Net Calcium Absorption Research Articles

Effects of lactose on calcium absorption and secretion by rat ileum.

The direct effects of lactose on net intestinal calcium absorption were determined by measuring unidirectional steady-state calcium fluxes in vitro under short-circuited conditions in segments of rat ileum. The isosmotic mucosal additions in segments of rat ileum. The isosmotic mucosal addition of lactose (160 mM) increased net calcium absorption (J net) by increasing the absorptive flux from mucosa to serosa (Jm----s) and reducing the secretory flux from serosa to mucosa (Js----m). Lactose also reduced tissue conductance and short-circuit current and reversed tissue polarity. 1,25-Dihydroxyvitamin D3 administration (50 ng/day for 4 days) increased J net from secretion to no net flux (Jm----s = Js----m), and lactose increased J net further to net absorption. Removal of sodium from the medium, like lactose addition, increased J net by increasing Jm----s and reducing Js----m. The replacement of medium sodium with choline abolished a further increase of J net by lactose. These results show that lactose increases net calcium absorption in the absence of transepithelial electrochemical or osmotic gradients. Transcellular calcium transport may be stimulated by lactose by hyperpolarization of the brush border as a result of reduced mucosal sodium.

An evaluation of the importance of gastric acid secretion in the absorption of dietary calcium.

Since calcium solubility is a prerequisite to calcium absorption, and since solubility of calcium is highly pH-dependent, it has been generally assumed that gastric acid secretion and gastric acidity play an important role in the intestinal absorption of calcium from ingested food or calcium salts such as CaCO3. To evaluate this hypothesis, we developed a method wherein net gastrointestinal absorption of calcium can be measured after ingestion of a single meal. A large dose of cimetidine, which markedly reduced gastric acid secretion, had no effect on calcium absorption in normal subjects, and an achlorhydric patient with pernicious anemia absorbed calcium normally. This was true regardless of the major source of dietary calcium (i.e., milk, insoluble calcium carbonate, or soluble calcium citrate). Moreover, calcium absorption after CaCO3 ingestion was the same when intragastric contents were maintained at pH 7.4 (by in vivo titration) as when intragastric pH was 3.0. On the basis of these results, we conclude that gastric acid secretion and gastric acidity do not normally play a role in the absorption of dietary calcium. Other possible mechanisms by which the gastrointestinal tract might solubilize ingested calcium complexes and salts are discussed.

Thyroparathyroidectomy exaggerates calciuric action of ammonium chloride in rats.

The effects of chronic ammonium chloride (NH4Cl) administration on urinary calcium, urinary hydroxyproline, and calcium and phosphate balances were studied in intact and thyroparathyroidectomized (TPTX) rats. NH4Cl (2 g/100 g diet) was administered for 16 days to growing rats consuming a low calcium (0.1% Ca) diet. NH4Cl increased urinary calcium and hydroxyproline. NH4Cl caused greater urinary calcium loss in TPTX than in intact rats, but hydroxyproline excretion in these groups was similar. Compensatory increases in net alimentary absorption of calcium and phosphate occurred in intact but not in TPTX rats. Urinary cAMP was depressed by thyroparathyroidectomy but was unaffected by NH4Cl. It is concluded that NH4Cl depresses renal tubular reabsorption of calcium and increases bone resorption in the presence and absence of the thyroid and parathyroid glands. However, our results suggest that parathyroid hormone plays an important calcium-sparing role, while calcitonin may act to limit the rate of bone resorption, in intact rats during chronic administration of NH4Cl.

Calcium flux in vivo in the rat duodenum and ileum during pregnancy and lactation.

Intestinal calcium transport was examined in nonpregnant, pregnant, lactating, and 100 ng 1,25-dihydroxyvitamin D3-dosed rats. Solutions containing 150 mM NaCl, 1 mM Ca, and 0.1 microCi 45Ca were recirculated in vivo through the duodenum and ileum to determine calcium efflux from the lumen, influx into the lumen, and net absorption. In the duodenum of the nonpregnant control rats, efflux of calcium was greater than influx, resulting in net absorption of calcium. In the ileum, calcium efflux and influx were equivalent and there was little net movement of calcium. 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] significantly increased efflux and net absorption of calcium in the duodenum without affecting influx. In the ileum 1,25(OH)2D3 had no effect on calcium flux. During pregnancy and lactation there was a marked increase in mucosal tissue, and expression of transport data per centimeter of intestine gave different results than transport expressed per gram of dry mucosal tissue. Per centimeter of intestine, duodenal efflux and net absorption, but not influx, increased significantly during pregnancy and to greater extent during lactation. Per gram of dry mucosa, duodenal efflux and net absorption were increased significantly only during lactation, and the increase was comparable with that observed in 1,25(OH)2D3-dosed rats. In the ileum of pregnant and lactating rats, efflux and influx expressed per centimeter of intestine increased to approximately the same extent, and net movement of calcium remained unchanged from the control level. Expression of ileal transport data per gram of dry mucosa resulted in no significant difference in calcium movement during pregnancy and lactation.(ABSTRACT TRUNCATED AT 250 WORDS)

Availability of Macro and Microelements from Edible Domestic Waste Fed to Sheep

Young Awassi rams fitted with rumen cannula and T-shaped cannulas in duodenum and terminal ileum were used to study net absorption of calcium, phosphorus, magnesium, zinc, manganese, and copper from edible domestic waste in comparison with a conventional concentrate diet. Mean net absorptions (% of intake) of calcium, phosphorus, and magnesium from edible domestic waste and control diets were 15.3,−.9; 54.4, 37.0; and 46.4, 58.8. The small intestine was the major site for calcium and phosphorus absorption and the rumen for absorption of magnesium. Net absorption of zinc, manganese, and copper was low in both diets, probably as a result of excessive supply and low physiological demands. There was a net gain of manganese and copper and a net loss of zinc in the stomachs. In addition to potential for energy and protein, edible domestic waste supplies macro and microelements for ruminants, and thus, it can be regarded as a complete concentrate diet.

Lactose Enhances Mineral Absorption in Infancy

SummaryTo determine if lactose promotes the intestinal absorption of calcium and other minerals by infants, metabolic balance studies were performed with infants fed two formulas nearly identical in composition except for carbohydrate. One contained only lactose and the other contained sucrose and corn starch hydrolysate. Each of six normal infants had two balance studies performed with each formula in alternating sequence. When lactose was the carbohydrate, net absorption and net retention of calcium were significantly greater than when lactose was not present in the formula. Absorptions of magnesium and manganese were also significantly enhanced by lactose. Absorptions of copper and zinc were somewhat greater (not statistically significant) when lactose was present, whereas absorption of iron was not affected. Absorption of phosphorus was not different, but urinary excretion was less when the lactose‐containing formula was fed and, hence, net retention of phosphorus was significantly enhanced. These results confirm findings from animal studies and previous human studies and show that, in infants, lactose has a significant and sustained promoting effect on absorption of calcium and other minerals.

Lactose Enhances Mineral Absorption in Infancy

To determine if lactose promotes the intestinal absorption of calcium and other minerals by infants, metabolic balance studies were performed with infants fed two formulas nearly identical in composition except for carbohydrate. One contained only lactose and the other contained sucrose and corn starch hydrolysate. Each of six normal infants had two balance studies performed with each formula in alternating sequence. When lactose was the carbohydrate, net absorption and net retention of calcium were significantly greater than when lactose was not present in the formula. Absorptions of magnesium and manganese were also significantly enhanced by lactose. Absorptions of copper and zinc were somewhat greater (not statistically significant) when lactose was present, whereas absorption of iron was not affected. Absorption of phosphorus was not different, but urinary excretion was less when the lactose containing formula was fed and, hence, net retention of phosphorus was significantly enhanced. These results confirm findings from animal studies and previous human studies and show that, in infants, lactose has a significant and sustained promoting effect on absorption of calcium and other minerals.

Effects of trifluoperazine, ouabain, and ethacrynic acid on intestinal calcium transport.

Bidirectional steady-state calcium fluxes were measured in vitro under short-circuited conditions in segments of rat duodenum and descending colon. The calcium-activated ATPase (Ca-ATPase) inhibitor trifluoperazine (TFP, 0.1 mM) reduced net calcium absorption in both tissues by decreasing the absorptive flux from mucosa to serosa (Jm leads to s) without consistently altering the secretory flux from serosa to mucosa. 1,25-Dihydroxyvitamin D3 administration (50 ng/day for 4 days) increased net calcium absorption by increasing Jm leads to s, and TFP reduced Jm leads to s to the same extent across tissues from vehicle- or 1,25-dihydroxyvitamin D3-treated animals. Na-K-ATPase inhibitors ouabain and ethacrynic acid both reduced short-circuit current without affecting calcium fluxes. These data suggest that Ca-ATPase, located in the basolateral membrane of intestinal epithelial cells, plays a role in the transepithelial transport of calcium. More general effects of TFP on intestinal epithelium may also contribute to the reduction in calcium fluxes. Duodenal and descending colon calcium transport appears independent of transcellular sodium transport mediated by Na-K-ATPase.

Regulation of calcium absorption by 1,25,dihydroxy-vitamin D--studies of the effects of a bisphosphonate treatment.

In 10 patients with Paget's disease of bone and 2 patients with osteoporosis, we studied the effects of hypocalcemia and hypophosphatemia induced by disodium-(3-amino-1-hydroxypropylidene)-1,-bisphosphonate (APD) treatment on the serum concentration of PTH and 1,25-dihydroxy-vitamin D [1,25(OH)2D3] and on calcium absorption and balance. The fall in serum calcium and phosphate was associated with a rise in the serum concentration of PTH and 1,25(OH)2D3, coupled with increases in net calcium absorption and calcium balance. The concentration of 1,25(OH)2D3 was significantly related (P less than 0.001) to the serum calcium (r = 0.66), the serum phosphate (r = 0.78), and the serum PTH (r = 0.71), confirming the interrelated control of these parameters on 1,25(OH)2D3 production. Moreover the rise in 1,25(OH)2D3 caused an appropriate rise in calcium absorption (r = 0.74) and calcium balance (r = 0.86), showing that this vitamin D metabolite contributes as a hormone to calcium homeostasis.

Availability of Macroelements from a Concentrate Diet Supplemented with Soybean Meal or Poultry Manure Fed to Sheep

Eight young Merino rams fitted with T-shape cannulas in the duodenum and terminal ileum were allotted to two equal groups to study net absorption of calcium, phosphorus, and magnesium from concentrate diets supplemented by ensiled manure from caged layers or by soybean meal. Mean net absorption (percent of intake) of calcium, phosphorus, and magnesium from manure from caged layers and soybean meal diets were 1.94, 14.8; 55.4, 55.8; and 44.2, 35.0.Irrespective of diet, the small intestine was the major site for calcium and phosphorus absorption and the rumen for absorption of magnesium. Secretion of phosphorus in rumen was 2.31 and 5.34 g/day for manure and soybean meal diets.Manure from caged layers supplies phosphorus and magnesium for sheep and can supplement deficient diets. Because of low calcium absorption from poultry manure, it appears that for limited inclusion, the extra calcium contributed by manure should not be detrimental.

Calcium Metabolism in Young Rats During Early Dietary Phosphate Deprivation

SummaryA series of in vivo experiments were designed to study the early effects of phosphate depletion on calcium metabolism in young adult (8–10 week old) rats with intact thyroid‐parathyroid glands. Hypercalciuria occurred within 24 h of dietary phosphate deprivation. It was the consequence of mobilization of calcium stores from the exchangeable calcium pool of the diaphysis of bone. Increased net intestinal absorption of calcium also occurred early. This was at least in part due to decreased net intestinal secretion of calcium into the gut lumen. Thus the early onset of hypercalciuria and hypercalcemia associated with Pi deprivation in normal growing young rats is the combined result of increased net intestinal calcium absorption and increased calcium mobilization from bone.

Effect of small doses of aluminum-containing antacids on calcium and phosphorus metabolism

The effect of small doses of four commercially available aluminum-containing antacids on calcium and phosphorus metabolism was investigated in adult males in 20 studies. During the use of these doses of antacids, urinary and fecal calcium increased significantly during a low calcium intake averaging 252 mg/day, and the calcium balances became distinctly more negative. There was a reversal of the normal pattern of phosphorus excretions, namely, the fecal phosphorus was high and the urinary phosphorus was low. During a normal calcium intake of 800 mg/day, these doses of antacids did not result in significant changes of the calcium excretions or balance. Despite the decrease in net intestinal absorption of calcium, the average 47Ca absorption remained unchanged, irrespective of the type and dose of antacid used. In three patients who received large therapeutic doses of antacids, 240 to 450 ml/day, the changes of calcium and phosphorus metabolism were intensified.

Effects of chlorothiazide on 1,25-dihydroxyvitamin D3, parathyroid hormone, and intestinal calcium absorption in the rat.

Previous studies have shown that thiazide diuretic agents reverse secondary hyperparathyroidism and reduce circulating 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and intestinal calcium absorption rates in patients with idiopathic hypercalciuria of the renal-leak variety. We have investigated whether thiazides can reverse the secondary increase in serum parathyroid hormone (PTH) and 1,25(OH)2D3 levels or intestinal calcium absorption induced by feeding rats a diet low in calcium (LCD, 0.02% calcium) but adequate in phosphorus and vitamin D. We found that LCD increased circulating immunoreactive PTH [chow vs. LCD, 0.52 +/- 0.06 vs. 1.06 +2- 0.1 (SE) ng/ml, P less than 0.001], 1,25(OH)2D3 (chow vs. LCD, 101 +/- 15 vs. 325 +/- 38 pg/ml, P less than 0.001), calcium uptake by everted gut sacs from duodenum, ileum, and descending colon, and net calcium absorption by descending colon studied in Ussing chambers in vitro. Chlorothiazide (CTZ) prevented the increase in PTH during LCD (chow + CTZ vs. LCD + CTZ, 0.69 +/- 0.07 vs. 0.73 +/- 0.06, NS) but not the increase in 1,25(OH)2D3 (chow + CTZ vs. LCD + CTZ, 88 +/- 10 vs. 277 +/- 31, P less than 0.002) or intestinal calcium transport. The drug caused no change in serum 1,25(OH)2D3 or intestinal calcium absorption in rats fed normal chow. In rats given exogenous 1,25(OH)2D3 to stimulate intestinal calcium absorption, CTZ reduced urine calcium excretion greatly but did not alter intestinal calcium absorption.

1,25-Dihydroxyvitamin D3: short- and long-term effects on bone and calcium metabolism in patients with postmenopausal osteoporosis.

We evaluated the effects of therapy with a small dose (0.5 microgram/day) of 1,25-dihydroxyvitamin D3 in 12 previously untreated patients with postmenopausal osteoporosis. Combined radiocalcium kinetic and balance studies showed that at base line, net calcium absorption (mean +/- SEM) was low (7 +/- 3%), calcium balance was negative (-59 +/- 22 mg/day), and bone resorption rate (297 +/- 40 mg/day) exceeded bone formation rate (239 +/- 36 mg/day). After short-term therapy (6-8 months), calcium absorption was normal (27 +/- 3%, P less than 0.001 for difference from base line), calcium balance had improved (+2 +/- 26 mg/day, P less than 0.05), and bone resorption rate (195 +/- 35 mg/day, P less than 0.01) had decreased more than bone formation rate (197 +/- 26 mg/day, NS). Compared with base-line values, after long-term therapy (2 years) the increased level of calcium absorption was maintained (27 +/- 3%, P less than 0.001), but calcium retention declined to a level intermediate (-27 +/- 24 mg/day, NS) between base-line and short-term treatment values, and both bone resorption rate (294 +/- 33 mg/day, NS) and bone formation rate (267 +/- 34 mg/day, NS) increased. Urinary hydroxyproline excretion was lower than before treatment (26.3 +/-2.0 mg/day) after both short-term (21.1 +/- 1.6 mg/day, P less than 0.001) and long-term (22.0 +/- 1.8 mg/day, P less than 0.01) treatments. Trabecular bone volume was 11.3 +/- 1.0% at base line and increased to 16.0 +/- 1.2% (P less than 0.01) after long-term treatment. By contrast, the eight patients studied before and after 6-8 months of placebo treatment had no significant change in any variable.

Magnesium transport in the cortical thick ascending limb of Henle's loop of the rabbit.

Isolated cortical thick ascending limbs of Henle's loop were perfused in order to directly evaluate magnesium transport in this segment. Transepithelial potential difference was altered by varying the NaCl concentration in perfusate and bath and adding 50 microM furosemide to the perfusate. Perfusion under standard conditions with isotonic solutions resulted in a mean transepithelial potential difference of +8.8 +/- 0.7 mV and net magnesium absorption at a rate of 0.32 +/- 0.06 pmol/mm per min. Perfusion with a hypotonic solution significantly increased potential difference and the net absorptive rate of magnesium, calcium, and potassium. Conversely, reversal of the polarity of the potential difference with low NaCl bath and luminal furosemide produced net secretion of magnesium, calcium, and potassium. Parathyroid hormone in a bath concentration of 1.0 U/ml increased magnesium absorption from 0.32 +/- 0.06 to 0.63 +/- 0.06 pmol/mm per min (P less than 0.001) and calcium from 0.52 +/- 0.08 to 0.97 +/- 0.08 pmol/mm per min (P less than 0.001). Dibutyryl cyclic AMP produced similar effects on both calcium and magnesium absorption. Increasing bath calcium concentration twofold significantly inhibited net calcium absorption from 0.79 +/- 0.27 to 0.16 +/- 0.02 pmol/mm per min but magnesium transport was unaffected. Increasing bath magnesium concentration twofold significantly inhibited net magnesium absorption from 0.56 +/- 0.14 to -0.09 +/- 0.13 pmol/mm per min but had no effect upon net calcium transport. Net absorption of magnesium was significantly increased with increased concentration in the perfusate but calcium transport was unchanged. Similarly, increasing perfusate calcium concentration produced an increase in net calcium transport but did not alter magnesium transport. These data indicate that this segment of the loop of Henle is an important site for magnesium transport. Transport is influenced by luminal and bath concentration and is stimulated by parathyroid hormone and cyclic AMP. The data do not provide support for the concept of an interactive process between calcium and magnesium, and suggest that the positive transepithelial voltage is an important driving force for net reabsorption of magnesium, as well as calcium and potassium in this segment.

Calcium balance in osteoporotic patients on long-term oral calcium therapy with and without sex hormones.

1. The effect of a high calcium intake (1 mmol day-1 kg-1) alone or with the concomitant administration of oestradiol and testosterone derivatives was monitored by serial calcium balances in 48 patients with osteoporosis of various types. 2. On high calcium alone there was an increase (P less than 0.001) in both total calcium balance and net calcium absorption as compared with values on a simulated home intake. This effect was sustained for periods up to 10 years (mean 3.5 years) by the addition of the hormones. 3. Discontinuation of the hormones (only) produced a significant decrease in both calcium balance and nett absorption but both were still increased when compared with the initial values under simulated home calcium intake. 4. No significant change in the urinary calcium output was observed under any of the regimens.

Absorption by sheep of calcium, phosphorus and magnesium from a poultry litter supplemented diet

SUMMARYEight Morino lambs equipped with simple cannulac in the duodenum and terminal ileum were divided into two equal groups and used to study the net absorption of calcium, phosphorus and magnesium from concentrate diets supplemented by soyabean meal or by ensiled poultry litter. Mean net absorption of Ca, P and Mg for the soya-bean meal and poultry-litter treatments, respectively, was 31·6, 10·1; 34·7, 35·4; and 51·5, 41·6.Irrespective of the diet, the small intestino was the major site for Ca and P absorption and the stomachs for the absorption of Mg. A large secretion of P in the stomachs was found with both diets; 8·50 and 9·32 g/day for the soya-bean meal and poultry-litter treatments, respectively.The results of the present study support the view that poultry litter is a good source of P and Mg for sheep and can be used with deficient diets. Owing to the low rate of Ca absorption from the poultry-litter diet, it appears that at limited levels of inclusion, the extra added Ca would not be a detrimental factor.

Calcium metabolism in young rats during early dietary phosphate deprivation.

A series of in vivo experiments were designed to study the early effects of phosphate depletion on calcium metabolism in young adult (8-10 week old) rats with intact thyroid--parathyroid glands. Hypercalciuria occurred within 24 h of dietary phosphate deprivation. It was the consequence of mobilization of calcium stores from the exchangeable calcium pool of the diaphysis of bone. Increased net intestinal absorption of calcium also occurred early. This was at least in part due to decreased net intestinal secretion of calcium into the gut lumen. Thus the early onset of hypercalciuria and hypercalcemia associated with Pi deprivation in normal growing young rats is the combined result of increased net intestinal calcium absorption and increased calcium mobilization from bone.

Effects of dietary phosphorus and calcium on the intestinal absorption of Ca in sheep.

1. Wether sheep were used, some of which were prepared with a Thiry-Vella loop of jejunum. The long-term use of these loops was ensured by regular perfusion with fresh nutrient solution. 2. The net calcium absorption rate from a Thirty-Vella loop of jejunum increased with increasing intraluminal Ca concentration and was increased by the addition of 1 alpha-hydroxy-cholecalciferol (3 micrograms/l) to the loop fluid. 3. When the diet of sheep was changed from one which was normal in Ca to a diet low in Ca there was an increase in the efficiency of net Ca absorption from the jejunal loop. This dietary change was accompanied by an increase in the circulating concentration of 1,25-dihydroxy-cholecalciferol (1,25(OH)2D3). 4. An intravenous infusion of the Ca chelating agent EDTA increased the efficiency of net Ca absorption from the jejunal loop. The use of 47Ca demonstrated that this reflected an increase in the true absorption rate of Ca. 5. Dietary phosphorus deficiency reduced the efficiency of intestinal Ca absorption and was associated with a reduction in the plasma concentration of 1,25(OH)2D3.

Calcium metabolism in postmenopausal and osteoporotic women consuming two levels of dietary protein

Eight postmenopausal women, four with osteoporosis diagnosed by their physician and four without, participated in a metabolic study to investigate the effects of level of protein intake on calcium metabolism; renal acid excretion; plasma total and ultrafiltrable calcium; 1,25-dihydroxyvitamin D; and serum immunoreactive parathyroid hormone. Radial bone mineral content was evaluated. Protein intake was 50 g/day during the 1st 15-day experimental period and 110 g during the 2nd period. Calcium, magnesium, and phosphorus intakes were held constant at 713, 323, and 1078 mg/day. The increase in protein intake significantly increased net calcium absorption and urinary calcium. A calcium intake of 713 mg was not sufficient for calcium balance for most of the women studied. The calciuretic effect of increased protein intake was associated with increased renal acid excretion. None of the plasma and serum measurements mentioned above was significantly affected by the level of protein intake except for a slight increase in plasma total calcium. No significant differences in radial bone mineral content or in any of the other measurements were observed between the osteoporotic and the normal group of women.