[Karl Note: This is a page describing a prescription drug -- in this case one intended for dogs. This drug is intended to make it easier for a body to utilize the calcium that comes into the body from food (or calcium tablets, for that matter).
The drug is NOT part of any Vibrant Life product, but the explanation here of how Vitamin D is processed in the body seems very helpful in understanding this vitamin, and calcium in the body.]
The drug is called: "Calcitriol"
BACKGROUND
Calcitriol and 1,25 dihydroxycholecalciferol are both
scientific names for active vitamin D. The biochemistry
of vitamin D is very different from that of other
vitamins; vitamin D is actually a hormone. To understand
how calcitriol is helpful in the treatment of chronic
renal failure, it is helpful to understand some
background infromation about vitamin D, its metabolic
opposite “Parathyroid hormone,” and the general calcium
and phosphorus balance within the body.
A REVIEW OF VITAMIN D
The story of vitamin D begins when a vitamin D precursor
is eaten. The precursor we get from plants is called
“ergosterol” and the precursor we get from eating animal
tissues is called “7-dehydrocholesterol.” These
substances are absorbed into the body when they are eaten
and transported to the skin for modification by sunlight
radiation (hence the popular terminology of vitamin D as
the “sunshine vitamin”). The animal origin substance is
converted to what is called “vitamin D3” or
“cholecalciferol” while the plant substance becomes
“vitamin D2” or “ ergocalciferol.” From here we will
follow the animal origin hormone as it is the most
metabolically active.
The next stop is the liver for modification (a hydroxyl group is added to the 25th carbon of the vitamin D3 molecule) thus forming 25-hydroxycholecalciferol. After this, 25-hydroxycholecalciferol (often abbreviated 25-D3 or “calcidiol”) circulates to the kidney for its final activation. Another hydroxyl group is added in the final activation to form 1,25 dihydroxylcholecalciferol, aka calcitriol.
ACTIONS OF VITAMIN D
Calcitriol has several important actions. It enhances
absorption of both calcium and phosphate from the GI
tract, promotes release of calcium and phosphate from the
bones where they are stored, and causes the kidney not to
excrete calcium and phosphate. In short, vitamin D is
designed to increase the amount of calcium and phosphorus
circulating in the bloodstream.
Calcitriol = Active Vitamin D and causes increased calcium and phosphorus in the bloodstream.
REVIEW OF PARATHYROID HORMONE
There are four tiny parathyroid glands around the thyroid
gland in the throat area. These glands produce a
biochemical called “parathyroid hormone” often
abbreviated “PTH.” When blood calcium drops, parathyroid
hormone is secreted heavily. Parathyroid hormones
encourages the activation of calcidiol into calcitriol by
the kidney. More active vitamin D helps drive the blood
calcium level up. Active vitamin D is able to shut off
the secretion of parathyroid hormone so that this cascade
does not get out of hand.
Parathyroid hormone is able to drive stored calicum and phosphorus from the bones as is vitamin D so these hormones are able to work in concert here but in the kidney they have different functions. In the kidney, while vitamin D saves both calcium and phosphate, parathyroid hormone causes only calcium to be saved and phophate to be dumped. There is a third hormone called “calcitonin” that keeps the blood calcium level from indefinitely rising. When blood calcium starts to get too high, calcitonin is released to begin storing calcium and phosphate back in the bones until it is needed again.
Parathyroid hormone causes
increased calcium and active vitamin D but reduced
phosphorus in the bloodstream.
Calcitonin causes reduced calcium and phosphorus in the
bloodstream.
WHAT HAPPENS IN KIDNEY FAILURE?
In early kidney failure, the kidney is not able to
activate vitamin D. Not only that but it becomes
inefficient at excreting phosphate. As a result active
vitamin D levels drop (which results in a drop in blood
calcium) and blood phosphate levels start to climb. The
calcium drop is seen by the parathyroid gland, PTH is
released, and hopefully the situation can be normalized.
But the kidney may not have enough capacity to activate vitamin D no matter how much parathyroid hormone is circulating. More and more parathyroid hormone is desparately secreted to get some vitamin D activated and to get the excess phosphorus dumped. But the kidney simply cannot respond. Soon there is so much circulating phosphate that it begins to combine with the blood calcium and calcium phosphate crystals begin to form in the body’s soft tissues. This removal of calcium from the circulation causes the bones to release all available calcium, possibly to a point where there is no calcium left to release. The bones becomes soft and bendable (this is classically most notable in the jaws leading to the condition in advanced kidney failure called “rubber jaw.”) The bone crystals in the soft tissues generate an inflammatory response. None of the tissues involved can function normally. A metabolic disaster has occurred.
HOW THIS MEDICATION IS USED
By giving active vitamin D in pill form, the above
disaster can hopefully be averted or reversed. (It is
more easily averted than reversed). It has been
established that parathyroid hormone is an important
toxin in kidney failure and we want to reduce its
secretion. This is best done with minute (measured in
units 1000 times smaller than the usual dosages)
quantities of vitamin D. These quantities are enough to
shut off parathyroid hormone secretion but are not high
enough to lead to elevated phosphorus levels.
If calcitriol is started early in kidney failure, parathyroid levels may be kept low enough that calcium/phosphorus imbalance never becomes an issue. If it is started later in failure, it is helpful but may not be able to provide as good a response.
Recently a survey of the owners and veterinarians of nearly 2000 pets in chronic renal failure has been formulated. These animals all received calcitriol. Approximately 80% of the owners reported that their pets were brighter and more social and had better appetites on calictriol. It was also felt that these animals had a substantially longer life span than patients not receiving calcitriol.
SIDE EFFECTS
Calcitriol is able to elevate serum calcium in
some cases which can further destabilize the calcium
phosphorus balance and can create further toxicity issues
to the kidney. Calicum is one of the parameters monitored
in kidney failure patients. If calcium should elevate,
calcitriol is discontinued for a week or so and calcium
is rechecked. Due to the complex nature of the above
hormone interactions, inadequate calcitriol can also
cause calcium elevation. Your veterinarian can direct you
further in this instance to determine a more appropriate
calcitriol dose.
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