Disturbances of mineral and bone metabolism following gastric antrectomy in the rat

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Lack of an acute effect of ghrelin on markers of bone turnover in healthy controls and post-gastrectomy subjects

BACKGROUND

Ghrelin is a gut-brain peptide that powerfully stimulates appetite and growth hormone secretion and is also known to directly regulate osteoblast cell function in vitro and in animal models. Little is known about the effects of ghrelin on bone turnover in humans. As the stomach is the main site of ghrelin synthesis, gastrectomy patients are deficient in ghrelin; they are also prone to osteopenia and osteomalacia.

HYPOTHESIS

Ghrelin may play a role in bone regulation in humans; ghrelin deficiency following gastrectomy is associated with the disrupted regulation of bone turnover seen in these subjects.

SUBJECTS AND METHODS

In a randomised, double-blind, placebo-controlled study 8 healthy controls and 8 post-gastrectomy subjects were infused with intravenous ghrelin (5 pmol/kg/min) or saline over 240 min on different days. Subjects were given a fixed energy meal during the infusion. Ghrelin, GH, type-1 collagen beta C-telopeptide (betaCTX), a marker of bone resorption, and procollagen type-1 amino-terminal propeptide (P1NP), a marker of bone formation, were measured.

RESULTS

Fasting ghrelin was significantly lower in the gastrectomy group during the saline infusion (226.1+/-62.0 vs. 762+/-71.1 ng/l p<0.001). Growth hormone was significantly higher at 90 min after the ghrelin infusion, compared to saline in both healthy controls (61.1+/-8.8 vs. 1.4+/-0.6 mIU/l p<0.001) and gastrectomy subjects (61.1+/-11.8 vs. 0.9+/-0.2 mIU/l p<0.001) confirming the ghrelin was bioactive. Gastrectomy subjects were significantly older and had significantly higher plasma betaCTX than healthy controls at all time points (ANOVA p=0.009). After adjustment for age and BMI ghrelin was found to be a significant predictor of baseline plasma betaCTX and was inversely correlated with baseline plasma betaCTX (beta=-0.54 p=0.03 R2=26%). However, there was no significant effect of the ghrelin infusion on plasma betaCTX or P1NP in either subject group.

CONCLUSIONS

Ghrelin infusion has no acute effect on markers of bone turnover in healthy controls and post-gastrectomy subjects, but is inversely correlated with bone resorption.

Ingestion of difructose anhydride III, a non-digestible disaccharide, improves postgastrectomy osteopenia in rats

OBJECTIVE

Total gastrectomy produces osteopenia with calcium malabsorption. We previously demonstrated that difructose anhydride III (DFAIII), a non-digestible disaccharide, stimulates intestinal calcium absorption in normal and ovariectomized rats. In the present study, we examined the effects of feeding DFAIII on gastrectomy-induced calcium malabsorption and osteopenia in rats. The potential of DFAIII to promote large intestinal calcium absorption was also evaluated through comparison with that of fructooligosaccharides (FOS).

MATERIAL AND METHODS

Male Sprague-Dawley rats were divided into two groups: totally gastrectomized and sham-operated rats. After a postoperative recovery period, rats from each group were divided into three subgroups and fed the control, DFAIII (30 g/kg), or FOS (30 g/kg) diet for 28 days.

RESULTS

Total gastrectomy severely reduced net calcium absorption, femoral calcium content and bone mineral density, resulting in fragility of the femur. DFAIII or FOS feeding partly and similarly restored the lowered calcium absorption and femoral variables, with an increase in the total short-chain fatty acid pool in the cecum. In gastrectomized rats, net calcium absorption was correlated with several cecal parameters, suggesting that cecal fermentation of DFAIII is associated with the improvement in gastrectomy-induced calcium malabsorption. Urinary excretion of deoxypyridinoline (D-Pyr) as a marker of bone resorption was increased by gastrectomy, and the elevated D-Pyr excretion was suppressed by feeding DFAIII.

CONCLUSIONS

Supplemental feeding of DFAIII partly prevents postgastrectomy osteopenia as a result of an improvement in calcium absorption. Our results suggest that the promotive effects of DFAIII on calcium absorption in the large intestine are comparable to those of FOS.

The pathophysiology of bone disease in gastrointestinal disease

Reduced bone mass and the increased risk of fracture in gastrointestinal diseases have a multifactorial pathogenesis. Undoubtedly, genetics play an important role, but other factors such as systemic inflammation, malnutrition, hypogonadism, glucocorticoid therapy in inflammatory bowel disease (IBD) and other lifestyle factors, such as smoking or being sedentary, may contribute to reduced bone mass. At a molecular level the proinflammatory cytokines that contribute to the intestinal immune response in IBD and probably also in coeliac disease are also known to enhance bone resorption. The discovery of the role of the receptor to activated NFkappaB (RANK) interaction with its ligand RANKL in orchestrating the balance between bone resorption and formation may link mucosal and systemic inflammation with bone remodelling, since RANK-RANKL are also involved in lymphopoiesis and T-cell apoptosis. Low circulating leptin in response to weight loss in any gastrointestinal disease may be an important factor in reducing bone mass. This report will summarize current concepts regarding gastrointestinal diseases (primarily IBD, coeliac disease and postgastrectomy states) and low bone mass and fracture.

Hyperphosphaturia after kidney transplantation in syngeneic rats: effects on nephrocalcinosis and bone metabolism?

BACKGROUND

Studies on kidney transplantation have thus far mainly dealt with surgical techniques, immunology, and transplant tolerance. Disturbed mineral metabolism after renal denervation has not received much attention. Basic physiological research in short-term experiments has shown that experimental renal denervation in rats leads to parathormone (PTH)-independent hyperphosphaturia (HPU). HPU and other metabolic complications also have been described after clinical kidney transplantation. Furthermore, there is an unexpected increase in the risk of bone fracture. However, these studies have examined an organism pre-damaged with regard to the parathyroid and immunosuppression. Experimental investigations in syngeneic rats were performed to see whether HPU also occurs after transplantation and thus after denervation and which target organs are involved.

METHODS

Thirty-six male Lewis rats subjected to laparotomy (n = 12), unilateral nephrectomy (n = 12), or unilateral transplantation and bilateral nephrectomy (n = 12) were observed for 18 weeks.

RESULTS

Animals that underwent transplantation had a significant loss of phosphate in the urine not associated with decreased calcium, phosphate, or magnesium in bone. Stability test showed no deterioration, despite a slight increase in the bone parameters of alkaline phosphatase, cyclic AMP, and hydroxyproline with unchanged calciotropic hormones. Nephrocalcinosis was not observed. Parallel to HPU, there was a compensatory reduction in fecal phosphate excretion.

CONCLUSIONS

The loss of phosphate after clinical kidney transplantation in the predamaged parathyroid hormone control system as well as immunosuppression and a surprising increase in the incidence of bone fractures may be explained by the denervation-related loss of phosphate. The lack of intestinal counter-regulation could be an important pathomechanism.

Ingestion of water-soluble soybean fiber improves gastrectomy-induced calcium malabsorption and osteopenia in rats

OBJECTIVE

Total gastrectomy produces calcium malabsorption and osteopenia. We examined the effects of feeding water-soluble soybean fiber (WSSF), a highly fermentable dietary fiber with low viscosity, on calcium absorption and bone characteristics in totally gastrectomized rats.

METHODS

Male Sprague-Dawley rats were assigned to two groups: sham operation and gastrectomy. After the postoperative recovery period, rats of each group were fed diets with or without WSSF (50 g/kg diet) for 4 wk.

RESULTS

Net calcium absorption and femoral dry weight and calcium content and maximum breaking force were lower in the gastrectomy groups than in the sham groups. In the gastrectomy groups, calcium absorption and these femoral parameters were higher in rats fed the WSSF diet than in rats fed the WSSF-free diet. Feeding WSSF increased the concentrations of short-chain fatty acids and soluble calcium and decreased the pH in cecal contents. Net calcium absorption in gastrectomized rats correlated positively with the concentration of total short-chain fatty acids (the sum of acetic, propionic, and butyric acids) in cecal contents (r = 0.545, P < 0.05) and negatively with cecal pH (r = -0.628, P < 0.05).

CONCLUSIONS

These findings indicate that ingestion of WSSF partly prevents the diminished calcium absorption after total gastrectomy in rats, resulting in the improvement of postgastrectomy osteopenia. The increase of calcium absorption may be associated with cecal fermentation of WSSF.

Calcium-regulating hormones, bone mineral content, breaking load and trabecular remodeling are altered in growing pigs fed calcium-deficient diets

Studies on calcium nutrition in appropriate large animal models can be directly relevant to humans. We have examined the effect of dietary Ca deficiency on various bone and bone-related variables, including plasma markers, histomorphometry, mineral content and breaking strength in pigs. Three groups of eight 38-d-old female pigs were fed adequate (0.9%; control), low (0.4%; LCa) or very low (0.1%; VLCa) Ca diets for 32 d. Plasma Ca significantly decreased over time only in the VLCa-deficient pigs. The concentrations of the parathyroid hormones (PTH) and calcitriol increased as Ca deficiency developed, and the plasma PTH and calcitriol levels varied inversely with dietary Ca. The total bone ash contents, bending moments, trabecular bone volume and the mineral apposition rate all decreased as the calcium intake decreased. The osteoclast surface areas were greater than those of controls in both Ca-deficient groups, whereas the osteoblast surface areas were greater only in the VLCa group. The plasma osteoblast-related markers (alkaline phosphatase, carboxy-terminal propeptide of type I procollagen and osteocalcin) were either greater or unaffected in the Ca-deficient pigs. The results indicate that deficient bone mineralization combined with an increased bone resorption led to bone loss and fragility. The differences in the changes in bone cells (number and activity) between LCa and VLCa groups might be due to differences (time and extent) of circulating PTH and calcitriol. The defective mineralization in both Ca-depleted groups resulted mainly from the lack of Ca because their osteoblast activity was either maintained or stimulated. The results also underline the progressive sensitivity of pigs to Ca supply and the usefulness of this model.

Secondary osteoporosis

Generalized osteoporosis currently represents a heterogeneous group of conditions with many different causes and pathogenetic mechanisms, that often are variably associated. The term "secondary" is applied to all patients with osteoporosis in whom the identifiable causal factors are other than menopause and aging. In this heterogeneous group of conditions, produced by many different pathogenetic mechanisms, a negative bone balance may be variably associated with low, normal or increased bone remodeling states. A consistent group of secondary osteoporosis is related to endocrinological or iatrogenic causes. Exogenous hypercortisolism may be considered an important risk factor for secondary osteoporosis in the community, and probably glucocorticoid-induced osteoporosis is the most common type of secondary osteoporosis. Supraphysiological doses of corticosteroids cause two abnormalities in bone metabolism: a relative increase in bone resorption, and a relative reduction in bone formation. Bone loss, mostly of trabecular bone, with its resultant fractures is the most incapacitating consequence of osteoporosis. The estimated incidence of fractures in patients prescribed corticosteroid is 30% to 50%. Osteoporosis is considered one of the potentially serious side effects of heparin therapy. The occurrence of heparin-induced osteoporosis appeared to be strictly related to the length of treatment (over 4-5 months), and the dosage (15,000 U or more daily), but the pathogenesis is poorly understood. It has been suggested that heparin could cause an increase in bone resorption by increasing the number of differentiated osteoclasts, and by enhancing the activity of individual osteoclasts. Hyperthyroidism is frequently associated with loss of trabecular and cortical bone; the enhanced bone turnover that develops in thyrotoxicosis is characterized by an increase in the number of osteoclasts and resorption sites, and an increase in the ratio of resorptive to formative bone surfaces, with the net result of bone loss. Despite these findings, the occurrence of pathological fractures in patients with hyperthyroidism is relatively low, and probably due to the fact that deficiencies in bone mass may be reversed by treatment of the thyroid disease. Most, but not all, studies on insulin-dependent diabetes mellitus (IDDM) report an association with osteopenia. In IDDM, the extent of bone loss is usually slight, which helps explain the discrepancy between the frequency of decreased bone mineral density, and the frequency of osteoporotic fractures in long-standing diabetes. Contradictory results have been obtained in non-insulin-dependent diabetes mellitus (NIDDM) patients. Increased rates of bone loss at the radius and lumbar spine were demonstrated either in patients with two-thirds gastric resection and Billroth II reconstruction, or in those with one-third resection and Billroth I anastomosis, and the metabolic bone disease following gastrectomy may consist also of osteomalacia or mixed pattern of osteoporosis-osteomalacia, with secondary hyperparathyroidism. Miscellaneous causes of secondary osteoporosis are also immobilization, pregnancy and lactation, and alcohol abuse.

Dietary fructooligosaccharides prevent osteopenia after gastrectomy in rats

Postgastrectomy osteopenia is observed generally in humans. Fructooligosaccharides increase the absorption of calcium from the large intestine of healthy rats. Thus, we have examined whether they stimulate calcium absorption and prevent osteopenia in rats following total gastrectomy. Rats were subjected to either a sham surgical operation or Billoth II gastrectomy. Seven rats from each surgical treatment group were fed a control diet, and another seven rats of each treatment group were fed a diet containing fructooligosaccharides (75 g/kg diet) for 4 wk. For 5 d each week, feces were collected, and the calcium and phosphorus contents were measured for calculation of the absorption of these minerals. At the end of the experiment, the rats were killed and bones were collected. The net calcium absorption, calcium content and bone mineral density of the femur and tibia in gastrectomized rats fed the control diet were significantly less than those in sham-operated rats fed control diet. The net calcium absorption in rats fed the fructooligosaccharides diet was greater than that in rats fed control diet. Moreover, dietary fructooligosaccharides prevented the decrease in the calcium content and bone mineral density in gastrectomized rats. Dietary fructooligosaccharides enhanced calcium absorption and prevented the changes indicative of postgastrectomy osteopenia such as decreases in bone calcium content and bone mineral density in gastrectomized rats.

Mineral bioavailability and bone mineral contents in pigs given calcium carbonate postprandially

We have further investigated the "meal effect" on mineral bioavailability in pigs by mineral balance studies and measurements of bone ash contents and bending moment. A group of seven pigs (CAA) was given all its dietary Ca as CaCO3 5 h after the first daily meal for 8 weeks. The control group of seven pigs received CaCO3 in the meal. Both groups were given normal P within the meals. Ca and P absorption and retention were evaluated by a 10-day balance trial. Several bones were collected at slaughter to determine bone ash, Ca, and P contents and bending moment (three-point bending test). Ingesting Ca after the meal did not affect Ca bioavailability or phosphorus absorption, but did reduce P retention, which in turn decreased the bone scores. Osteopenia, indicated by decreased total mineral contents of bones (and decreased ash:bone volume ratio), was associated with elevated plasma osteocalcin in the CAA group. Thus, CaCO3 need not be incorporated into a meal for high Ca absorption, provided that Ca is given after a meal, but simultaneous intakes of Ca and P are required for the best mineral retention.

Nuclear receptors for 1,25-dihydroxy-22-oxavitamin D3 (OCT) and 1,25-dihydroxyvitamin D3 in gastric gland neck mucous cells and gastrin enteroendocrine cells

22-Oxacalcitriol the analog with low calcemic effect and the original hormone 1,25(OH)2 vitamin D3 were localized by autoradiography in mouse stomach at different time intervals after intravenous injection. Both compounds showed a distinct nuclear concentration and retention in neck mucous cells of gastric and pyloric glands, and in dispersed endocrine cells in the antrum region. When the nuclear binding of radioactively labelled compound was compared between gastric neck cells and duodenal absorptive cells, binding was low but sustained in neck cells. Peak uptake after the injection was between 8 and 12 h in neck cells, but between 15 min and 30 min in duodenal villous epithelium. In the duodenum, weak nuclear labelling appeared at 8 h and was undetectable at 12 h under the conditions of the experiment. Nuclear labelling of neck cells remained detectable at 12 h and even after 24 h, similarly for both OCT and 1,25(OH)2 vitamin D3. These results suggest that the stomach is an important target tissue for vitamin D and its analog OCT. Regulation of neck cell functions is suggested, such as proliferation and differentiation of surface epithelium and gastric gland epithelium, and neck cell secretion of acidic mucus. Regulation is also indicated of G-cell gastrin secretion associated with gastrin paracrine effects on parietal cell HCl and intrinsic factor secretion, chief cell pepsinogen secretion, neck cell proliferation, as well as endocrine effects on systemic calcium homeostasis.

Relationship between fractional calcium absorption and gastric emptying

The relationship between calcium absorption and gastric emptying and the precision of measurement of fractional calcium absorption using a single isotope technique were evaluated in 14 normal postmenopausal women (age range 61-72 years). On two occasions separated by between 5 and 15 days, each subject was given 250 mL water containing 0.2 MBq of 45Ca in 20 mg of calcium carrier as the chloride, 20 mg kg-1 paracetamol and 9 MBq of 99mTc sulphur colloid. Venous blood samples were taken at -2, 15, 30, 45, 60, 90, 120, 150 and 180 min after consumption of the drink, and gastric emptying (GE) was monitored with a gamma camera. Fractional calcium absorption in the first hour (alpha 6) was calculated from the blood samples obtained at 15, 30, 45, 60, 90 and 120 min. An absorption rate was also derived from the 60 min sample using only a calibration curve (alpha 1). There were close correlations between radiocalcium absorption on the two study days (r = 0.89, P < 0.001 for both alpha 1 and alpha 6) and between alpha 1 and alpha 6 (r = 0.93, P < 0.001). Plasma paracetamol concentrations at 15 min were directly related to the early phase of GE (r = 0.42, P < 0.05). In contrast, calcium absorption was inversely related to GE (r = 0.45, P < 0.05). We conclude that radiocalcium absorption is not greatly influenced by gastric emptying rate and that the single blood sample procedure has similar precision to the six-blood sample test.

Highly selective vagotomy in the rat: effects on bone and mineral metabolism

Highly selective vagotomy (HSV) or sham operation was performed in male rats. Fifteen weeks later bone mineralization, fractional intestinal absorption and balance, urinary excretion, and serum levels of calcium, magnesium and phosphorus, together with serum gastrin, parathyroid hormone, calcitonin, vitamin D metabolites, osteocalcin, isoenzymes of alkaline phosphatase, and the urinary excretion of cyclic AMP and hydroxyproline were assessed. HSV induced chronic hypergastrinemia and enhanced the weight of the fundus, antrum, and pancreas. Body weight, food intake, intestinal absorption, mineral balance, and bone mineralization were unaffected by HSV, whereas serum parathyroid hormone levels and urinary hydroxyproline excretion were increased. It is concluded that in the rat 1) HSV has a trophic effect on gastric and extragastric tissues; 2) gastric acid production is not a major determinant of intestinal calcium absorption; and 3) normal bone mass in the presence of signs of hyperparathyroidism indicates an intrinsic capacity of HSV to interfere with calcium metabolism, probably via hypergastrinemia, gastrin being an element of the gastro-parathyroid axis. Our present findings underscore the fact that osteopenia after HSV in man may be a rare finding, but it cannot be ruled out that bone disease found after partial or total gastrectomy may be due in part to concomitant vagotomy.