We have previously shown that the channels through which calcium enters parathyroid cells under normal physiological conditions are voltage~independent and have single~channel conductance of about 0.6 pS. We have now determined that the steady~state influx of calcium ions through these channels is about 0.9 picoamperes per cell. The calcium pump rate required to balance this current is about 30,000 ions per square micrometer per second. This implies that the density of calcium pumps in parathyroid cells is more than twice that reported for any other cell. We are determining the effect of fragments of parathyroid hormone (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 RIA 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 RIA. Surprisingly, this quantity sometimes appeared to be reduced. A possible explanation for this effect is that some of the fragments bind to intact PTH, resulting in a reduced affinity for antibody. This could appear as a reduction in the amount of cold PTH.
