Calcium

Calcium Homeostasis

Importance of Analyzing Calcium

  • Calcium levels are maintained within a very narrow range for optimal functions:

    • Nerve transmission

    • Muscular contraction

    • Blood coagulation

    • Hormone secretion

    • Intercellular adhesion

  • Any imbalance can lead to serious consequences.

Calcium Distribution in the Body

  • The body contains approximately 1,000 grams of calcium.

  • 99% of this calcium is found as salt, providing structural rigidity along with collagen.

  • The remainder exists in extracellular fluids.

    • Ionized components: About 50%

    • Protein-bound components: About 40%

    • Complex components: Found within the blood

  • Note: Ionized calcium is the only relevant fraction for cellular function.

Sources of Calcium

  • Individuals obtain calcium primarily through the diet.

  • The skeleton serves as a mineral repository, releasing calcium into the blood on demand.

  • Prolonged oral intake or absorption maintains blood levels typically at the expense of skeletal stores, with significant losses primarily through urine.

Hormones Regulating Calcium

  • Three main hormones help regulate calcium:

    1. Parathyroid Hormone (PTH)

    2. Vitamin D

    3. Calcitonin

  • Vitamin D and PTH are the most important for calcium and phosphate homeostasis.

Function of Calcitonin

  • Calcitonin is secreted by the thyroid when calcium levels increase.

  • It inhibits the actions of both PTH and vitamin D.

Vitamin D Regulation

  • Vitamin D enhances calcium absorption in small bowel cells and is the dominant mechanism for calcium absorption in humans.

  • It is a steroid hormone derived from cholesterol, synthesized in the skin.

    • Factors affecting synthesis include exposure to sunlight, sunlight availability, type of skin covering, and an individual's age.

  • High levels of vitamin D decrease PTH secretion.

  • Elevated phosphate levels reduce vitamin D formation, although much phosphate is passively absorbed and less dependent on vitamin D.

PTH Regulation

  • PTH is secreted from four parathyroid glands adjacent to the thyroid.

  • These glands possess specialized Calcium Sensing Receptors (CSR) that respond to calcium level changes by increasing or decreasing PTH secretion.

  • PTH effects on calcium homeostasis include:

    • Mobilizing calcium from bone through increased bone resorption.

    • Increasing renal tubular calcium resorption.

    • Increasing phosphate excretion.

    • Enhancing hydroxylation of vitamin D.

  • Summary: Low blood calcium is sensed by CSR in parathyroid glands, leading to PTH secretion to restore normal levels.

Organs Involved in Calcium Metabolism

  • Gastrointestinal Tract: Requires normal function for calcium reabsorption.

    • Issues affecting absorption include:

      • Short bowel syndromes

      • Gastric weight loss surgeries

      • Genetic defects

      • Bowel fistulas

  • Kidneys: Key role in metabolism; disease can impair phosphate excretion leading to elevated serum phosphate levels, thus affecting calcium levels and stimulating PTH secretion.

Calcium Disorders

Hypocalcemia

  • Definition: Lowered calcium levels in the blood.

  • Primary Cause: Primary hypoparathyroidism (destruction/removal of parathyroid glands leading to low PTH levels).

    • Other causes include:

      • Acute pancreatitis

      • Vitamin D deficiency

      • Renal diseases

      • Rhabdomyolysis

      • Pseudohypoparathyroidism (rare hereditary disorder, decreased target tissue response to PTH).

  • Symptoms: Neuromuscular irritability, cardiac irregularities.

  • Treatment: Administer oral or parenteral calcium and vitamin D.

Hypercalcemia

  • Definition: Elevated calcium levels in the blood.

  • Primary Cause: Primary hyperparathyroidism (excess PTH secretion).

    • Other causes include malignancies (tumors producing PTH-related peptides), hyperthyroidism, diuretics.

  • Symptoms: Neurological (drowsiness, depression), gastrointestinal (constipation, nausea, vomiting), renal (nephrocalcinosis).

  • Treatment: Parathyroidectomies, estrogen replacements, increased salt and water intake.

Measuring Calcium

  • Specimens of choice: serum or lithium heparin.

  • Urine can be used if collected in a timed manner and acidified with hydrochloric acid.

  • Dye binding reactions may be used, but AAS is considered the reference method.

Role of Phosphate in Calcium Homeostasis

  • Phosphate is vital for biochemical processes (DNA/RNA formation, stored in ATP).

  • Regulation by the kidney influenced by PTH, vitamin D, calcitonin, growth hormone, and acid-base status.

    • PTH: Lowers phosphate concentration by increasing renal secretion. High PTH = low phosphate.

    • Vitamin D: Increases phosphate levels in the blood.

    • Growth Hormone: Raises blood phosphate concentrations.

    • Calcitonin: Moves phosphate into the bone, reducing calcium in the blood.

  • Most phosphate stored in bones.

Phosphate Disorders
Hypophosphatemia

  • Causes: Hyperparathyroidism, vitamin D deficiency. Risk factors include diabetic ketoacidosis, asthma, sepsis.

  • Treatment: Replacement therapy.

Hyperphosphatemia

  • Causes: Acute/chronic renal failure, increased phosphate intake, immature PTH/vitamin D metabolism in neonates, increased cell breakdown, intensive exercise, neoplastic disorders.

  • Notable Condition: Lymphoblastic leukemia, immature lymphoblasts have four times the phosphate concentration.

    • Low PTH in hypoparathyroidism can also cause elevated phosphate levels.

  • Analyzing Phosphate:

    • Use serum or lithium heparin while avoiding hemolysis (phosphate is found in red cells).

    • Timed urine specimens can be collected since phosphate is excreted through the kidneys.

    • Colorimetric reactions are commonly used as a methodology for analysis.