With regards to the article by Dane Frazier and the importance of Calcium
in the development of "thumps"-SDF, I would like to express my agreement
with his review of the role of calcium as a second messenger in many cell
physiological processes. Calcium is an absolutely essential electrolyte
and the number of roles that it plays can not be completely listed to this
date. However, even the cellular concentration of calcium is linked to
potassium. For any of you biochemists out there:
1) The intracellular concentrations of Na+ and K+ are main-
tained by an energy requiring pump known as the Na+/K+ ATPase.
2) The Na+/K+ ATPase pumps Na+ out of cells and K+ into cells.
Therefore, intracellular Na+ concentrations are low and K+
concentrations are high, with respect to outside the cells.
3) The rate at which the Na+/K+ ATPase works is dependent on how
much K+ is available on the outside of the cell. When extra-
cellular K+ is low the pump slows down, leading to an increase
in the amount of Na+ that is inside the cell. The cells must
somehow get rid of this Na+,
4) Na+ is then eliminated through 2 alternate routes. The first
is through Na+/Ca2+ exchange. So that intracellular Ca2+ may
actually increase, as intracellular Na+ is returned to normal.
The second alternate pathway for Na+ is in exchange for
protons (H+). This results in a movement of H+ into the cells,
making them acidic, but leaving a deficit of H+ outside the
cells, possibly(?) adding to the alkalosis of the plasma.
5) The extra calcium that is brought in through Na/Ca2+ exchange must
be stored inside the cells, because if it is free, "floating"
around it starts to turn on many biochemical processes as it
works as a second messenger. This could be spontaneous
muscle contractions (from tremors to cramping), or
contraction of the smooth muscle of blood vessels, reducing blood
flow and leading to further muscle cramping. In the heart it
can lead to many different forms of arrhythmias and can be life
threatening.
I do not want to be misinterpreted as down playing the role of calcium in
the development of thumps - SDF. Normal extracellular calcium is
essential for maintaining the integrity of cell membranes, for stabilizing
the electrical charges across membranes, and also extracellular calcium
helps modulate how much Na+ enters cells during action potentials (when
the nerve fires and tells the muscles to contract). When extracellular
Ca2+ is low the nerve cells become "irritable" and fire more easily. But
because K+ is low, they are also more "irritable", since K+ determines
where those cells are at rest. It is really a combination of many
factors that leads to thumps.
I haven't even addressed what part
alkalosis (low plasma H+ concentration) plays. To make a very long story
short, during alkalosis the connections between the nerve cells all down
the length of the nerve fiber are called gap junctions. In order for a
signal to be transmitted down the entire length of a nerve fiber the
electrical impulse must pass from one cell to the next through gap
junctions. When these gap junctions are narrowed, it is difficult for
the signal to continue - it is meeting a high degree of resistance. When
they are opened, electrical impulses can travel easily since resistance
is low. Alkalosis opens gap junctions and lowers the resistance to
electrical activity. Therefore, even a very small impulse can travel
farther, recruit more channels and activity and lead to firing of the
nerve. This is most likely also involved in thumps. By the way,
alkalosis will also cause the kidneys to eliminate more potassium -
complicating everything starting from ground zero. Conversely, potassium
depletion (like from dehydration) will cause alkalosis, leading to
decreased resistance to electrical activity. A double whammy!
I think that's enough for tonight. There is no simple explanation of
what is occuring in the endurance horses. Although I do not ride, I
always crew. Electrophysiology and cardiovascular pharmacology are my
obsessions. If you are interested I can supply references regarding the
basics of what I have been discussing. My personal opinion regarding
thumps in the endurance horse, is that it is an extremely
unpredictable condition and the thumping horse should not continue. There is
absolutely no way at a vet check for a veterinarian to know what plasma K+
levels, Ca2+ levels and pH of blood are. Electrolyte supplements may
temporarily suppress the situation, but what is going to happen over the
next 10 miles. Calcium alone may suppress the problem, but the
underlying causes are very involved. I do not feel it is worth risking
the horse. I would be interested in others opinions.
Lynn
Davie, FL
P.S. - CONGRATULATIONS WENDY ON YOUR NEWBORN!
L. M. Crespo
Associate Professor and Chair
Department of Pharmacology
NOVA-Southeastern University
lynn@alpha.acast.nova.edu