FIGURE 8.

Calyculin A attenuates Ca²⁺_o-induced suppression of PTH secretion in normal human parathyroid cells. Human parathyroid cells were prepared and perifused as described under “Experimental Procedures.” Cells were first perifused in 1.2 mm Ca²⁺_o corresponding to the midpoint of the normal range (1.1–1.3 mm). Lowering Ca²⁺_o concentration to 0.8 mm reversibly stimulated PTH secretion, whereas subsequent 1.4 mm Ca²⁺_o exposure suppressed PTH. However, in the continued presence of 1.4 mm Ca²⁺_o, cotreatment with 100 nm calyculin A elicited a progressive increase in PTH secretion to levels similar to or even exceeding those observed at 0.8 mm Ca²⁺_o. Upon the removal of calyculin A, PTH secretion levels slowly returned to their control levels. The result shown is representative of three independent experiments.