Intestinal Ca2+ absorption revisited: A molecular and clinical approach

Abstract

Ca²⁺ has an important role in the maintenance of the skeleton and is involved in the main physiological processes. Its homeostasis is controlled by the intestine, kidney, bone and parathyroid glands. The intestinal Ca²⁺ absorption occurs mainly via the paracellular and the transcellular pathways. The proteins involved in both ways are regulated by calcitriol and other hormones as well as dietary factors. Fibroblast growth factor 23 (FGF-23) is a strong antagonist of vitamin D action. Part of the intestinal Ca²⁺ movement seems to be vitamin D independent. Intestinal Ca²⁺ absorption changes according to different physiological conditions. It is promoted under high Ca²⁺ demands such as growth, pregnancy, lactation, dietary Ca²⁺ deficiency and high physical activity. In contrast, the intestinal Ca²⁺ transport decreases with aging. Oxidative stress inhibits the intestinal Ca²⁺ absorption whereas the antioxidants counteract the effects of prooxidants leading to the normalization of this physiological process. Several pathologies such as celiac disease, inflammatory bowel diseases, Turner syndrome and others occur with inhibition of intestinal Ca²⁺ absorption, some hypercalciurias show Ca²⁺ hyperabsorption, most of these alterations are related to the vitamin D endocrine system. Further research work should be accomplished in order not only to know more molecular details but also to detect possible therapeutic targets to ameliorate or avoid the consequences of altered intestinal Ca²⁺ absorption.

Keywords: Ca2+ absorption; Dietary calcium; Hormones; Paracellular pathway; Pathological alterations; Physiological conditions; Transcellular pathway.

Figure 1

Effects of hormones on intestinal Ca²⁺ absorption. Calcitriol stimulates the transcellular and paracellular absorptive pathways by inducing the expression of genes and proteins involved in Ca²⁺ transport and modifying the permeability of tight junctions. Thyroid hormones enhance the genomic actions of calcitriol whereas glucocorticoids inhibit the transcellular pathway by affecting the expression of Ca²⁺ transporting proteins. Fibroblast growth factor inhibits the intestinal Ca²⁺ absorption antagonizing 1,25(OH)₂D₃ action. Growth hormone enhances the intestinal Ca²⁺ absorption through vitamin D dependent and independent mechanisms. PMCA_1b: Plasma membrane Ca²⁺-ATPase; CB_9k: Calbindin 9k; Ca_v1.3: Ca²⁺ channel voltage-dependent L type alpha 1D subunit; TRPV5: Transient receptor potential vanilloid 5; TRPV6: Transient receptor potential vanilloid 6; CLDNs 2, 12 and 15: Claudins 2, 12 and 15; NCX1: Na⁺/Ca²⁺ exchanger; 1,25(OH)₂D3: Calcitriol; FGF-23: Fibroblast growth factor; GH: Growth hormone; GC: Glucocorticoids; T4: Thyroxine; TJ: Tight junction; VDR: Vitamin D receptor.