Page 130 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 6

Disorders of Calcium: Hypercalcemia

and Hypocalcemia

Patricia A. Schenck, Dennis J. Chew, Larry A. Nagode, and Thomas J. Rosol

Calcium is required in the body for many vital intracellu- NORMAL PHYSIOLOGY
lar and extracellular functions, as well as for skeletal sup-
port. Ionized calcium (iCa or Ca 2þ) is required for OVERVIEW OF CALCIUM
enzymatic reactions, membrane transport and stability, HOMEOSTASIS
blood coagulation, nerve conduction, neuromuscular
transmission, muscle contraction, vascular smooth mus- Regulation of serum calcium concentration is complex
cle tone, hormone secretion, bone formation and resorp- and requires the integrated actions of PTH, vitamin D
tion, control of hepatic glycogen metabolism, and cell metabolites, and calcitonin (Fig. 6-1). PTH and calcitriol
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growth and division. Intracellular calcium ions are (1,25-dihydroxyvitamin D 3 ) are the main regulators of
one of the primary regulators of the cellular response to calcium homeostasis and have major regulatory effects
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many agonists and serve as “an almost universal ionic on each other. PTH is largely responsible for the min-
messenger,” conveying signals received at the cell surface ute-to-minute control of serum iCa concentration,
to the inside of the cell. 463 In addition to serving as an whereas calcitriol maintains day-to-day control. In the
intracellular messenger, the iCa concentration in the fetus, the parathyroid glands and placenta produce
extracellular fluid (ECF) regulates cell function in many PTHrP, which binds to PTH receptors and regulates cal-
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organs, including the parathyroid gland, kidneys, and cium balance. After birth, the parathyroid glands mod-
thyroid C cells by binding to a newly identified cell ify their pattern of hormone secretion and produce
membrane-bound calcium-sensing receptor. 80 Normal predominantly PTH. Other hormones, including adrenal
homeostatic control mechanisms usually maintain the corticosteroids, estrogens, thyroxine, growth hormone,
serum calcium concentration within a narrow range and glucagon, and prolactin, have less influence on calcium
guarantee an adequate supply of calcium for intracellular homeostasis but may play a role during growth, lactation,
function. These mechanisms must be disrupted for hyper- or certain disease states.
calcemia or hypocalcemia to develop. Abnormal serum The intestine, kidneys, and bone are the major target
calcium concentrations are of diagnostic value and con- organs affected by calcium regulatory hormones. These
tribute to the development of lesions and clinical signs. interactions allow conservation of calcium in the ECF
Technological advances in the measurement of serum by renal tubular reabsorption, increased intestinal trans-
iCa concentration, parathyroid hormone (PTH), parathy- port of calcium from the diet, and internal redistribution
roid hormone-related protein (PTHrP), and vitamin D of calcium from bone (Fig. 6-2). The intestine and
metabolites have provided tools that allow greater diag- kidneys are the major regulators of calcium balance in
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nostic accuracy in the investigation of calcium disorders. health. Normally, dietary calcium intake equals the
Veterinarians must frequently interpret abnormal serum amount of calcium lost in urine and feces. The enteric
calcium concentrations. Large deviations of serum calcium absorption of calcium depends on the physiologic status
concentration from normal occur infrequently, but small of the intestines (e.g., acidity, presence of other dietary
deviations may be equally important because they also components, integrity of the villi or presence of small
provide diagnostic clues to an underlying disease. The intestinal disease, and degree of enterocyte stimulation
magnitude of altered serum calcium concentration often by calcitriol). Non–protein-bound calcium is filtered by
does not suggest a specific diagnosis or the extent of dis- the glomerulus and undergoes extensive renal reabsorp-
ease. Furthermore, a normal serum calcium concentration tion. This process results in reclamation of more than
146,482
does not eliminate a disorder of calcium homeostasis. 98% of the filtered calcium in health.

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