We have found calcium channels in parathyroid cells that have a single-channel conductance of about 0.6 pS under normal physiological conditions. Independent of membrane potential or calcium concentration, the channels are open about 8% of the time. These channels insure a strong correlation between intracellular and extracellular calcium concentrations. Such a correlation is important to insure proper functioning of parathyroid cells, since the intracellular calcium concentration controls secretion of parathyroid hormone (PTH), and secretion of PTH, in turn, affects the extracellular calcium concentration. We have determined the effect of fragments of PTH on measurements of PTH secretion. We measured the amount of intact PTH in solution by radioimmunoassay (RIA). We then added known quantities of PTH fragments, and used RlA to remeasure the amount of intact PTH. The presence of the fragments had a significant impact on the apparent quantity of intact PTH measured by RlA. Depending on the specific concentrations of the several molecules involved, the apparent quantity of intact PTH sometimes appeared to increase and sometimes appeared to decrease. The experimental results could be simulated by a model wherein fragments can not only bind to antibody, but can also bind to intact PTH, resulting in a reduced affinity for antibody.
