Interaction of alkaline metal ions with Ca(2+)-binding sites of Ca(2+)-ATPase of sarcoplasmic reticulum: 23Na-NMR studies

Microsecond molecular dynamics simulations of Mg²⁺- and K⁺-bound E1 intermediate states of the calcium pump

We have performed microsecond molecular dynamics (MD) simulations to characterize the structural dynamics of cation-bound E1 intermediate states of the calcium pump (sarcoendoplasmic reticulum Ca²⁺-ATPase, SERCA) in atomic detail, including a lipid bilayer with aqueous solution on both sides. X-ray crystallography with 40 mM Mg²⁺ in the absence of Ca²⁺ has shown that SERCA adopts an E1 structure with transmembrane Ca²⁺-binding sites I and II exposed to the cytosol, stabilized by a single Mg²⁺ bound to a hybrid binding site I'. This Mg²⁺-bound E1 intermediate state, designated E1•Mg²⁺, is proposed to constitute a functional SERCA intermediate that catalyzes the transition from E2 to E1•2Ca²⁺ by facilitating H⁺/Ca²⁺ exchange. To test this hypothesis, we performed two independent MD simulations based on the E1•Mg²⁺ crystal structure, starting in the presence or absence of initially-bound Mg²⁺. Both simulations were performed for 1 µs in a solution containing 100 mM K⁺ and 5 mM Mg²⁺ in the absence of Ca²⁺, mimicking muscle cytosol during relaxation. In the presence of initially-bound Mg²⁺, SERCA site I' maintained Mg²⁺ binding during the entire MD trajectory, and the cytosolic headpiece maintained a semi-open structure. In the absence of initially-bound Mg²⁺, two K⁺ ions rapidly bound to sites I and I' and stayed loosely bound during most of the simulation, while the cytosolic headpiece shifted gradually to a more open structure. Thus MD simulations predict that both E1•Mg²⁺ and E•2K+ intermediate states of SERCA are populated in solution in the absence of Ca²⁺, with the more open 2K+-bound state being more abundant at physiological ion concentrations. We propose that the E1•2K⁺ state acts as a functional intermediate that facilitates the E2 to E1•2Ca²⁺ transition through two mechanisms: by pre-organizing transport sites for Ca²⁺ binding, and by partially opening the cytosolic headpiece prior to Ca²⁺ activation of nucleotide binding.

Time-resolved charge translocation by sarcoplasmic reticulum Ca-ATPase measured on a solid supported membrane

Sarcoplasmic reticulum vesicles were adsorbed on an octadecanethiol/phosphatidylcholine mixed bilayer anchored to a gold electrode, and the Ca-ATPase contained in the vesicles was activated by ATP concentration jumps both in the absence and in the presence of K(+) ions and at different pH values. Ca(2+) concentration jumps in the absence of ATP were also carried out. The resulting capacitive current transients were analyzed together with the charge under the transients. The relaxation time constants of the current transients were interpreted on the basis of an equivalent circuit. The current transient after ATP concentration jumps and the charge after Ca(2+) concentration jumps in the absence of ATP exhibit almost the same dependence upon the Ca(2+) concentration, with a half-saturating value of approximately 1.5 microM. The pH dependence of the charge after Ca(2+) translocation demonstrates the occurrence of one H(+) per one Ca(2+) countertransport at pH 7 by direct charge-transfer measurements. The presence of K(+) decreases the magnitude of the current transients without altering their shape; this decrease is explained by K(+) binding to the cytoplasmic side of the pump in the E(1) conformation and being released to the same side during the E(1)-E(2) transition.

Multi-dose crystalloid cardioplegia preserves intracellular sodium homeostasis in myocardium

The goal of this study was to assess the effect of multi-dose St Thomas' cardioplegia on intracellular sodium homeostasis in a rat heart model. A new magnetic resonance method was applied which enable us to detect intracellular Na changes without chemical shift reagents. Three groups of isolated rat hearts were subjected to 51 min of ischemia and 51 min of reperfusion at 37 degrees C: Group 1-three infusions of St Thomas' cardioplegia every 17 min for 2 min (n=7); Group 2-single-dose infusion of cardioplegia at the beginning of stop-flow ischemia (n=8); and Group 3-clamp ischemia (n=3) without cardioplegia administration. Performance of the heart was assessed by rate-pressure product relative to the pre-ischemic level (RPP). An NMR method was applied which continuously detects the Na(i) concentration in the heart, using the ability of bound sodium to exhibit triple-quantum transitions and the growth of the corresponding signal when sodium ions pass from extracellular to intracellular space. Clamp ischemia without cardioplegia and 50 min of reperfusion left the heart dysfunctional, with Na(i) growth from the pre-ischemic level of 13.9+/-1.2 mM to 34.9+/-1.3 mM and 73. 9+/-1.9 mM at the end of ischemia and reperfusion, respectively. During single-dose cardioplegia the corresponding values for Na(i) were 30.2+/-1 mM and 48.5+/-1.7 mM (RPP=29%). Multiple infusions of cardioplegic solution resulted in a remarkable preservation of the heart's intracellular Na concentration with a non-significant increase in Na(i) during ischemia and only 16.7+/-1 mM, (P=0.01), after subsequent reperfusion (RPP=85%). The time course of Na(i) changes in the rat heart model demonstrates a prominent potential of multi-dose St Thomas' cardioplegia in preserving intracellular sodium homeostasis at 37 degrees C. The growth of Na(i) concentration during ischemia, as an indicator of the viability of the myocytes, can have a prognostic value for the heart's performance during reperfusion.

Effects of K+ on the binding of Ca2+ to the Ca(2+)-ATPase of sarcoplasmic reticulum

Equilibrium and kinetic fluorescence methods have been used to characterize the interactions between K+ and the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum. K+ shifts the E2-E1 equilibrium of the ATPase towards E1 and increases the rate of Ca2+ binding to the ATPase, as detected by changes in tryptophan fluorescence intensity, suggesting that K+ increases the rate of the E2-E1 transition. The data are consistent with binding of K+ at the inner Ca(2+)-binding site on the ATPase in competition with H+ and Mg2+, with a higher affinity in the E1 than in the E2 conformation. K+ has no effect on the affinity for Mg2+, as detected by changes in tryptophan fluorescence intensity; since it has been proposed that the changes in tryptophan fluorescence follow from binding to Mg2+ at the outer Ca(2+)-binding site, this suggests that K+ is unable to bind at the outer Ca(2+)-binding site. K+ increases the rate of dissociation of Ca2+ from the Ca(2+)-bound ATPase and reduces the effect of Mg2+ on the fluorescence intensity of the ATPase labelled with 4-(bromomethyl)-6,7-dimethoxycoumarin. It is suggested that these effects of K+ are the result of binding at a 'gating' site on the ATPase, in competition with binding of H+. Binding of K+ at the inner Ca(2+)-binding site and at the gating site account for the observed effects of K+ on the affinity of the ATPase for Ca2+.