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.

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.

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.

Pharmacological analysis of 5-hydroxytryptamine actions on guinea-pig ileal mucosa.

The possibility that 5-hydroxytryptamine (5-HT) is involved in the chloride secretory response evoked by electrical stimulation of submucosal neurons was investigated in guinea-pig ileum set up in Ussing flux chambers. When electrical stimulus pulses of 0.5 ms duration, amplitude of 3.2 mA and frequency of 10 Hz were applied repetitively in the plane of the tissue, a biphasic increase in short-circuit current (Isc) occurred. After tachyphylaxis to 5-HT or when the 5-HT antagonist cisapride was present in the bathing solutions, electrical stimulation of enteric nerves still evoked a biphasic change in Isc that was similar in magnitude to the response before either treatment. Cisapride (5 microM) prevented the mucosal response to exogenous 5-HT without altering the functional integrity of the tissues. Addition of 100 microM 5-HT to the submucosal side of the tissue evoked a biphasic increase in Isc that reflected primarily chloride secretion. Tetrodotoxin and atropine significantly reduced but did not abolish the change in Isc and chloride secretion. These results suggest that the mucosal response evoked by electrical field stimulation is not mediated by release of 5-HT at neuro-enterocyte junctions or from endocrine elements. 5-Hydroxytryptamine activated enteric cholinergic neurons within the submucosal plexus to stimulate chloride secretion.

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.

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.

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.

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.

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.

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.

Submucosal nerves and cholera toxin-induced secretion in guinea pig ileum in vitro.

The effects of cholera toxin on intestinal transport parameters were examined in muscle-stripped flat sheets of guinea pig ileum in order to determine whether the effects were partly mediated by stimulation of the mucosal innervation. Cholera toxin evoked an increase in short-circuit current that reflected active ion secretion. Tetrodotoxin completely blocked neurally mediated responses to electrical field stimulation, but it did not prevent the effects of cholera toxin. In the absence of tetrodotoxin, electrical stimulation of submucosal neurons evoked a biphasic increase in short-circuit current that was produced by chloride secretion. The first phase, which was known to be cholinergic, was enhanced by cholera toxin. These results suggest that cholera toxin may mediate intestinal secretion by direct action on the enterocytes as well as by enhancing cholinergically mediated intestinal secretory processes.

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.

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)

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.

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.