Absorption of short-chain fatty acids, sodium and water from the forestomach of camels.

In camelids the ventral parts of compartments 1 and 2 (C1/C2) and the total surface of compartment 3 of the forestomach are lined with tubular glands, whereas in ruminants the surface of the forestomach is composed entirely of stratified, squamous epithelium. Thus, differences in absorption rates between these foregut fermenters can be expected. In five camels C1/C2 was temporarily isolated, washed and filled with buffer solutions. Absorption of short-chain fatty acids (SCFA) and net absorption of sodium and water were estimated relative to Cr-ethylenediaminetetraacetic acid as a fluid marker. SCFA were extensively absorbed in the forestomach; clearance rates of SCFA with different chain lengths were equal. After lowering the pH of solutions SCFA absorption rates increased, but much less than the increase of the non-ionized fraction. Absorption of propionate was lower when acetate had been added. Findings suggest that most of the SCFA in camels are transported in the ionized form, most likely via an anion exchange mechanism. Net water absorption is closely related to net sodium absorption. Apparently water absorption results from an iso-osmotic process. Differences between absorption mechanisms of SCFA from the forestomach of camelids and ruminants are discussed.

Distribution patterns of the glucose transporters GLUT4 and GLUT1 in skeletal muscles of rats (Rattus norvegicus), pigs (Sus scrofa), cows (Bos taurus), adult goats, goat kids (Capra hircus), and camels (Camelus dromedarius).

Earlier studies demonstrated that forestomach herbivores are less insulin sensitive than monogastric omnivores. The present study was carried out to determine if different distribution patterns of the glucose transporters GLUT1 and GLUT4 may contribute to these different insulin sensitivities. Western blotting was used to measure GLUT1 and GLUT4 protein contents in oxidative (masseter, diaphragm) and glycolytic (longissimus lumborum, semitendinosus) skeletal muscle membranes of monogastric omnivores (rats and pigs), and of forestomach herbivores (cows, adult goats, goat kids, and camels). Muscles were characterized biochemically. Comparing red and white muscles, the isocitrate dehydrogenase (ICDH) activity was 1.5-15-times higher in oxidative muscles of all species, whereas lactate dehydrogenase (LDH) activity was 1.4-4.4-times higher in glycolytic muscles except in adult goats. GLUT4 levels were 1.5-6.3-times higher in oxidative muscles. GLUT1 levels were 2.2-8.3-times higher in glycolytic muscles in forestomach herbivores but not in monogastric animals. We conclude that GLUT1 may be the predominant glucose transporter in glycolytic muscles of ruminating animals. The GLUT1 distribution patterns were identical in adult and pre-ruminant goats, indicating that GLUT1 expression among these muscles is determined genetically. The high blood glucose levels of camels cited in literature may be due to an "NIDDM-like" impaired GLUT4 activity in skeletal muscle.

Feed intake, forestomach fluid volume, dilution rate and mean retention of fluid in the forestomach during water deprivation and rehydration in camels (Camelus sp.).

Camels were deprived of water for 11 days. Before and during water deprivation and during rehydration changes in body weight, feed and water intake were measured. Using the liquid marker Cr-EDTA forestomach fluid volume, mean fluid retention and fluid dilution in the forestomach were estimated. At the eleventh day of water deprivation hay intake had decreased to only 9.6% of controls, dilution rates had decreased to 31%, mean retention time of fluid in the forestomach had increased to 189%. At the end of dehydration flow of saliva of 2 l/h mainly contributed to the still rather high dilution rates. Thereby buffering capacity and flow of fluid into the forestomach for microbial digestion as well as the outflow from the forestomach were maintained. At the beginning of rehydration camels drank 97 l within a few minutes, and animals thereby replaced all the water lost. Following this first huge water intake water is rapidly absorbed from the forestomach, and forestomach volume decreased again to dehydration values. At the third day of rehydration control values were reached again. Although feed intake decreased dramatically during water deprivation, functions of the forestomach can be maintained sufficiently mainly due to saliva inflow. This explains the mostly rapid recovery of camels when water is available again.

Chewing activities and oesophageal motility during feed intake, rumination and eructation in camels.

It was the aim of this study to characterize rumination behaviour, eructation and oesophageal motility in camels to identify similarities and differences between camels and domestic ruminants. Recordings were carried out in five camels fed on a hay-based diet. On an average, the duration of rumination, feeding and resting was 8.3, 5.6 and 10.1 h per 24 h, respectively. Rumination activity peaked in the morning between 9:00 and 11:00 and in the night between 02:00 and 04:00 a.m. During rumination periods, on an average 67 boluses were regurgitated per hour. Each bolus was chewed for an average of 45 s with 68 chews per min. The pause between two rumination cycles lasted on an average 9 s. Hay intake took 61 min/kg dry matter (DM), rumination lasted 71 min/kg DM of hay consumed. The regurgitation of a bolus started with a contraction of cranial compartment 1 (C 1) during a B-sequence, followed by a deep inspiration with closed glottis. Digesta enters the oesophagus, and an antiperistaltic wave transported the bolus orally. Eructation starts with a contraction of the caudal C1 during a B-sequence when the cranial C1 is relaxed. After entering the oesophagus, a rapid antiperistaltic wave transports the gas orally. Results revealed that the parameter values obtained in the camels were remarkably similar to those in domestic ruminants despite profound morphological differences and different patterns of forestomach motility.

[Large intestinal epithelium and short-chain fatty acids. Expected, unexpected, unexplained findings]. / Dickdarmepithel und kurzkettige Fettsäuren Erwartetes, Unerwartetes, Ungeklärtes.

Short-chain fatty acids (SCFA) are present in hindgut contents in high concentrations. SCFA are generated and also absorbed rapidly in the large intestine. Absorption results from diffusion of the undissociated and lipid soluble form or in exchange for bicarbonate. The controversial concepts concerning the extent of diffusion or the exchange for bicarbonate are partly due to contradictory findings and unequal mechanisms in different species and in different segments of the large intestine as well as in the different methods used. An almost neutral pH microclimate at the luminal surface is of importance for absorption of SCFA. The apical membranes of the epithelial cells in caecum and in proximal colon of guinea pigs are an substantial barrier for the diffusion of SCFA. After withdrawal of butyrate for only one hour from the perfusion or incubation solutions a massive apoptosis had been observed during the in situ perfusion of segments of guinea pig large intestine and also in in vitro studies with isolated epithelia in Ussing-chambers. In vitro apoptotic bodies and cells are emitted at the epithelial surface. However, in vivo induced by butyrate deprivation resident macrophages were tightly clustered below the surface epithelium. Aided by cytoplasmatic projections these macrophages phagocytose and transport apoptotic material from the epithelial intercellular spaces into their bodies. Obviously macrophages can be overloaded by this massive apoptosis, and some of the undigested apoptotic fragments are emitted into the lamina propria. Formation of a colitis ulcerosa may originate from these released undigested apoptotic bodies. Furthermore hints indicate aetiological interrelations between deprivation of butyrate and colon cancer. Butyrate-paradox denotes the contrarian effects on apoptosis and cell proliferation after addition or deprivation of butyrate in cultures of large intestinal tumour-cell-lines in comparison with the healthy, intact epithelium.

Apoptosis induction in vivo and fate of apoptotic material in the colon of the guinea pig.

Short-chain fatty acids (SCFA) in particular butyrate are regarded as an energy source acting in beneficial, protective manner on the colonic mucosa. Previous investigations showed that the colonic mucosa bathed in Ussing chamber with a solution lacking butyrate induced massive apoptosis of epithelial cells. The apoptotic material (bodies and cells) was shed at the mucosa surface. In the present study we aimed to investigate the effects caused in vivo on the colonic mucosa by the absence of butyrate. For this purpose the colon of guinea pigs was perfused in situ with solutions either containing or lacking butyrate. The results show that within 2h of perfusion without butyrate a large amount of epithelial cells underwent apoptosis as in the in vitro experiments. However, apoptotic material instead to be extruded at the epithelial surface accumulates into the intercellular spaces from which it becomes removed by an unusual high number of macrophages. These, engorged with phagocytozed material, lie assembled in a layer below the epithelium. Similar alterations have not been observed after perfusion in the presence of butyrate. The results suggest that this SCFA may protect the colonic mucosa in that it prevents apoptosis. The alterations occurring during 2h of its absence allow to assume that a protracted butyrate deprivation may lead to a breakdown of the integrity of the mucosa thus influencing differently the activity of the macrophages.

Insulin responsiveness of sheep, ponies, miniature pigs and camels: results of hyperinsulinemic clamps using porcine insulin.

It had been suggested that marked species differences in glucose tolerance tests were due to differences in insulin resistance. To compare insulin responsiveness, euglycemic hyperinsulinemic clamps were carried out in sheep, ponies, miniature pigs and camels. Porcine insulin was infused as primed-continuous infusions for 2 h (6 mU x kg(-1) x min(-1)). The steady state glucose infusion rates in the pigs, sheep, ponies and camels were 96.0, 18.6, 7.1 and 6.1 micromol x kg(-1) x min(-1), respectively. The maximal plasma insulin concentrations during the insulin infusions were 2,700 microU x ml(-1) in the camels, 1,400 microU x ml(-1) in the sheep and ponies and 600 microU x ml(-1) in the pigs. The rate of insulin removal from plasma was lowest in the camels as compared to the sheep, ponies and pigs (0.019, 0.038, 0.035 and 0.070 min(-1), respectively). In all species the concentrations of plasma non-esterified fatty acids dropped significantly 10-30 min after the start of the insulin infusion. However, the rates of non-esterified fatty acid reduction were higher in the pigs and sheep than in the camels and ponies. Results confirm a considerably higher insulin responsiveness in the pigs as compared to the sheep. The ponies and camels were found to be even more insulin-resistant than the sheep.

Surface pH at the basolateral membrane of the caecal mucosa of guinea pig.

Since the major mechanisms responsible for regulation of intracellular pH of enterocytes are located in the basolateral membrane, respective effects may be expected on pH in the compartment near the basolateral membrane. A method was established to estimate the pH at the basolateral membrane (pHb) of isolated caecal epithelia of guinea pig using pH-sensitive fluorescein attached to lectin (lens culinaris). In the presence of bicarbonate and a perfusion solution-pH of 7.4, pHb was 7.70 +/- 0.15. In the absence of bicarbonate or chloride as well as by inhibition of the basolateral Cl--HCO-3 exchange with H2-DIDS, pHb was reduced near to solution-pH. Inhibition of the basolateral Na+-H+ exchanger by adding a sodium- and bicarbonate-free, low-buffered solution increased pHb. Decrease of pH of serosal perfusion solution to 6.4 provoked a similar decrease of pHb to solution pH. Short-chain fatty acids (SCFA) added to the mucosal solution caused a slight decrease of pHb. SCFA added to the serosal side alkalized pHb. However, in the presence of bicarbonate pHb returned quickly to the initial pHb, and after removal of SCFA a transient acidification of pHb was seen. These responses could not be inhibited by MIA or H2-DIDS. We conclude that no constant pH-microclimate exists at the basolateral side. The regulation of the intracellular pH of enterocytes reflects pHb. The slightly alkaline pHb is due to the bicarbonate efflux. Data support the presence of an SCFA--HCO-3 exchange.

Isolation and lipid composition of apical and basolateral membranes of colonic segments of guinea Pig.

In adapting several methods of membrane isolation we established a successful way to purify apical and basolateral membranes of guinea pig colon in a parallel procedure. The conventional purification control by marker enzymes was applied. In addition, luminal membrane proteins were stained with Texas Red. Apical and basolateral enterocyte membranes were enriched 10- to 12-fold by differential precipitation and via a continuous sorbitol gradient. The membrane fractions were examined with regard to their phospholipid (PL) and fatty acid patterns and to their cholesterol content. Fluorescence polarization studies were carried out using 1,6-diphenyl-1,3, 5-hexatrien. Remarkable differences in the fatty acid pattern of the proximal and the distal colon were seen. Due to a higher content of oleic acid the saturation index of the apical membranes of the proximal colon is lower compared to that of the apical membranes of the distal colon (0.34 +/- 0.03 vs 0.42 +/- 0.05). The cholesterol content of the apical membranes of the proximal colon is markedly higher than that of the apical membranes of the distal colon (3.42 +/- 0.14 vs 1.88 +/- 0.29 mol/mol PL). There are no differences in the fluidity of these apical membranes. We assume a balancing mechanism between the cholesterol content and the amount of saturated PL-fatty acids.

Role of short-chain fatty acids in the hind gut.

Short-chain fatty acids (SCFA) are produced by microbial fermentation in the hindgut in considerable amounts. Most of the anions in hindgut contents are SCFA, mainly acetate, propionate and butyrate. SCFA are rapidly absorbed. Mechanisms involved in the transepithelial transport are discussed. Besides the contribution to the overall energy metabolism of animals or men, SCFA have a number of further important effects on the colonic mucosa. Factors affecting the pH of compartments in the mucosa, cell swelling, stimulation of mucin release and of mucosal blood flow are mentioned. Controversial reports are known on the role of SCFA in the metabolism of colonocytes. In spite of the conflicting opinions on the interaction between SCFA metabolism and the development of colitis ulcerosa, diverticulosis and colorectal cancer seems to exist. The obscure differences between the effects of SCFA on cell proliferation, differentiation and apoptosis of colonocytes in vivo and in vitro indicate that besides direct effects of SCFA systemic effects such as neural and humoral factors are of crucial importance. The opposing effects of SCFA on proliferation and apoptosis in normal colonocytes and in colonic cancer cells may open possibilities for prevention and/or therapy of patients with colonic diseases.

Forestomach motility in llamas and camels.

Major aspects of forestomach anatomy in llamas and camels are described. The pattern of forestomach motility is a succession of motoric cycles, consisting of A- and B-contraction sequences and a pause. Respective differences between llamas and camels are discussed. Observations on regulation of motility are mentioned. Occurrences and courses of rumination and eructation within the motility cycles are emphasized. Motility of the forestomach achieves a selective retention of feed particles in the forestomach; a long retention time is a prerequisite for an effective microbial digestion, especially of poor quality feed.

Effect of SCFA on intracellular pH and intracellular pH regulation of guinea-pig caecal and colonic enterocytes and of HT29-19a monolayers.

The response of the intracellular pH (pHi, measured with BCECF) of the caecal and distal colonic epithelium of guinea pig and of monolayers of HT29 clone 19a cells on the addition of short-chain fatty acids (SCFA) was assessed. Addition of SCFA to the luminal side of these cells had no major effect on pHi, independent of whether the apical Na+/H+ exchange or the apical K+/H+ ATPase was inhibited or not. Addition of SCFA to the serosal side, on the other hand, caused a marked decrease of pHi, followed by an effective regulation back to basal values, and after removal of the acid, the cells became alkalinized. Intracellular pH is mainly regulated by mechanisms in the basolateral membrane. The basolateral Na+/H+ exchanger and the Cl-/HCO3- exchanger were mainly responsible for pHi regulation. Inhibition studies are consistent with a NHE-1 type Na+/H+ exchanger in the basolateral membranes. The apical Na+/H+ exchanger of caecal enterocytes and in HT29 cells, and the apical K+/H+ ATPase in the apical membrane of the distal colon have no or little influence on pHi regulation. The comparison shows that the HT29-19a cell line is an adequate model for studying pHi phenomena of hind gut epithelial cells.

Surface pH of the distal colonic epithelium of guinea-pigs.

A method for the continuous measurement of proton activity at the luminal surface of the gastro intestinal mucosa was developed. The pH-sensitive fluorescent dye 5-N-hexadecanoyl-amino fluorescein (HAF) was used to monitor the surface pH (pHs). HAF integrates in the apical membrane of the colonic cells because of its amphophilic character. By excitation at two different wavelengths, the ratio of the two fluorescence intensities corresponds to the pHs. pHs in the presence of bicarbonate is relatively independent of the luminal pH. In bicarbonate-free, HEPES-buffered solutions, such a constant "microclimate" was not present. Addition of 113 mmol.l-1 butyrate to the luminal solution caused an increase of the surface pH by 0.14 +/- 0.07 (n = 18) pH units in bicarbonate-containing medium. This increase was not observed in bicarbonate-free medium, indicating that the increase in pHs is due to a gain of bicarbonate during butyrate absorption.

Comparative aspects of glucose tolerance in camels, sheep, and ponies.

The aim of the study was to gain informations about factors responsible for the higher level of plasma glucose in camels as compared to sheep and ponies. An intravenous glucose tolerance test was carried out with four camels, four ponies, and four sheep by infusing 1 mmol glucose per kg body weight intravenously within 3 min. Concentrations of glucose, insulin, and non-esterified fatty acids (NEFA) were estimated in venous plasma samples taken before and within 6 hr after infusion. Basal glucose values were higher in camels (7.1 +/- 0.3 mmol/l) than in ponies (4.2 +/- 0.4 mmol/l) and sheep (3.4 +/- 0.2 mmol/l). The rate of glucose elimination was markedly lower in camels (0.270 +/- 0.018 hr-1) than in sheep (0.804 +/- 0.036 hr-1) and ponies (0.858 +/- 0.084 hr-1). The insulin response after glucose infusion was more pronounced in ponies and sheep than in camels. Concentrations of NEFA in plasma dropped 30 min after the infusion in all species, however, NEFA level decreased slower in camels than in sheep and ponies. It is concluded that the markedly higher plasma concentration of glucose in camels compared to sheep and ponies may be caused by a poorer insulin response and/or a reduced tissue sensitivity to insulin.

Mass spectrometric determination of HCO3- permeability and carbonic anhydrase activity in intact guinea-pig colon epithelium.

1. A mass spectrometric method originally used in red blood cells was applied to suspensions of isolated colonocytes and intact colonic epithelium to measure the exchange of 18O between HCO3-, CO2 and H2O to determine intracellular carbonic anhydrase activity (Ai) and membrane bicarbonate permeability (P). 2. In suspensions of isolated guinea-pig colon epithelial cells, colonocytes, we found significantly higher values of Ai and P for cells derived from the proximal colon than for cells from the distal colon. In the case of Ai, this confirms earlier reports. 3. When the 18O exchange process was observed across the mucosal (apical) side of intact colon mucosa, the estimated values of Ai were identical to those obtained for isolated colonocytes, for both the proximal and the distal part of the colon. This is considered to be strong evidence that this method can be applied to a layer of intact epithelium as well as to cell suspensions. 4. The values of P obtained from the apical side of intact colon mucosa were 6 times higher than those estimated from measurements with isolated colonocytes. This indicates that the basolateral membrane of colon epithelium, which participates in the 18O exchange process in isolated colonocytes but not in the 18O exchange process across the apical side of intact mucosa, has a markedly lower bicarbonate permeability than the apical membrane. 5. When the 18O exchange process was observed across the serosal (basolateral) side of intact colon mucosa, the P values, as expected, were low compared with the apical side of intact mucosa. However, rather unexpectedly, the Ai values derived from these measurements were 2-3 times lower than those obtained with isolated colonocytes. It appears possible that the latter finding is an artifact due to the submucosal tissue markedly slowing down CO2 diffusion from the bathing medium into the epithelial cells, thus causing an apparent fall in Ai. 6. Ai decreased and P increased with increasing temperature, as expected, when studied on the mucosal side of intact colon. This provides additional support for the validity of the method.

Transport of propionate across the distal colonic epithelium of guinea pig in the presence and absence of bicarbonate and of chloride.

Short-chain fatty acids (SCFA) are rapidly absorbed in the large intestine. However, the mechanisms involved have not been fully delineated. Findings indicate that absorption of SCFA occurs in the undissociated form as well as by an anion exchange, whereby marked species and segmental differences are present. The guinea pig distal colon has certain peculiarities. Unidirectional fluxes of propionate across guinea pig distal colon were studied under short-circuit current conditions in Ussing chambers. Removal of bicarbonate caused reduction of mucosal-to-serosal fluxes by 30%, serosal-to-mucosal fluxes were little affected. In chloride-free solution unidirectional fluxes were also reduced. However, in the presence of transepithelial chloride-gradients with the Cl(-)-free solution only at the luminal side, no such effects were seen. Findings support the presence of SCFA(-)-HCO3- exchange in guinea pig distal colon. Chloride seems not of major importance for SCFA transport.

Withdrawal of butyrate from the colonic mucosa triggers "mass apoptosis" primarily in the G0/G1 phase of the cell cycle.

Butyrate exerts a trophic effect on the colonocytes and plays a protective role in ulcerative colitis. In the present study, we investigated the effect of butyrate withdrawal on the colonic mucosa of the guinea-pig. The samples were mounted in Ussing chambers and bathed for 45, 60, 90 and 150 min with standard Ringer solution with or without sodium butyrate. Light and electron microscopy for morphology, electrophysiological methods for testing tissue function, histochemistry using the TUNEL method for localization of apoptotic cells and flow cytometry for cell cycle analysis were applied. Morphological observations revealed that butyrate deprivation caused a time-dependent hypoplasia and a rapid triggering of massive apoptosis as substantiated by the TUNEL assay. The epithelium, however, did not show discontinuities at any time. Electrophysiological data confirmed that no leakage of the epithelium had occurred. In the control specimens, the mucosa underwent a moderate reduction in height; apoptotic epithelial cells were infrequently observed. Cell cycle analysis of colonocytes isolated from the mucosa deprived of butyrate revealed a decrease in the percentage of cells occupying each phase of the cycle, especially the G0/G1 phase. Thus, in the absence of butyrate, apoptosis was enhanced and cell renewal reduced. The trophic protective action exerted by butyrate in both physiological and pathological conditions could derive from its capacity to modulate survival and death of colonocytes.

Metabolism of the macrolide immunosuppressant, tacrolimus, by the pig gut mucosa in the Ussing chamber.

1. The macrolide tacrolimus (FK506), used as an immunosuppressant, is a cytochrome P450 (CYP) 3A substrate in the liver. The metabolism of tacrolimus and the transport of its metabolites in the pig gut was studied in the Ussing chamber. Tacrolimus and its metabolites were quantified by h.p.l.c./mass spectrometry. 2. In the Ussing chamber, demethyl, didemethyl, hydroxy and hydroxy-demethyl tacrolimus were generated. Their formation was concentration- and time-dependent. The metabolite pattern was not different from that after incubation of tacrolimus with human small intestinal microsomes. 3. The metabolite formation was highest in the duodenum and declined in the order duodenum > jejunum > ileum > colon > stomach. 4. Since tacrolimus metabolism was inhibited by the specific CYP3A inhibitors, troleandomycin and ketoconazole, we concluded that these enzymes are involved in intestinal metabolism of tacrolimus. 5. Tacrolimus metabolites re-entered the mucosa chamber (> 90%) and passed through the small intestinal preparation into the serosa chamber. 6. It is concluded that tacrolimus is metabolized in the intestine, that the metabolites are able to re-enter the gut lumen and also enter into the portal vein and that small intestinal metabolism and transport is at least in part responsible for the low oral bioavailability of tacrolimus.

Sodium transport across the caecal and colonic epithelium of germfree and specific-pathogen free rats.

It was assumed that the enlarged caecum and the accumulation of semiliquid contents in germfree rats is accompanied by changes in sodium absorption. Transepithelial sodium fluxes were studied under Ussing chamber conditions across epithelial sheets of the caecum and colon of germfree (GF) and specific-pathogen free (SPF) rats. Net sodium transport was highest in the proximal colon and in the proximal segment of the distal colon; it was considerably lower in the caecum and in the distal segment of the distal colon. In the caecum and proximal colon of the GF rats, the electroneutral sodium absorption was increased as compared to the SPF rats. In the proximal segments of the distal colon, no differences were seen. In the distal segment of the distal colon, the mainly electroneutral sodium transport in the SPF rats was changed into electrogenic transport in the GF rats. These differences may be due to the increased aldosterone levels of the GF rats.

Comparative views of electrophysiological parameters of large intestinal segments in pig, sheep, pony, guinea pig and rat.

Short circuit current (ISC) and transepithelial conductance (gt) across sheets of epithelia were measured in the caecum, the proximal and the distal colon of pig, sheep, pony, rat and guinea pig. The electrical parameters underline the basic segmental and species differences. The diversity of ISC demonstrates the different nature of electrogenic transport mechanisms, and data clearly show the heterogeneity with respect to transport mechanisms along the large intestine in the various species. The great differences in amiloride sensitive ISC indicate the variabilities in the electrogenic Na transport. Whereas in the pig, sheep and pony caecum, in the guinea pig proximal colon and in all segments of the rat hindgut no indications for a major electrogenic Na transport was seen, in all other segments amiloride caused a marked decrease in ISC. Electrogenic Na transport seems to be highest in sheep distal colon and in pig proximal and distal colon, somewhat less in guinea pig and in pony distal colon. The epithelium with the lowest Powest transepithelial conductance clearly is that of the pony caecum. Except in sheep, gt-values are not much different from those in pony and also pig and guinea pig. By far the epithelium with the highest conductance is that of the rat proximal colon. gt was similar in the proximal and the distal colon of pig, sheep and pony; in guinea pig values were slightly, in rat much lower.