Calcium absorption across epithelia

JGJ Hoenderop, B Nilius… - Physiological …, 2005 - journals.physiology.org
JGJ Hoenderop, B Nilius, RJM Bindels
Physiological reviews, 2005journals.physiology.org
Ca2+ is an essential ion in all organisms, where it plays a crucial role in processes ranging
from the formation and maintenance of the skeleton to the temporal and spatial regulation of
neuronal function. The Ca2+ balance is maintained by the concerted action of three organ
systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1
g Ca2+ daily from which 0.35 g is absorbed in the small intestine by a mechanism that is
controlled primarily by the calciotropic hormones. To maintain the Ca2+ balance, the kidney …
Ca2+ is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca2+ balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca2+ daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca2+ balance, the kidney must excrete the same amount of Ca2+ that the small intestine absorbs. This is accomplished by a combination of filtration of Ca2+ across the glomeruli and subsequent reabsorption of the filtered Ca2+ along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca2+ fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D3) and parathyroid hormone. Until recently, the mechanism by which Ca2+ enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca2+ channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca2+ channels constitute the rate-limiting step in Ca2+-transporting epithelia. They form the prime target for hormonal control of the active Ca2+ flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca2+ transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca2+ channels accumulating in a comprehensive model for epithelial Ca2+ absorption.
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