Calcium and Phosphorus Homeostasis

Abstract

Calcium and phosphorus homeostasis relies on a complex, tightly regulated system involving many ions and hormones. The regulation of calcium and phosphorus is controlled by the actions of these ions and hormones on the intestine, kidneys and bone. Disturbances in the serum level of calcium and/or phosphorus can lead to significant pathology, including kidney stones and bone disease. In addition to parathyroid hormone and vitamin D, recently identified factors such as fibroblast growth factors and klotho play an important role in maintaining mineral ion homeostasis. The identification of subfamily V transient receptor potential cation channels (TRPV channels), Na/P_i cotransporters, the vitamin D receptor and the calcium-sensing receptor have further advanced our understanding of this complex physiology. In this review we discuss the current understanding of the relationships between the ions, hormones, and transporters that maintain calcium and phosphorus homeostasis.

Nutritional and Micronutrient Status of Elderly People Living in a Rural Community of Bangladesh

“…Due to limited exposure to sunlight, consequent to lesser outdoor activities might be a contributor to vitamin D deficiency among the elderly. On the other hand, vitamin D is one of the major calcium regulatory components of the body [40]. Thus, its imbalance generally reflects in calcium deficiency as well [41], nonetheless, our observations correspond to previous findings.…”

Parathyroid hormone (PTH) rapidly enhances CFTR-mediated HCO3− secretion in intestinal epithelium-like Caco-2 monolayer: a novel ion regulatory action of PTH

“…In mammals, the direct PTH actions, prominently confined to the kidney and bone, include inhibition of proximal tubular HCO 3 Ϫ and phosphate reabsorption, stimulation of distal tubular Ca 2ϩ reabsorption, and enhancement of bone resorption (6,54). However, direct effect of PTH on the intestine remained elusive, and whether PTH could exert an ion regulatory action in the intestinal epithelium was not known.…”

“…Although the finding in chick duodenum was consistent with the calciotropic role of PTH, the PTH-enhanced rapid Ca 2ϩ absorption was not observed in Caco-2 monolayer (S. Laohapitakworn and N. Charoenphandhu, unpublished observations). Generally, in mammals including humans and rodents, PTH enhances the intestinal Ca 2ϩ absorption indirectly by stimulating renal synthesis of 1,25(OH) 2 D 3 (54). Since PTH increased the intracellular cAMP levels in most target cells (35), and the cAMP-regulated ion transporters (e.g., CFTR, NHE1, NHE3, NBCe1, and Na ϩ -K ϩ -ATPase) were abundant in mammalian intestinal cells (30,51,64), it was possible that PTH could directly activate these transporters, leading to ion transport across mammalian intestinal epithelium.…”

“…Unlike the well-buffered apical solution in a Ussing chamber (i.e., the estimated PTH-induced HCO 3 Ϫ flux of ϳ135 pmol·s Ϫ1 ·cm Ϫ2 was not large enough to shift the [HCO 3 Ϫ ]-to-PCO 2 ratio of the buffer solution), the unbuffered ileal fluid would have been alkalinized by the robust HCO 3 Ϫ secretion. Since the ileum is the principal site for phosphate absorption (45), this PTH action could be of physiological significance because phosphate should precipitate as nonabsorbable complexes in the ileal alkaline lumen (11,24), thus fulfilling the classical hypophosphatemic action of PTH (54). On the other hand, in lower vertebrates, the PTH function with regard to HCO 3 Ϫ secretion and the responsive gastrointestinal segment may differ from that in mammals.…”

The enigmas of bone without osteocytes

“…Extracellular and intracellular calcium levels are extremely tightly controlled by elaborate physiological pathways. 82 The skeleton serves as a major reservoir of calcium, which osteoclasts can access when calcium plasma levels decrease below a given threshold, by resorbing bone and releasing free calcium ions into the circulation. Conversely, when high calcium levels are encountered, calcium is deposited into the skeleton for storage.…”