PARACRINE INTERACTIONS AMONG PARATHYROID CELLS - EFFECT OF CELL-DENSITY ON CELL SECRETION

Cell-cell interactions are important in the regulation of endocrine ce ll secretion. To investigate the possibility that cell communication m ay alter the regulation of parathyroid cell secretion, we utilized the reverse hemolytic plaque assay (RHPA) to measure parathyroid hormone (PTH) release from individual cells. Bovine parathyroid cells were dis persed and plated with protein A-conjugated erythrocytes at cell densi ties ranging from 0.9 to 36 x 10(2) cells/cm(2) in 0.2 mM calcium. Cel l populations were greater than 98% homogeneous as determined by immun ocytochemistry and in situ hybridization for PTH mRNA. Plaques were de veloped and data analyzed for the amount of PTH per cell released (pla que area in mu m(2) x 10(4)) and the determination of cell recruitment (% plaques formed). A positive correlation existed between parathyroi d cell density and the amount of PTH released. As the distance between cells increased, the plaque area (amount of PTH released per cell) de creased (ranging from 1.0 x 10(4) mu m(2) at 0.9 x 10(2) cells/cm(2) v ersus 1.6 x 10(4) mu m(2) at 36 x 10(2) cells/cm(2)). The percentage o f cells releasing PTH (recruitment) also decreased (16% at 0.9 x 10(2) cells/cm(2) versus 47% at 36 x 10(2) cells/cm(2)). These data suggest that parathyroid cells in close proximity are stimulated to secrete m ore hormone than those at lesser densities. In addition, parathyroid c ells are recruited to secrete PTH when plated at high density. Factor( s) released by the parathyroid cell may increase cell responsiveness a nd stimulate secretion in a paracrine fashion.