Parathyroid hormone release calcium regulation

Parathyroid hormone (PTH) plays a crucial role in regulating concentrations of calcium and phosphorus in the extracellular fluid. The major signal for parathyroid hormone release is low extracellular levels of free calcium ion. PTH acts on 3 major targets—small intestine, kidney, and bone— to restore calcium ion concentrations in the extracellular fluid to the normal range if they fall too low. 

B30. Brown, E. M., Wilson, R. E., Eastman, R. C., et al., Abnormal regulation of parathyroid hormone release by calcium in secondary hyperparathyroidism due to chronic renal failure. J. Clin. Endocrinol. Metab. 54, 172-179 (1982). 

Lithium is one of the group lA alkali metals (like potassium and sodium) and is not normally present in the body. It acts predominantly through the phosphatidylinositol (PI) second messenger system, causing alterations in calcium- and protein kinase C (PKC)-mediated processes. Lithium can also alter the adenylate cyclase (AC) system, but this action is probably related to its toxic effects. Many calcium-dependent systems may be affected by lithium, among them regulation of receptor sensitivity, parathyroid hormone release, and proper functioning of intracellular microtubule structures. - ... 

The parathyroid glands secrete parathyroid hormone (PTH) that regulate calcium levels in the blood. A decrease in serum calcium stimulates the release of PTH. A decrease of PTH is called hypoparathyroidism and an increase in PTH is hyperparathyroidism. 

In mammals, both a thyroid and a parathyroid hormone exist to regulate the level of circulating calcium. Concentrations of calcium, inside and outside of cells are about 10 and 10 M respectively. Release of calcium, whether from outside the cell or from internal stores, can trigger a wide range of biol( ical responses. These concentration levels are controlled by the plasma membrane, membranes of the endoplasmic reticulum (in muscle the sarcoplasmic reticulum), and the inner membrane of mitochondria. One important function of calcium is to control the permeability of semi-permeable membranes which maintain their integrity in its presence and become porous and leaky in its absence. This control follows from the abundance of this ion, its divalent character, and the presence of phosphate anions in the membranes (Ames, Tsukada and Nesbett, 1967). 

Regulation of 25-hydroxycholecalciferol 1-hydroxylase 1,25-diOH D3 is the most potent vitamin D metabolite. Its formation is tightly i regulated by the level of plasma phosphate and calcium ions (Figure 28.24). 25-Hydroxycholecalciferol1 -hydroxylase activity is I increased directly by low plasma phosphate or indirectly by bw I plasma calcium, which triggers the release of parathyroid hormone I... 

The major location of calcium in the body is in the skeleton, which contains more than 90% of the body calcium as phosphate and carbonate. Bone resorption and formation keeps this calcium in dynamic equilibrium with ionized and complexed calcium in blood, cellular fluids and membranes. Homeostasis is mainly regulated by the parathyroid hormone and vitamin D which lead to increased blood calcium levels, and by a thyroid hormone, calcitonin, which controls the plasma calcium concentration J5 Increasing the concentration of calcitonin decreases the blood calcium level, hence injections of calcitonin are used to treat severe hyperalcaemia arising from hyperparathyroidism, vitamin D intoxication or the injection of too high a level of parathyroid extract. High levels of calcitonin also decrease resorption of calcium from bone. Hypocalcaemia stimulates parathyroid activity, leading to increased release of calcium from bone, reduction in urinary excretion of calcium and increased absorption of calcium from the intestine. Urinary excretion of phosphate is enhanced. 

The thyroid and parathyroid glands serve a number of vital endocrine functions. The thyroid gland synthesizes and secretes the thyroid hormones T3 and T4. These hormones are important regulators of cellular metabolism and metabolic rate. Thyroid hormones also interact with other hormones to facilitate normal growth and development. The parathyroid glands control calcium homeostasis through the release of PTH. This hormone is crucial... 

Parathyroid hormone (PTH) is an 84-amino acid peptide secreted by the parathyroid glands, and is the principal regulator of extracellular calcium levels [44, 45]. The effects of PTH on extracellular calcium are mediated directly or indirectly through effects on bone, kidney, and intestine. A decrease in extracellular calcium causes an increase in PTH secretion. As a consequence, the rise in PTH levels causes increased bone resorption and the release of calcium from bone, decreased calcium excretion by the kidney, and increased intestinal calcium absorption. The therapeutic application of PTH has centered on the bone effects as an anabolic treatment for osteoporosis. PTH increases the activity of both osteoblasts (which form bone) and osteoclasts (which mediate bone resorption). The desirable anabolic effects of PTH on osteoblasts appear to be highly dependent on dose schedule and the duration of daily exposure. 

The concentration of ionized calcium is closely regulated by the interactions of parathyroid hormone (PTH), phosphorus, vitamin D, and calcitonin (Fig. 49-9). Parathyroid hormone increases serum calcium concentrations by stimulating calcium release from bone, reducing renal excretion, and enhancing absorption in the gastrointestinal tract secondary to increased renal production of 1,25-... 

Abnormalities in calcium or parathyroid hormone homeostasis may be a factor in depression. Significant fluctuations in calcitonin, a calcium-regulating hormone, and low plasma calcium levels during the menstrual cycle may play a part in the etiology of PMS. Calcium influx into brain cells is involved with the release of many neurotransmitters. Calcium supplementation (e.g., 1200-1600 mg/day of calcium carbonate in two divided doses) has been shown to reduce premenstrual symptoms such as anxiety, depression, irritability, mood swings, headache, fluid retention, and cramps. " " Calcium supplementation may help to prevent osteoporosis later in life, and it is a relatively safe and inexpensive treatment. ... 

The regulation of calcitonin synthesis and release from the parafollicular C cells of the thyroid gland is calcium dependent. Rising serum calcium is the principal stimulus responsible for calcitonin synthesis and release. Other hormones, such as glucagon, gastrin, and serotonin, also stimulate calcitonin release. Calcitonin has been isolated in tissues other than the parafollicular C cells (parathyroid, pancreas, thymus, adrenal), but it is not known whether this material is biologically active. 

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