CA-2+ DEPENDENCE OF INOSITOL 1,4,5-TRISPHOSPHATE-INDUCED CA-2+ RELEASE IN RENAL EPITHELIAL LLC-PK1 CELLS

We have studied arginine vasopressin (AVP)-, thapsigargin- and inositol 1,4,5-trisphosphate (InsP3)-mediated Ca2+ release in renal epithelial LLC-PK, cells. AVP-induced changes in the intracellular free calcium concentration ([Ca2+]i) were studied in indo-1 loaded single cells by confocal laser cytometry. AVP-mediated Ca2+ mobilization was also observed in the absence of extracellular Ca2+, but was completely abolished after depletion of the intracellular Ca2+ stores by 2 muM thapsigargin. Using Ca-45(2+) fluxes in saponin-permeabilized cell monolayers, we have analysed how InsP, affected the Ca2+ content of non-mitochondrial Ca2+ pools in different loading and release conditions. Less than 10% of the Ca2+ was taken up in a thapsigargin-insensitive pool when loading was performed in a medium containing 0. 1 muM Ca2+. The thapsigargin-insensitive compartment amounted to 35% in the presence of 110 muM Ca2+, but Ca2+ sequestered in this pool could not be released by InsP3. The thapsigargin-sensitive Ca2+ pool, in contrast, was nearly completely InsP3 sensitive. A submaximal [InsP3], however, released only a fraction of the sequestered Ca2+. This fraction was dependent on the cytosolic as well as on the luminal [Ca2+]. The cytosolic free [Ca2+] affected the insP3-induced Ca2+ release in a biphasic way. Maximal sensitivity toward InsP, was found at a free cytoSoliC [Ca2+] between 0.1 and 0.5 muM, whereas higher cytosolic [Ca2+] decreased the InsP3 sensitivity. Other divalent cations or La3+ did not provoke similar inhibitory effects on InSP3-induced Ca2+ release. The luminal free [Ca2+] was manipulated by varying the time of incubation of Ca2+-loaded cells in an EGTA-containing medium. Reduction of the Ca2+ content to one-third of its initial value resulted in a fivefold decrease in the InSP3 sensitivity of the Ca2+ release.