Calcium homeostasis. III: The bone membrane potential and mineral dissolution.

Active ionic transport through the bone membrane appears to be involved in the regulation of calcium level in the bloodstream. This transport process can be monitored by the transmembrane electrical potential difference, which increases in the presence of parathyroid hormone. The present work is an evaluation of the constraints placed on the system by the solubility limit of the mineral phase. A thermodynamic analysis demonstrates that control of mineral dissolution can only occur when the transported species is one of the mineral phase constituents. A combination of previous experimental results with the present development limits the possible active transport mechanism responsible for the adjustment of mineral dissolution/deposition to: an outward-directed pump for hydroxyl ion, an outward-directed pump for phosphate ion, or an inward-directed pump for hydrogen ion.

A theoretical model for calcium absorption from the intestinal lumen.

A theoretical model is developed for the absorption of calcium in the human gut and a mathematical description of the model is written. For simplicity, only three compartments are considered to derive a kinetic equation which can be fit to plasma activity measurements following oral ingestion of radiotracer calcium. Initial tests show good success in correlating total fractional absorption of calcium.

Calcium homeostasis: the effect of parathyroid hormone on bone membrane electrical potential difference.

Although it is generally accepted that calcium ion homeostasis is performed primarily by bone, the mechanism by which this regulation is accomplished remains unclear. The recent demonstration of a metabolism-related electrical potential difference across bone membrane implies an active transport process within this layer of cells lining bone surfaces. This work presents a determination of the effect of parathyroid hormone on the measured potential difference to demonstrate whether this potential is involved in the homeostatic mechanism. A thermodynamic evaluation is performed based on a measurement of the distribution of charged and uncharged tracers between bone extra-cellular fluid and the bathing medium. For embryonic chick calvaria whose viability was assured by the measurement of oxygen consumption, parathyroid hormone at dosage levels up to pharmacological caused an increase in the measured potential difference and thus in the inferred activity level of ionic transport through the membrane. This result is shown to be consistent with an indirect regulation of Ca++ ion through a variable active transport of K+ ion into the interior of bone and thus provides support for the argument that the membrane potential is part of the calcium homeostatic mechanism.

Electrical potential difference across bone membrane.

The control of ion fluxes to and from bone has important implications to mineralization and calcium homeostasis. Since ionic transport frequently results in an electrical potential difference across the layer of cells lining bone surfaces, knowledge of this potential is critical to understanding the means of regulation of ionic concentrations in the interior bone fluid phase. This work presents a determination of a metabolism-related electrical potential difference by a thermodynamic argument based on the distribution of charged and uncharged tracers between the bone extracellular fluid compartment and the bathing medium. For embryonic chick calvaria whose viability was assured by oxygen consumption measurements, an electrical potential of 4 mV was determined, positive with respect to the bone fluid compartment. This measurement is shown to be consistent with the active transport of K+ ion into the interior of bone; the potential developed by this transport process will then exclude from the interior phase a passively leaked Ca2+ ion.