The role of vitamin D in the endocrinology controlling calcium homeostasis

Abstract

Vitamin D and its' metabolites are a crucial part of the endocrine system that controls whole body calcium homeostasis. The goal of this hormonal control is to regulate serum calcium levels so that they are maintained within a very narrow range. To achieve this goal, regulatory events occur in coordination at multiple tissues, e.g. the intestine, kidney, bone, and parathyroid gland. Production of the vitamin D endocrine hormone, 1,25 dihydroxyvitamin D (1,25(OH)₂ D) is regulated by habitual dietary calcium intake and physiologic states like growth, aging, and the menopause. The molecular actions of 1,25(OH)₂ D on calcium regulating target tissues are mediated predominantly by transcription controlled by the vitamin D receptor. A primary role for 1,25(OH)₂ D during growth is to increase intestinal calcium absorption so that sufficient calcium is available for bone mineralization. However, vitamin D also has specific actions on kidney and bone.

Keywords: Absorption; Endocrinology; Excretion; Homeostasis; Parathyroid hormone.

Figure 1. Whole body calcium balance and its control by habitual dietary calcium intake

(A) Calcium kinetic studies show that calcium homeostasis is controlled by the balance between calcium absorption (Va) and excretion (urine, Vu; feces, VF, endogenous fecal losses, Vf) as well as the rate of bone formation (Vo+) and resorption (Vo−). Calcium balance = calcium intake (Vi) – (VF+Vu), Net calcium absorption = Va-Vf; bone balance = bone formation (Vo+) – bone resorption (Vo−). (B) Habitual dietary calcium intake alters calcium homeostasis (using data reported in Bronner and Aubert (Bronner and Aubert, 1965).

Figure 2. Models for intestinal calcium absorption and renal calcium reabsorption

(A) Kinetic modeling of intestinal mineral absorption shows that both saturable and non-saturable pathways exist. Total transport is the sum of a linear, concentration-dependent, non-saturable transport process (defined by a straight line) and a saturable component that can be defined by the Michaelis-Menton equation. [Ca] = luminal concentration of calcium; D = the slope of the non-saturable linear component assuming the intercept equals zero; Vmax = the maximum transport rate seen for the saturable transport component; Km = the luminal concentration of calcium at ½ the Vmax. (B) A summary of models for calcium absorption across the intestinal barrier. Potential mechanisms for entry, A, intracellular movement, B, and exit, C, from the cells are provided in the table for the intestine and kidney. D = the paracellular movement; N/A = not applicable.