Thapsigargin stimulates intracellular calcium mobilization and inhibits parathyroid hormone release

Ca 2+ and other divalent cations like Sr 2+ , Ba 2+ , and Mg 2+ stimulate rapid and sustained increases in intracellular Ca 2+ ([Ca 2+ ] i ) and 1,4,5‐inositol trisphosphate (1,4,5‐InsP 3 ) presumably by interacting with recently identified parathyroid cell membrane Ca 2+ receptors. We used thapsigargin (THAPS), an inhibitor of the microsomal Ca 2+ ‐ATPase, to deplete InsP 3 ‐sensitive intracellular Ca 2+ stores to determine whether sustained increases in [Ca 2+ ] i due to divalent cations require intact cytosolic Ca 2+ pools. In Fura 2‐loaded parathyroid cells, THAPS produced a gradual increase in [Ca 2+ ] i which reached a steady‐state level by 2–3 minutes. The effect of THAPS (3 × 10 −6 M) was substantial with [Ca 2+ ] i , rising from 281 ± 27 nM at 0.5 mM Ca 2+ to a peak value of 684 ± 30 nM ( p < 0.0001). The addition of Sr 2+ to cells at 0.5 mM extracellular Ca 2+ induced an immediate 2‐to 3‐fold increase in [Ca 2+ ] i which stabilized at a [Ca 2+ ] i above baseline for ≥10 minutes. THAPS (3 × 10 −6 M) pretreatment for ≥5 minutes blocked this sustained‐phase increment in [Ca 2+ ] i due to Sr 2+ . In the absence of extracellular Ca 2+ , there was a slight but nonsignificant effect of THAPS on [Ca 2+ ] i . Incubation of cells with THAPS did not change the levels of 3 H‐inositol phosphates (InsP 3 , InsP 2 , and InsP 1 ) or alter Sr 2+ ‐induced accumulation of InsP 3 , InsP 2 , and InsP 1 . THAPS substantially reduced parathyroid hormone secretion at 1.0 mM Ca 2+ by 20 ± 16, 57 ± 8, 75 ± 10, and 83 ± 9% at 10 −7 , 3 × 10 −7 , 10 −6 , and 3 × 10 −6 M THAPS, respectively. We conclude that depletion of intracellular Ca 2+ stores by THAPS stimulates Ca 2+ mobilization, presumably from extracellular sources, and that this agent and divalent cations such as Sr 2+ activate the same pathway for sustained Ca 2+ mobilization. The inhibition of secretion by THAPS supports the idea that increases in [Ca 2+ ] i play a suppressive role in the control of hormone release in the parathyroid.