Anion effects on in vitro sarcoplasmic reticulum function. Co-transport of anions with calcium

Luminal Ca2+ regulation of RyR1 Ca2+ channel leak activation and inactivation in sarcoplasmic reticulum membrane vesicles

In this study, we tested the hypothesis that the RyR1 Ca2+ channel closure is sensitive to outward trans-SR membrane Ca2+ gradients established by SERCA1 pumping. To perform these studies, we employed stopped-flow rapid-kinetic fluorescence methods to measure and assess how variation in trans-SR membrane Ca2+ distribution affects evolution of RyR1 Ca2+ leaks in RyR1/ CASQ1/SERCA1-rich membrane vesicles. Our studies showed that rapid filling of a Mag-Fura-2-sensitive free Ca2+ pool during SERCA1-mediated Ca2+ sequestration appears to be a crucial condition allowing RyR1 Ca2+ channels to close once reloading of luminal Ca2+ stores is complete. Disruption in the filling of this pool caused activation of Ruthenium Red inhibitable RyR1 Ca2+ leaks, suggesting that SERCA1 pump formation of outward Ca2+ gradients is an important aspect of Ca2+ flux control channel opening and closing. In addition, our observed ryanodine-induced shift in luminal Ca2+ from free to a CTC-Ca+-sensitive, CASQ1-associated bound compartment underscores the complex organization and regulation of SR luminal Ca2+. Our study provides strong evidence that RyR1 functional states directly and indirectly influence the compartmentation of luminal Ca2+. This, in turn, is influenced by the activity of SERCA1 pumps to fill luminal pools while synchronously reducing Ca2+ levels on the cytosolic face of RyR1 channels.

The influence of Mg2+ on anion binding to sarcoplasmic reticulum membranes as detected by 35Cl-NMR

35Cl-NMR spectroscopy has been used to study the competition between anions, including nucleotides, on skeletal muscle sarcoplasmic reticulum membranes. Different chloride binding sites can be distinguished according to their Mg2+ sensitivity. Phosphate binding is enhanced by Mg2+ whereas the anion transport inhibitor pyridoxalphosphate-6-azophenyl-2'-sulfonic acid (PPAPS) binding is not. The affinity of the enzyme for the Mg-adenylyl imidodiphosphate (MgAMP-PNP) complex is decreased whereas that for MgATP is increased. Three sets of binding sites can be discriminated from which chloride is displaced by different anions with varying efficiency. High affinity binding of AMP-PNP and PPAPS occurs at the same site, that can also be occupied by phosphate. Low-affinity binding of PPAPS and AMP-PNP also coincides, but in a site where phosphate binding is negligible. ATP and ADP bind to both sites. In the presence of Mg2+ a third anion binding site can be occupied by phosphate but neither by AMP-PNP nor PPAPS.

Effects of simulated ischemia and reperfusion on the sarcoplasmic reticulum of digitonin-lysed cardiomyocytes

ATP-dependent, inorganic phosphate-supported 45Ca2+ uptake by digitonin-lysed adult rat ventricular cardiomyocytes was used to evaluate the effects of simulated ischemia and reperfusion on the physically intact sarcoplasmic reticulum. Mitochondrial reactions were inhibited with rotenone and oligomycin. 45Ca2+ accumulation in the presence of the calcium efflux inhibitors, procaine (10 mM) and ruthenium red (30 microM), was used to characterize unidirectional uptake kinetics. A decrease in pH from 7.2 to 6.6 increased the [Ca2+] K0.5 from 0.5 to 2.0 microM and reduced the apparent Vmax by 28%. In the absence of procaine and ruthenium red, at a free [Mg2+] of 0.5 mM, maximum net uptake occurred at pCa 6.2 when pH was 7.2 and at pCa 6.0 when pH was 6.6. At lower pCa, net Ca2+ accumulation declined. Increasing free [Mg2+] from 0.5 to 1 mM at pH 6.6 or to 2.5 mM at pH 7.2 increased net 45Ca2+ accumulation in the absence of procaine and ruthenium and shifted maximum uptake to pCa 5.6 and 6.0, respectively. Increases in cytosolic free [Mg2+] thought to occur during myocardial ischemia are therefore capable of inhibiting calcium efflux from the sarcoplasmic reticulum. Reducing [ATP] from 10 to 1 mM reduced maximum net 45Ca2+ uptake by 30% both in the presence and absence of efflux inhibitors. Preincubation of intact myocytes under conditions designed to simulate ischemia and reperfusion decreased 45Ca2+ uptake greater than or equal to 50%. The data indicate that myocardial ischemia and reperfusion can alter both Ca2+ accumulation and calcium release by the sarcoplasmic reticulum.

Studies on the anion binding selectivity of sarcoplasmic reticulum membranes by 35Cl-NMR

Anion binding sites on the membranes of sarcoplasmic reticulum vesicles can be characterized with the aid of 35Cl-NMR. Titration experiments with a series of different anions reveal that multivalent, phosphate-like anions bind much stronger to SR vesicles than monovalent anions like halides whereas oxalate seems to have an intermediate position. The binding strength decreases with decreasing ionic radius according to the following sequence: vanadate greater than phosphate greater than sulfate much greater than iodide greater than oxalate greater than bromide greater than chloride much greater than fluoride. This is also reflected by increasing dissociation constants. Although vanadate in absolute terms replaces much more chloride than either, phosphate or sulfate, their dissociation constants are very similar. This implicates a special binding mechanism for vanadate. Phosphate analoguous compounds like pyridoxalphosphate-6-azophenyl-2'-sulfonic acid and its 4'-nitroderivative show the strongest binding.

Inhibition of dicarboxylic anion transport by fluorescein isothiocyanate in skeletal sarcoplasmic reticulum

It has been demonstrated previously that dicarboxylic anions are cotransported during ATP-dependent Ca2+ transport by skeletal muscle sarcoplasmic reticulum (SR) membranes, and that anion cotransport stimulates Ca2+ transport. In the current study, we present evidence that dicarboxylic anion cotransport and Ca2+ transport are kinetically distinct in SR, but both functions are mediated by the CaATPase protein. Preincubation of SR with 40 microM fluorescein isothiocyanate (FITC) (pH 7.0) inhibited essentially all of the Ca2+ ATPase activity, as well as active oxalate-supported and oxalate-independent 45Ca2+ accumulation. The addition of 1 mM beta, gamma-methyleneadenosine 5'-triphosphate (AMP-PCP) to the preincubation media fully protected the dicarboxylic anion-independent Ca2+ ATPase activity and the oxalate-independent active 45Ca2+ accumulation from the inhibitory effects of FITC; however, the ATP-associated [14C]oxalate accumulation, the oxalate-dependent 45Ca2+ accumulation, and the oxalate- and maleate-dependent stimulation of Ca2+ ATPase activity were not protected by AMP-PCP. Thus, the dicarboxylic anion accumulation and the stimulation of Ca2+ uptake by dicarboxylic anions could be functionally separated from the ATP-dependent, anion-independent Ca2+ translocation. FITC bound exclusively to the 100-kDa (CaATPase) and 92-kDa (phosphorylase) proteins in the SR membranes and to purified CaATPase in sodium dodecyl sulfate-polyacrylamide gel electrophoresis; 1 mM AMP-PCP inhibited 50-55% of the FITC fluorescence on the 100-kDa protein, but did not significantly alter fluorescence on the 92-kDa protein. Two-dimensional gel analysis demonstrated a single 100-kDa protein in longitudinal SR membranes. FITC appears to inhibit ATP-dependent Ca2+ transport, and dicarboxylic anion translocation through interaction at separate domains of the CaATPase protein.

Covalent and non-covalent inhibitors of the phosphate transporter of sarcoplasmic reticulum

The sarcoplasmic reticulum (SR) of skeletal muscle contains a Pi transporter which transports Pi into the lumen of the SR, increasing the level of accumulation of Ca2+ by SR by forming insoluble salts with Ca2+. Phosphonocarboxylic acids inhibit the transport of Pi by the transporter, phosphonoformic acid itself being transported into the SR increasing the level of accumulation of Ca2+. Phenylphosphonic acid also inhibits Pi transport, distinguishing the Pi transporter of SR from the Na+/Pi transporter of brush-border membranes. Oxalate transport is also inhibited by the phosphono-carboxylic acids, consistent with the suggestion that oxalate and phosphate are carried on the same transporter. The effects of maleate are, however, not inhibited, suggesting a separate carrier for the dicarboxylic acids. Acetic anhydride and phenylglyoxal inhibit the transporter, Pi providing protection against the effects of acetic anhydride, suggesting the presence of a lysine residue at the Pi binding site. ATP provides protection against the effects of acetic anhydride and phenylglyoxal, suggesting the presence of an ATP binding site on the transporter.

Characterization of anion binding sites of sarcoplasmic reticulum vesicles by 35Cl-NMR

Sarcoplasmic reticulum vesicles were prepared from rabbit skeletal muscle. It could be demonstrated that the anion binding sites on this membrane can be studied by 35Cl-NMR spectroscopy. Titration of sarcoplasmic reticulum vesicles with the sulfate exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) revealed specific binding of this compound to the sarcoplasmic reticulum membrane. A new inhibitor, pyridoxalphosphate-6-azophenyl-2'-sulfonic acid (PPAPS) was introduced and proved to displace chloride equally well from its binding sites. Two binding sites could be distinguished by titration with inorganic phosphate in the presence and absence of the inhibitors. Because of the insensitivity of 35Cl-NMR spectroscopy these anion binding sites have to be located on a protein being present in considerable amount in the sarcoplasmic reticulum membrane.

Characterization of calcium uptake into rough endoplasmic reticulum of rat pancreas

ATP-dependent Ca2+ uptake into isolated pancreatic acinar cells with permeabilized plasma membranes, as well as into isolated endoplasmic reticulum prepared from these cells, was measured using a Ca2+ -specific electrode and 45Ca2+. Endoplasmic reticulum was purified on an isopycnic Percoll gradient and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the rough endoplasmic reticulum RNA was enriched threefold and the typical marker for the plasma membrane Na+,K+(Mg2+)ATPase was decreased 20-fold. When different fractions of the Percoll gradient were compared, 45Ca2+ uptake correlated with the RNA content and not with the Na+,K+(Mg2+)ATPase activity. The characteristics of nonmitochondrial Ca2+ uptake into leaky isolated cells and 45Ca2+ uptake into isolated endoplasmic reticulum were very similar: Calcium uptake was maximal at 0.3 and 0.2 mmol/liter free Mg2+, at 1 and 1 mmol/liter ATP, at pH 6.0 and 6.5, and free Ca2+ concentration of 2 and 2 mumol/liter, respectively. Calcium uptake decreased at higher free Ca2+ concentration. 45Ca2+ uptake was dependent on monovalent cations (Rb+ greater than K+ greater than Na+ greater than Li+ greater than choline+) and different anions (Cl- greater than Br- greater than SO4(2-) greater than NO3- greater than I- greater than cyclamate- greater than SCN-) in both preparations. Twenty mmol/liter oxalate enhanced 45Ca2+ uptake in permeabilized cells 10-fold and in vesicles of endoplasmic reticulum, fivefold. Calcium oxalate precipitates in the endoplasmic reticulum of both preparations could be demonstrated by electron microscopy. The nonmitochondrial Ca2+ pool in permeabilized cells characterized in this study has been previously shown to regulate the cytosolic free Ca2+ concentration to 0.4 mumol/liter. Our results provide firm evidence that the endoplasmic reticulum plays an important role in the regulation of the cytosolic free Ca2+ concentration in pancreatic acinar cells.