Vitamin D, Parathyroid, and BONES
VITAMIN D
main purpose is reabsorption of Ca+ and phos in jejunum-->MINERALIZE BONE and cartilage
osteoblasts have receptors for vitamin D-->when bound release alpha 1 phosphatase
expect increased alpha 1 phosphatase with fractures, bone healing
cholesterol converted to early form of vitamin D in skin with sunlight
dietary vitamin D absorbed in jejunum
next, 25 hydroxylated in liver
1 hydroxylated in kidney by enzyme: 1 alpha hydroxylase
PTH causes synthesis of 1 alpha hydroxylase in proximal tubule
vitamins D and K not in breast milk
infants are given vitamin K in shot at birth
infants need sun exposure for D
DEFICIENCY
usudt no sun, poor diet, liver disease (can't 25 hydroxylate)
def mb caused by increased liver p450 system dt drugs (barbiturates, phenytoin) or alc
deb mbdt renal dz or low PTH: no enzyme
vitamin D deficiency causes
rickets in kids; cranial tabes = soft skull, also rachitic rosaries
two types of rickets: 1 is no 1 hydroxylation, 2 is no receptor
osteomalacia in adults, pathological fractures
VITAMIN D TOXICITY
high-->high Ca++-->kidney stones
PARATHYROID
reabsorbs Ca+ where thiazides block Na+ reabsorption (distal and collecting tubule)
same transporter for both, they "take turns" or more like Ca+ waits for Na+ to clear
when thiazides in use to block Na resorption more Ca is resorbed
risk of thiazides is hypercalcemia
parathyroid increases Ca resorption by same transporter
PTH also reduces phos reabsorption in proximal tubule
breaks down bone, receptor for PTH is on osteoblast (different receptor than vit d)
when receptor bound osteoblast releases IL-1 which activates osteoclasts
IL-1 also known as osteoclast activating factor-->break down bone to maintain blood Ca+
IL-1 also related to fever, increasing B-cell numbers and activity: generation of Abs
IL-1 production limited by estrogen production in females, testosterone in males
post-menopausal osteoporosis caused by low est-->high IL-1-->osteoclasts eat bone
CALCITONIN
has receptor on osteoclase-->inhibits clast
used as Tx for osteoporosis, hypercalcemia