Anion dependence of Ca2+ transport and (Ca2+ + K+)-stimulated Mg2+-dependent transport ATPase in rat pancreatic endoplasmic reticulum

Sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) inhibitors identify a novel calcium pool in the central nervous system

Ca2+ uptake into the endoplasmic reticulum (ER) is mediated by Ca2+ ATPase isoforms, which are all selectively inhibited by nanomolar concentrations of thapsigargin. Using ATP/Mg2+-dependent 45Ca2+ transport in rat brain microsomes, tissue sections, and permeabilized cells, as well as Ca2+ imaging in living cells we distinguish two ER Ca2+ pools in the rat CNS. Nanomolar levels of thapsigargin blocked one component of brain microsomal 45Ca2+ transport, which we designate as the thapsigargin-sensitive pool (TG-S). The remaining component was only inhibited by micromolar thapsigargin, and thus designated as thapsigargin resistant (TG-R). Ca2+ ATPase and [32P]phosphoenzyme assays also distinguished activities with differential sensitivities to thapsigargin. The TG-R Ca2+ uptake displayed unique anion permeabilities, was inhibited by vanadate, but was unaffected by sulfhydryl reduction. Ca2+ sequestered into the TG-R pool could not be released by inositol-1,4,5-trisphosphate, caffeine, or cyclic ADP-ribose. The TG-R Ca2+ pool had a unique anatomical distribution in the brain, with selective enrichment in brainstem and spinal cord structures. Cell lines that expressed high levels of the TG-R pool required micromolar concentrations of thapsigargin to effectively raise cytoplasmic Ca2+ levels. TG-R Ca2+ accumulation represents a distinct Ca2+ buffering pool in specific CNS regions with unique pharmacological sensitivities and anatomical distributions.

ATP inhibition and rectification of a Ca2+-activated anion channel in sarcoplasmic reticulum of skeletal muscle

We describe ATP-dependent inhibition of the 75-105-pS (in 250 mM Cl-) anion channel (SCl) from the sarcoplasmic reticulum (SR) of rabbit skeletal muscle. In addition to activation by Ca2+ and voltage, inhibition by ATP provides a further mechanism for regulating SCl channel activity in vivo. Inhibition by the nonhydrolyzable ATP analog 5'-adenylylimidodiphosphate (AMP-PNP) ruled out a phosphorylation mechanism. Cytoplasmic ATP (approximately 1 mM) inhibited only when Cl- flowed from cytoplasm to lumen, regardless of membrane voltage. Flux in the opposite direction was not inhibited by 9 mM ATP. Thus ATP causes true, current rectification in SCl channels. Inhibition by cytoplasmic ATP was also voltage dependent, having a K(I) of 0.4-1 mM at -40 mV (Hill coefficient approximately 2), which increased at more negative potentials. Luminal ATP inhibited with a K(I) of approximately 2 mM at +40 mV, and showed no block at negative voltages. Hidden Markov model analysis revealed that ATP inhibition 1) reduced mean open times without altering the maximum channel amplitude, 2) was mediated by a novel, single, voltage-independent closed state (approximately 1 ms), and 3) was much less potent on lower conductance substates than the higher conductance states. Therefore, the SCl channel is unlikely to pass Cl- from cytoplasm to SR lumen in vivo, and balance electrogenic Ca2+ uptake as previously suggested. Possible roles for the SCl channel in the transport of other anions are discussed.

ATP-sensitive voltage- and calcium-dependent chloride channels in sarcoplasmic reticulum vesicles from rabbit skeletal muscle

Chloride channels in the sarcoplasmic reticulum (SR) are thought to play an essential role in excitation-contraction (E-C) coupling by balancing charge movement during calcium release and uptake. In this study the nucleotide-sensitivity of Cl- channels in the SR from rabbit skeletal muscle was investigated using the lipid bilayer technique. Two distinct ATP-sensitive Cl- channels that differ in their conductance and kinetic properties and in the mechanism of ATP-induced channel inhibition were observed. The first, a nonfrequent 150 pS channel was inhibited by trans (luminal) ATP, and the second, a common 75 pS small chloride (SCl) channel was inhibited by cis (cytoplasmic) ATP. In the case of the SCl channel the ATP-induced reversible decline in the values of current (maximal current amplitude, Imax and integral current, I') and kinetic parameters (frequency of opening FO, probability of the channel being open PO, mean open TO and closed Tc times) show a nonspecific block of the voltage- and Ca2+-dependent SCl channel. ATP was a more potent blocker from the cytoplasmic side than from the luminal side of the channel. The SCl channel block was not due to Ca2+ chelation by ATP, nor to phosphorylation of the channel protein. The inhibitory action of ATP was mimicked by the nonhydrolyzable analogue adenylylimidodiphosphate (AMP-PNP) in the absence of Mg2+. The inhibitory potency of the adenine nucleotides was charge dependent in the following order ATP4- > ADP3- > > > AMP2-. The data suggest that ATP-induced effects are mediated via an open channel block mechanism. Modulation of the SCl channel by [ATP]cis and [Ca2+]cis indicates that (i) this channel senses the bioenergetic state of the muscle fiber and (ii) it is linked to the ATP-dependent cycling of the Ca2+ between the SR and the sarcoplasm.

Ca2+ uptake by endoplasmic reticulum of renal cortex. I. Ionic requirements and regulation in vitro

A subcellular fraction enriched in cytochrome c reductase (7.9-fold) and relatively de-enriched (0.64-fold) in Na+/K(+)-ATPase was prepared from canine kidney cortex by sucrose density gradient ultracentrifugation. It was shown by electron microscopy to consist primarily of a light fraction of endoplasmic reticulum (LER). LER vesicles displayed ATP-dependent 45Ca2+ uptake that was insensitive to 10 mM KCN or NaN3, and was promptly released by 20 microM A23187 or ionomycin. Inositol-1,4,5-trisphosphate (IP3) appeared to produce a time-dependent release of 45Ca2+. Vanadate inhibited 45Ca2+ uptake with a Ki approximately 0.3 mM, further suggesting that the activity resided in the ER rather than the plasma membrane. 45Ca2+ uptake by LER, at 5 microM total [Ca2+], displayed a strong dependence on divalent cations (Mg2+ greater than Co2+ greater than Mn2+ much greater than Ba2+ greater than or equal to Cd2+ greater than or equal to Sr2+, present at 2 mM) as well as on monovalent cations (Na+ greater than or equal to K+ + Na+ greater than K+ greater than Li+ greater than choline +), and anions (Cl- greater than acetate- greater than or equal to NO3- greater than or equal to F- greater than H2PO4- much greater than gluconate- greater than or equal to oxalate= much greater than SO4=). It had a fairly narrow pH optimum (7.25-7.50). Preincubation (10 min) of LER vesicles with 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated LER Ca2+ uptake; this effect was enhanced in the presence of renal cytosol [5% (vol/vol)].(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of caffeine-, IP3- and vanadate-sensitive Ca2+ pools in acinar cells of the exocrine pancreas

Previous studies have shown the existence of functionally distinguishable inositol 1,4,5-trisphosphate- (IP3) sensitive and IP3-insensitive nonmitochondrial intracellular Ca2+ pools in acinar cells of the exocrine pancreas. For further characterization of Ca2+ pools, endoplasmic reticulum (ER) membrane vesicles were separated by Percoll gradient centrifugation which allowed us to distinguish five discrete fractions designated P1 to P5 from the top to the bottom of the gradient. Measuring Ca2+ uptake and Ca2+ release with a Ca2+ electrode, we could differentiate three nonmitochondrial intracellular Ca2+ pools: (i) an IP3-sensitive Ca2+ pool (IsCaP), vanadate- and caffeine-insensitive, (ii) a caffeine-sensitive Ca2+ pool (CasCaP), vanadate- and IP3-insensitive, and (iii) a vanadate-sensitive Ca2+ pool (VasCaP), neither IP3- nor caffeine-sensitive, into which Ca2+ uptake is mediated via a Ca2+ ATPase sensitive to vanadate at 10(-4) mol/liter. A fourth Ca2+ pool is neither IP3- nor caffeine- or vanadate-sensitive. Percoll fraction P1 contained essentially the IsCaP, CasCaP and VasCaP and was mainly used for studies on Ca2+ uptake and Ca2+ release. When membrane vesicles were incubated in the presence of caffeine (2 x 10(-2) mol/liter), Ca2+ uptake up to the steady state [Ca2+] did not appear to be altered as compared to the control Ca2+ uptake. However, in control vesicles spontaneous Ca2+ release occurred after the steady state had been reached, whereas caffeine-pretreated vesicles did not spontaneously release Ca2+. Addition of IP3 at steady state [Ca2+] induced similar Ca2+ release followed by Ca2+ reuptake in both caffeine-pretreated and control vesicles. However, when caffeine was acutely added at steady state, Ca2+ was released from all Ca2+ pools including the IsCaP. Following Ca2+ reuptake after IP3 had been added, a second addition of IP3 to control vesicles induced further but smaller Ca2+ release, and a third addition resulted in a steady Ca2+ efflux by which all Ca2+ that had been taken up was released. This steady Ca2+ release started at a Ca2+ concentration between 5.5-8 x 10(-7) mol/liter and could also be induced by the IP3 analogue inositol 1,4,5-trisphosphorothioate (IPS3) or by addition of Ca2+ itself. Ruthenium red (10(-5) mol/liter) inhibited both caffeine-induced as well as Ca2(+)-induced but not IP3-induced Ca2+ release. Heparin (100 micrograms/ml) inhibited IP3- but not caffeine-induced Ca2+ release. The data indicate the presence of at least three separate Ca2+ pools in pancreatic acinar cells: the IsCaP, CasCaP and VasCaP. During Ca2+ uptake these Ca2+ pools appear to be separate.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of GTP and Ca2+ on inositol 1,4,5-trisphosphate induced Ca2+ release from permeabilized rat exocrine pancreatic acinar cells

The effects of Ca2+ and GTP on the release of Ca2+ from the inositol 1,4,5-trisphosphate (IP3) sensitive Ca2+ compartment were investigated with digitonin permeabilized rat pancreatic acinar cells. The amount of Ca2+ released due to IP3 directly correlated with the amount of stored Ca2+ and was found to be inversely proportional to the medium free Ca2+ concentration. Ca2+ release induced by 0.18 microM IP3 was half maximally inhibited at 0.5 microM free Ca2+, i.e. at concentrations observed in the cytosol of pancreatic acinar cells. GTP did not cause Ca2+ release on its own, but a single addition of GTP (20 microM) abolished the apparent desensitization of the Ca2+ release which was observed during repeated IP3 applications. This effect of GTP was reversible. GTP gamma S could not replace GTP. Desensitization still occurred when GTP gamma S was added prior to GTP. The reported data indicate that GTP, stored Ca2+ and cytosolic free Ca2+ modulate the IP3 induced Ca2+ release.

Voltage-dependent InsP3-insensitive calcium channels in membranes of pancreatic endoplasmic reticulum vesicles

Stimulus-secretion coupling in exocrine glands involves Ca2+ release from intracellular stores. In endoplasmic reticulum vesicle preparations from rat exocrine pancreas, an inositol 1,4,5-trisphosphate(InsP3)-sensitive, as well as an InsP3-insensitive, Ca2+ pool has been characterized. But Ca2+ channels in the endoplasmic reticulum of rat exocrine pancreas have not been demonstrated at the level of single-channel current. We have now used the patch-clamp technique on endoplasmic reticulum vesicles fused by means of the dehydration-rehydration method. In excised patches, single Ba2(+)- and Ca2(+)-selective channels were recorded. The channel activity was markedly voltage-dependent. Caffeine increased channel open-state probability, whereas ruthenium red and Cd2+ blocked single-channel currents. Ryanodine, nifedipine and heparin had no effect on channel activity. The channel activity was not dependent on the free Ca2+ concentration, the presence of InsP3, or pH. We conclude that this calcium channel mediates Ca2+ release from an intracellular store through an InsP3-insensitive mechanism.

Chloride and potassium channels in U937 human monocytes

Ionic channels in a human monocyte cell line (U937) were studied with the inside-out patch-clamp technique. A Ca2(+)-activated K+ channel and three Cl- -selective channels were observed. The Ca2(+)-activated K+ channel had an inward-rectifying current-voltage relationship with slope conductance of 28 pS, and was not dependent on membrane potential. Among the three Cl- channels, an outward-rectifying 28-pS channel was most frequently observed. The permeability ratio (Cl-/Na+) was 4-5 and CH3SO4- was also permeant. The channel became less active with increasing polarizations in either direction, and was inactive beyond +/- 120 mV. The channel, observed as bursts, occasionally had rapid events within the bursts, suggesting the presence of another mode of kinetics. Diisothiocyanatostilbene-disulfonic acid (DIDS) blocked the channel reversibly in a dose-dependent manner. The second 328-pS Cl- channel had a linear current-voltage relationship and permeability ratio (Cl-/Na+) of 5-6. This channel became less active with increasing polarizations and inactive beyond +/- 50 mV. DIDS blocked the channel irreversibly. The channel had multiple subconductance states. The third 15-pS Cl- channel was least frequently observed and least voltage sensitive among the Cl- channels. Intracellular Ca2+ or pH affected none of the three Cl- channels. All three Cl- channels had a latent period before being observed, suggesting inhibitory factor(s) present in situ. Activation of the cells with interferon-gamma, interferon-alpha A or 12-O-tetradecanoylphorbol-13-acetate (TPA) caused no change in the properties of any of the channels.

The calcium signal and neutrophil activation

The cytosolic free calcium concentration, [Ca2+]i in phagocytic cells (e.g. neutrophils, human leukemic cell line HL-60) is an important determinant of cellular activity. In resting phagocytes [Ca2+]i is low (approximately 100 nM), but in response to occupation of cell surface receptors, it rises to micromolar levels, thereby activating a variety of cellular functions. The increases in [Ca2+]i consist of two components: an immediate that is independent of extracellular Ca2+, and a more delayed that is abolished by the removal of extracellular Ca2+. These two components reflect the involvement of two subcellular structures in intracellular Ca2+ homeostasis: an intracellular Ca2+ store, referred to as the calciosome; and the plasma membrane. The function of the intracellular Ca2(+)-store depends on a Ca2(+)-pump, functionally and immunologically related to the cardiac sarcoplasmic reticulum Ca2(+)-ATPase, a Ca2(+)-storage protein, similar to muscle calsequestrin, and a Ca2(+)-release channel, which is sensitive to inositol 1,4,5-trisphosphate. The Ca2(+)-regulatory function of the plasma membrane depends on a Ca2+ pump, similar to the erythrocyte-type Ca2(+)-ATPase, and a Ca2+ channel; the activity of the Ca2+ channel is closely coupled to phosphatidylinositol turnover.

Calciosome, a sarcoplasmic reticulum-like organelle involved in intracellular Ca2+-handling by non-muscle cells: studies in human neutrophils and HL-60 cells

Calciosomes are intracellular organelles in HL-60 cells, neutrophils and various other cell types, characterized by their content of a Ca2+-binding protein that is biochemically and immunologically similar to calsequestrin (CS) from muscle cells. In subcellular fractionation studies the CS-like protein copurifies with functional markers of the inositol 1,4,5-trisphosphate (IP3) releasable Ca2+-store. These markers (ATP-dependent Ca2+-uptake and IP3-induced Ca2+-release) show a subcellular distribution which is clearly distinct from the endoplasmic reticulum and other organelles. In morphological studies, antibodies against rabbit skeletal muscle CS protein specifically stained hitherto unrecognized vesicles with a diameter between 50 and 250 nm. Thus both, biochemical and morphological studies indicate that the calsequestrin containing intracellular Ca2+-store, now referred to as the calciosome, is distinct from other known organelles such as endoplasmic reticulum. Calciosomes are likely to play an important role in intracellular Ca2+-homeostasis. They are possibly the intracellular target of inositol 1,4,5-trisphosphate and thus the source of Ca2+ that is redistributed into the cytosol following surface receptor activation in non-muscle cells.

Characterization of inositol 1,4,5-trisphosphate-sensitive (IsCaP) and -insensitive (IisCaP) nonmitochondrial Ca2+ pools in rat pancreatic acinar cells

We have measured Ca2+ uptake and Ca2+ release in isolated permeabilized pancreatic acinar cells and in isolated membrane vesicles of endoplasmic reticulum prepared from these cells. Ca2+ uptake into cells was monitored with a Ca2+ electrode, whereas Ca2+ uptake into membrane vesicles was measured with 45Ca2+. Using inhibitors of known action, such as the H+ ATPase inhibitors NBD-Cl and NEM, the Ca2+ ATPase inhibitor vanadate as well as the second messenger inositol 1,4,5-trisphosphate (IP3) and its analog inositol 1,4,5-trisphosphorothioate (IPS3), we could functionally differentiate two nonmitochondrial Ca2+ pools. Ca2+ uptake into the IP3-sensitive Ca2+ pool (IsCaP) occurs by a MgATP-dependent Ca2+ uptake mechanism that exchanges Ca2+ for H+ ions. In the absence of ATP Ca2+ uptake can occur to some extent at the expense of an H+ gradient that is established by a vacuolar-type MgATP-dependent H+ pump present in the same organelle. The other Ca2+ pool takes up Ca2+ by a vanadate-sensitive Ca2+ ATPase and is insensitive to IP3 (IisCaP). The IsCaP is filled at "higher" Ca2+ concentrations (approximately 10(-6) mol/liter) which may occur during stimulation. The low steady-state [Ca2+] of approximately 10(-7) mol/liter is adjusted by the IisCaP. It is speculated that both Ca2+ pools can communicate with each other, the possible mechanism of which, however, is at present unknown.

Ionic and osmotic dependence of secretion from permeabilised acini of the rat pancreas

Many types of secretory granule have been observed to swell as a result of cell stimulation implying a degree of osmotic control, although the regulation of granule fusion with the apical plasma membrane is not clearly understood. In the present study we have investigated the ionic and osmotic dependency of basal and stimulated 3H-protein release from rat pancreatic acini, permeabilised by either digitonin or high voltage electric discharge. Acini were stimulated with either cholecystokinin-pancreozymin octapeptide (CCK-8), carbachol (CCh), or with phorbol ester (TPA) plus cAMP. Stimulated secretion was significantly reduced when 130 mmol/l Cl- in the buffer was replaced by I-, NO3-, SCN- or cyclamate-. Secretion in Cl- buffers was inhibited by the anion transport inhibitor 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS), by 40% of the control response. Neither Na+ nor N-methyl-D-glucamine+ could replace K+ in the buffer. Ba2+ and quinine, which block K+ conductance pathways, inhibited stimulated secretion by 50%. Finally, stimulated secretion from leaky cells was nearly abolished by doubling buffer osmolarity. The data suggest that when the cell is stimulated, a Cl- and a K+ permeability appear in the zymogen granule membrane and the ions enter down their electrochemical gradients. The increased intragranular osmolarity results in granular swelling which is intimately associated with secretion.

Anion channels in giant liposomes made of endoplasmic reticulum vesicles from rat exocrine pancreas

Using the method of dehydration and rehydration, rough endoplasmic reticulum (RER) vesicles, isolated by differential centrifugation, can be enlarged to giant liposomes with diameters ranging from 5 to 200 micron. Patch-clamp studies on these giant RER liposomes revealed the existence of a channel with a mean conductance of 260 +/- 7 pS (n = 23; 140 mmol/liter KCl on both sides). The channel is about four times more permeable for Cl- than for K+. Its activity is strongly voltage regulated. At low potentials (+/- 20 mV) the channel is predominantly in its open state with an open probability near 1.0, whereas it closes permanently at high positive and negative voltages (+/- 70 mV). The channel activity is not influenced by changing the free Ca2+ concentration from 1 mmol/liter to less than 10(-9) mol/liter on either side, and is also not affected by typical Cl- -channel blockers like diphenylamine-2-carboxylate (DPC, 1 mmol/liter) or 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS, 1 mmol/liter). Another chloride channel with a single-channel conductance of 79 +/- 6 pS (n = 4) was less frequently observed. In the potential range of -80 to +40 mV this channel displayed no voltage-dependent gating. We assume that these anion channels are involved in the maintenance of electroneutrality during Ca2+ uptake in the RER.

Inhibitor of anion transport, DIDS, releases Ca2+ from hepatic microsomes

Addition of 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS) to Ca2+ loaded hepatic microsomal vesicles evoked a dose-dependent release of the accumulated Ca2+. Ca2+ uptake was also inhibited. The effects of DIDS do not seem to be due to the inhibitions of either Cl- or proton fluxes. The results indicate that DIDS inhibits Ca2+ uptake and releases Ca2+ by inhibiting the Ca2+-ATPase and the formation of the phosphorylated intermediate of the enzyme, and that it might interact with a specific site on the vesicle which is involved in the translocation of Ca2+ across the microsomal and mitochondrial membranes.