Page 284 - Anatomy and Physiology of Farm Animals, 8th Edition

Endocrinology / 269

calcium phosphate. The action of PTH on release is the concentration of ionized cal­
The only significant regulator of PTH
osteocytes releases calcium and phosphate
VetBooks.ir from stores associated with the extracellular cium in the blood plasma. Normally, about
fluids in and around bone. PTH stimulates
50% of plasma calcium is bound to plasma
the degradation of bone by osteoclasts proteins (primarily albumin), and this
while it inhibits bone formation by bound calcium is not biologically active.
osteoblasts. The overall effect of PTH is to The other 50% is unbound and exists as
increase blood calcium concentration and calcium ions. Chief cells detect decreases
lower blood phosphate concentration by in the concentration of ionized calcium,
increasing urinary excretion of phosphate. and they respond by increasing their secre­
The forms of vitamin D consumed in tion of PTH. The resulting rise in plasma
the diet or produced in the skin by the calcium has a negative feedback effect on
action of ultraviolet light on precursors is further PTH secretion. Recall from earlier
not the most active form of the vitamin. in this chapter that calcitonin from the thy­
These forms are further metabolized in the roid gland also functions to decrease serum
liver to a second precursor form that is fur­ calcium if it is in excess. A summary of the
ther metabolized in the kidneys to the relationship between PTH, the kidneys,
most biologically active form. It is this final and calcium absorption from the intestine
form (1,25‐dihydroxycholcalciferol or cal- is shown in Figure 13‐10.
citriol) that functions as the true vitamin Diets insufficient in calcium are
D. PTH promotes the formation of this uncommon in domestic animals other
final metabolite within the kidney. The two than carnivores, but they can occur,
major functions of vitamin D are to especially with diets formulated primarily
increase the rate of calcium absorption on grain products, which typically have
from the gastrointestinal tract and to high phosphorus and low calcium. In
reduce the loss of calcium in the urine. such cases, the chronically low intake of
Thus, the overall effect of vitamin D is to dietary calcium stimulates increased
retain calcium in the body. secretion of PTH to keep blood calcium

Kidneys

Vitamin D PTH
Parathyroid
glands
Active Vitamin D

2+
Ca absorption (intestine)

Figure 13-10. Relationship of parathyroid hormone (PTH), the kidneys, and calcium ion homeostasis
in the cow. PTH from chief cells in the parathyroid gland activates vitamin D in the kidney; activated
vitamin D promotes calcium (Ca ) absorption from the intestine. Source: Reece, 2009. Reproduced with
2+
permission of John Wiley & Sons, Inc.