Ruthenium red selectively depletes inositol 1,4,5-trisphosphate-sensitive calcium stores in permeabilized rabbit pancreatic acinar cells

Inhibitory effects of ruthenium red on inositol 1,4, 5-trisphosphate-induced responses in rat megakaryocytes

The effects of ruthenium red (RR) on inositol 1,4,5-trisphosphate (InsP(3))-induced responses were studied in rat bone marrow megakaryocytes with the patch-clamp whole-cell recording technique in combination with fura-2 microfluorometry. Internal application of InsP(3) (100 microM) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) and activated the Ca(2+)-dependent K(+) current. Administering InsP(3) together with RR (100-500 microM) inhibited InsP(3)-induced responses (both Ca(2+) and current responses) in a dose-dependent fashion. Pretreatment of megakaryocytes with extracellular RR (50 microM) also inhibited InsP(3)-induced responses. Intracellular and extracellular application of RR reduced ADP-induced increases in [Ca(2+)](i). In contrast, in isolated single pancreatic acinar cells, RR had no effect on InsP(3)-induced responses. Taken together, these results suggest that the site of the inhibitory action of RR is at the InsP(3) receptor, or its closely associated proteins. In addition, we have shown that RR is a useful pharmacological tool with which to examine the InsP(3)-mediated responses of megakaryocytes.

Studies on human porin XVIII: the multicompartment effector ruthenium red reduces the voltage dependence of human VDAC in planar lipid bilayers

The initial data on the effect of ruthenium red on mature human type-1 VDAC are presented. Highly enriched human type-1 porin in planar lipid bilayers shows lowered voltage-dependence whenever a commercially available ruthenium red preparation is applied. The hexavalent polycationic dye ruthenium red affects different functions in varying cell compartments. Concerning the plasma membrane of cells the actual data, together with our former measurements on the interaction of VDAC and the polycationic synthetic polyamine Compound 48/80, refer to a second VDAC opener, which is relevant for studies on the stimulation of exocytotic processes of different cell types.

The endoplasmic reticulum can act as a functional Ca2+ store in all subcellular regions of the pancreatic acinar cell

Stimulation of pancreatic acinar cells raises [Ca2+]i via Ca2+ release from inositol-1,4,5-trisphosphate (InsP3)-sensitive intracellular Ca2+ stores, generally considered to reside within the endoplasmic reticulum (ER). However, with physiological doses of cholinergic agonists, the [Ca2+]i increase is localized to the apical (secretory) pole of the cell, leading to suggestions that zymogen (secretory) granules themselves may constitute an InsP3-sensitive Ca2+ store responsible for localized Ca2+ release. We have therefore re-investigated whether the ER in pancreatic acinar cells is capable of acting as a functional Ca2+ store in all, or only some, cellular regions. In streptolysin O-permeabilized cells, the ER accumulated up to 25 mmol of 45Ca2+ per liter ER volume by an ATP-dependent, thapsigargin-sensitive, process. This tracer Ca2+ uptake was dependent on ambient (loading) [Ca2+], as was the intra-ER free [Ca2+], assessed by imaging the fluorescence of Magfura-2 within the Ca2+ stores. Comparison of free and total intra-ER [Ca2+] indicated that 200-300 Ca2+ ions are bound within the ER lumen for every Ca2+ ion remaining free. Subcellular analysis showed that ER stores in all regions of the permeabilized cell took up Ca2+ at loading [Ca2+] between 60 nM and 1 microM. Thapsigargin released Ca2+ from stores in all cellular regions, as did InsP3. Immunofluorescence with antibodies against sarco(endo)plasmic reticulum-2b type Ca2+,Mg2+-ATPase or calreticulin confirmed that ER Ca2+ stores were present throughout the cytoplasm. In summary, these results clearly show that the endoplasmic reticulum can act as a functional Ca2+ store in all regions of the acinar cell, including the apical pole.

Imaging of intracellular calcium stores in individual permeabilized pancreatic acinar cells. Apparent homogeneous cellular distribution of inositol 1,4,5-trisphosphate-sensitive stores in permeabilized pancreatic acinar cells

Several lines of evidence suggest that the existence of a heterogeneous population of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-sensitive Ca2+ stores underlies the polarized agonist-induced rise in cytosolic Ca2+ concentration ([Ca2+]i) in pancreatic acinar cells (Kasai, H., Li, Y. X., and Miyashita, Y. (1993) Cell 74, 669-677; Thorn, P., Lawrie, A. M., Smith, P. M., Gallacher, D. V., and Petersen, O. H. (1993) Cell 74, 661-668). To investigate whether the apical pole of acinar cells contains Ca2+ stores which are relatively more sensitive to Ins(1,4,5)P3 than those in basolateral areas, we studied Ca2+ handling by Ca2+ stores in individual streptolysin O (SLO) permeabilized cells using the low affinity Ca2+ indicator Magfura-2 and an in situ imaging technique. The uptake of Ca2+ by intracellular Ca2+ stores was ATP-dependent. A steady-state level was reached within 10 min, and the free Ca2+ concentration inside loaded Ca2+ stores was estimated to be 70 microM. Ins(1,4,5)P3 induced Ca2+ release in a dose-dependent, "quantal" fashion. The kinetics of this release were similar to those reported for suspensions of permeabilized pancreatic acinar cells. Interestingly, the permeabilized acinar cells showed no intercellular variation in Ins(1,4,5)P3 sensitivity. Although SLO treatment is known to result in a considerable loss of cytosolic factors, permeabilization did not result in a redistribution of zymogen granules, as judged by electron microscope analysis. These results suggest that Ins(1,4,5)P3-sensitive Ca2+ stores are unlikely to be redistributed as a result of SLO treatment. The effects of Ins(1,4,5)P3 were therefore subsequently studied at the subcellular level. Detailed analysis demonstrated that no regional differences in Ins(1,4,5)P3 sensitivity exist in this permeabilized cell system. Therefore, we propose that additional cytosolic factors and/or the involvement of ryanodine receptors underlie the polarized pattern of agonist-induced Ca2+ signaling in intact pancreatic acinar cells.

Differences in uptake, storage and release properties between inositol trisphosphate-sensitive and -insensitive Ca2+ stores in permeabilized pancreatic acinar cells

Rabbit pancreatic acinar cells, permeabilized by saponin treatment, were used to study the kinetics of ATP-dependent Ca2+ uptake and release in inositol 1,4,5-trisphosphate (Ins-1,4,5-P3)-sensitive and -insensitive stores. Permeabilized acinar cells rapidly accumulated Ca2+ to steady-state. At steady state, approximately 60% of actively stored Ca2+ resided in the Ins-1,4,5-P3-sensitive store. Kinetic analysis of the Ca2+ uptake process revealed that the initial Ca2+ uptake rate was 1.7 times higher in the Ins-1,4,5-P3-insensitive store as compared to the Ins-1,4,5-P3-sensitive store. On the other hand, the Ca2+ uptake capacity was 1.6 times higher in the Ins-1,4,5-P3-sensitive store as compared to the Ins-1,4,5-P3-insensitive store. The Ca2+ uptake rate in the Ins-1,4,5-P3-sensitive store remained virtually constant for at least 4 min, whereas in the Ins-1,4,5-P3-insensitive Ca2+ store this rate progressively declined with time. These observations are compatible with: (i) an Ins-1,4,5-P3-sensitive store containing relatively few Ca2+ pumps but possessing a relatively high Ca2+ uptake capacity, which may reflect the presence of a substantial amount of Ca2+ binding protein; and (ii) an Ins-1,4,5-P3-insensitive Ca2+ store containing relatively many Ca2+ pumps but possessing a relatively low Ca2+ uptake capacity, which may reflect the presence of little if any Ca2+ binding protein. The data presented are consistent with the idea of a heterogeneous distribution of Ca2+ pumps, Ca2+ binding proteins and Ca2+ release channels between intracellular Ca2+ storage organelles.

Induction of Ca2+ oscillations by selective, U73122-mediated, depletion of inositol-trisphosphate-sensitive Ca2+ stores in rabbit pancreatic acinar cells

The effect of the putative inhibitor of phospholipase C activity, U73122, on the Ca2+ sequestering and releasing properties of internal Ca2+ stores was studied in both permeabilized and intact rabbit pancreatic acinar cells. U73122 dose dependently inhibited ATP-dependent Ca2+ uptake in the inositol (1,4,5)-trisphosphate-[Ins(1,4,5)P3]-sensitive, but not the Ins(1,4,5)P3-insensitive, Ca2+ store in acinar cells permeabilized by saponin treatment. In a suspension of intact acinar cells, loaded with the fluorescent Ca2+ indicator, Fura-2, U73122 alone evoked a transient increase in average free cytosolic Ca2+ concentration ([Ca2+]i,av), which was largely independent of external Ca2+. Addition of U73122 to cell suspensions prestimulated with either cholecystokinin octapeptide or JMV-180 revealed an inverse relationship in size between the U73122- and the agonist-evoked [Ca2+]i,av transient. Moreover, thapsigargin-induced inhibition of intracellular Ca(2+)-ATPase activity resulted in a [Ca2+]i,av transient, the size of which was not different following maximal prestimulation with either U73122 or agonist. These observations suggest that U73122 selectively affects the Ins(1,4,5)P3- casu quo agonist-sensitive internal Ca2+ store, whereas thapsigargin affects both the Ins(1,4,5)P3-sensitive and -insensitive Ca2+ store. Digital-imaging microscopy of Fura-2-loaded acinar cells demonstrated that U73122, in contrast to thapsigargin, evoked sustained oscillatory changes in [Ca2+]i. The U73122-evoked oscillations were abolished in the absence of external Ca2+. The ability of U73122 to generate external Ca(2+)-dependent Ca2+ oscillations suggests that depletion of the agonist-sensitive store leads to an increase in Ca2+ permeability of the plasma membrane and that the Ins(1,4,5)P3-insensitive Ca2+ pool is necessary for the Ca2+ oscillations.