RUTHENIUM RED SELECTIVELY DEPLETES INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE CALCIUM STORES IN PERMEABILIZED RABBIT PANCREATIC ACINAR-CELLS

Rabbit pancreatic acinar cells, permeabilized by saponin treatment, ra pidly accumulated 3.5 nmol of Ca2+/mg protein in an energy-dependent p ool when incubated at an ambient free Ca2+ concentration of 100 nm. Ma ximal loading of the internal stores was reached at 10 min and remaine d unchanged thereafter. Complete inhibition of the Ca2+ pump with thap sigargin revealed that this plateau was the result of a steady-state b etween slow Ca2+ efflux and ATP-driven Ca2+ uptake. Sixty percent of t he pool could be released by Ins(1,4,5)P3, whereas GTP released anothe r twenty percent. The striking finding of this study is that the energ y-dependent store could also be released by ruthenium red. Uptake expe riments in the presence of ruthenium red revealed that the dye, at con centrations below 100 mum, selectively reduced the size of the Ins(1,4 ,5)P3-releasable pool. Ruthenium red had no effect on the half-maximal stimulatory concentration of Ins(1,4,5)P3. At concentrations beyond 1 00 mum, the dye also affected the GTP-releasable pool. Comparison with thapsigargin revealed that ruthenium red released Ca2+ from stores lo aded to steady-state at a rate markedly faster than can be explained b y inhibition of the ATPase alone. From the data presented, we conclude d that ruthenium red selectively releases Ca2+ from the Ins(1,4,5)P3-s ensitive store by activating a Ca2+ release channel, whereas Ca2+ rele ase from the GTP-sensitive store is predominantly caused by inhibition of the Ca2+ pump. The postulated ruthenium red-sensitive Ca2+ release channel might be similar to the ryanodine-receptor in muscle.