Re: Parathyroid glands

[Susan Evans Garlinghouse <suendavid@worldnet.att.net>]

jtaylor@pyramid.net wrote:
> 
> There was a article in endurance news a awhile back about the
> parathyroid glands.  The article stated that horses fed alfalfa excrete
> huge amounts of calcium, and their parathyroid glands "go to sleep"

This is the hypercalcitoninism that Heidi, I and others have talked
about when horses are fed alot of calcium-rich feeds.


> Therefore, train endurance horses on grass or oat hay, to keep the
> parathyroid alert and active, and give alf just before and during
> competition.

Yup, the "take-home" point is that parathyroid hormone and calcitonin
are "inducible" hormones, meaning that their specific levels in the body
can fluctuate according to the internal environment---ie, how much
calcium is available at the time.  This is as opposed to non-inducible
(there's another term for this and if I wasn't stoned on painkillers I'd
know what it was) hormones that stay at a steady-state "set-point" no
matter what you do.

BTW, I agree to feed primary grass hays, but also be careful of not
making drastic changes in diet just before or at a competition.  Having
higher plasma calcium levels won't help you much if you have a case of
colic brewing because the night before the horse got twenty pounds of
alfalfa when he's used to only a few pounds a day.  Just a friendly word
of caution.


 My question is, what and where is the parathyroid how does
> it work and how does a horse react if this thing is active or not?

The parathyroid is, not surprisingly, right next to the thyroid gland,
as a matter of fact sort of embedded in it, and is located in the neck
alongside the trachea.  Up around where the head attaches to the neck. 
Don't go looking for bumps or anything, it's very tiny.  The only
function of the parathyroid gland (that's been identified so far---there
are some other cells in the gland that they're still trying to figure
out) is to secrete parathyroid hormone (PTH), and the only purpose of
PTH is to regulate blood calcium levels.  Calcium is EXTREMELY important
in all kinds of metabolic pathways and functions, so regulating the
levels is a big deal.

Specifically, parathyroid responds when blood calcium levels get too
low, and release the hormone PTH.  PTH affects different parts of the
body to help raise calcium levels---1) it stimulates osteoclasts (the
cells that break down mineralized bone tissue) to remove calcium from
storage in the bones and deposit it into the blood; 2) it tells the
kidneys to stop excreting calcium and retain all it can; 3) stimulates
the intestines to absorb more calcium from any food source moving
through the gut at the time; 4) put the production of any new bone into
hiatus until there's not such an immediate demand for calcium; 5) and
stimulates the production of calcitriol, the active form of vitamin D3,
which also has a role in calcium absorption.

The fact that PTH is an inducible hormone is where you can get into
trouble with endurance horses.  If you feed excessive calcium, the
parathyroid gland figures, fine, why make PTH to mobilize and conserve
calcium when there's ALL this calcium lying around all the time.  So PTH
levels tend to be pretty low.  Then you take your horse into an
endurance ride, there's a big demand for calcium, all the calcium in the
blood gets used up and the parathyroid gland gets metaphorically left
with it's pants down because it doesn't have enough PTH around to
provide more calcium from storage.  Hence (geez, I can't believe I use
words like "hence") the blood calcium levels get low enough that the
horse gets into trouble, depending on the exact circumstances---either
with increased muscle fatigue and soreness, or by coming down with
thumps.  Thumps is just a hyperirritability of the phrenic nerve caused
by, among other things, low calcium levels.

So if you feed a ration that is adequate but not excessive in calcium,
then the parathyroid gland stays "alert" by happily producing PTH and
keeping a close eye on blood calcium levels and stays ready for any
drops in blood calcium that may come along during an endurance ride. If
a big, sudden demand comes along, there's enough PTH around to quickly
mobilize sufficient calcium from storage depots.  If, on the other hand,
you feed alot of calcium all the time, PTH levels are low and the body
can't either mobilize enough calcium for the sudden demands of sustained
exercise, and also cannot synthesize PTH at a moment's notice in order
to be able to mobilize more calcium.  That's the "sleepy" parathyroid
gland you read about.

Hope this answers your questions.

Susan Garlinghouse