Calcium homeostasis and bone surface proteins, a postulated vital process for plasma calcium control.

This report is a more in-depth explanation of a recently reported hypothesis for controlling the ionic calcium content of plasma and extracellular fluids (ECF). The hypothesis proposes a two-step process for returning calcium to the ECF against the established gradient continuously moving calcium from plasma to bone surfaces. The first step in this process is the predicted transfer of calcium directly from bone surfaces to the non-collagenous proteins, which are in contact with bone mineral. This calcium would be complexed to existing proteins and a portion would automatically become available for equilibration with ionic calcium in the ECF. The basis of the hypothesis is that the equilibration level helps to set the ionic calcium concentration of plasma. The gradient toward bone and the proposed two-step return occur in the ECF of bone and would be considered normal physiochemical processes. Thus, these processes are critical for mineral ion homeostasis in mammals. In this hypothesis, parathyroid hormone (PTH) is not required for the basic process. However, PTH works within the process to raise and set a precise plasma calcium concentration. The report to follow describes the process and discusses its relationship to normal and pathological conditions affecting human health.