BISC 307L 2nd Edition Lecture 20 Current Lecture Calcium in the Body Roles As Ca2 1 Intracellular signal for enzymes contraction exocytosis etc Necessary for adhesion at tight junctions Cofactor for blood coagulation Affects neuronal excitability hypocalcemia hyperexcitability side note hypercalcemia leads to hypoexcitability All those functions of Ca2 really only amount to 1 of the calcium in the body Most of the rest of it is in bones Not as free Ca2 ions but as a type of Ca2 phosphate called hydroxyapatite which forms the mineral portion of bones As calcium phosphate hydroxyapatite 99 forms the mineral portion of bones So calcium exists in the body not only as the most important structural component of bone but as a storage pool where excess Ca2 can be deposited or liberated to maintain the reservoir of Ca2 in the body at appropriate levels Hormones that Regulate Body Calcium The hormones that regulate plasma Ca2 come from 2 main places the thyroid and the parathyroid Pictured to the right is a thyroid gland seen from the back Calcitonin is synthesized by the C cells in between the thyroid follicles And embedded within the thyroid gland are the PT glands pink nodules There are 3 main hormones that regulate plasma Ca2 1 Parathyroid hormone released from the PT glands embedded in the posterior surface of the thyroid gland pink nodules PT hormone is essential for life it raises plasma calcium 2 The second hormone has 3 names calcitriol vitamin D3 or 1 25 dihydroxy cholecalciferol It is a steroid hormone made from vitamin D Vitamin D can be made in the skin or can be taken in through diet 3 The liver cells take that circulating vitamin D and change it into an intermediate which is processed in the kidney PTH will stimulate the conversion of the intermediate into calcitriol which also raises plasma calcium 4 Calcitonin secreted by C cells in the thyroid gland in between the thyroid follicles in the thyroid gland Ca2 Balance The small intestine is the only source of dietary Ca2 and is the source of new Ca2 being added to the body new Ca2 comes from absorption across the wall of the SI Dietary Ca2 in the SI has two fates stay and be passed onto the larger intestine to be excreted out in feces or be absorbed into the ECF The ECF is the box in the middle and it shows the normal level of free Ca2 concentration And the circle at the bottom is the intracellular fluid in cells where the free calcium is much less a thousandth or lower relative to the ECF we ve talked about how active transport creates a strong inward gradient for Ca to enter cells active transport pumps it out Bone is a very important reservoir of Ca2 Ca2 can be taken from the ECF and deposited into the bones shown in the upper left corner of the diagram above Ca2 deposited in the bone causes new bone formation and the cells that do that are called osteoblasts Ca 2 can also be released from bones by osteoclasts which break or resorb the bone and liberate Ca 2 On the right is the kidney and Ca2 being a small ion is freely filtered into the tubular fluid of the kidney where it passes along the kidney tubule That Ca2 in the kidney tubule can be reabsorbed back into the blood or left to pass out with the urine These hormones achieve Ca2 balance by acting on the rates of different processes The first hormone we are looking at is PTH it raises plasma Ca 2 in 3 ways 1 Strongly stimulates bone reabsorption of calcium PTH on top of the arrow pointing from bone to EF indicates that it stimulates the activity of osteoclasts in breaking down bone and liberating Ca2 2 PTH on the right arrow pointing from kidney to ECF stimulates reabsorption of Ca 2 from the kidney tubule back into the blood and ECF Ca2 has two effects on ion transport across the kidney tubule wall it stimulates Ca2 reabsorption back into the blood and stimulates phosphate excretion This is important because if Ca2 and phosphate concentration were to go up simultaneously than insoluble calcium phosphate salts would form and precipitate out of the blood forming Ca2 deposits in the tissue which would have negative consequences 3 The third effect is indirect PTH on the arrow pointing from SI to ECF stimulates the synthesis of calcitriol which raises plasma calcium So PTH also indirectly raises plasma calcium The second hormone we are looking at is calcitriol whose main effect is to stimulate Ca 2 absorption from the small intestine up at the top So the PTH in parenthesis enhances that action by stimulating calcitriol synthesis That s the main effect of calcitriol bringing new Ca 2 into the body Another effect is to stimulate bone resorption like PTH does shown on left Another anterior pituitary hormone we are looking at is Prolactin which also raises plasma Ca 2 Its main functions are centered around the production of milk in mammary glands Here Prolactin also stimulates Ca2 reabsorption from the digestive tract which important in lactating women because this ensures that the Ca2 necessary for the milk from lactation comes out of ECF not out of bone Lastly we have calcitonin it lowers plasma Ca2 It does this mainly by counteracting the effects of PTH If PTH stimulates bone resorption calcitonin stimulates bone deposition If PTH stimulates Ca2 reabsorption in the kidney calcitonin inhibits it So calcitonin lowers Ca 2 levels by depositing it into the bone or allowing it to pass out through the urine In adults calcitonin doesn t t play much of a role in Ca2 maintenance in body But in children and lactating women it is important In children where bones are growing rapidly stimulation of osteoblast activity by calcitonin is important for growth And in lactating women and during pregnancy there is extra demand for Ca2 in the body for adding to the milk and for the fetus respectively Calcitonin secretion is relatively high in pregnant and lactating women it stimulates bone growth to protect the mother s bones from weakening from osteoporosis But calcitonin does not actually lower plasma calcium levels it is important for these levels to be maintained at normal This is achieved because there is a simultaneous increase in secretion of calcitriol and prolactin which raise plasma Ca2 So you have no net effect on plasma Ca2 concentration but you have a protection of the bones because they are taken out of the equation as a source of Ca2 Ca2 reabsorption in the kidney and absorption in the digestive tract become the source of that Ca Topic Reproductive Figs Overview of Sexual