Calcium physiology and clinical evaluation

Calcium physiology

Approximately 99% of body calcium resides in the skeleton, of which a small percentage is freely exchangeable with the extracellular calcium. Although only 1% of the calcium in the body is present in the extracellular and intracellular spaces, the exchange and balance of calcium ions are critical in these spaces. The calcium concentrations in blood plasma are tightly regulated, with calcium ions playing a vital role in nerve impulse transmission, muscular contraction, blood coagulation, hormone secretion, and intercellular adhesion ( ) .

Overall calcium balance is regulated by the concerted actions of calcium absorption in the intestine, reabsorption in the kidney, and exchange from bone. All of these processes are controlled by hormones that are part of the complex mechanism regulating both the total body and extracellular calcium.

The normal plasma concentration of total calcium is approximately 2.20–2.55 mmol/L. While all calcium in blood plasma are calcium ions, much of these ions are bound to anions, with about 40%–45% bound to proteins (mostly albumin), and about 10%–15% complexed to smaller anions, mostly bicarbonate, citrate, lactate, and phosphate. The remaining 45%–50% are free calcium ions ( , ) .

The concentration of calcium in the cytoplasm of cells is about 15,000–20,000 times lower than the concentrations in extracellular plasma ( ) . This extracellular-to-intracellular gradient is maintained by the high-energy phosphates powering various ion pumps. The ionized calcium in the blood is the physiologically active portion of calcium and controls many neuromuscular and secretory processes ( ) .

As an example, ionized calcium plays a vital role in cardiovascular function. The inward flow of calcium ions into cardiac cells helps control cardiac rhythm by binding to contractile proteins in myocardial cells, which initiates the contractile process. The rate of flow of calcium ions into smooth muscle cells influences the tension of arterioles that regulate blood pressure. Calcium channels in the cell membrane regulate this flow by opening when the membrane is depolarized. Magnesium ions help stabilize the calcium channels. Cardiotropic drugs such as epinephrine and isoproterenol facilitate the transport of calcium ions through these channels promoting more rapid or stronger contractions increasing cardiac work. Calcium channel blockers, such as nifedipine, verapamil, and diltiazem lower blood pressure by relaxing blood vessels and slowing the heart rate which increases blood flow and oxygen supply to the heart and reduces cardiac work ( , ) .

Calcium ions enter cells through calcium channels, triggering the release of calcium ions from both the sarcoplasmic reticulum and the inner cell membrane. Regulation of calcium entry through these voltage-gated channels is essential for the control of physiological processes such as secretion, synaptic transmission, and excitation–contraction coupling ( ) . ATPases remove the excess calcium ions from inside the cell and transport calcium ions back into the sarcoplasmic reticulum for future release. In addition to these voltage-dependent calcium channels, other phosphorylation-dependent calcium channels are opened by a protein kinase activated by cyclic adenosine monophosphate (cAMP) ( ) . Several substances affect these channels: the cardiotropic drugs epinephrine and isoproterenol facilitate the transport of calcium ions; acetylcholine and calcium channel blockers hinder the transport of calcium ions ( ) .

Calcium ions are also important as second messengers. After stimulation at the cell surface by a specific molecule (first messenger), the inward flux of calcium ions (second messenger) initiates cellular actions, such as the production of hormones, such as insulin, aldosterone, vasopressin, and renin. This is done by keeping cytoplasmic Ca ²⁺ concentration low at rest and by mobilizing Ca ²⁺ in response to stimulus, which in turn activates the cellular reaction ( ) .

Regulation in the blood

Intestinal (GI) absorption of calcium . Most GI absorption of calcium is in the duodenum, jejunum, and ileum. Absorption occurs by either paracellular pathways through “tight junction” channels between cells, or transcellular pathways through cells. The paracellular route is partly regulated by 1,25-dihydroxy vitamin D (calcitriol) that increases the permeability of these tight junctions to Ca ions. Calcitriol also stimulates the intestinal cells to increase the synthesis of calbindin, an intracellular protein that accepts Ca ions from the intestinal microvilli, then ultimately releases the Ca ions into blood ( ) .

Renal regulation of calcium . As shown in Fig. 5.1 , 60%–70% of filtered Ca ions are reabsorbed in the proximal convoluted tubule (PCT), mostly by passive mechanisms, but small amounts are reabsorbed by active mechanisms primarily regulated by PTH and calcitonin ( ) . In the thick ascending limb of the loop (TAL), paracellular Ca ion absorption is driven by the Na–K–2Cl (NKCC2) cotransporter and the renal outer medullary K ion (ROMK) channel. Cotransporters are carrier proteins that simultaneously transport two different molecules or ions from one side of a membrane to the other at a rate of several thousand molecules per second. By using a favorable electrical or concentration gradient to transport one ion across a membrane, another ion is transported against its concentration gradient. The NKCC2 and ROMK processes transport Na, K, and Cl ions in a manner that yields a net positive charge in the lumen of the TAL, which drives the reabsorption of Ca ions by a paracellular route.

The cells in the TAL also express several “tight junction” proteins called claudins. Claudins are membrane proteins in these tight junctions between cells that regulate the flow of molecules and ions through these paracellular (between-cell) spaces. In the TAL, claudins reabsorb a small but important amount of Ca ions. Claudins are closely regulated by the calcium-sensing receptors (CaSR) that inhibit their ability to reabsorb Ca ions ( ) .

Hormonal regulation of calcium

Three hormones regulate serum calcium by altering their secretion rate in response to changes in ionized calcium: PTH, vitamin D, and calcitonin, although the role of calcitonin is less clear. Their actions are shown in Table 5.1 . A molecule with PTH-like actions, called PTH-related peptide (PTHrP), appears to have hypercalcemic actions similar to those of PTH, and may be especially important in hypercalcemia of malignancy ( , ) .

Parathyroid hormone . When calcium levels in blood fall, cells in the parathyroid glands respond by synthesizing and secreting PTH, which acts synergistically with vitamin D to increase blood calcium levels. The parathyroid glands respond rapidly to a decrease in ionized calcium concentration in blood, with a fourfold increase in PTH secretion stimulated by a 5% decrease in ionized calcium ( , , ) and is stopped by an increase in ionized calcium ( , ). PTH is synthesized in the parathyroid gland as a precursor molecule of 115 amino acids. After cleavage in the gland, intact PTH is secreted into the blood, where it circulates as an intact hormone and also as amino-terminal, carboxy-terminal, and midmolecular fragments that have varying biological activity. Both intact PTH and amino-terminal PTH interact with PTH receptors on bone and renal cell membranes ( ). In bone, PTH activates osteoclasts to release cytokines that enhance the breakdown of bone to release calcium and phosphate. In kidneys, PTH conserves calcium by increasing tubular reabsorption of Ca ions and lowers phosphate by inhibiting tubular reabsorption of phosphate. PTH also activates a specific 1-α-hydroxylase enzyme in the kidney that increases the production of active vitamin D.

Vitamin D. Inactive forms of vitamin D ₃ are obtained either from the diet or from exposure of skin to sunlight. These are converted in the liver to 25-hydroxycholecalciferol (25-OH-D ₃ ), then activated by the enzyme 1-α-hydroxylase in the kidney to form 1,25-dihydroxycholecalciferol [1,25-(OH) ₂ -D ₃ ; calcitriol], the biologically active form. As noted earlier, PTH promotes this conversion to active vitamin D by activating the production of the 1-α-hydroxylase enzyme, and phosphate inhibits this production. The metabolism of vitamin D is shown in Fig. 5.2 .

Active vitamin D ( ) :

• Enhances the activity of calcium pumps and calcium channels in intestinal cells to increase calcium and phosphate absorption

• Has a short-term enhancement of PTH-activated bone resorption to release calcium into the blood

• Acts with PTH to minimize renal excretion of calcium

• Contributes to the negative feedback regulation of PTH through specific receptors for 1,25-(OH) ₂ -D ₃ on the parathyroid glands

Calcitonin . If calcium levels in the blood increase, several mechanisms act to reverse the increase: (1) C cells in the thyroid gland synthesize calcitonin, which exerts its calcium-lowering effect by inhibiting the osteoclasts in bone, which promotes deposition of calcium into bone and lowers calcium concentrations in blood. Calcitonin may also promote this effect by inhibiting the actions of both PTH and vitamin D on these cells. It is still not clear if calcitonin contributes to normal calcium homeostasis, because (a) persons without thyroid glands are still able to regulate their calcium levels in the blood, and (b) calcitonin does not change in response to small (1%–2%) increases in ionized calcium. A study on hemodialysis patients showed that ionized calcium had to increase by ∼10% to stimulate a calcitonin response ( , ) . Another study noted that a 0.4 mmol/L increase was required to stimulate a response ( ) .

Distribution in cells and blood

More than 99% of calcium in the body is in the bone as hydroxyapatite, a complex molecule of calcium and phosphate. The remaining 1% is mostly in the blood and extracellular fluid. The amount of intracellular calcium is much lower than in blood, and this is especially true of cardiac and smooth muscle cells, where the concentration of free ionized calcium in the cytosol is 10,000–20,000 times less than that of blood. Having this large gradient ensures the rapid inward flux of calcium ions necessary to trigger muscle contraction. Maintaining this gradient also requires efficient mechanisms for pumping out calcium ions. Loss of this very large gradient will lead to cell death ( ) .

Calcium in the blood is distributed among several forms: free calcium ions, bound to protein, and bound to smaller anions ( , ) . As shown in Fig. 5.3 , bound forms of calcium are in equilibrium with the free calcium ions. This distribution can change in many diseases, but major changes may occur during surgery or in critically ill patients because of changes in citrate, bicarbonate, lactate, and phosphate. These changes, in addition to the effect of pH on Ca ²⁺ binding to proteins, are the principal reasons why ionized calcium cannot be reliably calculated from total calcium measurements in acutely ill individuals. As a rough guide, pH changes influence the concentration of ionized calcium, with a decrease of 0.1 pH unit increasing ionized calcium by ∼0.05 mmol/L (0.2 mg/dL) ( ).

Signs and symptoms of hypocalcemia

Hypocalcemia is considered to be a total calcium <8.5 mg/dL (<2.12 mmol/L) or an ionized calcium <1.00 mmol/L. As ionized calcium decreases, concentrations of 0.50–0.75 mmol/L may require administration of calcium and concentrations less than 0.50 mmol/L may produce tetany or life-threatening complications ( ) . Ionized calcium concentrations below 0.70 mmol/L have been associated with higher morbidity and mortality ( , ) .

Symptoms of hypocalcemia are often manifested as cardiovascular disorders such as cardiac insufficiency, hypotension, and arrhythmias. Hypocalcemia can also cause irregular muscle spasms, called tetany, and numbness around the mouth. The rate of fall in ionized calcium initiates symptoms as much as the absolute concentration of ionized calcium ( ) .