Relationship between H+, anion, and monovalent cation movements and Ca2+ transport in sarcoplasmic reticulum: further proof of a cation exchange mechanism for the Ca2+-Mg2+-ATPase pump

Phosphate, nitrendipine and valinomycin increase the Ca2+/ATP coupling ratio of rat skeletal muscle sarcoplasmic reticulum Ca(2+)-ATPase

Nitrendipine and valinomycin act synergistically to stimulate ATP-dependent Ca2+ accumulation by rat skeletal muscle sarcoplasmic reticulum vesicles 3-fold. The stimulation is not caused by activation of the Ca(2+)-ATPase or by inhibition of the sarcoplasmic reticulum Ca2+ channel, but is due to an increased efficiency of transport by Ca(2+)-loaded vesicles. At low Ca2+ concentrations, nitrendipine+valinomycin inhibits Ca2+ uptake by increasing the Ca2+ KM but does not effect equilibrium Ca2+ binding to the Ca(2+)-ATPase (Kd = 0.75 microM). In the presence of 50 mM phosphate, nitrendipine+valinomycin increases the steady-state coupling ratio (Ca2+ accumulated per ATP hydrolyzed) from 0.6 to 1.9 by decreasing the rate of ATP hydrolysis by 72%, while reducing the Ca2+ accumulation rate by only 13%. The rates of both passive and Ca(2+)-ATPase-mediated Ca2+ release are reduced by nitrendipine+valinomycin. The data indicate that nitrendipine and valinomycin act directly on the Ca(2+)-ATPase to decrease the ATP hydrolysis rate, increase the Ca2+ KM, decrease Ca2+ efflux, and increase the Ca2+/ATP coupling ratio of Ca(2+)-loaded vesicles.

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.

Electrogenic pump current of sarcoplasmic reticulum Ca(2+)-ATPase reconstituted at high lipid/protein ratio

When Ca(2+)-ATPase from sarcoplasmic reticulum was reconstituted with excess phospholipid (at a 1:800 weight ratio) in a monomeric state and activated by Ca2+ and ATP a transmembrane potential developed which could be continuously recorded by the fluorochrome oxonol VI. The results demonstrate the electrogenicity of active Ca2+ transport during continuous turnover. The fluorescence signal can be quantified in terms of net current electrical flow through the vesicular membranes and compared to the ATP hydrolysis to give the number of electrostatic charges transferred during Ca2+ transport. From such measurements a stoichiometry of 1.8 +/- 0.4 Ca2+ per ATP hydrolyzed at pH 7.1 can be obtained. The method is also convenient for determination of the kinetics of Ca(2+)-ATPase activation by ATP and free Ca2+.

Calcium regulation in neurons: transport processes

The ionic stoichiometry of the major Ca2+ transport mechanisms in neurons is still a matter for debate. The past year has seen some particularly interesting developments in this field, not least the finding that the neuronal Na(+)-Ca2+ exchange may be able to transport K+.

The pH dependence of the cardiac sarcolemmal Ca2(+)-transporting ATPase: evidence that the Ca2+ translocator bears a doubly negative charge

The pH dependence of the Ca2(+)-transporting ATPase of bovine cardiac sarcolemma was determined in a membrane vesicle preparation. The maximal velocity (Vmax) at saturating external Ca2+ showed a sigmoidal pH dependence with maximal values in the 6.0-6.5 range, a half-maximal value at 7.2 and minimal (less than or equal to 15%) values at pH greater than or equal to 8.0. The apparent affinity for Ca2+ (1/Km) varied over 10(4)-fold for 6.0 less than or equal to pH less than or equal to 8.5, increasing with increasing pH. Plots of log(1/Km) vs. pH were biphasic. In the acid range (6.0 less than or equal to pH less than or equal to 7.2), a slope of 2.6 was observed for the calmodulin-activated form of the pump. For 7.2 less than or equal to pH less than or equal to 8.5, a slope of 0.5 was observed. At pH 7.4, the Km is approx. 48 +/- 19 nM. The Ca2+ pump of cardiac sarcoplasmic reticulum in the same preparation had a Km of 304 +/- 115 nM and showed a similar pH dependence except that the slope in the acid range was 1.7. When calmodulin was removed from the sarcolemmal pump, its Km was raised to approx. 1.0 microM, the slope in the acid range was reduced to 1.7 and the Vmax was markedly reduced. The results are explicable in terms of a model in which each of the two Ca2+ binding sites on the pump contains two buried COO- groups responsible for high affinity. The Km effect is explained by 2 H+ vs. 1 Ca2+ competition for occupation of each of the two cytoplasmically-oriented translocators (4 H+ vs. 2 Ca2+). The Vmax effect is explained by counter-transport of H+. The findings are considered in terms of the published amino acid sequence of the cardiac sarcolemmal pump and recent site-directed mutagenesis vs. function studies identifying the Ca2+ binding site in the skeletal sarcoplasmic reticulum pump. The kinetic data are also applied to pump behavior under conditions of ischemia and acidosis.

The calmodulin-activated form of the Ca2(+)-pumping ATPase of the cardiac sarcolemmal membrane produces Ca2+ gradients with a thermodynamic efficiency of 100%

The thermodynamic efficiency of the calmodulin-activated form of the Ca2+-pumping ATPase of the bovine cardiac sarcolemma (SL) was evaluated in sealed vesicles under reversible conditions. The free internal Ca2+ concentration ([Ca2+]i) established in the SL vesicle lumen by action of the ATPase was determined as a function of the [ATP]/([ADP][Pi]) ratio for the following experimental conditions: 250 mM sucrose, 100 mM KCl, 0.1 mM Mg2+, 25 mM HEPES, 25 mM Tris, pH 7.40, at 37 degrees C, [Ca2+]o = 50 nM (1 mM Ca/EGTA buffer), 0.75 mM Mg-ATP, 0.1 mM Pi, variable [ADP]. Under these conditions, with the pump working near its Km of 64 nM, the [Ca2+]i achieved was less than or equal to 18 mM, decreasing with increasing [ADP] for [ADP] greater than or equal to 0.84 mM. A plot of the square of the [Ca2+]i/[Ca2+]o ratio against [ATP]/([ADP][Pi]) gave a straight line with a slope of 1.5 x 10(7) M. This was in agreement, within the experimental error, with the equilibrium constant for ATP hydrolysis under these conditions (1.09 x 10(7) M). These results demonstrate (1) tight coupling between Ca2+ transport and ATP hydrolysis with a stoichiometry of 2 Ca2+ moved per ATP split and (2) a low degree of passive leakage. Analysis at low [ADP] (less than 0.83 mM) showed the unexpected result that ADP increases the rate of the forward reaction of the pump. The maximal effect on the initial rate is a 96 +/- 5% increase, with an EC50 of approximately 0.4 mM (ADP). Similar but lesser stimulation was observed with CDP. The implications of the above results for the energetics of the pump and for its physiological function in the beating heart are discussed.

Ca2+ pumping ATPase of cardiac sarcolemma is insensitive to membrane potential produced by K+ and Cl- gradients but requires a source of counter-transportable H+

The sensitivity of the Ca2+ pumping ATPase of bovine cardiac sarcolemma (SL) to changes in membrane potential was studied in a preparation of sealed SL vesicles. Membrane potential was imposed by preincubating the vesicles in media of defined ion composition (K+, Cl-, choline+ and gluconate-) and diluting into media of differing ion composition. The durations of the ion gradients and relative ion permeabilities were determined in separate experiments by the dependence of the half time for net K+ (or choline+) movement coupled with these anions (Cl- or gluconate-), registered by the fluorescence of 1-anilino-8-naphthalene sulfonate (Chiu, V.C.K., Haynes, D.H. 1980. J. Membrane Biol. 56:203-218). Relative permeabilities were: 1.0, K+; greater than or equal to 10.0, 1 microM valinomycin-K+; 4.0, Cl-; 0.66, choline+; 0.38, gluconate-. Durations of the gradients ranged between 17 sec (KCl, valinomycin) to 195 sec (K(+)-gluconate-). In separate experiments, active Ca2+ uptake was monitored using chlorotetracycline (CTC) fluorescence, a technique validated by 45-Ca2+ measurements (Dixon, D., Brandt, N., Haynes, D.H. 1984. J. Biol. Chem. 259:13737-13741). Active Ca2+ uptake was initiated in the presence of monovalent ion gradients. The values of the membrane potentials (Em) imposed by the monovalent ion gradients were calculated using the ion concentrations, their relative permeabilities and the Goldman-Hodgkin-Katz equation. No effect of membrane potential on transport rate was observed (less than or equal to 4%, for 5-7% SD) for imposed potentials as extreme as greater than or equal to +71 and less than or equal to -67 mV. Formal analysis shows that the above observations are not compatible with models in which the Ca2+ pumping ATPase functions in an electrogenic or charge-uncompensated fashion. Further experimentation showed that the pump rate is slowed when uptake is measured at less-than-adequate concentrations of buffer (5 vs. 25 mM HEPES/Tris). This, together with further control experiments using nigericin and FCCP, gave evidence that the pump requires a source of counter-transportable H+ in the vesicle lumen. The above experimentation also underlines the need for control of internal pH to obviate erroneous interpretation of ion perturbation experiments. The results are compared with results obtained with the Ca2+ ATPase pump of skeletal sarcoplasmic reticulum.

ATP-dependent calcium transport in rat parotid basolateral membrane vesicles is modulated by membrane potential

The ATP-dependent Ca2+ transport activity (T. Takuma, B.L. Kuyatt and B.J. Baum, Biochem. J. 227:239-245, 1985) exhibited by inverted basolateral membrane vesicles isolated from rat parotid gland was further characterized. The activity was dependent on Mg2+. Phosphate (5 mM), but not oxalate (5 mM), increased maximum Ca2+ accumulation by 50%. Half-maximal Ca2+ transport was achieved at approximately 70 nM Ca2+ in EGTA-buffered medium while maximal activity required greater than 1 microM Ca2+ (Vmax = 54 nmol/mg protein/min). Optimal rates of Ca2+ transport were obtained in the presence of KCl, while in a KCl-free medium (mannitol or sucrose) approximately 40% of the total activity was achieved, which could not be stimulated by FCCP. The initial rate of Ca2+ transport could be significantly altered by preimposed membrane potentials generated by K+ gradients in the presence of valinomycin. Compared to the transport rate in the absence of membrane potential, a negative (interior) potential stimulated uptake by approximately 30%, while a positive (interior) potential inhibited uptake. Initial rates of Ca2+ uptake could also be altered by imposing pH gradients, in the absence of KCl. When compared to the initial rate of Ca2+ transport in the absence of a pH gradient, pHi = 7.5/pHo = 7.5; the activity was approximately 60% higher in the presence of an outwardly directed pH gradient, pHi = 7.5/pHo = 8.5; while it was approximately 80% lower when an inwardly directed pH gradient was imposed, pHi = 7.5/pHo = 6.2. The data show that the ATP-dependent Ca2+ transport in BLMV can be modulated by the membrane potential, suggesting therefore that there is a transfer of charge into the vesicle during Ca2+ uptake, which could be compensated by other ion movements.

Determination of the rate of rapid pH equilibration across isolated sarcoplasmic reticulum membranes

The rate of the transmembrane pH equilibration in the isolated vesicles of sarcoplasmic reticulum of skeletal muscle after extravesicular pH jump was determined for the first time. A highly water-soluble pH sensitive fluorescent dye, 8-hydroxy-1,3,6-pyrenetrisulphonic acid (pyranine), was used as intravesicular pH indicator in the stopped-flow fluorophotometry. The pH of the medium was controlled with 20 mM HEPES-K or PIPES-K. The fluorescence intensity changed monophasically as much as expected from its pH dependency for imposed delta pH. The half time for initial pH of 7.53 or 6.83 was about 6 msec. The H+/OH- permeability was 11 cm/sec. The results suggested that each vesicle contained large numbers of H+/OH- channels.

Intracellular calcium storage and release in the human platelet. Chlorotetracycline as a continuous monitor

The calcium-sensitive fluorescent probe chlorotetracycline was used to monitor calcium movement in human platelets. The chlorotetracycline fluorescence signal is a linear measure of the level of free calcium in the dense tubules and in the mitochondria, with probe sensitivity in the millimolar range. Experiments perturbing the system with the calcium ionophore A23187 shows that the level of free internal calcium in the organelle depends upon the cytoplasmic level, which, in turn, depends upon the passive permeability of the plasma membrane. Chlorotetracycline in the cytoplasmic compartment does not respond to changes in the cytoplasmic calcium concentration, which is held in the micromolar to submicromolar range by an extrusion system. The calcium concentration in the cytoplasmic compartment can be directly manipulated by the calcium ionophore A23187 and is measured in parallel experiments with Quin 2, a high-affinity indicator. The calcium transport systems of the organelles are shown to be less susceptible to short circuit by A23187. Analysis shows that mitochondrial uptake is slow (t 1/2 = 20 minutes), produces a large increase in chlorotetracycline fluorescence, and is inhibited by sodium azide plus oligomycin. Uptake by the dense tubules is more rapid (t 1/2 = 2 minutes), produces a smaller increase in chlorotetracycline fluorescence, is inhibited by trifluoperazine, and is less sensitive to A23187. The Km is estimated as 1 microM or lower. Studies show that the chlorotetracycline technique is useful for the monitoring of calcium uptake and release by the platelet organelles, and suggests that the Quin 2/chlorotetracycline technique will be useful as a diagnostic of both physiological and pathological activation mechanisms.

The effects of valinomycin on ion movements across the sarcoplasmic reticulum in frog muscle

The effects of valinomycin on the elemental composition and the fractional volume of the terminal cisternae (t.c.) of the sarcoplasmic reticulum (s.r.) were determined in rapidly frozen frog semitendinosus muscles. The concentrations of valinomycin used for the electron probe studies (5 microM) had no effect on tetanus tension or t.c. volume (2.% of fibre volume). Mitochondria were markedly swollen and their K content was significantly increased in both the resting and the tetanized valinomycin-treated muscles. Valinomycin had no effect on the concentration of Na, Mg, P, Cl, K and Ca in the t.c. of resting muscles. In untreated, tetanized muscles, Ca2+ release was accompanied by the uptake of K and Mg into the t.c. in an amount that was significantly less than the positive charge removed through Ca2+ release, confirming previous observations showing an apparent charge deficit (Somlyo, Gonzalez-Serratos, Shuman, McClellan & Somlyo, 1981). Valinomycin abolished the apparent charge deficit: in tetanized, valinomycin-treated muscles, the uptake of K into the t.c. was significantly (P less than 0.001) greater than in the untreated muscles and Mg uptake also remained highly significant. It is suggested that Ca2+ release from activated muscle is an electrogenic process and that the K+ conductance of the s.r. in untreated frog muscles is insufficient to allow charge neutralization of the Ca2+ current during release. The increase in K+ permeability caused by valinomycin permits the greater counter movement of K+ under the combined influence of the electrical potential generated by outward Ca2+ movement and the acidic cation binding proteins in the lumen of the s.r. The results are consistent with the proposal (Somlyo et al. 1981) that in normal frog muscles not treated with valinomycin, the apparent positive charge deficit observed after a tetanus reflects the movement of protons and, possibly, organic cations.

The Ca2+ permeability of sarcoplasmic reticulum vesicles. II. Ca2+ efflux in the energized state of the calcium pump

Ca2+ efflux from sarcoplasmic reticulum vesicles was studied by measurements of net Ca2+ uptake, 45Ca2+ flux and hydrolysis of energy-rich phosphate. The maximal Ca2+ uptake capacity (150-200 nmol/mg protein at pH 6.7, 10 mM MgCl2 and mu = 0.26) was independent of the nature and concentration of the energy-donating substrate (ATP or carbamyl phosphate) and of temperature (15-35 degrees C), suggesting coupling between influx and efflux of Ca2+. In the presence of high concentrations of ATP, this efflux of Ca2+ was much higher than the passive Ca2+ permeation, measured after ATP or Ca2+ depletion of the reaction medium. Ca2+ efflux was imperceptible at vesicle filling levels below 35-40 nmol Ca2+/mg protein, and uncorrelated to the inhibition of the Ca2+-ATPase by high intravesicular Ca2+ concentrations. Analysis of the data indicated that Ca2+ efflux under our conditions probably is associated with one of the Ca2+-ATPase partial reactions, occurring after dephosphorylation, rather than with a reversal of the Ca2+ translocation step in the phosphorylated state of the enzyme. Furthermore, passive Ca2+ permeation may be concurrently reduced during the enzymatically active state. It is proposed that both Ca2+ efflux and passive Ca2+ permeation (Ca2+ outflow) proceed via the same channels which are closed (occluded) during part of the Ca2+-ATPase reaction cycle.

Action of mercurials on the active and passive transport properties of sarcoplasmic reticulum

The effect of Hg2+ and CH3-Hg+ on the passive and active transport properties of the Ca2+-Mg2+-ATPase-rich fraction of skeletal sarcoplasmic reticulum (SR) is reported. The agents abolish active transport, at 10(-5) and 10(-4) M concentrations, respectively. Addition of the mercurials was also shown to release actively accumulated Ca2+. The mercurials increase the passive Ca2+ and Mg2+ permeability in the absence of ATP at the same concentrations at which they inhibit transport. It is proposed that both effects are the result of direct binding of the mercurials to the SH groups of the Ca2+-Mg2+-ATPase pump, altering the conformational equilibria of the pump. The agents were also shown to increase the passive KCl permeability. The SR preparation consists of two vesicle populations with respect to K+ permeability, one with rapid KCl equilibration faciliated by a monovalent cation channel function and one with slow KCl equilibration. The mercurials increase the rates of KCl equilibration in both fractions, but produce higher rates in the fraction containing the channel function. The results are discussed in terms of pump and channel function and are compared with results for the electrical behavior of the CA2+-Mg2+-ATPase and other SR proteins in black lipid membranes, as presented by others.

Evidence that trypsin digestion exposes a channel in the sarcoplasmic reticulum membrane

We present evidence that proteolytic digestion of sarcoplasmic reticulum membranes with trypsin exposes an ionophore that is capable of translocating calcium across the membrane of preloaded vesicles. Net transport of calcium appears to stop when the chemical potential of the ion on both sides of the membrane is equal. The temperature coefficient of steady-state leakage suggests that the ionophore is of the channel or pore type. We suggest that tryptic digestion exposes a channel in sarcoplasmic reticulum membranes through which calcium, and perhaps other ions as well, can diffuse down concentration gradients.