WHERE IS THE INVERTING FACTOR IN HORMONE-SECRETION FROM PARATHYROID CELLS

Secretion of hormones and transmitters in the body fall into two gener al categories. In the majority of the secreting cells, including the p resynaptic terminals in the nervous system, an increase in the extrace llular calcium causes an increase in secretion (4, 9, 22, 23). There a re two notable exceptions to this general rule: the parathyroid cells (3, 15) and the renal juxtaglomerular cells (5), where an increase in extracellular calcium leads to a decrease in secretion. Because these two cell types have a cardinal role in a wide variety of physiological and pathophysiological functions, it is of great importance to unders tand the regulation of their hormone secretion process. A key element to such an understanding is the identification of the location of the ''inverting step,'' which makes the parathyroid cells behave in a fash ion contrary to most other secretary cells. Whole cell imaging studies strongly suggested that the inversion factor is between the changes i n intracellular calcium concentration ([Ca2+](i)) and the secretion of the hormone (24). Surprisingly, confocal calcium imaging of the parat hyroid cells did not support this dogma. It revealed that the interior of the parathyroid cell is a nonhomogeneous medium and that an increa se in the extracellular calcium concentration produces changes in [Ca2 +](i), in both the same and apposite directions, in different parts of the parathyroid cell.