Controlling PTH secretion through calcium ion concentration
Decreasing the extracellular fluid calcium ion concentration inhibits this pathway, and stimulates PTH secretion, a process in contrast to many endocrine tissues, in which hormone secretion is stimulated when these pathways are activated.
Even a slight decrease in the concentration of calcium ions in the extracellular fluid can cause the parathyroid glands to increase secretion after only a few minutes. If this condition persists, the glands will enlarge, sometimes up to 5 times or more. For example, the parathyroid glands are enlarged in a patient with rickets, whose blood calcium is consistently low. The parathyroid glands also become significantly enlarged during pregnancy, although reductions in maternal extracellular fluid calcium ion concentrations are difficult to measure, and they are significantly enlarged during breastfeeding. because calcium is used for milk formation.
In contrast, conditions that raise calcium ion levels above normal reduce the activity and size of the parathyroid glands. These conditions include: (1) excess dietary calcium, (2) increased dietary vitamin D, and (3) bone loss caused by non-PTH agents (eg, non-PTH bone loss). use). Changes in extracellular fluid calcium ion concentrations are sensed by calcium-sensing receptors in the parathyroid cell membranes. The calcium receptor is a glycoprotein receptor that, when stimulated by calcium ions, activates phospholipase C and increases the intracellular synthesis of inositol 1,4,5triphosphate and diacylglycerol.
This activity stimulates the release of calcium from intracellular sources, resulting in a decrease in PTH secretion. In contrast, a decrease in extracellular fluid calcium ion concentrations inhibits this pathway and stimulates PTH secretion, a process in contrast to many endocrine tissues, in which hormone secretion is stimulated when these pathways are activated.
Figure. Estimation of effects on plasma calcium concentrations of the hormones PTH and calcitonin. Note that long-term changes in calcium concentration of only a few % can cause up to 100% change in PTH concentration.
Figure estimating the relationship between plasma calcium concentration and plasma PTH concentration. The red curve shows the acute effect when the calcium concentration changes over a period of a few hours. This effect indicates that calcium concentrations that are only a very small decrease from normal values also double or triple plasma PTH. Chronic effects where calcium ion concentrations vary over a period of many weeks, with time for the glands to enlarge, shown by the red dashed line, which indicates a small decrease in calcium levels of only a few minutes. % milligrams/deciliter in plasma can double PTH secretion. This is characteristic of a powerful regulatory mechanism that controls calcium ion concentration.