Calcium has essential physiological functions that include muscle contraction, hormone secretion, enzyme activation, cell division, cell membrane stability, neuromuscular excitability, and blood coagulation. Calcium also participates in pathological processes such as cell necrosis, apoptosis, free radical production, cytokine release, protease activation and vasoconstriction. There is a fine line between the physiological and pathological roles of calcium, thus, maintaining extracellular calcium concentrations within a narrow limit is extremely important. To regulate extracellular calcium concentrations there is homeostatic system that includes 3 body systems: gastrointestinal tract, kidney, bone; 3 hormones: parathyroid hormone (PTH), calcitonin (CT), and 1,25-dihydroxyvitamin D (1,25-D); and a calcium-sensing receptor (CaR) in the chief cells of the parathyroid gland, C-cells of the thyroid gland, and renal tubular cells. Another endocrine factor that has minimal calcium-regulatory functions in the adult but is important in fetal calcium homeostasis is parathyroid hormone-related protein (PTHrP). In the extracellular compartment total calcium consists of calcium bound to protein, complexed to anions (phosphate, lactate, sulfate), and in an active, free, ionized form (Ca~(2+)). A decrease in plasma pH increases Ca~(2+) while the opposite occurs with alkalosis. In horses ionized calcium represents -55% of the total calcium. A decrease in Ca~(2+) concentrations increase PTH secretion while an increase in Ca~(2+) decreases PTH and increases CT secretion. Ca~(2+) has minimal effect on PTHrP secretion. Disorders of calcium homeostasis include hypocalcemia and hypercalcemia.
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