Duane L. Miller

Total Recovery Studies of Nonabsorbable Indicators in the Rat Small Intestine

Summary Polyethylene glycol, phenol red, polyethylene glycol- 14 C, inulin-carboxyl- 14 C, and mannitol-1- 14 C were compared as nonabsorbable indicators in the rat small intestine. The indicator solution was perfused through the lumen of the in situ intestine. The indicator content of collected perfusion solution and luminal washings was used to calculate total recovery. Mean total recoveries for all indicators were more than 95 % except for tracer mannitol-1- 14 C. When carrier mannitol was added, recovery of mannitol-1- 14 C was satisfactory. Even the indicators polyethylene glycol and phenol red were only 97 to 98 % recovered. Total indicator recoveries from serum and urine could not explain the 2 to 3 % loss of indicator. If recovery data are a valid measure of indicator absorption, indicators underestimate net water absorption by 2 to 3 %. The radioactively labeled indicators are analyzed readily and should prove to be increasingly useful.

Nonabsorbed Indicators: A Comparison of Phenol Red and Inulin-14C and Effects of Perfusion Technique

Nonabsorbed indicators should be completely recoverable from the intestinal lumen and have an intraluminal distribution pattern identical to the unabsorbed residue of the substance(s) whose transport is being studied. Since small amounts of all nonabsorbed indicators are lost, an error is introduced when indicators are used in transport studies. We examined this error with phenol red and inulin-carboxyl-14C in the small intestine of the rat using two common techniques: (1) single pass perfusion, in which a relatively large volume and amount of indicator traversed the lumen once; and (2) recirculation, in which a relatively small volume and amount of indicator was repeatedly passed through the lumen. We found no difference between techniques in the absolute rate of loss of indicator. However, when a smaller quantity of indicator was recirculated, the percentage loss of indicator was significantly greater. Since indicator concentration is used to calculate net water flux, any loss of indicator causes spuriously low final concentrations (spuriously high estimates of secretion). The greater percentage loss of indicator with recirculation doubled this error. Moreover, the indicators differed significantly: more phenol red was lost than inulin, and about 20% more was washed from the mucusal surface (adsorption). Thus, experimental design as well as the behavior of individual indicators significantly influence the results of transport studies.

Effects of Experimental Diabetes on Intestinal Strontium Absorption in the Rat

Summary Control and streptozotocin diabetic rats were studied at 5 and 12 days after induction of diabetes. Strontium absorption was measured by in situ perfusion of duodenum and ileum. Duodenal absorptive capacity (absorption per unit length) and absorptive specific activity (absorption per gram of dry weight mucosa) were depressed. Depression was present both at 5 days, when mucosal growth is similar in controls and diabetics, and at 12 days, when mucosal growth is 50% greater in diabetics. Effects of diabetes on ileal absorption were minimal in comparison with effects on duodenum. This depression of duodenal strontium absorption in the diabetic rat is analogous to effects of diabetes on calcium absorption and may be mediated by abnormal vitamin D metabolism.

Rat small intestine: Development, composition and effects of perfusion

Development of the small intestine is well underway by birth, but it contributes a meager 1.4% (0.07 g) to total body weight. At 25 g (five times the birth weight), it has more than doubled in length (from 20 to 45 cm), is about nine times as heavy, and represents 2.4% (0.6 g) of the body weight. During adolescence, the small intestine undergoes rapid gains until it reaches 75% of its adult length (80 cm), and attains a maximum percentage of nearly 5.0% (3.0 g) of the body weight (60 g). By 125 g, it is nearly fully developed: 95 cm in length, 5 g in weight, and 4% of the body weight. From this point, it only lengthens 10–20 cm and increases 1–2 g in weight. However, the percent of intestine to body weight continues to decrease throughout adulthood to reach only 1.4% (500 g), which was the percentage at birth. Regression formulas show that in the adult rat, the small intestine lengthens 5 cm and gains 0.5 g for each 100-g increase in body weight. Variation within each of the intestinal values measured was considerable but correlated positively with the body weight. Tissue water content was greatest in the duodenum, and varied according to both site and age.Development of the small intestine is well underway by birth, but it contributes a meager 1.4% (0.07 g) to total body weight. At 25 g (five times the birth weight), it has more than doubled in length (from 20 to 45 cm), is about nine times as heavy, and represents 2.4% (0.6 g) of the body weight. During adolescence, the small intestine undergoes rapid gains until it reaches 75% of its adult length (80 cm), and attains a maximum percentage of nearly 5.0% (3.0 g) of the body weight (60 g). By 125 g, it is nearly fully developed: 95 cm in length, 5 g in weight, and 4% of the body weight. From this point, it only lengthens 10–20 cm and increases 1–2 g in weight. However, the percent of intestine to body weight continues to decrease throughout adulthood to reach only 1.4% (500 g), which was the percentage at birth. Regression formulas show that in the adult rat, the small intestine lengthens 5 cm and gains 0.5 g for each 100-g increase in body weight. Variation within each of the intestinal values measured was considerable but correlated positively with the body weight. Tissue water content was greatest in the duodenum, and varied according to both site and age.

Kinetics of Calcium Transport in the Hamster Duodenum and Ileum

Dietary calcium is absorbed chiefly from the small intestine, and characteristics of the transport process differ depending on site. Because transport behavior is crucial to calcium homeostasis, we characterized kinetics of duodenal and ileal calcium transport in vivo in the hamster. We measured net calcium transport with 40Ca and flux into the animal (influx) with 45Ca over the concentration range of 0- 25 mM. Nonmediated transport was twice as great in duodenum as in ileum. Mediated transport showed Michaelis-Menten kinetics. Kt for mediated transport (4.3 mM) was the same in duodenum and ileum, whereas Vmax (mumol/h per g) was twice as great in ileum (290) as in duodenum (137). Thus, Kt defines similar carrier function at both sites, but Vmax suggests calcium transport capacity is doubled in ileum. The lower nonmediated transport in ileum limits secretion and the higher Vmax absorbs calcium at a greater rate, permitting the ileum to move luminal calcium against a concentration gradient more effectively than the duodenum. Thus, ileal calcium transport, by its site and functional characteristics, makes a crucial contribution to calcium homeostasis in the hamster.

Proximal to distal secretory and absorptive gradients of the rat small intestine.

Summary Secretion of sodium, chloride, and water was most rapid in the proximal small intestine and showed a progressive distal decline when osmotic and concentration gradients were imposed by intraluminal 300 mM mannitol. Like secretion, absorption of sodium, chloride, and water from 152 mM sodium chloride solution was highest proximally and showed a progressive distal decrease. The pattern of transmucosal potential differences showed no correlation with absorption and secretion rates.

Effects of Dietary Calcium and Phosphorus on Vitamin D Metabolism and Calcium Absorption in Hamster

Abstract We studied the following responses to restriction of dietary calcium and phosphorus in the growing hamster: (i) serum concentrations of calcium, inorganic phosphorus, magnesium, and vitamin D metabolites; and (ii) calcium transport by ileum. Diets fed were normal calcium with normal or low phosphorus or low calcium with normal or low phosphorus. We found serum 1α,25-dihydroxycalciferol (1,25-[OH]2D) concentration did not differ significantly among the diet groups. Calcium absorption, measured as serosal/mucosal calcium concentration ratio produced by everted ileal sac, was greater in the low calcium, normal phosphorus group than in all other groups. The other groups did not differ from one another in calcium absorption. Feeding the low calcium, normal phosphorus diet increased inorganic phosphorus and magnesium but decreased calcium concentration in serum in comparison with the other three diets. Both low phosphorus diets were without effect on serum calcium, but the low calcium, low phosphorus diet increased serum inorganic phosphorus and magnesium above that of the normal calcium, low phosphorus diet. Ileal calcium absorption in hamster (i) was independent of serum 1,25-(OH)2D concentration; (ii) increased in response to low dietary calcium if dietary phosphorus was normal; and (iii) was independent of dietary calcium, if dietary phosphorus was low. Despite increased calcium absorption, serum calcium was decreased in the low calcium-normal phosphorus group as compared with all other groups. Feeding low calcium diets increased serum inorganic phosphorus and magnesium as compared with feeding the corresponding normal calcium diets (i.e., independently of whether dietary phosphorus content was normal or low). These studies demonstrate that the interrelationships between calcium absorption and vitamin D and mineral metabolism in hamster differ from other mammals.

Fat and mineral balance in the spontaneously hypertensive (SH) and Wistar-Kyoto (WKy) rat

Abstract Calcium metabolism and hypertension may be interrelated in the SH/WKy rat model. Results of calcium balance studies in this hypertensive model have been conflicting. Malabsorptive disease causing steatorrhea can cause variability in calcium balance. To test this hypothesis we measured balances of fat, calcium, magnesium, and phosphorus in ad lib fed 12 week old males from the two strains. Balances of fat, calcium and magnesium was the same in SH and WKy, but phosphorus balance was lower in SH. We conclude that despite the differences in calcium metabolism between SH and WKy, fat and calcium balance do not differ, and homeostasis is maintained in both strains. The conflicting results of balance studies cannot be attributed to malabsorption.

A new feeder for powdered diets.

A new feeder for powdered diets was developed for use with small rodents. The feeder has unique advantages over commercially available feeders. Wastage is minimal, food intake can be measured, and it is adaptable to various rodent species and different types of powdered diets. This study describes and evaluates the new feeder using three animal groups - weanling rats, adult rats, and adult hamsters. In the rat studies, the rate of growth of weanling and adult rats fed commercial rat chow in pellet form using conventional feeders was the same as that of rats fed the same chow in meal form using the new feeders. Likewise in the hamster studies, the rate of growth of adult hamsters fed powdered semi-synthetic diet from conventional glass jars was the same as those fed the same diet from the new feeders. The new feeders supplied the animals with a continuous supply of meal free of contamination from feces or urine. Unrecoverable spillage (wastage) was 1-2% of the amount of food given. The new feeders were easy to handle and clean, had ample capacity, and needed little attention.

Alpha-Aminoisobutyric Acid Loss by Rat Small Intestine: Role of Intestinal Site and Luminal Contents

Loss of amino acid into the lumen may be important in diarrheal states. Labeled alpha-aminoisobutyric acid (AIB) was given parenterally, and concentration gradients developed between lumen, intestinal tissue and plasma. After 24 h, we measured AIB loss into the intestinal lumen by recirculating isolated segments in situ under conditions of either net water absorption or net water secretion. Final 14C-AIB concentrations in intestinal tissue were 200-800 times that of lumen, 6--15 times that of serum. Despite increased total 14C-AIB loss during net water secretion, and based on the low luminal 14C-AIB concentrations, the contribution of amino acid loss to fecal nitrogen during osmotic diarrhea appears to be minimal.