Electrolyte disorders in horses with colic. Part 2: calcium, sodium, chloride and phosphate

Calcium abnormalities are common in horses with colic (Dart et al. 1992; Garcia-Lopez et al. 2001). Calcium is involved in excitation and contraction of cardiac muscle and the maintenance of vascular tone. It is a positive inotrope, causing an increase in smooth muscle contractility (Gasthuys et al. 1991; Grubb et al. 1996; Garcia-Lopez et al. 2001). It also has important effects on the action potential, which is shortened with hypercalcaemia and prolonged with hypocalcaemia (Atkins 1999). Other functions of calcium in the body include neuromuscular transmission, enzyme and hormone production and coagulation (Grubb et al. 1996). It is also involved in cell messaging and receptor coupling (Zaloga et al. 1992). Calcium is primarily extracellular, with 99% being found in bones and teeth (Dart et al. 1992). Only 1% is found in the extracellular fluid, of which roughly half is protein bound, and approximately 47% ionised in the plasma (Dart et al. 1992). Many commonly used analysers measure total plasma calcium, which may not be related to ionised calcium, the biologically active form (Garcia-Lopez et al. 2001). Acid-base alterations will affect the protein binding of calcium; with acidosis increasing ionised calcium as it is displaced from protein bound sites by hydrogen ions (Cooper et al. 1992; Dart et al. 1992; Garcia-Lopez et al. 2001). Hypocalcaemia