Calcium binding by parathyroid cell plasma membranes

Parathyroid hormone (PTH) secretion from parathyroid glands is controlled mainly by extracellular calcium both in vivo and in vitro. In this study, the Ca2+ binding properties of bovine parathyroid cells have been investigated on a highly purified plasma membrane preparation with flow dialysis techniques. Scatchard plot analysis of the data shows the existence of at least two different binding sites: the high affinity have an apparent Kd1 of 6.6 X 10(-5) M and a capacity (n1) of 20.1 nmol/mg protein. The low affinity sites have an apparent Kd2 of 2.6 X 10(-4) M and a capacity (n2) of 37.7 nmol/mg protein. Furthermore, at higher total Ca2+ concentrations, additional bindings with an apparent Kd3 in the millimolar range and a capacity (n3) of 118 nmol/mg protein are detectable. Neither the apparent Kd's nor n's of these calcium binding sites was affected by isotonic substitution of the medium with NaCl or KCl, while the number of binding sites increased in Choline Cl. On the other hand, the presence of 5-10 mM LiCl in isotonic Na-K medium caused a marked decrease in calcium binding from these sites. LiCl is the only monovalent cation that both in vitro and in vivo is able to enhance PTH release regardless of high or low extracellular Ca2+ concentrations. Therefore the observed displacement of calcium from calcium binding sites seems to be rather specific and could correlate with the observed enhanced PTH secretion in parathyroid cells. It is proposed that the first event in the regulation of PTH secretion by extracellular Ca2+ is the binding of Ca2+ to the low affinity, high capacity sites present on the surface of parathyroid cells and that calcium occupancy of those sites is the necessary event to initiate intracellular signals leading to inhibition of PTH secretion.