Renal Tubular Calcium Research Articles

Disorders of the calcium-sensing receptor

The human calcium-sensing receptor (CaSR) is a 1078-amino-acid cell surface protein which is expressed in the parathyroids, thyroid cells and the kidney, and is a member of the family of G protein-coupled receptors. The CaSR allows regulation of parathyroid hormone (PTH) secretion and renal tubular calcium reabsorption in response to alterations in extracellular calcium concentrations. The human CaSR gene is located on chromosome 3q13.3–q21, and loss of function CaSR mutations have been reported in the hypercalcaemic disorders of familial benign (hypocalciuric) hypercalcaemia (FBH or FHH) and neonatal severe primary hyperparathyroidism (NSHPT). In addition, gain of function CaSR mutations have been observed in a novel familial syndrome of hypocalcaemia with hypercalciuria. The human CaSR gene on chromosome 3q13.3–q21 is likely to be one of several, as two other loci for FBH have been located on chromosome 19p and 19q13. Cloning and characterisation of these genes will help to further elucidate the mechanisms regulating extracellular calcium.

Reply : unitary model of osteoporosis revisited

Liel et al. have raised an important issue relating to our recent unitary model for osteoporosis. This model(1) attributes virtually all of involutional bone loss in women and much of it in men to estrogen (E) deficiency. Exempted from this hypothesis were the roles of environmental and behavioral factors in accelerating bone loss in some individuals and the influence of differences in peak bone mass in determining the amount of bone that can be lost before the fracture threshold is reached. In our paper, we cited evidence that the rapid phase of bone loss in early postmenopausal women was caused mainly by loss of direct restraining effects of E on bone cell activity, whereas the slow phases in late postmenopausal women and in aging men were mainly caused by loss of the calcium-retaining effects of E on the gut and the kidney. Although the direct effects of E on bone cells are readily apparent, we agree with Liel et al. that the effects of E on intestinal and renal calcium transport are less well documented. Because the negative calcium balance associated with postmenopausal bone loss may be only 50–100 mg/day,(2) however, it is difficult to detect an effect this small in humans with measurements of intestinal or renal tubular calcium transport. Although intestinal epithelial cells contain E receptors,(3) this does not necessarily establish that the action of E on enhancing intestinal calcium transport is direct. Thus, the suggestion of Liel et al. that the E effect may be mediated by up-regulating the numbers of vitamin D receptors (VDRs) is of considerable interest. As supporting evidence, they cite studies of their own and of others demonstrating that E increases VDR concentrations both in osteoblastic cells and in intestinal cells. Moreover, their hypothesis is consistent with the findings of Genari et al.(4) that oophorectomy blunted the stimulatory effect of exogenously administered 1,25-dihydroxyvitamin D on intestinal calcium absorption. Although Liel et al. did not explicitly suggest that a similar mechanism may also underlie the effect of E on renal calcium conservation, this could well be so. E administration to early postmenopausal women increases parathyroid hormone–independent tubular reabsorption of calcium,(5) renal tubules contain VDR,(6) and 1,25-dihydroxyvitamin D enhances renal calcium conservation.(7) Although an action of E to enhance extraskeletal calcium conservation appears to be the only plausible explanation for the findings that increased bone resorption is associated with secondary hyperparathyroidism in late postmenopausal women and that both abnormalities are reversed by E replacement,(8, 9) further studies are needed to document the actions of E on extraskeletal calcium metabolism unequivocally and to elucidate the underlying mechanism.

Pathophysiology and treatment of idiopathic hypercalciuria.

Nearly 50 years after its initial description by Dr. F. Albright, the term idiopathic hypercalciuria (IH) is still in use. The exact mechanism of hypercalciuria is still unknown despite extensive pathophysiologic investigations; recent advances represent the focus of this review. A precise definition of true IH is proposed, taking into account the various nutritional conditions influencing calcium excretion. The potential pathogenic mechanisms are discussed, and the limits of the classical Pak's pathophysiological classification are recalled. The evidence supporting the role of an increased intestinal calcium absorption, a defect in renal tubular calcium reabsorption, or an increased bone loss as a primary mechanism in IH are successively examined. Since overall available human data indicates that all three mechanisms may be found in IH, the hypothesis that a broader disorder encompassing all these various abnormalities may be involved in IH is discussed. Three global hypotheses to account for IH physiopathology are examined: a diffuse defect in fatty acid content of cell membranes, an increased expression of the vitamin D receptor of the 25(OH) vitamin D 1 alpha-hydroxylase, or of the calcium sensor receptor and a monocyte disease. Finally, the available clinical data justifying the therapeutic approaches are reviewed, and guidelines for dietary recommendations regarding calcium and also animal protein, sodium chloride, alcohol, carbohydrate, phosphate, and potassium intakes are proposed, and drug therapy indications are discussed.

Vitamin D receptor gene polymorphism is associated with urinary calcium excretion but not with bone mineral density in postmenopausal women.

Polymorphism of vitamin D receptor (VDR) gene has been found to be associated with serum osteocalcin (OC) levels and bone mineral density (BMD) in Caucasian identical twins and unrelated postmenopausal women. Being ethnically different and living in a geographic area with adequate vitamin D status due to abundant sunshine exposure, it is unclear whether VDR gene polymorphism will affect bone mass in Thai population. In the present study, we investigated the association between VDR gene polymorphism and bone metabolism in Thai postmenopausal women. Subjects consisted of 84 postmenopausal women. Bsm I, Taq I and Apa I polymorphisms of VDR gene were determined by PCR-RFLP. B, T and A represent the absence of the corresponding restriction sites while b, t and a indicate the presence of the restriction sites. Data were expressed as mean +/- SE. Sixty-six subjects (78.6%) had bb genotype while 18 (21.4%) had Bb genotype. None of the subjects was found to have BB genotype. Taq I restriction site was in linkage disequilibrium to the Bsm I site. For Apa I polymorphism, 33 (39.3%), 42 (50.0%) and 9 (10.7%) of the subjects had aa, Aa and AA genotypes, respectively. There was no significant difference in serum intact OC levels and BMD at various skeletal sites among subjects with different genotypes. Despite the lack of difference in BMD and intact OC levels, subjects with bb genotype had higher 24-hour urinary calcium excretion than those with Bb genotype (bb, 6.1 +/- 0.3 mmol/day; Bb, 4.4 +/- 0.6 mmol/day; p < 0.05). The effect of Bsm I VDR genotype was still significant (p < 0.05) after dietary calcium intake was controlled using analysis of covariance. Despite the difference in urinary calcium levels, there was no significant difference in fractional excretion of calcium among subjects with different Bsm I-related genotypes, suggesting that the effect of the VDR gene polymorphism on urinary calcium excretion is more likely due to the effect on intestinal calcium absorption rather than renal tubular calcium reabsorption. We conclude that VDR genotype distributions in Thai postmenopausal women are different from those reported in Caucasians. VDR gene polymorphism does not appear to be associated with BMD or bone turnover in Thai postmenopausal women. However, Bsm I VDR polymorphism may have physiologic role in calcium homeostatasis by modulating intestinal calcium absorption.

Parathyroid hormone-related protein is not an autocrine growth factor for normal prostatic epithelial cells.

Parathyroid hormone-related protein (PTHrP) is the primary factor responsible for humoral hypercalcemia of malignancy. The hypercalcemic actions of PTHrP occur via stimulation of renal distal tubular calcium reabsorption and increased osteoclastic bone resorption. These effects of PTHrP are thought to be mediated through a common parathyroid hormone (PTH)/PTHrP receptor. In addition to the well-established role of PTHrP in bone remodeling, PTHrP is believed to be an important mediator of cellular growth and differentiation in a number of nonbony tissues. We recently demonstrated abundant expression of PTHrP in normal and malignant human prostatic tissues, and in cultured prostatic epithelial cells. In vitro assays were used to test growth-regulatory activity of synthetic and endogenous PTHrP peptides on normal prostatic epithelial cells. No growth-regulatory activity could be demonstrated. PTHrP is not an autocrine growth factor for normal prostatic epithelial cells.

Les hypercalciuries: étiologies et traitement

Hypercalciuria is a rare biological symptom with multiple possible etiologies in children. Normal calcium excretion rate in children is defined as lower than 4 mg/kg per day, significantly higher values being observed in infants. When using urinary calcium: creatinine ratio, normal values are below 0.22 mg/mg in children, and below 0.6 to 0.8 mg/mg in infants. In our experience half patients with hypercalciuria have idiopathic hypercalciuria. Idiopathic hypercalciuria can be hereditary with a dominant autosomal mode of inheritance. Its pathophysiology is unclear, increased calcium intestinal absorption and impaired renal tubular calcium reabsorption being the two main underlying anomalies. Patients with hypercalciuria should be informed about the risk of urolithiasis and its possible prevention by a high water intake. In those patients with nephrocalcinosis or recurrent episodes of lithiasis, hydrochlorothiazide can be effective in reducing hypercalciuria. However, adverse effects of hydrochlorothiazide on serum lipids have been recently reported and make this treatment questionable in the long term.

How is plasma calcium concentration held constant?

Summary: Extracellular or plasma calcium ion concentration is held constant at 5 mg/dL through the actions of parathyroid hormone (PTH), vitamin D and calcitonin, on their target organs, kidney and bone. the thresholds of renal tubular calcium reabsorption and bone resorption and formation are both set at 5 mg/dL. the set‐point of PTH secretion is also set at 5 mg/dL plasma calcium ion. Thus, the sensing system (parathyroid cell) and the effectors, kidney and bone, are all set to maintain plasma calcium at 5 mg/dL, and such setpoint in each organ may be determined by the membrane bound calcium sensor proteins. the effectiveness of this system depends upon the presence of bone remodelling, which allows a swift move of plasma calcium from and to bone in response to PTH and calcitonin, respectively. In this regard, directing haematopoiesis to bone marrow which provides bone resorbing osteoclasts is critical. It is likely that this shift of haematopoiesis occurs through evolution at the transition from the aquatic to the terrestrial life and this event is directed by expression of ‘homing molecule’ in bone marrow stromal cells. This brief review provides a factual and conceptual framework of the current understanding how plasms calcium is held constant at 5 mg/dL.

How is plasma calcium held constant? Milieu interieur of calcium

Extracellular or plasma calcium ion concentration is held constant at 5 mg/dl through the actions of parathyroid hormone (PTH), vitamin D, and calcitonin, on their target organs, kidney and bone. The thresholds of renal tubular calcium reabsorption and bone resorption and formation are both set at 5 mg/dl. The set point of PTH secretion is also set at 5 mg/dl plasma calcium ion. Therefore, the sensing system (parathyroid cell) and the effectors, kidney and bone, are all set to maintain plasma calcium at 5 mg/dl, perhaps through membrane-bound calcium sensor proteins. The effectiveness of this system depends upon the presence of bone remodeling, which allows a swift shift of plasma calcium from and to bone in response to PTH and calcitonin, respectively. In this regard, directing hematopoiesis to bone marrow that provides bone resorbing osteoclasts is critical. It is likely that this shift of hematopoiesis occurs through evolution at the transition from the aquatic to the terrestrial life, and this event is directed by expression of "homing molecule" in bone marrow stromal cells. This brief review provides a factual and conceptual framework of the current understanding of the milieu interieur of the calcium ion.

Disorders of calcium ion sensing.

T HE PARATHYROID glands secrete parathyroid hormone (PTH) at a rate that is dependent upon and responsive to the extracellular calcium ion concentration (Ca*‘,) (1). Recent advances in molecular techniques have enabled the characterization of an extracellular calcium ionsensing receptor (Can) that initiates the afferent limb of the cellular pathways necessary for regulating this Ca2’0-responsive PTH secretion (2). Renal tubular calcium reabsorption, secretion of calcitonin by the thyroidal C-cells, and probably many other Ca2t0responsive cellular functions are dependent upon the CaR to permit the regulation of extracellular calcium homeostasis and other diverse cellular functions unrelated to calcium homeostasis, such as found in the brain. We will review the biochemical and molecular characteristics of the CaR as well as three disorders: familial benign hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia, which are caused by abnormalities of CaR function (3, 4).

Determination of circulating parathyroid hormone levels and differential diagnosis of hypercalcemia

Primary hyperparathyroidism is the most frequent cause of hypercalcemia in outpatients. In contrast, this electrolyte disorder is very often associated with cancer when detected in hospital, particularly in the frame of tumors of breast, lung or lympho-hematopoietic system. Hypercalcemia results from an imbalance between the fluxes of calcium entering and leaving the extracellular space. Theses fluxes, mainly those at the levels of bone and kidney, are the main regulators of calcium homeostasis. Depending on the etiology, increases in either bone resorption or renal tubular calcium reabsorption can predominate as the cause of elevated calcemia. Thus, an increment of renal tubular reabsorption of calcium plays a prominent role in hypercalcemia resulting from increased serum concentrations of parthyroid hormone, but can also be detected in 50% of malignant hypercalcemias. The ectopic production of authentic parathyroid hormone has convincingly been demonstrated in very few cases. The syndrome of pseudohyperparathyrodism encountered in malignant hypercalcemia can be accounted for by the tumoral secretion of an analog of parathyroid hormone, parathyroid hormone-related protein. Both proteins, wich are produced by different genes located on different chromosomes, interact with the same cell membrane receptors and display identical spectrum of actions. Since they are immunologically quite distinct, there is no cross-reactivity in the various competititive or radiometric immunoassays actually available. The determination of circulating levels of parathyroid hormone is an essential step in the differential diagnosis of hypercalcemias, provided the assays offer adequate sensitivity and specificity. Nowadays, this appears to be generally the case.

Down-regulation of the PTH/PTHrP receptor in uremia

Resistance to the action of PTH has been well characterized in the setting of chronic renal failure. Most evidence points to post-receptor abnormalities in its pathogenesis. The recent cloning of the PTH/PTHrP receptor (PTH-R) has permitted us to examine whether in a 5/6 nephrectomy rat model (CRF) the expression of the PTH-R gene is modified. First, we have found that the renal PTH-R mRNA expression is markedly decreased in CRF compared to normal rats. Diminished PTH-R transcripts were associated with a lower PTH-induced cAMP production in renal membranes in CRF suggesting a decrease in the PTH-R number or post-receptor modifications. Second, thyroparathyroidectomized (TPTX) rats with normal renal function had no change in the renal PTH-R expression whereas TPTX-CRF rats still showed a decreased renal PTH-R mRNA expression suggesting that high plasma PTH levels were not etiologically important in the observed down-regulation. Despite the renal PTH-R down-regulation, CRF rats had a normal renal handling of calcium. They also had a higher phosphate excretion than control rats. TPTX-CRF rats showed a decrease in renal tubular calcium reabsorption and a phosphate retention when compared with CRF animals with intact parathyroid glands. This suggests that a few available PTH-R in the kidney allow PTH to exert, to a certain extent, its physiological role in this experimental model of uremia. In conclusion, these findings indicate a down-regulation of the renal PTH-R expression in CRF which appears to be independent of parathyroid gland function. The relevance of this phenomenon in the setting of the secondary hyperparathyroidism of uremia remains to be elucidated.

Parathyroid hormone-related protein and calcium phosphate metabolism.

There is marked homology between the parathyroid hormone (PTH) and PTH-related protein (PTHrP) molecules at the amino terminal but the rest of the molecules are quite different, providing immunologically distinct peptides. However, they interact with the same receptor. Thus, PTHrP mediates biological actions reminiscent of PTH. PTHrP gene is a single copy gene, producing one to three mRNA transcripts through alternative splicing of the carboxy terminal, encoding peptides of 139, 141 or 173 amino acids. Having been recently isolated from malignant tumours, PTHrP is now considered to be the major mediator of humoral hypercalcaemia of malignancy (HHM). The PTH-like effects of PTHrP on the kidney and bone have been well characterized. The increase in renal tubular calcium reabsorption and the reduction in tubular phosphate reabsorption with a concomitant rise in nephrogenous cyclic AMP constitute the pathophysiological changes in the renal handling of calcium and phosphate in HHM. The osteotropic contribution to the malignant hypercalcaemia has been validated by enhanced osteoclastic bone resorption--an indirect effect of the amino terminal portion of the PTHrP molecule on osteoblasts. However, PTHrP has also been detected in a large number of normal adult tissues/organs as well as in human and animal fetuses. Fetal plasma calcium is higher than maternal and this is achieved by active transport of calcium across the placenta. Using ovine placental perfusion models, PTHrP, which is believed to originate from fetal parathyroid glands and the placenta itself, has been demonstrated to sustain this calcium gradient. Active placental transport of magnesium, but not phosphate, was also shown to be enhanced by PTHrP.(ABSTRACT TRUNCATED AT 250 WORDS)

Management of hypocalcemic effects of WR2721 administered on a daily times five schedule with cisplatin and radiation therapy. The New York Gynecologic Oncology Group.

To determine the effects of the chemoprotective agent, WR2721, administered on a daily x 5 schedule with cisplatin and radiation therapy, on calcium and parathyroid hormone (PTH) levels. Twenty women with cervical cancer were enrolled in a clinical trial to determine the maximal safe dose of WR2721 plus radiation therapy and cisplatin on a novel daily x 5 schedule. Detailed studies of the effects of WR2721 on calcium and PTH levels were initiated after a patient developed symptomatic hypocalcemia. Treatment with WR2721 resulted in a rapid decline in serum PTH levels within 4 hours, which fell below the lower limits of normal at 24 hours, then returned to within normal limits at 48 hours. In contrast, serum levels of ionized calcium were not affected acutely, and declined by only 7% within 24 hours. However, this small decrease persisted for the 5 days of treatment. Hypocalcemic effects were successfully managed with oral calcium carbonate and calcitriol supplements. In one patient, particularly sensitive to the effects of WR2721, serum levels of ionized calcium decreased to less than 3.0 mg/dL despite oral calcium supplements. The effects of WR2721 on serum ionized calcium levels are mediated by direct inhibition of PTH activity; other effects such as inhibition of renal tubular calcium reabsorption cannot be excluded. We recommend that patients treated with WR2721, cisplatin, and radiation therapy receive routine oral calcium and calcitriol supplementation and that serum ionized calcium levels be monitored frequently.

Increased intestinal vitamin D receptor in genetic hypercalciuric rats. A cause of intestinal calcium hyperabsorption.

In humans, familial or idiopathic hypercalciuria (IH) is a common cause of hypercalciuria and predisposes to calcium oxalate nephrolithiasis. Intestinal calcium hyperabsorption is a constant feature of IH and may be due to either a vitamin D-independent process in the intestine, a primary overproduction of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], or a defect in renal tubular calcium reabsorption. Selective breeding of spontaneously hypercalciuric male and female Sprague-Dawley rats resulted in offspring with hypercalciuria, increased intestinal calcium absorption, and normal serum 1,25(OH)2D3 levels. The role of the vitamin D receptor (VDR) in the regulation of intestinal calcium absorption was explored in 10th generation male genetic IH rats and normocalciuric controls. Urine calcium excretion was greater in IH rats than controls (2.9 +/- 0.3 vs. 0.7 +/- 0.2 mg/24 h, P < 0.001). IH rat intestine contained twice the abundance of VDR compared with normocalciuric controls (536 +/- 73 vs. 243 +/- 42 nmol/mg protein, P < 0.001), with no difference in the affinity of the receptor for its ligand. Comparable migration of IH and normal intestinal VDR on Western blots and of intestinal VDR mRNA by Northern analysis suggests that the VDR in IH rat intestine is not due to large deletion or addition mutations of the wild-type VDR. IH rat intestine contained greater concentrations of vitamin D-dependent calbindin 9-kD protein. The present studies strongly suggest that increased intestinal VDR number and normal levels of circulating 1,25(OH)2D3 result in increased functional VDR-1,25(OH)2D3 complexes, which exert biological actions in enterocytes to increase intestinal calcium transport. Intestinal calcium hyperabsorption in the IH rat may be the first example of a genetic disorder resulting from a pathologic increase in VDR.

Altered renal calcium handling in hypercalcemia of malignancy.

It has been controversial whether increased renal tubular calcium reabsorption contributes to hypercalcemia in patients with malignancies. Moreover, whether this abnormality is associated with volume depletion, a parathyroid hormone-like effect, or other mechanisms has not been clarified. Eight consecutive patients with hypercalcemia due to a variety of tumor types were studied in detail. The glomerular filtration rate (iothalamate clearance) was reduced in all patients (0.98 +/- 0.10 (mean +/- SE) mL/s.1.73 m2; P less than 0.001) compared with normal controls (N = 9) (1.93 +/- 0.08 mL/s.1.73 m2), but it was similar to that in controls matched for renal insufficiency (N = 6) (1.15 +/- 0.05 mL/s.1.73 m2). During hypercalcemia produced by calcium infusion, urinary calcium excretion (millimoles of calcium per liter of glomerular filtrate) was increased in controls with renal insufficiency compared to those with normal renal function (P = 0.028). In all patients with hypercalcemia of malignancy, urinary calcium excretion was decreased compared with controls with renal insufficiency, but it was low in only five of eight patients compared with normal controls. Extracellular fluid volume (iothalamate volume of distribution) was not decreased in any patient, and urinary cAMP and/or plasma parathyroid hormone-like bioactivity were increased in six of eight patients. After treatment with an inhibitor of bone resorption, aminopropylidene 1,1 diphosphonate, abnormal renal calcium handling was not detected if the serum calcium normalized. It was concluded that increased renal tubular calcium reabsorption was consistently present in patients with hypercalcemia of malignancy compared with controls matched for renal insufficiency, but the proportion with the abnormality was underestimated if normal controls were used.(ABSTRACT TRUNCATED AT 250 WORDS)

Editorial: Thyroid Hormones and Bone Mass

Thyrotoxicosis increases bone turnover. This accentuation of the normal remodelling sequence increases the number of osteoclasts and resorption sites, and alters the ratio of resorptive to formative bone surfaces. One result of this increase in bone resorption is hypercalcemia, which has been reported to occur in approximately 20% of patients with thyrotoxicosis. Ionized calcium levels appear to be increased in as many as 50% of patients with thyrotoxicosis (1). Since serum PTH and 1,25-dihydroxyvitamin D3 levels are decreased and intestinal calcium absorption blunted in patients with thyrotoxicosis, the increased calcium levels are thought to directly reflect the accelerated bone resorption. Hypercalciuria is also common in patients with thyrotoxicosis, occurring in individuals with and without hypercalcemia. The suppression of PTH secretion in patients with thyrotoxicosis, and the resultant decrease in renal tubular calcium reabsorption mediated by PTH, are thought to be the mechanisms responsible for h...

Immunofluorescent Study on the Interaction between Collagen and Calcium Oxalate Crystals in the Renal Tubules

The interaction of calcium oxalate crystals and renal tubular cells was studied. Rat renal collecting tubular cells were cultured and immunologically stained with anti-type-IV collagen antiserum (type-IV collagen exists in renal tubular basement membrane). When renal tubular cells and calcium oxalate crystals were mixed, clumps were formed. These clumps were examined by immunological staining with anti-type-IV collagen antiserum. In another series of experiments, calcium-containing crystals were found to be adsorbed onto mucous threads and cast-like materials, although no such adsorption was observed on squamous cells. These absorbed materials interacted with anti-type-IV collagen antiserum. These results suggest that collagen in the renal tubular basement membrane may act as matrix in urinary stone formation.

Intranasal Salmon Calcitonin

Salmon calcitonin, a polypeptide hormone secreted by the parafollicular C cells of the thyroid gland, lowers serum calcium levels by decreasing bone resorption and renal tubular calcium reabsorption. An analgesic action, possibly mediated via beta-endorphins, is also evident. In the past, parenteral formulations of salmon calcitonin have been used in the management of metabolic bone disorders, but their routine use has been limited by the inconvenience of this route of administration and by poor tolerability. The development of an intranasal preparation of salmon calcitonin will provide a more convenient means of administering the drug. In clinical trials published to date intranasal salmon calcitonin has been effective and well tolerated in small numbers of recently postmenopausal women at risk of developing osteoporosis, and in patients with established osteoporosis, Paget's disease, or osteoporosis secondary to corticosteroid usage, multiple myeloma or ovariectomy. For periods of up to 2 years the drug reduces bone resorption and improves bone architecture, relieves pain and increases functional status. Further research is needed to confirm longer term efficacy (in particular, effects on fracture rate), optimal dosage schedules and the role of intermittent and combination treatment regimens.

Ultrastructural and functional abnormalities of intestinal and renal epithelium in the SHR

Intestinal calcium transport, renal tubular calcium reabsorption, and plasma 1.25 (OH)2 vitamin D3 (calcitriol) levels have all been reported to be diminished in the spontaneously hypertensive rat (SHR) compared with its genetic control the Wistar Kyoto rat (WKY). In the present study, absorptive duodenal and renal tubular epithelia of 12- to 14-week-old male SHR and WKY were examined by electron microscopy to determine whether such disturbances could be related to structural abnormalities. Patchy loss of microvilli in both duodenal and proximal tubular epithelia was observed in the SHR, whereas brush border membrane was entirely normal in the WKY. Irregular spaces were observed between the basal aspects of SHR intestinal epithelial cells and their basement membrane. In addition, the average height of duodenal and renal microvilli was reduced in the SHR. Two specific markers of the brush border membrane, alkaline phosphatase and villin, as well as the cytoplasmic vitamin-D dependent calcium-binding proteins, CaBP9K and CaBP28K were determined. Duodenal alkaline phosphatase activity was reduced in the SHR, compared with the WKY: 0.145 +/- 0.002 vs. 0.186 +/- 0.002 IE/min.microns 3 x 10(3) brush border, mean +/- SEM, N = 10 pairs, P less than 0.001. However, duodenal villin expression was not different from that of the WKY. Duodenal CaBP9K and renal CaBP28K content was diminished in the SHR: 21.0 +/- 0.80 vs. 29.9 +/- 2.19 micrograms/mg protein, N = 6 pairs, P less than 0.01 for duodenum, and 4.47 +/- 0.39 vs. 7.67 +/- 0.54 micrograms/mg protein, N = 6 pairs, P less than 0.001 for kidney. These data showing structural and functional abnormalities of intestinal and kidney cells in the SHR appear to reflect a disorder of transporting epithelia which may be either intrinsic or related to reduced circulating calcitriol.

Corticosteroid-induced Osteoporosis

Glucocorticoid-induced bone loss has been recognized for almost 50 years, yet it remains an important medical problem and a common cause of fracturing. Loss of skeletal mass may occur early in the course of glucocorticoid therapy and appears to be related to the cumulative dose of steroid, as well as to the usual risk factors for osteoporosis. The pathophysiology of this disorder is poorly understood, but available information suggests that glucocorticoids exert direct and indirect actions on bone and calcium homeostasis that may contribute to osteopenia. Direct effects appear to occur soon after the initiation of glucocorticoid therapy and include (1) suppression of intestinal calcium absorption, (2) decreased renal tubular calcium resorption with increased urinary calcium excretion, and (3) suppressed osteoblast function and decreased bone formation. Ultimately, renal and intestinal losses of calcium result in the development of secondary hyperparathyroidism, which, in concert with stimulation of parathyroid secretion and an increased sensitivity of bone cells to PTH, produces an increase in bone resorption and bone turnover. Although effective forms of prevention and treatment have not been identified, available information provides a rationale for the use of calcium supplements, maintenance of vitamin D nutrition, and the use of agents that suppress bone resorption and bone turnover. The use of sodium fluoride to stimulate bone formation has not been fully tested in this condition and it remains an investigational agent. Although recent advances in our understanding of the glucocorticoid receptor are likely to lead us to new therapeutic approaches, present methods of prevention and treatment have not been adequately evaluated and await prospective study.