greater in fasted compared with fed or hibernating squirrels. When responses were normalized to crypt area, neurally evoked changes in Isc were greatest in the hibernators. Carbachol and serotonin evoked dose-dependent changes in Isc that were greater in fasted compared with fed squirrels at all concentrations. Histamine (100 microM) and theophylline (1 mM) also produced greater increases in Isc in fasted than in fed squirrels. The results suggest that a 3-day fast, or the extended fast of hibernation, results in enhanced secretory capacity in the ground squirrel jejunum.

Intestinal secretion after jejunal bypass in the ground squirrel

The preceding paper [H. V. Carey, Am. J. Physiol. 263 (Regulatory Integrative Comp. Physiol. 32): R1202-R1208, 1992] demonstrated that a 3-day fast or the extended fast of hibernation enhanced the chloride secretory responses to a variety of agonists in the ground squirrel jejunum. Here we examined the effect of jejunal bypass on intestinal secretory capacity in squirrels that either remained active and continued to feed (ACT), or were induced to hibernate (HIB). Mucosal wet weights were reduced in jejunal segments that had little or no exposure to the luminal stream (all segments from HIB and bypassed segments from ACT) compared with segments in contact with luminal contents (sham and in continuity segments of ACT). Tissue conductances were greater in those segments not exposed to luminal contents. Short-circuit current (Isc) responses to electrical stimulation of submucosal neurons, or to serosal carbachol, were greater in segments not exposed to luminal contents when responses were normalized to serosal surface area. Normalization of secretory responses to crypt area produced a similar pattern. Isc responses to mucosal alanine, which reflect electrogenic Na+ absorption, were greatest in segments with little or no exposure to luminal contents. Tissue wet weights, conductances and secretory responses of bypassed segments of HIB and ACT squirrels were always similar. These results suggest that luminal contents directly influence jejunal secretory capacity.

Maintenance of intestinal nutrient transport during hibernation

We studied nutrient absorption across the brush-border membrane in jejunal tissues from active 13-lined ground squirrels and in hibernating squirrels that had not eaten for at least 6 wk. Body weights and jejunal wet weights per centimeter were significantly reduced in the hibernators. Rates of total and carrier-mediated uptake of 3-O-methylglucose (3-OMG) per centimeter intestinal length were greater in the active squirrels, but 3-OMG uptakes per milligram intestinal weight were greater in the hibernators, due to a significantly greater maximum rate of uptake (Jmax) per milligram with no differences in apparent Michaelis constant (Km). Passive permeabilities to L-glucose were similar in both groups. Total uptake of L-proline per centimeter was greater in active squirrels, but total proline uptake per milligram was greater in the hibernators due to a significantly greater Jmax per milligram with no difference in apparent Km. Na(+)-independent proline uptake accounted for a greater proportion of total proline uptake in active compared with hibernating squirrels. As a consequence, Na(+)-dependent proline uptake was greater in the hibernators when uptake was normalized either to intestinal length or intestinal weight. Thus hibernation is associated with an increase in the Jmax per milligram for 3-OMG and proline transport, as well as a shift in the Na+ dependency of proline uptake. We conclude that nutrient absorption is selectively retained in mammalian hibernators to maintain transport function after the extended winter fast.

Effect of hibernation and jejunal bypass on mucosal structure and function

Intestinal mucosal structure and function may be regulated by systemic factors associated with oral feeding, as well as local responses initiated by contact of the mucosa with food. This study compared the relative effects of these factors in an animal model that undergoes seasonal long-term fasting. Jejunal bypass operations or sham surgeries were performed on active fed ground squirrels or on squirrels that subsequently ceased feeding and hibernated. Mucosal wet weight, protein content, villus height, and surface area were reduced in jejunal segments that had minimal exposure to luminal contents (bypassed segments of active squirrels and all segments of hibernators) compared with segments exposed to the luminal stream (incontinuity and sham segments of active squirrels). When normalized to mucosal weight, transepithelial absorption of 3-O-methylglucose and alanine-dependent sodium flux were greater in jejunal segments with minimal exposure to luminal nutrients. Altered structure in bypassed segments of active and hibernating squirrels paralleled changes in functional parameters despite the presence of different systemic factors in the two groups. Thus, in this animal model, contact of the mucosa with food, and not systemic factors associated with oral feeding, is the primary factor maintaining mucosal mass. The absence of mucosal contact with nutrients enhances specific absorptive function by mechanisms that have yet to be determined.

Seasonal changes in mucosal structure and function in ground squirrel intestine

Seasonal variations in mucosal structure and transport function were examined in active and hibernating ground squirrel jejunum. Mucosal wet weight and protein content, villus height, and mucosal surface area were lowest in hibernators, increased in spring, peaked in summer, and declined in fall. Transepithelial electrical parameters and net Na+ and 3-O-methylglucose (3-OMG) absorption measured in flux chambers were greatly reduced in hibernator tissues tested at 7 degrees C compared with tissues from hibernators or active squirrels studied at 37 degrees C. At 37 degrees C, net Na+ and 3-OMG absorption normalized to serosal surface area was similar in tissues from all squirrels. When normalized to mucosal surface area, absorption in hibernators was greater than in spring or summer squirrels but was not significantly different from fall values. Absorption normalized to mucosal area was similar among spring, summer, and fall squirrels. Increases in short-circuit current evoked by mucosal addition of D-glucose or L-alanine were significantly greater in hibernators than in fall squirrels. These results demonstrate seasonal variations in mucosal structure and transport function in captive ground squirrels. Enhanced area-specific absorption in hibernator tissues at 37 degrees C may represent a compensatory response to pronounced mucosal atrophy that occurs during hibernation.

Adrenergic inhibition of neurally evoked secretion in ground squirrel intestine

The role of norepinephrine in the mucosal response to neural stimulation was examined in the ground squirrel jejunum. Electrical field stimulation of jejunal sheets in flux chambers evoked a tetrodotoxin-sensitive increase in short-circuit current (42 +/- 6 microA/cm2) that declined during the stimulus period, followed by a second rise in current after the stimulus was terminated. The initial increase in current was inhibited by bumetanide and significantly reduced in chloride-free solutions, suggesting neurotransmitter stimulation of electrogenic chloride secretion during electrical stimulation. Atropine reduced the neurally evoked secretory response to 35% of control values. Blockade of alpha 2-adrenoceptors with phentolamine or yohimbine, or prevention of norepinephrine release with bretylium enhanced the atropine-resistant portion of the response and abolished the post-stimulus rise in current. The neurally evoked response was not affected by prazosin or propranolol. Catecholamine immunoreactivity was detected in nerve fibers of submucosal and myenteric plexuses but none was found in cell bodies of either plexus. The results suggest that electrical stimulation of the ground squirrel jejunum evokes active chloride secretion that is mediated by acetylcholine and other secretory transmitter(s), and simultaneously inhibited by norepinephrine released from adrenergic fibers in the mucosa.

Intestinal transport in megacolonic mice. Alterations in sugar absorption

This study examined jejunal sugar absorption in piebald mice with congenital megacolon and in normal littermates. Transmural potential difference, short-circuit current, and tissue conductance of flat sheets of jejunum set up in flux chambers were significantly greater in the diseased mice compared to normal siblings. In piebald mice, net absorption of 3-o-methylglucose was enhanced due to a significant increase in mucosal-to-serosal flux compared with normal littermates. Stimulation of electrogenic sodium absorption by alanine (10 mM) increased basal short-circuit currents more in piebald tissues than in tissues from normal mice, whereas stimulation of ion transport by carbachol (10 microM) evoked an increase in short-circuit current that was similar in the two groups. Alterations in intestinal mass, morphology, or Na+, K+-ATPase activity could not account for the increase in absorptive function characteristic of piebald mice.

Effects of enteric neural stimulation on chloride transport in human left colon in vitro

Nonpathological segments of muscle-stripped left colon from patients undergoing surgery for carcinoma, diverticulitis, or other causes were set up in flux chambers in order to determine the influence of intrinsic neurons on ion transport. In the basal state, both sodium and chloride were actively absorbed, and there was no significant residual ion flux. Electrical field stimulation of the intrinsic mucosal innervation evoked an increase in short-circuit current that was dependent on stimulus frequency and strength. The response was mimicked by scorpion venom, which is known to depolarize neurons, and was nearly abolished by tetrodotoxin, which prevents neurotransmitter release. Atropine reduced, but did not abolish, the response to neural stimulation. Carbachol, aminophylline, and theophylline significantly increased short-circuit current above basal levels. Electrical field stimulation evoked an increase in short-circuit current that could be accounted for by a decrease in net chloride absorption without any significant effect on sodium absorption or residual ion flux. These results suggest that ion transport in the human left colon is regulated by intrinsic submucosal cholinergic neurons as well as other neuronal types, and activation of these nerves could provide the basis for certain diarrheal disorders.

Neuromodulation of intestinal transport in the suckling mouse

The influence of enteric nerves on intestinal ion and sugar transport was investigated in 3- to 4-wk-old suckling mice. Whole thickness segments of jejunum were mounted as flat sheets in flux chambers equipped to electrically stimulate nerves in the intestinal wall. Tetrodotoxin significantly reduced basal short-circuit current in a subset of tissues containing 3-O-methylglucose in the mucosal bath. Electrical field stimulation of intrinsic nerves evoked an increase in short-circuit current of 86 +/- 15 microA/cm2 that was due to an increase in active chloride secretion with no significant change in the net movement of other ions. The secretory response to neural stimulation was abolished by tetrodotoxin and reduced by the muscarinic antagonist, atropine. Mucosal-to-serosal fluxes of 3-O-methylglucose were not altered by the cholinergic agonist carbachol. These results show that ion transport in the suckling mouse jejunum is regulated by the enteric nervous system. Neural stimulation evokes a chloride secretory response that is mediated by acetylcholine and other noncholinergic transmitters. Sugar transport in the murine small intestine does not appear to be influenced by muscarinic cholinergic agonists.

Thyrotropin-releasing hormone (TRH) evokes chloride secretion via cholinergic pathways in the guinea-pig ileum

The effect of thyrotropin-releasing hormone (TRH) on ion transport in the guinea-pig ileum was investigated in flux chambers. TRH evoked a concentration-dependent increase in short-circuit current that was due to active anion secretion. The response to TRH was prevented by tetrodotoxin and nearly abolished by atropine, but was not affected by mecamylamine. The results suggest that TRH plays a role as a neuromodulator or neurotransmitter that influences secretory processes by directly or indirectly activating cholinergic motor neurons.

Tonic activity of submucosal neurons influences basal ion transport

The influence of tonically active submucosal neurons on basal ion transport was studied using sheets of guinea pig ileum set up in flux chambers. Tetrodotoxin evoked an immediate and sustained decrease in short-circuit current that was sustained for 60 minutes compared with control tissues in which basal currents gradually decreased over time. Time-dependent changes in basal short-circuit currents in tissues treated with atropine were not significantly different from control tissues. The decrease in short-circuit current after tetrodotoxin resulted from a greater increase in net chloride absorption than sodium absorption. Changes in net sodium and chloride transport were due to an increase in the mucosal-to-serosal fluxes of these ions. The results suggest that tonic activity of submucosal neurons limits the absorptive capacity of the guinea pig ileum.

Muscarinic receptor subtypes mediating the mucosal response to neural stimulation of guinea pig ileum

Muscarinic receptors involved in the secretory response evoked by electrical stimulation of submucosal neurons were investigated in muscle-stripped flat sheets of guinea pig ileum set up in flux chambers. Neural stimulation produced a biphasic increase in short-circuit current due to active chloride secretion. Atropine and 4-diphenylacetoxy-N-methylpiperadine methiodide (4-DAMP) (10(-7) M) were more potent inhibitors of the cholinergic phase of the response than was pirenzepine. Dose-dependent increases in base-line short-circuit current were evoked by carbachol and bethanechol; 4-hydroxy-2-butynyl trimethylammonium chloride (McN A343) produced a much smaller effect. Tetrodotoxin abolished the effects of McN A343 but did not alter the responses of carbachol and bethanechol. McN A343 significantly reduced the cholinergic phase of the neurally evoked response and caused a rightward shift of the carbachol dose-response curve. All muscarinic compounds inhibited [3H]quinuclidinyl benzilate binding to membranes from mucosal scrapings, with a rank order of potency of 4-DAMP greater than pirenzepine greater than McN A343 greater than carbachol greater than bethanechol. These results suggest that acetylcholine released from submucosal neurons mediates chloride secretion by interacting with muscarinic cholinergic receptors that display a high binding affinity for 4-DAMP. Activation of neural muscarinic receptors makes a relatively small contribution to the overall secretory response.