The endurance horse is alkalotic, as has been noted in previous posts by
Tom Ivers, Susan Evans, and others. They can get this way, as has also
been noted, by blowing of carbon dioxide accessory to the attempt to cool
themselves during hot, humid rides. This is termed respiratory alkalosis
for the fall in body concentration of hydrogen ions increases the pH,
which is alkalosis.
Endurance horses can also become alkalotic in other ways. The first of
the two most common ways is to retain bicarbonate (HCO3-) because of the
loss of chloride (Cl-) in sweat. The retention of a base to accomodate
electrical neutrality is the most common cause of metabolic alkalosis.
The second common manner in which the endurance horse becomes alkalotic
is by the loss of potassium (K+) in sweat resulting in hypokalemia or low
blood potassium. Normaly, potassium is exchanged for sodium in the
tubules of the kidney under the influence of the hormone aldosterone.
When potassium is not present sufficiently, the body will exchange
hydrogen for the sodium. (Hydrogen is lost in the urine as sodium is
returned to the blood.) The loss of an acid, the hydogen ion, causes
the body to become alklotic. This is consistent with the concept of
survival
priority, in that it is more important to maintain blood volume than fine
tune electrical activity on the membranes of cells.
Lastly, endurance horses may occassionaly become alkalotic through
inappropraite administraion of alkalyzing intravenous fluids by
veterinarians or by
riders giving horses electrolytes containing sodiun bicarbonate.
OK, get back into your chair seats before you fall off of the edge and
quit yawning. What
is the practical implication of alkalosis to tying up? Alkalosis has an
effect on the availability of calcium and potassium. Unavailable calcium
may predispose the horse to Thumps (Synchronous Diaphragmatic Flutter)
which is not the subject at hand. Alkalosis causes potassium to be driven
into cells (acidosis causes potassium to be driven into the blood). The
result of alkalosis causes a double whammy to the endurance horse -- it
is losing potassium in the sweat and the remaining blood levels decline
further as postassium is driven into the cells.
ONE of the functions of potassium is to dilate capillary beds to tissues
requiring oxygen, energy, and waste removal. In the working endurance
horse, the hardest working tissues are the skeletal muscles, especially
those driving muscles of the croup and hamstrings. If lowered blood
potassium diminishes perfusion of these muscles with blood (and therefore
oxygen), the muscles start forming energy anaerobically with the end product
being the formation and accumulation of lactic acid. If exercise
continues, tying up is the end result.
Therefore, tying up may be the result of an error in carbohydrate
metabolism, but its occurrence may also be the result of many other factors,
of which the above is only one. Tying up may also be proceeded by
alkalosis which may be a prime effector of the condition under some
circumstances (ty ups occuring late in the ride especially for horses
underconditioned or with a history of repeated episodes ,- the recurrent
tying up candidate. LOW BLOOD POTASSIUM IS PROBABLY NOT THE UNDERLYING
CAUSE OF THE HORSE THAT TYES UP AFTER A FEW MILES OF EXERCISE.
I hope this post has not muddied the waters further as my intention as
been to shed a small amount of light in a small corner of the tying up
syndrome. The discussion on this list have been fascinating. Happy
Thanksgiving.
Regards
Dane L. Frazier DVM