Barton S. Levine

Magnesium, the Mimic/Antagonist of Calcium

Magnesium, the fourth most abundant metal in living organisms, is distributed in three major compartments in the body: 65 per cent in the mineral phase of skeleton, 34 per cent in the intracellular...

Milk Alkali Syndrome and the Dynamics of Calcium Homeostasis

Intensive treatment with calcium-containing antacids and milk first was used in the early 20th century for the treatment of peptic ulcer disease and sometimes was associated with toxicity, eventually known as the milk alkali syndrome. Despite the introduction of H2 blockers and proton pump inhibitors for the treatment of peptic ulcer disease, the milk alkali syndrome continues to occur but is seen more frequently in older women who are receiving treatment for osteoporosis. The milk alkali syndrome provides a unique opportunity to discuss calcium homeostasis in a setting in which the primary calcium regulatory hormones, parathyroid hormone (PTH) and calcitriol, are not overtly abnormal. A thorough understanding of the pathophysiology of the milk alkali syndrome, including its generation and maintenance, requires knowledge of intestinal calcium absorption, bone influx and efflux of calcium, and renal calcium excretion and also how these processes change with age. In this review, the pathophysiology of the milk alkali syndrome is discussed in light of recent advances in our understanding of calcium homeostasis, particularly the role of the calcium-sensing receptor (CaSR) and epithelial calcium channels that are present in various tissues such as the parathyroid gland, kidney, and intestine. The contributions of alkalosis, per se , to the generation and maintenance of hypercalcemia are discussed in detail. Almost 100 years ago, Sippy (1) developed a calcium-laden milk and antacid regimen for the treatment of peptic ulcer disease. His rationale was to neutralize the hyperacidity that was deemed responsible for peptic ulcer disease. The Sippy regimen was used for the treatment of peptic ulcer disease, a disorder that was most common in middle-aged men, until the 1970s, when nonantacid treatment first was introduced (2–4). The original recommendation by Sippy consisted of the hourly administration of milk and cream together with what became known as …

Erythropoiesis-stimulating protein therapy and the decline of renal function: a retrospective analysis of patients with chronic kidney disease

ABSTRACT Background/Aims: Previous studies have hinted at possible associations between anemia and progression of renal disease. The study objective was to determine whether treatment with erythropoiesis-stimulating proteins (ESPs) can curb the rate of decline in renal function in pre-dialysis patients with chronic kidney disease (CKD). Methods: Observational, before/after analysis using electronic medical records from the Veterans Administration (VA). Included patients had at least two measurements of serum creatinine levels before and after ESP treatment initiation. The Cockcroft–Gault formula was used to derive estimates of glomerular filtration rate (GFR). Rate of renal function decline prior to and following initiation of therapy were compared. Results: One hundred and twenty two patients with renal impairment levels of Stage 3 (moderate) or Stage 4 (severe) at ESP treatment initiation were identified. Over 80% of patients initiated therapy with either Grade 1 or Grade 2 anemia. The rate of renal function decline was calculated as the slope of the least-squares linear regression line of the inverse serum creatinine over time during the pre-treatment initiation and post-treatment initiation time periods. Overall, patients experienced a slowing in the rate of renal function decline after treatment was initiated (mean pre-treatment initiation rate of –0.094 dL/mg/yr versus mean post-treatment initiation rate of –0.057 dL/mg/yr). Conclusion: Renal function declined at a slower rate following ESP initiation. Results are consistent with prior studies indicating delayed dialysis initiation in patients treated with ESPs. Analyses were limited by the observational study design and lack of information regarding some potential confounders. Longer-term, prospective trials are needed to determine whether ESPs slow progression of renal disease and the potential magnitude of such an effect.

Approach to Treatment of Hypophosphatemia

Hypophosphatemia can be acute or chronic. Acute hypophosphatemia with phosphate depletion is common in the hospital setting and results in significant morbidity and mortality. Chronic hypophosphatemia, often associated with genetic or acquired renal phosphate-wasting disorders, usually produces abnormal growth and rickets in children and osteomalacia in adults. Acute hypophosphatemia may be mild (phosphorus level, 2-2.5 mg/dL), moderate (1-1.9 mg/dL), or severe (<1 mg/dL) and commonly occurs in clinical settings such as refeeding, alcoholism, diabetic ketoacidosis, malnutrition/starvation, and after surgery (particularly after partial hepatectomy) and in the intensive care unit. Phosphate replacement can be given either orally, intravenously, intradialytically, or in total parenteral nutrition solutions. The rate and amount of replacement are empirically determined, and several algorithms are available. Treatment is tailored to symptoms, severity, anticipated duration of illness, and presence of comorbid conditions, such as kidney failure, volume overload, hypo- or hypercalcemia, hypo- or hyperkalemia, and acid-base status. Mild/moderate acute hypophosphatemia usually can be corrected with increased dietary phosphate or oral supplementation, but intravenous replacement generally is needed when significant comorbid conditions or severe hypophosphatemia with phosphate depletion exist. In chronic hypophosphatemia, standard treatment includes oral phosphate supplementation and active vitamin D. Future treatment for specific disorders associated with chronic hypophosphatemia may include cinacalcet, calcitonin, or dypyrimadole.

Pathophysiology of Calcium, Phosphorus, and Magnesium Dysregulation in Chronic Kidney Disease

Calcium, phosphorus, and magnesium homeostasis is altered in chronic kidney disease (CKD). Hypocalcemia, hyperphosphatemia, and hypermagnesemia are not seen until advanced CKD because adaptations develop. Increased parathyroid hormone (PTH) secretion maintains serum calcium normal by increasing calcium efflux from bone, renal calcium reabsorption, and phosphate excretion. Similarly, renal phosphate excretion in CKD is maintained by increased secretion of fibroblast growth factor 23 (FGF23) and PTH. However, the phosphaturic effect of FGF23 is reduced by downregulation of its cofactor Klotho necessary for binding FGF23 to FGF receptors. Intestinal phosphate absorption is diminished in CKD due in part to reduced levels of 1,25 dihydroxyvitamin D. Unlike calcium and phosphorus, magnesium is not regulated by a hormone, but fractional excretion of magnesium increases as CKD progresses. As 60–70% of magnesium is reabsorbed in the thick ascending limb of Henle, activation of the calcium‐sensing receptor by magnesium may facilitate magnesium excretion in CKD. Modification of the TRPM6 channel in the distal tubule may also have a role. Besides abnormal bone morphology and vascular calcification, abnormalities in mineral homeostasis are associated with increased cardiovascular risk, increased mortality and progression of CKD.

The Journey From Vitamin D–Resistant Rickets to the Regulation of Renal Phosphate Transport

In 1937, Fuller Albright first described two rare genetic disorders: Vitamin D resistant rickets and polyostotic fibrous dysplasia, now respectively known as X-linked hypophosphatemic rickets (XLH) and the McCune-Albright syndrome. Albright carefully characterized and meticulously analyzed one patient, W.M., with vitamin D-resistant rickets. Albright subsequently reported additional carefully performed balance studies on W.M. In this review, which evaluates the journey from the initial description of vitamin D-resistant rickets (XLH) to the regulation of renal phosphate transport, we (1) trace the timeline of important discoveries in unraveling the pathophysiology of XLH, (2) cite the recognized abnormalities in mineral metabolism in XLH, (3) evaluate factors that may affect parathyroid hormone values in XLH, (4) assess the potential interactions between the phosphate-regulating gene with homology to endopeptidase on the X chromosome and fibroblast growth factor 23 (FGF23) and their resultant effects on renal phosphate transport and vitamin D metabolism, (5) analyze the complex interplay between FGF23 and the factors that regulate FGF23, and (6) discuss the genetic and acquired disorders of hypophosphatemia and hyperphosphatemia in which FGF23 plays a role. Although Albright could not measure parathyroid hormone, he concluded on the basis of his studies that showed calcemic resistance to parathyroid extract in W.M. that hyperparathyroidism was present. Using a conceptual approach, we suggest that a defect in the skeletal response to parathyroid hormone contributes to hyperparathyroidism in XLH. Finally, at the end of the review, abnormalities in renal phosphate transport that are sometimes found in patients with polyostotic fibrous dysplasia are discussed.

Intestinal Absorption of Calcium: Its Assessment, Normal Physiology, and Alterations in Various Disease States

Publisher Summary The metabolism of calcium (Ca) is regulated in part via changes in its intestinal absorption. This chapter discusses the general aspects of Ca absorption, the methods for measuring overall intestinal absorption and the transport of Ca, and various hormonal and other factors that may alter Ca absorption. It also discusses the common physiologic events and clinical disorders that modify the intestinal Ca absorption. Unlike other elements in the diet, such as sodium and potassium, which are almost entirely absorbed, only a fraction of dietary Ca is absorbed by the intestine of adult humans. The net absorption of Ca represents the vectoral sum of two processes, the transfer of Ca from the intestinal lumen into the blood and the transfer of Ca from the plasma into the lumen. There are a number of factors that have a major effect on intestinal Ca absorption; thus, Ca absorption is much greater in children during periods of skeletal growth; on the other hand, Ca absorption decreases with advancing age. The fraction of intestinal Ca absorbed is known to increase in patients ingesting a diet low in Ca content; also, there are several hormones, agents, and constituents in the diet that may alter the intestinal absorption of Ca.

Calcitonin, the forgotten hormone: does it deserve to be forgotten?

Calcitonin is a 32 amino acid hormone secreted by the C-cells of the thyroid gland. Calcitonin has been preserved during the transition from ocean-based life to land dwellers and is phylogenetically older than parathyroid hormone. Calcitonin secretion is stimulated by increases in the serum calcium concentration and calcitonin protects against the development of hypercalcemia. Calcitonin is also stimulated by gastrointestinal hormones such as gastrin. This has led to the unproven hypothesis that postprandial calcitonin stimulation could play a role in the deposition of calcium and phosphate in bone after feeding. However, no bone or other abnormalities have been described in states of calcitonin deficiency or excess except for diarrhea in a few patients with medullary thyroid carcinoma. Calcitonin is known to stimulate renal 1,25 (OH)2 vitamin D (1,25D) production at a site in the proximal tubule different from parathyroid hormone and hypophosphatemia. During pregnancy and lactation, both calcitonin and 1,25D are increased. The increases in calcitonin and 1,25D may be important in the transfer of maternal calcium to the fetus/infant and in the prevention and recovery of maternal bone loss. Calcitonin has an immediate effect on decreasing osteoclast activity and has been used for treatment of hypercalcemia. Recent studies in the calcitonin gene knockout mouse have shown increases in bone mass and bone formation. This last result together with the presence of calcitonin receptors on the osteocyte suggests that calcitonin could possibly affect osteocyte products which affect bone formation. In summary, a precise role for calcitonin remains elusive more than 50 years after its discovery.

Serum calcium and bone: effect of PTH, phosphate, vitamin D and uremia.

Hyperparathyroidism develops in chronic kidney disease (CKD). A decreased calcemic response to parathyroid hormone (PTH) contributes to the development of hyperparathyroidism and is presumed due to reduced calcium efflux from bone. Contributing factors to the decreased calcemic response to PTH in CKD include: 1) hyperphosphatemia; 2) decreased serum calcitriol; 3) downregulation of the PTH1 receptor; 4) large, truncated amino-terminal PTH fragments acting at the carboxy-PTH receptor; and 5) uremic toxins. Also, prolonged high dose calcitriol administration may decrease the exchangeable pool of bone calcium independent of PTH. The goal of the review is to provide a better understanding of how the above cited factors affect calcium efflux from bone in CKD. In conclusion, much remains to be learned about the role of bone in the regulation of serum calcium.

Fuller Albright: The Consummate Clinical Investigator

The authors have been involved in the study of mineral metabolism for a good part of our academic careers. As such, we have admired, studied, and benefited from the scientific work and writings of Fuller Albright, whose productive career at Harvard Medical School and the Massachusetts General Hospital spanned almost 30 yr from the late 1920s until 1956. The senior author, Charles Kleeman, was a house officer at Boston City Hospital in 1948 when he first read Fuller Albrights remarkable book Parathyroid Glands and Metabolic Bone Disease (1). This landmark publication summarized Albrights many contributions to mineral metabolism during the previous two decades. Our goal in this historical review is: 1) to describe Albright, the man and his life; 2) review some of his major research accomplishments; and 3) conclude by citing an appreciation of Albright by several of his coworkers and trainees. Fuller Albright was born in Buffalo, New York, on January 12, 1900. His father was a wealthy industrialist and philanthropist. The major art museum in Buffalo is known today as the Albright-Knox Art Gallery. Albright attended the Nichols School in Buffalo, which was founded by his father. He not only excelled academically, but also was captain of the football team. During his childhood, the Albright family made frequent visits to Wilmurt Lake in the Adirondacks, where he became an avid fly fisherman and developed woodsmans skills. During his academic years in Boston, Albright would spend summer vacations at Wilmurt Lake with his family. It was at Wilmurt Lake where he directed that his ashes be scattered after his death. Fuller Albright attended Harvard College, but after only 18 mo, he falsified his age and enlisted in the Army after Americas entry into World War I. It was also the time of the great influenza pandemic, which has …