Rapid Recycling of Ca(2+) between IP(3)-Sensitive Stores and Lysosomes

Inositol 1,4,5-trisphosphate (IP(3)) evokes release of Ca(2+) from the endoplasmic reticulum (ER), but the resulting Ca(2+) signals are shaped by interactions with additional intracellular organelles. Bafilomycin A(1), which prevents lysosomal Ca(2+) uptake by inhibiting H(+) pumping into lysosomes, increased the amplitude of the initial Ca(2+) signals evoked by carbachol in human embryonic kidney (HEK) cells. Carbachol alone and carbachol in combination with parathyroid hormone (PTH) evoke Ca(2+) release from distinct IP(3)-sensitive Ca(2+) stores in HEK cells stably expressing human type 1 PTH receptors. Bafilomycin A(1) similarly exaggerated the Ca(2+) signals evoked by carbachol or carbachol with PTH, indicating that Ca(2+) released from distinct IP(3)-sensitive Ca(2+) stores is sequestered by lysosomes. The Ca(2+) signals resulting from store-operated Ca(2+) entry, whether evoked by thapsigargin or carbachol, were unaffected by bafilomycin A(1). Using Gd(3+) (1 mM) to inhibit both Ca(2+) entry and Ca(2+) extrusion, HEK cells were repetitively stimulated with carbachol to assess the effectiveness of Ca(2+) recycling to the ER after IP(3)-evoked Ca(2+) release. Blocking lysosomal Ca(2+) uptake with bafilomycin A(1) increased the amplitude of each carbachol-evoked Ca(2+) signal without affecting the rate of Ca(2+) recycling to the ER. This suggests that Ca(2+) accumulated by lysosomes is rapidly returned to the ER. We conclude that lysosomes rapidly, reversibly and selectively accumulate the Ca(2+) released by IP(3) receptors residing within distinct Ca(2+) stores, but not the Ca(2+) entering cells via receptor-regulated, store-operated Ca(2+) entry pathways.