Calcium, Phosphorus, Magnesium and Related Disorders (By Basil OM Saleh) Objective: 1. Calcium & Phosphorus homeostasis, Hypercalcaemia, and Hypocalcaemia. 2. Magnesium homeostasis,Hypo- and Hyper-Magnesiemia. References: 1.Clinical chemistry & Metabolic Medicine Tietz Text book of Clinical Chemistry &Molecular Diagnostics

Calcium is the most abundant mineral in the human body; 1 Kg in a 70 Kg adult human. Approximately 99 % of it is found in the bone in combination with phosphate (as hydroxyapatite). Phosphate is also one of abundant mineral; approximately 600 mg (0.60 Kg) is present in 70 Kg adult human mainly (85 %) is in the bone. The small fraction (~1 %) of body’s calcium is found in the extra-osseous compartment that is very important for normal physiology of human body because of its effect on neuromuscular excitability and cardiac muscle physiology.

In blood, calcium is found in three forms: 1. biologically and functionally active fraction; the ionized or Free calcium Ca ++ (50-65 %), 2. Calcium-protein bound-mainly albumin (30-45 %), 3. Calcium Complexed with anions like citrate, oxalate,.., (5-10 %). The measured serum or plasma Total Ca is the summation of these three forms, but the physiologically active one is the Free Ca. Terms: Free Ca (Ca++), Hypocalcaemia (abnormal low circulated Ca++), Hypercalcaemia (abnormal high circulated Ca++), active vitamin D (Dihydroxy cholecalciferol [Dihydroxy D3 (DOHD3)].

The intracellular calcium-binding protein is Calmodulin. This protein is very important to regulate the intracellular calcium and its release from the sarcoplasmic reticulum (Muscle’ s mitochondria) into the sarcoplasm (cytoplasm of muscle) and subsequent muscle contraction and relaxation. Important Notes Plasma albumin concentration: Change in serum or plasma protein (in particular albumin) leads to parallel change in serum calcium-protein bound fraction and so total calcium concentration, but the free calcium (the ionized Ca++) concentration remains unchanged.

Therefore, measurement of serum Free calcium Ca++ is the correct choice, if technically difficult, measurement of BOTH Total Calcium and albumin (or total protein) is preferred rather than the total Ca alone to correct for protein concentration abnormalities.Changes in Hydrogen ion concentration [H+] is accompanied by parallel changes in Free calcium concentration, but the total calcium remains unchanged. In alkalosis, [H+] is decreased, the Ca++ is decreased and tetany may occur even the measured total Ca level is normal.

In case of acidosis the free Ca++ is increased due to increasing of [H+] ions and also because of buffering of the bone by acid media (increase Ca solubility) and so increasing release of Ca from the bones into the ECF. Homeostasis of Blood Calcium: Control of Blood (plasma or serum) Ca is maintained in the presence of several factors and organs; The factors are: -Parathyroid Hormone PTH -Active Vitamin D3 [1,25 dihydroxy cholecalciferol (or dihydroxy Vitamin D3, DOHVD3)]

Normal Functioning of Organs, include: 1. Intestine 2. Parathyroid Gland 3. Kidneys. First, about Factors: DOHVD3. There are two types of vitamin D; D2 (ergocalciferol) the plant source and D3 the animal source. In our body, the animal Vitamin D (D3) is the important source and it is synthesized from cholesterol in skin under UV effect of the sun light. However, when the requirements of vitamin D is high, as during growth & pregnancy, and when the animal source is low because of low sunlight exposure as in elderly and chronically sick subjects who are confined indoor and in infants, the plant dietary sources are important. The synthesized vitamin in skin is cholecalciferol (D3) which is biologically inactive. This D3 is transported by blood to

the liver where it is converted by hepatic 25- hydroxylase enzyme to 25 OH-cholecalciferol (25OHD3) which still inactive till it is transported to the Kidneys (proximal renal tubular cells) where it is activated to active vitamin D3 (DOHD3 or DOHcholecalciferol) by renal enzyme 1-alpha-hydroxylase. This latter enzyme is stimulated by hypophosphataemia and high plasma PTH levels, while it is inhibited by hyperphosphataemia & Hypercalcaemia. So, kidney is endocrine organ synthesizing and secreting active D3 (considered as hormone), impairment of this hydroxylation (1-alpha hydroxylation) leads to hypocalcaemia as in chronic renal failure.

The predominant form of D3 in circulation and which is usually measured in cases of vitamin D3 deficiencies is 25OHD3, but the active form that is required for Ca homeostasis is DOHD3 which its measurement is indicated in cases of vitamin D3 metabolism defect or impairment (kidney failure). The actions of active D3 (DOHD3) in Ca homeostasis involved: 1. It is essential for intestinal lumen absorption of dietary Ca and PO It acts synergistically in combination with PTH to release of Ca and PO4– from bone (bone resorption) in response to decreased circulated Ca++ (hypocalcaemia).

PTH. It is a single chain poly peptide (protein) hormone with 84 amino acids and the first N-terminal 34-amino acids the biologically active peptide sequence. It is secreted from parathyroid gland in response to hypocalcaemia. The normal actions of this hormone in maintaining of normal blood Ca levels are 1. indirect intestinal absorption of Ca and inorganic phosphate; this is achieved by PTH stimulation of renal 1-alpha-hydroxylase enzyme and the consequent formation of active D3 (DOHD3).

2. increased the Bone resorption of Ca and PO4– by stimulation of osteoclasts activities with resultant increased blood concentrations of these two elements. 3. PTH increases the renal reabsorption of Ca and renal excretion of PO4– - (phosphaturia), this step is important in determining the NET action of PTH as (this latter action step- 3 overcome the sum of the previous above actions 1 & 2). SO, the net of PTH action is: increases the circulated levels of Ca and decreases that of PO4--. Control of PTH secretion is depend on the blood Ca++ concentration.

A fall in blood Ca++ increases the secretion (and so the blood) PTH levels till the correction of Ca++ blood levels, then in the presence normal physiology, the PTH secretion (and blood levels) returns to normal. PTH secretion is also enhanced by hypomagnesaemia. The borderline increases in PTH blood levels in the presence of Hypercalcaemia is an indication of abnormal Parathyroid gland function. The combination of PTH and DOHD3 actions is revealed when there is hypocalcaemia. In this case, a fall in plasma Ca++ level stimulates PTH synthesis and secretion. The PTH enhances 1- alpha-hydroxylase enzyme activity and therefore increases the synthesis of active D3 (DOHD3) by kidney.

These mechanisms act tiil the correction of plasma Free Ca, hence both PTH and DOHD3 secretion is suppressed. So, both PTH & DOHD3 act synergistically on the osteoclasts of bone, releasing Ca into the blood, DOHD3 also increases Ca intestinal lumen absorption. Human body also contains what is known the Parathyroid related protein PTHrP. This protein is structurally biologically similar to PTH (contains the active N-terminal 34 amino acids sequence) and therefore activating the PTH receptors mimicking the hormone action in respect to Ca metabolism. It is widely distributed throughout the body but it is repressed (inhibited).

In certain tumors as that of bronchus, head tumors is derepressed (activated) and may leads to abnormal condition of Hypercalcaemia. Another, but less important factor is Calcitonin hormone that is secreted from parafollicular cells of Thyroid gland (Thyroid medulla). This hormone has an opposite action to blood PTH and so decreasing the bone release of Ca (reduce bone resorption of ca) and may be used in treatment of Hypercalcaemia.