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In addition to calcium's structural role in bone, the concentration of Calcium Ions within the body is critical for normal muscle contraction.

Muscle fibres are connected to the nervous system via neuromuscular junctions; these junctions release a substance called ACETYLCHOLINE, the released acetylcholine binds to special receptors on the muscle fibres.

This produces electrical activity by stimulating the movement of SODIUM and POTASSIUM IONS through the cell membrane, although this binding occurs for only a few milliseconds, it creates far more electrical activity than is required to stimulate muscle contraction.

This electrical activity generated on the surface of the muscle fibre is conducted into the cell, where it causes the release of CALCIUM IONS.

The calcium ions then modulate the acetylcholine and stimulate contraction of the muscle fibres. (MYOFIBRILS)
A pump within the muscle cell returns the calcium ions to the cell, keeping the concentration of calcium ions in the myofibrils at an extremely low level, except when required for the next muscle contraction.

Calcium is present in the extra cellular fluid in three forms, IONISED, bound to ALBUMIN, or combined with CHLORIDE or BICARBONATE.

Only ionised calcium is biologically active in muscle contraction.
Most laboratory methods for measuring SERUM CALCIUM simply measure the total serum calcium, however this provides no real guide to the availability of IONISED CALCIUM.

Nor does simply measuring total serum calcium take into consideration changes in calcium value's depending on blood Ph.
ALKALOSIS decreases ionised calcium, while ACIDOSIS has the opposite effect.

Special equipment is needed to measure ionised calcium.

Low Serum Calcium (Hypocalcemia)
The initial symptom of low serum calcium may simply be an episodic weakness after a hard run.

If severe, the symptoms may include muscle tremors, panting, nervousness, seizures, and increased body temperature.
These signs may be sporadic instead of constant.

HYPOCALCEMIA is potentially life threatening and should be treated without delay.

High Serum Calcium (Hypercalcemia)
Although high serum calcium in greyhounds is rare, depending on the cause, the symptoms may include: depression, anorexia, constipation, weakness, polydipsia / polyuria (excessive water drinking and urine production) and cardiac arrhythmia's.

When high serum calcium is coupled with elevated SERUM PHOSPHORUS levels it may cause soft tissue mineralization and severe kidney damage due to kidney calcification.

Severe HYPERCALCEMIA may be fatal.
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Calcuim & Muscle Function