GTP-sensitivity of the energy-dependent Ca2+ storage pool in permeabilized pancreatic acinar cells.
Isolated rabbit pancreatic acinar cells, permeabilized by saponin treatment and incubated in the presence of 0.1 microM free Ca2+, accumulated 3.3 nmol of Ca2+/mg of acinar protein in an energy-dependent pool. Part of this energy-dependent pool could be released by GTP in a polyethylene glycol-dependent manner. The kinetics of GTP-induced release of Ca2+ showed a biphasic pattern with an initial rapid phase followed by a sustained slower phase. In contrast, IP3-induced release of Ca2+ was completed within 30 s following addition of IP3. No reuptake of Ca2+ was observed following GTP- or IP3-induced release of Ca2+. The GTP effect was independent of IP3 and not inhibited by Ca2+, indicating that the IP3-operated Ca2+ channel is not involved in GTP-induced release of Ca2+. The size of the IP3-releasable pool was not affected by GTP, indicating that GTP, when added to permeabilized acinar cells, does not promote the coupling between IP3-insensitive and IP3-sensitive Ca2+ accumulating organelles. Thus, in permeabilized acinar cells, GTP and IP3 act on different Ca2+ sequestering pools. Interestingly, however, comparison of the size of the GTP-releasable pool with that of the IP3-releasable pool for the cell preparations used in the present study, revealed an inversed relationship, indicating that at the time of permeabilization the GTP-releasable pool can be coupled to a greater or lesser extent to the IP3-releasable pool. This suggests that, in the intact cell, a GTP-dependent mechanism may exist that controls the size of the IP3-releasable pool by coupling IP3-insensitive to IP3-sensitive organelles. Moreover, this suggests that the extent of coupling is preserved during permeabilization.
van de Put
F H
FH
Department of Biochemistry, University of Nijmegen, The Netherlands.
De Pont
J J
JJ
Willems
P H
PH
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002463:Cell Membrane Permeability; D004305:Dose-Response Relationship, Drug; D006160:Guanosine Triphosphate / Q000378:metabolism / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism / Q000494:pharmacology*; D007700:Kinetics; D010179:Pancreas / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D011092:Polyethylene Glycols / Q000494:pharmacology*; D011817:Rabbits; D051381:Rats; D012503:Saponins / Q000494:pharmacology
D012503:Saponins; D011092:Polyethylene Glycols; D015544:Inositol 1,4,5-Trisphosphate; D006160:Guanosine Triphosphate; D002118:Calcium