she is correct about this. When the osteoclasts stop knocking on the bone
to get calcium because there is enough already in circulation, production of
PTH slows down and when the cow then calves, the sudden need for CA is not
met by the feed, the PTH can't get the Ca out of the bone fast enough, so
the cow experiences hypocalcemia. If this is corrected the cow will be ok
once the bone gets stimulated enough (aobut 24 hours later). THis is
probalby exactly what's happening in the endurance horse. We need the excess
calciun during the ride for an acute effort, but since the horse isn't
getting it from alfalfa while he is being ridden, he ties up. I guess the
theory behind it is that if the horse is being stimulated by lack of Ca then
there is always a "background level" of stimulation going on which won't
occur when Ca rich feed is being given.
Here is what it says from my CLin Pathology book:
"Bone is made up of a matrix of collagen fibers that is mineralized by
hydroxyapatite crystals. THese bone salts are largely composed of Calcium
and Phosphorus. A portion of theses salts is readily mobilized, which forms
the so-called exchangeable calcium adn phos which are in equiloibrium with
the serum [blood]. Theses exchangeable ions are responsible for buffering
and evening out sudden changes in serum calcium to keep it within normal
limits.
" Bone is constantly being remodeled as it is torn down by osteoclasts
[cells that eat bone] and rebuilt by osteoblasts [bone making cells]. HTe
presence of PTH (parathyroid hormone) enhances the absorption of Ca and O
from the bone and increases the concentration of theses electrolytes in the
serum. THe presence of Calcitonin shifts teh balance to mineralizing and
laying down bone. THerefore, CT causes a decrease in serum calcium.
{skip some}
PARATHYROID HORMONE
"PTH is responsible for the minute to minute regulation of Ca concentration
in the blood. Hypocalcemia (not enough Ca) causes the parathyroid glands to
secrete PTH and conversely, hypercalcemia (too much Ca) causes the glands
to cease secretion of PTH. PTH also has an indirect effecton the intestine
as it promotes formation of active metabolite of Vitamin D in the kidney.
HTe presence of Vitamin D increases absorption of both calcium and
phospohorus in the intestine (now you know why they put Vitamin D in milk!).
The net effect of PTH is to increase the concentraion of Ca and decrease P
in the extracellular fluids becasue the renal effects are so profound.
{skip some}
HYPOCALCEMIA
"MOst of the disturbances that result in hypocalcemia cna be attributed to
disturbances of PTH, CT or Vitamin D. Depending on the hormoine that is
disturbed, the effect on circulating calcium and phosphorus can generally be
predicted by its physiologic actiona s described above.
DISORDERS OF PTH
{skip some}
"MOre commonly, we see functional hypothyroidism when a sudden heavy demand
for calcium accompanies lactation. THis is most often seen in cattle, but
similar conditions are found in bitches, mares, and ewes. Parturient
hypocalcemia is a complex metabolic disease of high producing dairy cows. It
is generally found in COWS THAT ARE FED A HIGH CALCIUM DIET during
pregnancy. THis causes calcitonin release and a chronic reduction of bone
remodeling. When parturition causes a sudden, heavy demand for lactation and
a drain on blood calcium, the bone resorptive cells are not present in large
enough quantities to respond to PTH. HYPOCALCEMIA, HYPOPHOSPHATEMIA, and
MUSCLE PARALYSIS result. Hyperglycemisa is often found as adequate blood
calcium is needed for insulin secretion.
Hmm, more interesting stuff...
"Nutritional secondary hypERparathyroidism occurs most often in young
growign dogs, cats, and HORSES that are fed a diet that is high in
phosphorus and low in calcium. ALl meat diets are most often the culprit in
dogs and cats and EXCESSIVE bran supplementation is often implicated in horses."
OK that's enough for today, but that's what my book said.
I also had to review a case of tying up for this class, and the mechanisms
are the same according to our professors. There may be other mechanisms
involved, but this is the main one and will cause all the major signs of
tying up in conjunction with muscle damage.!
HOpe this clarifies and sorry about the big words but I am confident most
everyone will understand, if not, write me...
Gwen
.Seems to me that an
>> analogy with distance horses wouldn't be too far-fetched.
>> (I mean, this is why horse fed diets high in alfalfa tie up.)
>>
>Someone please stop me before I hurt myself, or get hurt! I just can't let
>this one go, and no, I'm not educated in animal husbandry, and I may be way
>off base here, but when, oh when, is a dairy cow like an endurance horse?
>I can just see me riding 'ol Bossie to my top ten finish! OK, maybe horse
>and cow physiology is similar, but I just have to check for myself on this
>one. Sorry.
>
Gwen Dluehosh
Desert Storm Arabians
1156 Hightop Rd, #89
Blacksburg, VA 24060
540/953-1792