Texas Equine Veterinary Association

www.texasequineva.com • 8 impulses and concentrations of calcium (Ca 2+ ) and magnesium (Mg 2+ ; Figure 3). During muscle contraction, nerve impulses stimulate the release of calcium (Ca 2+ ) from the sarcoplasmic reticulum resulting in a conformational change of tropomyosin releasing troponin allowing actin and myosin to bind for contraction. During relaxation, however, magnesium (Mg 2+ ) blocks the effects of calcium (Ca 2+ ) preventing further muscle contraction. Although these two electrolytes work closely in conjunction with one another to regulate muscle contractions, similar to sodium (Na+) and (K+), their absorption and bioavailability from the gastrointestinal tract differ. Calcium (Ca 2+ ) and magnesium (Mg 2+ ) actually compete for absorption in the gastrointestinal tract making the ratio of Ca:Mg the most important consideration in supplementation. An inappropriate ratio of supplementation of the two minerals can lead to implications associated with energy metabolism, muscle contraction, and bone structure/formation. When to Consider Supplementation Supplementation should be considered anytime there is a suspected disturbance to normal hydration status, acid-base balance, or gastrointestinal function. Hydration status is largely influenced by intensity of exercise and climate. Increasing intensity of exercise, increases sweat losses, expanding the need for electrolyte replenishment. Equine sweat, unlike many other species, is hypertonic, meaning the electrolyte concentrations of sweat are greater than blood electrolyte concentrations. us, electrolyte losses are greater in a sweating horse compared to a sweating human. e role of sweat is to cool the body from the heat produced by increased metabolic activity during exercise. In hot and humid climates, however, the evaporation of sweat is limited resulting in even greater sweat production, compounding the already present effects of exercise. Alternatively, in cold climates, water consumption oen declines, especially following the first cold snap of the year. is can result in dehydration and an increased risk of impaction colic oen caused by an increase in hay intake for thermoregulation and low water consumption. e supplementation of electrolytes, particularly salt (NaCl), stimulates thirst. e thirst stimulation from electrolyte supplementation encourages water consumption, thereby avoiding dehydration and the risk of impaction colic in cold, winter climates. During gastrointestinal disturbances, sodium (Na + ) concentrations in the large intestine are oen compromised resulting in abnormal water absorption. In the instance of diarrhea, for example, sodium (Na + ) loss into the large intestine is amplified. When sodium (Na + ) concentrations are increased in the large intestine, the desired osmotic gradient is lost, and water absorption is inhibited resulting in diarrhea. Replacement of electrolytes in this case is critical to hydration status. In more severe cases, this oen requires intravenous replacement of electrolytes and body fluids until the gastrointestinal tract can be repaired. Electrolyte Concentrations and Supplemental Forms As with most nutrient requirements, supplemental concentrations are determined by normal physiological concentrations and Continues on page 11 Figure 1. Periodic table of elements highlighting electrolytes involved in maintaining fluid balance, acid-base balance, and membrane potential. Figure 2. Coordinated movement of sodium (Na+) and potassium (K+) through the s o d i u m - p o t a s s i u m pump located on the membrane of most cells (Karp et al., 2015). Figure 3. e coordination of calcium (Ca2+) and magnesium (Mg2+) in skeletal muscle contraction and relaxation.

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