Macronutrient signals for adaptive modulation of intestinal digestive enzymes in two omnivorous Galliformes.

According to the adaptive modulation hypothesis, digestive enzyme activities are matched to their respective dietary substrate level so that ingested nutrients are not wasted in excreta due to insufficient digestive capacity, and so membrane space or expenditures building/maintaining the intestinal hydrolytic machinery are not wasted when substrate levels are low. We tested predictions in juvenile northern bobwhites (Colinus virginianus) and juvenile and adult domestic chickens (Gallus gallus domesticus) by feeding them on diets varying in starch, protein, and lipid composition for 7-9 d (bobwhites) or 15 d (chickens). Birds were euthanized, intestinal tissue harvested, and enzyme activities measured in tissue homogenates from proximal, medial and distal small intestine. We found that (1) α-glucosidase (AG; maltase and sucrase) activities were induced by dietary starch in both juvenile and adult chickens but not in northern bobwhites; (2) aminopeptidase-N (APN) activities were induced by dietary protein in both bobwhites and juvenile but not adult chickens; (3) AG activities were suppressed by an increase in dietary lipid in both bobwhites and juvenile but not adult chickens; and (4) APN activities were not suppressed by high dietary lipid in any birds. We review findings from 35 analogous trials in 16 avian species. 100% of avian omnivores modulate at least one enzyme in response to change in dietary substrate level. AG induction by dietary carbohydrate occurs in more members of Galloanserae than in Neoaves, and all omnivorous members of Neoaves tested so far increase APN activity on high dietary protein, whereas fewer of the Galloanserae do.

Differential transcriptional responses underlie dietary induction of intestinal carbohydrase activities in house sparrow nestlings.

Many species show diet-induced flexibility of activity of intestinal enzymes; however, molecular and genetic mechanisms responsible for such modulation are less known, particularly in altricial birds. The goal of our study was to test whether a diet-induced increase in activity of intestinal maltase and sucrase in house sparrow nestlings is matched with an increase in maltase-glucoamylase (MG) and sucrase-isomaltase (SI) complex mRNAs respectively. Both enzyme activities were significantly higher in mid-intestine of nestlings fed a medium-starch (MS) diet compared to those fed a starch-free (SF) diet. In contrast to the similar pattern of dietary induction for both enzyme activities, diet MS elevated significantly only the level of MG mRNA, but not SI mRNA. The coordinated increase in activity of maltase and in MG mRNA is consistent with the hypothesis that dietary induction of this enzyme is under transcriptional control. In contrast, the lack of such coordination for changes in activity of sucrase and SI mRNA suggests that upregulation of this enzyme may be achieved by post-translational factor(s). We conclude that genetic mechanisms responsible for diet-induced flexibility of digestive enzymes in birds may differ from that observed in mammals.

Isolation of epithelial cells, villi and crypts from small intestine of pigeons (Columba livia)

The isolation of viable enterocytes, villi and crypts from the small intestine of a feral bird (Columba livia) is important for performing physiological experiments in ecologically relevant processes of membrane transport. The effectiveness of mechanical disruption, enzymatic digestion and chelating agents were compared. The objectives were to isolate enterocytes, villi and crypts from the small intestine of young pigeons; to evaluate the viability of the isolated intestinal epithelial cells isolated; and to verify the integrity of enterocytes by biochemical features. Enzymatic and mechanical methods yielded both elongated columnar and spherical cells. With the chelating method villi and crypts were obtained. All methods produced a high yield of intestinal epithelial cells with about 50% viability. Brush border enzymes (sucrase-isomaltase and alkaline phosphatase) activities were high and, as reported in chickens, they did not differ along the intestinal villus-crypt axis. Although the three methods have good viabilities, the enzymatic technique gives the best yield in cell number, while the chelating method provides the highest populations of morphologically distinctive villi and crypts.

Isolation of epithelial cells, villi and crypts from small intestine of pigeons (Columba livia)

The isolation of viable enterocytes, villi and crypts from the small intestine of a feral bird (Columba livia) is important for performing physiological experiments in ecologically relevant processes of membrane transport. The effectiveness of mechanical disruption, enzymatic digestion and chelating agents were compared. The objectives were to isolate enterocytes, villi and crypts from the small intestine of young pigeons; to evaluate the viability of the isolated intestinal epithelial cells isolated; and to verify the integrity of enterocytes by biochemical features. Enzymatic and mechanical methods yielded both elongated columnar and spherical cells. With the chelating method villi and crypts were obtained. All methods produced a high yield of intestinal epithelial cells with about 50% viability. Brush border enzymes (sucrase-isomaltase and alkaline phosphatase) activities were high and, as reported in chickens, they did not differ along the intestinal villus-crypt axis. Although the three methods have good viabilities, the enzymatic technique gives the best yield in cell number, while the chelating method provides the highest populations of morphologically distinctive villi and crypts.(AU)

L-glucose absorption in house sparrows (Passer domesticus) is nonmediated.

We previously demonstrated in intact house sparrows substantial absorption in vivo of L-glucose, the stereoisomer of D-glucose that is assumed not to interact with the intestine's D-glucose transporter. Results of some studies challenge this assumption for other species. Therefore, we tested it in vitro and in vivo, based on the principle that if absorption of a compound (L-glucose) is mediated, then absorption of its tracer will be competitively inhibited by high concentrations of either the compound itself or other compounds (e.g., D-glucose) whose absorption is mediated by the same mechanism. An alternative hypothesis that L-glucose absorption is primarily paracellular predicts that its absorption in vivo will be increased (not decreased) in the presence of D-glucose, because the permeability of this pathway is supposedly enhanced when Na(+)-coupled glucose absorption occurs. First, using intact tissue in vitro, we found that uptake of tracer-radiolabeled L-glucose was not significantly inhibited by high concentrations (100 mM) of either L-glucose or 3-O-methyl-D-glucose, a non-metabolizable but actively transported D-glucose analogue. Second, using intact house sparrows, we found that fractional absorption of the L-glucose tracer was significantly increased, not reduced, when gavaged along with 200 mM 3-O-methyl-D-glucose. This result was confirmed in another experiment where L-glucose fractional absorption was significantly higher in the presence vs. absence of food in the gut. The greater absorption was apparently not due simply to longer retention time of digesta, because no significant difference was found among retention times. Our results are consistent with the idea that L-glucose is absorbed in a non-mediated fashion, largely via the paracellular pathway in vivo.

Intestinal passive absorption of water-soluble compounds by sparrows: effect of molecular size and luminal nutrients.

We tested predictions that: (1) absorption of water-soluble probes decreases with increasing molecular size, consistent with movement through effective pores in epithelia, and (2) absorption of probes is enhanced when measured in the presence of luminal nutrients, as predicted for paracellular solvent drag. Probes (L-arabinose, L-rhamnose, perseitol, lactulose; MW 150.1-342.3 Da) were gavaged in nonanesthetized House sparrows ( Passer domesticus), or injected into the pectoralis, and serially measured in plasma. Bioavailability was calculated as F=AUC by gavage/AUC by injection, where AUC is the area under the curve of plasma probe concentration vs. time. Consistent with predictions, F declined with probe size by 75% from the smallest to the largest probe, and absorption of probes increased by 40% in the presence of luminal glucose or food compared to a mannitol control. Absorption of water-soluble probes by sparrows is much higher than in humans, which is much higher than in rats. These differences seem mainly attributable to differences in paracellular solvent flux and less to differences in effective paracellular pore size.

Developmental changes in digestive physiology of nestling house sparrows, Passer domesticus.

Six decades of studies have speculated that digestive capacity might limit avian growth rate or that developmental changes in the gut might determine developmental changes in digestive efficiency. However, there are no studies on digestive enzymes during avian development, except for studies on mainly domestic birds that exhibit the precocial mode of development. We studied alimentary organ masses, intestinal enzyme activities (sucrase, maltase, isomaltase, aminopeptidase-N), and pancreatic enzyme activities (amylase, trypsin, chymotrypsin) during development of a wild passerine bird exhibiting the altricial mode of development. Wild nestling house sparrows were studied immediately after removal from the nest (days 0, 3, 6 of age; day 0=hatch), whereas captives were raised in the laboratory beginning day 3 on a formulated casein/starch-based diet until fledging age (after day 12). Digestive biochemistry was dynamic. Tissue-specific activities of some digestive enzymes continued to increase through fledging, by >10 times in some cases (e.g., sucrase and maltase in midintestine). Total pancreatic amylase activity increased 100 times between hatch and day 12 through a combination of increases in tissue-specific activity and pancreas mass. House sparrows differ from poultry, in whom after about 2 wk of age the specific activity of intestinal and pancreatic digestive enzymes is generally constant or declines during development. The data on intestinal and pancreatic enzymes help explain why digestive efficiency of nestling house sparrows improves with age, and the data seem consistent with the idea that digestive capacity might limit feeding rate and hence growth rate.

Passive absorption of hydrophilic carbohydrate probes by the house sparrow Passer domesticus.

To evaluate the permeability of the intestine of the house sparrow Passer domesticus to hydrophilic compounds, we applied a pharmacokinetic technique to measure in vivo absorption of two carbohydrate probes, l-arabinose and d-mannitol. Probes were fed or injected, and blood and excreta were subsequently collected and analyzed by gas chromatography/mass spectrometry. Following injection, plasma probe concentration decreased in a log-linear fashion, implying single-compartment, first-order kinetics. Following oral administration, plasma probe concentrations increased, reached a maximum at 10 min and then decreased in log-linear fashion. Mannitol and arabinose absorption were calculated from the areas under the post-absorption plasma curve and the respective distribution spaces and elimination constants. The amounts absorbed increased linearly with the concentration administered (range 1-1000 mmol x l(-1)), implying a passive process. The mouth-to-cloaca retention time of digesta, measured using the non-absorbable compound potassium ferrocyanide, was independent of probe concentration. On average, 69% of the oral dose of probe was absorbed and this was independent of the concentration of probe administered. This paper supports an earlier report of substantial passive glucose absorption in house sparrows and offers a method to study the extent of hydrophilic solute absorption, which has importance for future research in areas as diverse as biomedical, ecological and evolutionary physiology.

Dietary modulation of intestinal enzymes of the house sparrow (Passer domesticus): testing an adaptive hypothesis.

Insectivorous/frugivorous passerine species studied so far lack the ability to modulate intestinal maltase activity, in contrast to galliformes. We tested for dietary modulation of small intestine (SI) enzymes including maltase in house sparrows to understand whether the difference between the galliformes on the one hand, and the passerines on the other, reflects a phylogenetic pattern (maltase modulated in galliformes but not passerines), a dietary pattern (maltase modulated in granivores but not insectivore/frugivores), some other pattern, or chance. We also tested the prediction that intestinal peptidase activity would be increased on a high protein (HP) diet. Birds were fed three diets high in starch, protein, or lipid for 10 days. For birds on the HP diet (60.3% protein) we observed the predicted upward modulation of aminopeptidase-N activity, as compared with the lower-protein, high starch (HS) (12.8% protein) diet. In contrast, birds eating the HS diet had similar maltase and sucrase activities, and only slightly higher isomaltase activity, compared with birds eating the high protein (HP), starch-free diet. Birds eating high lipid (HL) diet had low activities of both carbohydrases and peptidase. Considering that the statistical power of our tests was adequate, we conclude that house sparrows show little or no increase in carbohydrases in response to elevated dietary carbohydrate. We cannot reject the hypothesis that maltase lability among avian species has a phylogenetic component, or that high dietary fat has a depressing effect on both carbohydrase and peptidase activities.

Digestive responses during food restriction and realimentation in nestling house sparrows (Passer domesticus).

We used nestling house sparrows (Passer domesticus) under laboratory conditions to test for modulation of digestive efficiencies during periods of low and high food intake and tested the hypothesis that nestlings would exhibit compensatory changes in digestive efficiency following a period of food restriction. During the low intake period, nestlings were held at constant body mass for 48 h beginning on either day 3 or day 6 of life by feeding them at 50% of control rations. After 48 h of food restriction, nestlings were fed as much as they could consume, allowing the nestlings restricted at day 6 (early restriction not assessed) to consume 14% more food than control nestlings. For nestlings restricted at day 6 apparent dry mass assimilation of the entire diet was found to be 5% and 8% lower during food restriction and realimentation, respectively, compared with control nestlings that were not under- or overfed. There were no significant differences in radiolabeled starch assimilation efficiencies between control and restricted nestlings. Starch assimilation efficiencies remained constant from 3 d of age onward in control nestlings. Total starch extracted was lower during food restriction but reached a rate similar to that of control nestlings during the realimentation period. Passage times (time of first defecation, mean retention time, and mode passage time) measured with an indigestible marker were longer during food restriction and shorter during realimentation, relative to control nestlings. During realimentation there was no difference in intestinal rates of hydrolysis or mediated uptake of L-leucine compared with control nestlings. The main effect of changing food intake was apparently to alter flow rate, and hence retention time, causing slight changes in digestive efficiency. Thus, nestlings did not exhibit compensatory changes in digestion rates as implied by the hypothesis. Our finding of a lower dry mass assimilation efficiency and similar total starch assimilation during realimentation (relative to controls) helps explain why nestling house sparrows do not display compensatory growth, despite higher food intake. Our results indicate that the gut has little spare capacity to deal with increased food intake during growth following food restriction.

Glucose and amino acid absorption in house sparrow intestine and its dietary modulation.

We acclimated house sparrows (Passer domesticus; 26 g) to high-starch (HS), high-protein (HP), and high-lipid (HL) diets and tested the predictions that uptake of D-glucose and amino acids will be increased with increased levels of dietary carbohydrate and protein, respectively. HS birds had lower mediated D-glucose uptake rate than HP birds. Total uptake of L-leucine at low concentration (0.01 mM), but not of L-proline at 50mM, was increased by dietary protein. Measures of D-glucose maximal mediated uptake (1.2 +/- 0.2 nmol.min-1.mg-1) and intestinal mass (1 g) indicated that the intestine's mediated uptake capacity was only approximately 10% of the D-glucose absorbed at the whole animal level. This implied that nonmediated glucose absorption predominated. We applied a pharmacokinetic technique to measure in vivo absorption of L-glucose, the stereoisomer that does not interact with the Na(+)-glucose cotransporter. At least 75% of L-glucose that was ingested was apparently absorbed. This adds to the increasing evidence that substantial passive glucose absorption occurs in birds and may explain why mediated D-glucose uptake does not increase on high-carbohydrate diets.

Thermal freedom of Graomys griseoflavus in a seasonal environment.

1. Thermal freedom, defined as the physiological potential to make full spatiotemporal use of the seasonal thermal macroclimate, was assessed in Graomys griseoflavus, a cricetid rodent inhabiting the Chaco Province in Argentina. 2. The extent of the physiological thermal range (58 degrees C) was larger than the seasonal thermal range (30-50 degrees C), favoring Graomys resistance to low winter temperatures. The upper physiological thermal limit was almost coincident with the highest extreme of summer temperatures. 3. Graomys thermal freedom is facilitated by a high evaporative water loss, high metabolic capability, wide thermal neutral zone and a 4 degrees C lability of body temperature. 4. Behavioral responses (thermal avoidance) are utilized by this species, but with no apparent loss in thermal freedom.