Biomedical subjects

M Keelan

Publications and source records attributed to M Keelan.

At least 91 records · Page 5Linked to original sources

PubMed1989

Rechallenge following an early life exposure to a high-cholesterol diet enhances diet-associated alterations in intestinal permeability.

This study was undertaken to test the hypothesis that the intestinal transport response to a high-cholesterol (2%) diet (HCD) is influenced by prior exposure to HCD at an earlier age. Uptake was assessed using an in vitro technique, and rabbits were fed one of four dietary regimens: chow for 20 weeks, chow for 14 weeks followed by HCD for 6 weeks, chow for 2 weeks followed by 4 weeks HCD followed by chow for 14 weeks, or 2 weeks chow followed by 4 weeks HCD followed by 8 weeks chow and then 6 further weeks with HCD. The passive jejunal uptake of octanoic, lauric, cholic, and taurocholic acids was greater in animals previously exposed at an earlier age to HCD, as compared with those first exposed to HCD at a later age. Furthermore, the effect of HCD on the passive permeability properties of the intestine lasted for as long as 14 weeks after the animals were switched from HCD back to chow. These alterations in permeability properties occurred in the absence of changes in the intestinal mucosal surface area and were not explained by differences in the rabbits' body weight gain, food consumption, or mucosal surface area of the jejunum or ileum. Thus, (a) early changes in animals' dietary content of cholesterol may have prolonged effects on intestinal transport function, and (b) this early feeding experience with cholesterol may alter the adaptive pattern of the intestine when animals are exposed to a dietary cholesterol challenge at a later age.

Animals↗

PubMed1988

Fatty acid desaturation in the intestinal mucosa.

Information as to the ability of the enterocyte to desaturate fatty acids is lacking. This is important in understanding whether the source of intestinal arachidonic (20:4(n-6) acid is biliary or from de novo synthesis. Delta 9- and delta 6-desaturase enzymes were assayed in homogenates of rat jejunum, ileum and liver. Rat small intestine possesses desaturase activity to convert palmitic (16:0) to palmitoleic (16:1) and linoleic (18:2(n-6) to linolenic (18:3(n-6) acid. Enzyme activities were highest in liver relative to activity in jejunal and ileal homogenates. It is concluded that delta 9- and delta 6-desaturase activities may have an important role in determining physico-chemical properties and thus transport properties of enterocyte membranes.

Animals↗

PubMed1988

Regional differences in lipid composition and incorporation of saturated and unsaturated fatty acids into microsomal membranes of rat small intestine.

Incorporation of [1-14C]palmitic (16:0) and [1-14C]linoleic (18:2 omega 6) acids into microsomal membranes of proximal (jejunum) and distal (ileum) regions of rat small intestine was investigated, and the lipid composition, including fatty acid profiles of membrane phospholipids, was determined. Jejunal microsomes contained significantly higher amounts of total phospholipids, phosphatidylcholine, and phosphatidylinositol, and lower amounts of cholesterol and sphingomyelin when compared with ileal microsomes. Jejunal microsomal phospholipids contained higher levels of stearic (18:0), 18:2 omega 6, and eicosapentaenoic (20:5 omega 3) acids followed by reduced levels of oleic (18:1 omega 9), arachidonic (20:4 omega 6), and docosahexaenoic (22:6 omega 3) acids when compared with those from the ileum, except for phosphatidylinositol where no significant difference between 20:4 omega 6 content of each site was observed. In both jejunal and ileal microsomes, incorporation of [1-14C]18:2 omega 6 was significantly higher than that of [1-14C]16:0. Incorporation of both [1-14C]16:0 and [1-14C]18:2 omega 6 was significantly higher in jejunal microsomal lipid fractions (phospholipids, diacylglycerols, triacylglycerols) when compared with the ileal microsomal fraction. These data suggest that (1) jejunal and ileal microsomal membranes differ from each other in terms of lipid composition and lipid synthesis, (2) site variations in the specificity of acyltransferases for different fatty acids exist, and (3) higher delta 9-, delta 6-, delta 5-, and delta 4-desaturase activities exist in ileal compared with jejunal enterocytes.

Animals↗

PubMed1988

Dietary effects of omega 3-fatty acids on intestinal transport function.

Animals were fed for 2 weeks on one of four isocaloric and isocholesterolic semisynthetic diets: high 18:3 omega 3, low 18:3 omega 3, high 20:5 omega 3, or low 20:5 omega 3. The weight of the intestine and the percentage of the wall consisting of mucosa was greater in high 20:5 omega 3 than in high 18:3 omega 3, and greater in low 20:5 omega 3 than in low 18:3 omega 3, although the mucosal surface area was 26% lower in high 20:5 omega 3 than high 18:3 omega 3. The jejunal uptake of 40 mM glucose and ileal uptake of 40 mM galactose was greater in high 18:3 omega 3 than in high 20:5 omega 3, jejunal uptake of fatty acid 12:0 was higher, but 18:0 was lower in high 18:3 omega 3 than in high 20:5 omega 3. The jejunal or ileal uptake of cholesterol was not affected by 20:5 omega 3. However, 20:5 omega 3 had a variable effect on the uptake of medium- and long-chain fatty acids. Alterations in the uptake of fatty acids and glucose were not explained by any difference in the animals' food consumption, body weight gain, or intestinal weight, but the reduced jejunal uptake of 40 mM glucose in rats fed the high 20:5 omega 3 diet was associated with reduced mucosal surface area. Thus, (i) varying the source of omega 3-fatty acids (vegetable, 18:3 omega 3 versus fish oil, 20:5 omega 3) altered the mucosal mass of the intestine, and (ii) the source of the dietary omega 3-fatty acid (18:3 omega 3 versus 20:5 omega 3) influenced intestinal hexose uptake, with fish oil having an anti-absorptive effect on the jejunal uptake of D-glucose.

Animals↗

PubMed1988

Use of polyunsaturated fatty acid diet to treat the enhanced intestinal uptake of lipids in streptozotocin diabetic rats.

Intestinal passive transport of lipids is enhanced in rats with streptozotocin-induced diabetes. The present study was undertaken to test the effect of semisynthetic isocaloric diets, varying in saturated (S) or polyunsaturated (P) fatty acid content, on intestinal lipid uptake when fed to rats previously rendered diabetic. In vitro uptake of fatty acids and cholesterol was higher in the jejunum and ileum of diabetic rats than in control rats fed rat chow. Feeding P prevented the enhanced jejunal and ileal uptake of 18:1 and 18:2, and feeding S prevented the enhanced jejunal uptake of 18:2 in diabetic rats fed rat chow. In diabetic rats, feeding P was associated with reduced jejunal uptake of 16:0 and 18:1 and reduced ileal uptake of 16:0, 18:0, and 18:2, as compared with diabetic rats fed S. Weight gain and blood glucose concentration was similar in diabetic animals fed chow, P or S. The percentage of the jejunal wall comprised of mucosa was greater in diabetic than in control rats fed chow or P but was similar in control and diabetic rats fed S. However, this diminution in uptake was not explained by these differences. The potential long-term role of feeding P in the diminution of the hyperlipidemia associated with human diabetes remains to be established.

Animals↗

PubMed1987

Treatment of the enhanced intestinal uptake of glucose in diabetic rats with a polyunsaturated fatty acid diet.

Intestinal absorption of most nutrients is enhanced in diabetic rats. We wished to test the hypothesis that manipulation of dietary fatty acids will modify enhanced uptake of glucose in rats with established streptozotocin-diabetes. Chow-fed control rats or animals with one week of streptozotocin-diabetes were continued on chow or were fed ad libitum for three weeks with semisynthetic isocaloric diets containing a high content of either essential polyunsaturated or non-essential saturated fatty acids. The jejunal and ileal in vitro uptake of varying concentrations of glucose was much higher in diabetic than control rats fed chow or the saturated fatty acid diet. In contrast, the enhanced uptake of this sugar was reduced or normalized in diabetic rats fed the polyunsaturated fatty acid diet. Feeding the polyunsaturated fatty acid diet was associated with increased brush-border membrane activity of alkaline phosphatase in diabetic jejunum and ileum, but neither the saturated fatty acid diet nor the polyunsaturated fatty acid diet altered brush-border membrane cholesterol or phospholipids in control or in diabetic rats. Mucosal surface area was similar in diabetic rats fed the saturated fatty acid diet or the polyunsaturated fatty acid diet. Thus, (1) feeding the polyunsaturated fatty acid diet diminishes the enhanced jejunal and ileal uptake of glucose in diabetic rats, and (2) the influence of the polyunsaturated fatty acid diet on uptake in diabetic rats was not explained by alterations in intestinal morphology or brush-border membrane content of cholesterol or phospholipids. This study suggests that manipulation of dietary lipids may play a role in the normalization of the enhanced intestinal glucose uptake in rats with established diabetes.

Animals↗

PubMed1987

Late effects of early feeding of a low cholesterol diet on the intestinal active and passive transport properties in the rabbit.

It has previously been demonstrated that a short period of feeding of a low cholesterol diet (LCD) alters the active and passive transport properties of the intestine. This study was undertaken to determine the late effect of early nutrition with LCD. An in vitro technique was used to determine the uptake of glucose, galactose, leucine, medium- and long-chain fatty acids, cholesterol and bile acids into the jejunum, ileum and colon of rabbits. Five dietary groups were used: group A was fed chow for 16 weeks; group B was fed chow for 14 weeks followed by LCD for 2 weeks; group C was fed chow for 2 weeks, LCD for 2 weeks, chow for 10 weeks, and LCD for 2 weeks; group D was fed chow for 2 weeks and LCD for 14 weeks; and group E was fed chow for 2 weeks, LCD for 2 weeks and chow for 14 weeks. Animals fed LCD for 2 weeks at an early age demonstrated different alterations in the active and passive transport of these solutes as compared with animals exposed to LCD for only 2 weeks at a later age (group C vs. group D). Feeding the animals chow for 12 weeks after 2 weeks of LCD (group E) did not result in a normalization of the altered intestinal transport. The transport changes were progressive, with qualitative and quantitative differences in animals fed LCD for 2 weeks (group B) as compared with animals fed LCD for 14 weeks (group D). These changes in intestinal transport were not due to differences in food consumption or mucosal weight but were likely of nutritional significance, since the animals' body weight gain differed in animals fed chow as compared with those fed LCD. Thus, (1) feeding LCD is associated with alterations in the active and passive jejunal, ileal and colonic uptake of nutrients; (2) these alterations are influenced by the duration of feeding LCD; (3) the effects of LCD persist for at least 10 weeks after the diet is stopped and feeding with chow is restored; and (4) rechallenge with LCD following an earlier exposure to LCD is associated with an accentuation of the intestinal effects of LCD. It is concluded that these late effects of early nutrition with a low cholesterol diet upon intestinal transport function may have an important impact on the nutritional status of the animal.

Aging↗

PubMed1987

Onset and persistence of changes in intestinal transport following dietary fat manipulation.

In this study we determined the time-course for the onset and the loss of the effect of short-term feeding rats isocaloric semisynthetic diets containing a high content of saturated (HS) or polyunsaturated (HP) fatty acids on the jejunal and ileal uptake of medium- and long chain fatty acids, cholesterol and glucose. Animals were fed HP or HS for 3, 7 or 14 days; then the diet was switched to standard Purina rat chow for a further 3, 7 or 14 days. The uptake of medium chain fatty acids was unchanged. The differences between HP and HS in glucose uptake occurred within 3 days, but persisted for 14 days, whereas there were qualitative as well as quantitative changes in the pattern of lipid uptake: differences in uptake of stearic, oleic, linoleic and linolenic acids and cholesterol occurred after 7 days of feeding HP or HS. Jejunal uptake of linoleic acid was greater in HP than HS on day 7, but HS was greater than HP on day 14. The effect of diet on lipid uptake was similar in the jejunum and ileum. The altered uptake of stearic and oleic acids persisted after the rats were switched back to chow, whereas the uptake of the other nutrients became similar. Thus, changes in dietary content of saturated and polyunsaturated fatty acids have early effects on intestinal transport function; some of these changes persist even when animals are returned to feeding on chow; and glucose transport is rapidly altered by dietary changes, whereas lipid uptake changes only after 7 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

Development of active and passive transport of bile acids in rabbit intestine.

Previous studies have indicated that saturable, Na+-bile acid coupled transport is absent in the ileum throughout most of the suckling period and that the maturation of ileal bile acid uptake which occurs during weaning results primarily through an increase in functional bile acid carriers within the ileal brush border membrane. This study was undertaken in weanling and adult rabbits to establish the effect of maturation on the active ileal and passive jejunal and colonic uptake of 8 bile acids. The in vitro uptake of a range of concentrations of cholic (C), taurocholic (TC), glycocholic (GC), chenodeoxycholic (CDC), taurochenodeoxycholic (TCDC), glycochenodeoxycholic (GCDC), deoxycholic (DC) and taurodeoxycholic (TDC) acid was determined. Active ileal uptake of C, TC, GC, DC and TDC was greater in adult than in weanling animals, whereas uptake of CDC, TCDC and GCDC was similar in both groups. The relative permeability for passive uptake of bile acids into the jejunum and colon was similar in young and adult rabbits. Jejunal mucosal surface area was similar in the two groups, but was greater in the ileum in the adult than in the weanling rabbits due to an increase in villus height, width, and number of villi per unit serosal length. However, the age-associated differences in active bile acid uptake were not explained simply on the basis of these differences in ileal mucosal surface area or the concentration of bile acids in the intestinal lumen. Thus, maturation of the rabbit from weanling to adulthood does not influence the relative passive permeability of the jejunum or colon to bile acids, but does increase active ileal uptake of conjugated and unconjugated cholic acid and deoxycholic acid, but not chenodeoxycholic acid.

Aging↗

PubMed1987

Diets alter jejunal morphology and brush border membrane composition in streptozotocin-diabetic rats.

Previous studies have demonstrated enhanced active and passive uptake of many nutrients in animals with experimental diabetes. These changes in absorption cannot be explained by differences in intestinal morphology, although the brush border membrane (BBM) phospholipids do change in diabetes. Manipulation of diet produces alterations in intestinal uptake of lipids and glucose. This study was undertaken to determine the effect of diet and diabetes on jejunal morphology and BBM lipid composition. Rats were rendered hyperglycemic with streptozotocin and were fed for 2 weeks on a diet that was high or low in carbohydrate, essential fatty acids, cholesterol, or protein. In both control and diabetic rats, these diets produced changes in villus height and BBM sucrase and alkaline phosphatase activities. In both control and diabetic rats, BBM phospholipids were unaffected by changes in the dietary content of essential fatty acids, cholesterol, or protein, but total BBM phospholipid content was reduced in animals fed low as compared with high carbohydrate diet. Total BBM phospholipid content was higher in diabetic than in control animals fed the low protein diet, whereas BBM phospholipid content was lower in diabetic than in control animals fed the high carbohydrate diet, and was even lower in diabetic animals fed the low as compared with the high carbohydrate diet. These changes in total phospholipids were due to alterations in the BBM content of phospholipids containing choline. In control animals, BBM cholesterol was higher in rats fed the low as compared with the high cholesterol diet, or the low as compared with the high protein diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

Spectrum of effects of dietary long-chain fatty acids on rat intestinal glucose and lipid uptake.

Isocaloric modification in the ratio of dietary polyunsaturated-to-saturated fatty acids influences intestinal uptake of actively and passively transported nutrients. This study was undertaken to determine which dietary fatty acid was responsible for these alterations in absorption. Adult female rats were fed isocaloric semisynthetic diets high in palmitic and stearic acids (SFA), oleic acid (OA), linoleic acid (LA), or linolenic acid (LNA). An in vitro technique was used to measure the uptake of varying concentrations of glucose as well as a series of fatty acids and cholesterol. Jejunal uptake of 40 mM glucose was highest in rats fed SFA and lowest in those fed LA; ileal glucose uptake was similar in OA, LA, and LNA, but was lowest in SFA. Jejunal uptake of medium-chain fatty acids (8:0-12:0) was higher in OA than in other diet groups; ileal uptake of medium-chain fatty acids was unaffected by diet. Jejunal and ileal uptake of 18:2 was higher in LNA than in SFA or OA; the uptake of the other long-chain saturated or unsaturated fatty acids was unchanged by diet. The ileal but not the jejunal uptake of cholesterol was increased in LA as compared with SFA or OA, and reduced in LNA as compared with LA. These transport changes were not explained by differences in the animals' food consumption, body weight gain, intestinal mass, or mucosal surface area. We postulate that these diet-induced transport alterations may be mediated via changes in brush border membrane phospholipid fatty acyl composition. Thus, intestinal transport of nutrients may be varied by isocaloric changes in the dietary content of individual fatty acids.

Animals↗

PubMed1987

A high linoleic acid diet diminishes enhanced intestinal uptake of sugars in diabetic rats.

Intestinal absorption is altered in rats by diabetes and by dietary manipulation. For 5 wk rats were fed chow (C) or semisynthetic diets containing isocaloric amounts of a high content of either polyunsaturated (P) or saturated (S) fatty acids; then half were rendered hyperglycemic with streptozotocin while the remaining half served as nondiabetic controls. Studies were performed after a total of 8 wk on C, P, or S. Jejunal villous surface area was greater in diabetic rats than in control rats fed C or S, whereas the jejunal mucosal surface area was significantly greater in diabetic animals than in control animals only when they were fed S. Ileal mucosal surface area was similar in diabetic and in control rats fed C, S, or P; although ileal villous surface area was greater in diabetic than in control rats only when they were fed C. The jejunal and ileal uptake of varying concentrations of glucose and galactose was higher in diabetic rats than controls fed C or S. In contrast, the enhanced uptake of these sugars was reduced or normalized in diabetic rats fed P. The jejunal brush-border membrane (BBM) content of phospholipids was higher in diabetic than in control rats fed chow, but there was no difference in diabetic vs. control animals fed P or S in BBM total free fatty acids, phospholipids, or cholesterol. Thus feeding a high linoleic acid diet diminishes the enhanced intestinal uptake of glucose and galactose in diabetic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

Feeding rats diets containing cheno- or ursodeoxycholic acid or cholestyramine modifies intestinal uptake of glucose and lipids.

We wished to test the hypothesis that variations in the luminal content of bile acid produced by feeding chenodeoxycholic acid (CDC), ursodeoxycholic acid (UDC) or cholestyramine (C) alter intestinal transport properties and morphology. Rats were fed standard chow pellets containing 0.5% CDC, 0.5% UDC or 2% C for a period of 2 weeks, and the in vitro uptake of glucose, cholesterol, bile acids and a homologous series of fatty acids was assessed. The food consumption was similar in animals fed chow, CDC, UDC and C, yet rats fed CDC or UDC gained less weight, and the weight of the jejunal and ileal mucosa was lower in animals fed CDC or C than in those fed chow or UDC. Ileal but not jejunal uptake of glucose was reduced in animals fed UDC, CDC or C. The active ileal uptake of bile acids was enhanced by UDC, CDC or C, whereas the jejunal passive permeability to bile acids was reduced by feeding C. Feeding C inhibited the jejunal and ileal uptake of cholesterol; C, UDC and CDC had a variable effect on the intestinal uptake of the fatty acids 10:0-18:0. The jejunal mucosal surface area was lower in groups fed CDC or UDC as compared with rats fed chow or C, and the ileal mucosal surface area was lower in rats fed CDC. However, the altered intestinal transport could not be explained by the altered morphology. Thus, (1) chronic variations in the intestinal luminal content of bile acids produced by the feeding of CDC, UDC or C resulted in alterations in the active and passive transport properties of the intestine, and (2) these changes differed between the jejunum and the ileum and were not explained simply by alterations in the animals' food intake or mucosal morphology. These studies suggest that chronic variations in the bile acids in the intestinal lumen may be one of the factors independently influencing the transport properties and mucosal surface area of the intestine. The long-term effect of changes in luminal bile acid content on intestinal function in man remains to be established.

Animals↗

PubMed1987

Early feeding of a high-cholesterol diet enhances intestinal permeability to lipids in rabbits.

This study was undertaken in young and growing rabbits to establish the of feeding a high- (2%) cholesterol diet (HC) on the intestinal in vitro uptake of a homologous series of saturated fatty acids, dodecanol, cholesterol, and bile acids. In the jejunum there was an increase in the uptake of myristic acid and dodecanol in HC as compared with animals fed Purina Chow, whereas in the ileum the uptake of stearic acid, cholesterol, and taurocholic acid was reduced. There was increased jejunal uptake of medium-chain length fatty acids, increased incremental change in free energy (integral of delta Fw----l), increased uptake of palmitic, taurocholic acid, and glycocholic acid and reduced uptake of myristic acid and dodecanol in animals fed the high-cholesterol diet for 10 days followed by Purina Chow for 18 days (HC-chow), as compared with HC. There was also increased ileal uptake of palmitic acid in HC-chow as compared with HC but reduced ileal uptake of stearic acid, cholesterol, taurocholic acid, and glycocholic acid. In the colon, there was increased uptake of stearic acid and dodecanol in HC-C as compared with HC, but reduced uptake of cholesterol, taurocholic acid and glycocholic acid. These changes in lipid uptake were not explained by alterations in the mucosal surface area or in the animals' body weight gain. It is proposed that early feeding experiences with a high-cholesterol diet may alter the normal development of intestinal transport of lipids with increased jejunal permeability to medium-chain fatty acids and increased effective resistance of the intestinal unstirred water layer.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

Isocal and Portagen alter intestinal uptake of sugars but not mucosal surface area in rabbits with an intact intestinal tract and following ileal resection.

This study was undertaken to examine the effect of feeding 2 defined-formula diets, Isocal and Portagen, on intestinal morphology and transport function in rabbits subjected to an ileal resection, and in control animals with an intact intestinal tract. Neither ileal resection nor feeding Isocal or Portagen significantly altered jejunal mucosal surface area. In animals with an intact intestinal tract, feeding Isocal was associated with decreased jejunal uptake of 2 mM glucose, whereas both Isocal and Portagen were associated with increased jejunal uptake of 40 mM glucose, and increased ileal uptake of 2 mM glucose, as compared with rabbits fed chow. In animals with an ileal resection, feeding Isocal was associated with increased jejunal and colonic uptake of galactose, whereas jejunal uptake of glucose was unaffected. Thus, (1) feeding defined-formula diets alters the intestinal active uptake of sugars without altering intestinal mucosal surface area; (2) sugar uptake rates were generally higher in animals fed Isocal than in those fed Portagen; (3) ileal resection obscured the differences in the jejunal uptake of higher concentrations of glucose observed in control rabbits (with an intact intestinal tract) fed these defined-formula diets; and (4) the differences in the effects of defined-formula diets on sugar uptake in control and resected animals was likely due to a direct effect on the glucose and galactose carrier(s), rather than to an indirect effect on food consumption, body weight gain, intestinal morphology, or the effective resistance of the intestinal unstirred water layer. It is suggested that the functional differences observed between animals fed Isocal versus Portagen may be due to the varying source of lipids and proteins in these diets, their different ratios of saturated-to-polyunsaturated fatty acids, and their different percent content of carbohydrate.

Animals↗

PubMed1986

Uptake of hexoses and lipids into rabbit jejunum and colon following ileal resection: effect of variations in the cholesterol and fat content of the diet.

After 6 weeks feeding on either a high-cholesterol/fat (H) or a low-cholesterol/fat (L) diet, jejunal and colonic uptake was measured using a previously validated in vitro technique in control rabbits with an intact intestinal tract (C) and in animals submitted to the surgical removal of the distal half of the small intestine (R). The uptake of hexoses and fatty acids was influenced by ileal resection and by diet. Dietary manipulation altered the passive and active transport properties of the intestine and had a different effect on intestinal transport in animals with an ileal resection than in animals with an intact small intestine.

Animals↗

PubMed1986

Fractionated low doses of abdominal irradiation alters jejunal uptake of nutrients.

Abdominal radiation is associated with changes in intestinal uptake of nutrients that begins within three days and persist for over 33 weeks. Clinically, fractionated doses of radiation (FDR) are used in an attempt to minimize the complications of this therapy, but the effects of fractionated doses of radiation on intestinal transport have not been defined. Accordingly, an in vitro technique was used to assess the jejunal and ileal uptake of varying concentrations of glucose and leucine, as well as the uptake of single concentrations of fatty acids and decanol in rats exposed 3, 7, and 14 days previously to a course of 200 cGy given on each of five consecutive days. FDR was associated with an increase in the uptake of decanol, and therefore a decrease in the effective resistance of the unstirred water layer. FDR had a variable effect on the uptake of glucose and leucine, with a decline in the value of the Michaelis constant (Km) and the passive permeability coefficient for glucose (Pd), whereas the Km for leucine was unchanged and the Pd for leucine was variably affected by FDR. The maximal transport rate (Jdm) for leucine progressively rose following FDR, whereas the Jdm for glucose initially rose, then fell. The uptake of galactose and medium chain-length fatty acids was unchanged by FDR, whereas the jejunal uptake of myristic acid rose, and the uptake of cholic acid declined, then returned to normal. FDR was associated with greater body weight gain and jejunal and ileal weight, but these changes did not adequately explain the variable alterations in the kinetics of uptake. The changes in nutrient uptake following FDR differed from the absorption changes occurring after a single dose of radiation. Thus, fractionated doses of abdominal radiation produce complex changes in the intestinal uptake of actively and passively transported nutrients, and these variable changes are influenced by the time following radiation exposure and by the solute studied.

Abdomen↗

PubMed1986

The development of the small intestine.

The remarkable degree of coordination between the development of various aspects of gastrointestinal function suggests that the process may be triggered by a single or a few central mechanisms, such as weaning and (or) hormones. Precocious development of enzyme and transport function can be induced by exogenous thyroxine and corticosteroids, while thyroidectomy and adrenalectomy abolish the normal pattern of postnatal development. These hormones may have a primary or a permissive role. Activation of the dormant hormonal mechanism could be controlled by a genetically coded biologic clock, such as chronologic age, or by a biological signal such as body size and oral intake. Generally speaking, shortly after birth, there are increases in the intestinal mucosal surface area, brush border membrane enzymes, and carrier-mediated transport. These adaptive changes occur as a result of the genetic endowment of the animal, but may be modified by environmental factors, particularly nutrient intake.

Aging↗