Ca2+ and other divalent cations like Sr2+, Ba2+, and Mg2+ stimulate rapid and sustained increases in intracellular Ca2+ ([Ca2+]i) and 1,4,5-inositol trisphosphate (1,4,5-InsP3) presumably by interacting with recently identified parathyroid cell membrane Ca2+ receptors. We used thapsigargin (THAPS), an inhibitor of the microsomal Ca(2+)-ATPase, to deplete InsP3-sensitive intracellular Ca2+ stores to determine whether sustained increases in [Ca2+]i due to divalent cations require intact cytosolic Ca2+ pools. In Fura 2-loaded parathyroid cells, THAPS produced a gradual increase in [Ca2+]i which reached a steady-state level by 2-3 minutes. The effect of THAPS (3 x 10(-6) M) was substantial with [Ca2+]i, rising from 281 +/- 27 nM at 0.5 mM Ca2+ to a peak value of 684 +/- 30 nM (p < 0.0001). The addition of Sr2+ to cells at 0.5 mM extracellular Ca2+ induced an immediate 2- to 3-fold increase in [Ca2+]i which stabilized at a [Ca2+]i above baseline for > or = 10 minutes. THAPS (3 x 10(-6) M) pretreatment for > or = 5 minutes blocked this sustained-phase increment in [Ca2+]i due to Sr2+. In the absence of extracellular Ca2+, there was a slight but nonsignificant effect of THAPS on [Ca2+]i. Incubation of cells with THAPS did not change the levels of 3H-inositol phosphates (InsP3, InsP2, and InsP1) or alter Sr(2+)-induced accumulation of InsP3, InsP2, and InsP1.(ABSTRACT TRUNCATED AT 250 WORDS)