Chloride-dependent sarcoplasmic reticulum Ca2+ release correlates with increased Ca2+ activation of ryanodine receptors

The biophysical properties of TRIC-A and TRIC-B and their interactions with RyR2

Trimeric intracellular cation channels (TRIC-A and TRIC-B) are thought to provide counter-ion currents to enable charge equilibration across the sarco/endoplasmic reticulum (SR) and nuclear membranes. However, there is also evidence that TRIC-A may interact directly with ryanodine receptor type 1 (RyR1) and 2 (RyR2) to alter RyR channel gating. It is therefore possible that the reverse is also true, where the presence of RyR channels is necessary for fully functional TRIC channels. We therefore coexpressed mouse TRIC-A or TRIC-B with mouse RyR2 in HEK293 cells to examine if after incorporating membrane vesicles from these cells into bilayers, the presence of TRIC affects RyR2 function, and to characterize the permeability and gating properties of the TRIC channels. Importantly, we used no purification techniques or detergents to minimize damage to TRIC and RyR2 proteins. We found that both TRIC-A and TRIC-B altered the gating behavior of RyR2 and its response to cytosolic Ca2+ but that TRIC-A exhibited a greater ability to stimulate the opening of RyR2. Fusing membrane vesicles containing TRIC-A or TRIC-B into bilayers caused the appearance of rapidly gating current fluctuations of multiple amplitudes. The reversal potentials of bilayers fused with high numbers of vesicles containing TRIC-A or TRIC-B revealed both Cl- and K+ fluxes, suggesting that TRIC channels are relatively non-selective ion channels. Our results indicate that the physiological roles of TRIC-A and TRIC-B may include direct, complementary regulation of RyR2 gating in addition to the provision of counter-ion currents of both cations and anions.

CLIC4 localizes to mitochondrial-associated membranes and mediates cardioprotection

Mitochondrial-associated membranes (MAMs) are known to modulate organellar and cellular functions and can subsequently affect pathophysiology including myocardial ischemia-reperfusion (IR) injury. Thus, identifying molecular targets in MAMs that regulate the outcome of IR injury will hold a key to efficient therapeutics. Here, we found chloride intracellular channel protein (CLIC4) presence in MAMs of cardiomyocytes and demonstrate its role in modulating ER and mitochondrial calcium homeostasis under physiological and pathological conditions. In a murine model, loss of CLIC4 increased myocardial infarction and substantially reduced cardiac function after IR injury. CLIC4 null cardiomyocytes showed increased apoptosis and mitochondrial dysfunction upon hypoxia-reoxygenation injury in comparison to wild-type cardiomyocytes. Overall, our results indicate that MAM-CLIC4 is a key mediator of cellular response to IR injury and therefore may have a potential implication on other pathophysiological processes.

The cardiac ryanodine receptor N-terminal region contains an anion binding site that is targeted by disease mutations

Ryanodine receptors (RyRs) are calcium release channels located in the membrane of the endoplasmic and sarcoplasmic reticulum and play a major role in muscle excitation-contraction coupling. The cardiac isoform (RyR2) is the target for >150 mutations that cause catecholaminergic polymorphic ventricular tachycardia (CPVT) and other conditions. Here, we present the crystal structure of the N-terminal region of RyR2 (1-547), an area encompassing 29 distinct disease mutations. The protein folds up in three individual domains, which are held together via a central chloride anion that shields repulsive positive charges. Several disease mutant versions of the construct drastically destabilize the protein. The R420Q disease mutant causes CPVT and ablates chloride binding. The mutation results in reorientations of the first two domains relative to the third domain. These conformational changes likely activate the channel by destabilizing intersubunit interactions that are disrupted upon channel opening.

Lack of CFTR in skeletal muscle predisposes to muscle wasting and diaphragm muscle pump failure in cystic fibrosis mice

Cystic fibrosis (CF) patients often have reduced mass and strength of skeletal muscles, including the diaphragm, the primary muscle of respiration. Here we show that lack of the CF transmembrane conductance regulator (CFTR) plays an intrinsic role in skeletal muscle atrophy and dysfunction. In normal murine and human skeletal muscle, CFTR is expressed and co-localized with sarcoplasmic reticulum-associated proteins. CFTR-deficient myotubes exhibit augmented levels of intracellular calcium after KCl-induced depolarization, and exposure to an inflammatory milieu induces excessive NF-kB translocation and cytokine/chemokine gene upregulation. To determine the effects of an inflammatory environment in vivo, sustained pulmonary infection with Pseudomonas aeruginosa was produced, and under these conditions diaphragmatic force-generating capacity is selectively reduced in Cftr(-/-) mice. This is associated with exaggerated pro-inflammatory cytokine expression as well as upregulation of the E3 ubiquitin ligases (MuRF1 and atrogin-1) involved in muscle atrophy. We conclude that an intrinsic alteration of function is linked to the absence of CFTR from skeletal muscle, leading to dysregulated calcium homeostasis, augmented inflammatory/atrophic gene expression signatures, and increased diaphragmatic weakness during pulmonary infection. These findings reveal a previously unrecognized role for CFTR in skeletal muscle function that may have major implications for the pathogenesis of cachexia and respiratory muscle pump failure in CF patients.

Effects of Mg(2+) and SR luminal Ca(2+) on caffeine-induced Ca(2+) release in skeletal muscle from humans susceptible to malignant hyperthermia

Regulation of the ryanodine receptor (RYR) by Mg(2+) and SR luminal Ca(2+) was studied in mechanically skinned malignant hyperthermia susceptible (MHS) and non-susceptible (MHN) fibres from human vastus medialis. Preparations were perfused with solutions mimicking the intracellular milieu and changes in [Ca(2+)] were detected using fura-2 fluorescence. At 1 mM cytosolic Mg(2+), MHS fibres had a higher sensitivity to caffeine (2-40 mM) than MHN fibres. The inhibitory effect of Mg(2+) on caffeine-induced Ca(2+) release was studied by increasing [Mg(2+)] of the solution containing 40 mM caffeine. Increasing [Mg(2+)] from 1 to 3 mM reduced the amplitude of the caffeine-induced Ca(2+) transient by 77 +/- 7.4 % (n = 8) in MHN fibres. However, the caffeine-induced Ca(2+) transient decreased by only 24 +/- 8.1 % (n = 9) in MHS fibres. In MHN fibres, reducing the Ca(2+) loading period from 4 to 1 min (at 1 mM Mg(2+)) decreased the fraction of the total sarcoplasmic reticulum (SR) Ca(2+) content released in response to 40 mM caffeine by 90.4 +/- 6.2 % (n = 6). However, in MHS fibres the response was reduced by only 31.2 +/- 17.4 % (n = 6) under similar conditions. These results suggest that human malignant hyperthermia (MH) is associated with reduced inhibition of the RYR by (i) cytosolic Mg(2+) and (ii) SR Ca(2+) depletion. Both of these effects may contribute to increased sensitivity of the RYR to caffeine and volatile anaesthetics.

Divergent effects of the malignant hyperthermia-susceptible Arg(615)-->Cys mutation on the Ca(2+) and Mg(2+) dependence of the RyR1

The sarcoplasmic reticulum (SR) Ca(2+) release channel (RyR1) from malignant hyperthermia-susceptible (MHS) porcine skeletal muscle has a decreased sensitivity to inhibition by Mg(2+). This diminished Mg(2+) inhibition has been attributed to a lower Mg(2+) affinity of the inhibition (I) site. To determine whether alterations in the Ca(2+) and Mg(2+) affinity of the activation (A) site contribute to the altered Mg(2+) inhibition, we estimated the Ca(2+) and Mg(2+) affinities of the A- and I-sites of normal and MHS RyR1. Compared with normal SR, MHS SR required less Ca(2+) to half-maximally activate [(3)H]ryanodine binding (K(A,Ca): MHS = 0.17 +/- 0.01 microM; normal = 0.29 +/- 0.02 microM) and more Ca(2+) to half-maximally inhibit ryanodine binding (K(I,Ca): MHS = 519.3 +/- 48.7 microM; normal = 293.3 +/- 24.2 microM). The apparent Mg(2+) affinity constants of the MHS RyR1 A- and I-sites were approximately twice those of the A- and I-sites of the normal RyR1 (K(A,Mg): MHS = 44.36 +/- 4.54 microM; normal = 21.59 +/- 1.66 microM; K(I,Mg): MHS = 660.8 +/- 53.0 microM; normal = 299.2 +/- 24.5 microM). Thus, the reduced Mg(2+) inhibition of the MHS RyR1 compared with the normal RyR1 is due to both an enhanced selectivity of the MHS RyR1 A-site for Ca(2+) over Mg(2+) and a reduced Mg(2+) affinity of the I-site.

Iodide and bromide inhibit Ca(2+) uptake by cardiac sarcoplasmic reticulum

Recent studies indicate that the Ca(2+) permeability of the sarcoplasmic reticulum (SR) can be affected by its anionic environment. Additionally, anions could directly modulate the SR Ca(2+) pump or the movement of compensatory charge across the SR membrane during Ca(2+) uptake or release. To examine the effect of anion substitution on cardiac SR Ca(2+) uptake, fluorometric Ca(2+) measurements and spectrophotometric ATPase assays were used. Ca(2+) uptake into SR vesicles was inhibited in a concentration-dependent manner when Br(-) or I(-) replaced extravesicular Cl(-) (when Br(-) completely replaced Cl(-), uptake velocity was approximately 70% of control; when I(-) completely replaced Cl(-), uptake velocity was approximately 39% of control). Replacement of Cl(-) with SO(2)(-4) had no effect on SR uptake. Although both I(-) and Br(-) inhibited net Ca(2+) uptake, neither anion directly inhibited the SR Ca(2+) pump nor did they increase the permeability of the SR membrane to Ca(2+). Our results support the hypothesis that an anionic current that occurs during SR Ca(2+) uptake is reduced by the substitution of Br(-) or I(-) for Cl(-).

Chloride channels and their functional roles in smooth muscle tone in the vasculature

Although evidence of important contributions by Cl- channels to agonist-induced currents have been reported in vascular smooth muscle cells, the functional roles played by Cl- channels in the smooth muscle contraction and in setting the membrane potential remain essentially obscure. All of the admittedly few papers published have focused on the physiological roles of Cl- channels in the contraction and membrane depolarization elicited by agonists. At present, it seems likely that in vascular cells: a) Cl- conductance contributes to membrane depolarization, with the subsequent contraction being due to Ca2+ release from the intracellular store sites, and b) Cl- movements through the membrane of the Ca2+ store sites also regulate Ca2+ release and Ca2+ uptake from/into the store sites. As a Ca2+-dependent Cl- current is most easily demonstrated under quasi-physiological conditions (by the perforated patch-clamp method), the contribution made by Cl- channels to smooth muscle function may be more important than previously thought. The development of the new, selective Cl--channel blockers as well as the identification and gene engineering of the channel molecules are essential if we are to advance our knowledge of the physiology and pharmacology of the Cl- channels residing in vascular smooth muscle cells.

Different Ca2+ releasing action of caffeine and depolarisation in skeletal muscle fibres of the rat

1. The relative abilities of caffeine and transverse tubular (T-) system depolarisation to induce Ca2+ release in mammalian skeletal muscle were compared in mechanically skinned fibres of the rat, in order to determine whether normal excitation-contraction (E-C) coupling is achieved by up-regulating the Ca2+-induced Ca2+ release process, as caffeine is known to do. 2. Caffeine triggered Ca2+ release in soleus (slow-twitch) fibres at much lower concentrations than in extensor digitorum longus (EDL) (fast-twitch) fibres when the sarcoplasmic reticulum (SR) of each type was loaded with Ca2+ at close to endogenous levels. The difference in caffeine sensitivity resulted at least in part from the SR being loaded endogenously at near maximal capacity in soleus fibres but at less than half of maximal capacity in EDL fibres. The caffeine sensitivity could be reversed by reversing the relative level of SR loading. 3. The ability of caffeine to induce Ca2+ release was markedly reduced by lowering the level of SR loading or by raising the free [Mg2+] from 1 to 3 mM. Caffeine, even at 30 mM, triggered little or no Ca2+ release in EDL fibres (a) at 1 mM (physiological) Mg2+ when the SR was loaded at two-thirds or less of the endogenous level, and (b) at 3 mM Mg2+ when the SR was loaded at close to the endogenous level. In contrast, depolarisation potently elicited Ca2+ release under these conditions in the same fibres. 4. The inability of 30 mM caffeine to induce Ca2+ release under certain conditions was not attributable to desensitisation or inactivation of the release channels, because there was no response even upon initial exposure to caffeine and depolarisation always remained able to trigger Ca2+ release. It instead appeared that caffeine was a relatively ineffectual stimulus in EDL fibres except under conditions where (a) the SR was heavily loaded, (b) the free [Mg2+] was low, or (c) a high [Cl-] was present. 5. These results show that the normal E-C coupling mechanism in mammalian skeletal muscle does not involve just enhancing Ca2+-induced Ca2+ release, and evidently requires the removal or bypassing of the inhibitory effect of Mg2+ on the Ca2+ release channels.

Differential Ca(2+) sensitivity of skeletal and cardiac muscle ryanodine receptors in the presence of calmodulin

Calmodulin (CaM) activates the skeletal muscle ryanodine receptor Ca(2+) release channel (RyR1) in the presence of nanomolar Ca(2+) concentrations. However, the role of CaM activation in the mechanisms that control Ca(2+) release from the sarcoplasmic reticulum (SR) in skeletal muscle and in the heart remains unclear. In media that contained 100 nM Ca(2+), the rate of (45)Ca(2+) release from porcine skeletal muscle SR vesicles was increased approximately threefold in the presence of CaM (1 microM). In contrast, cardiac SR vesicle (45)Ca(2+) release was unaffected by CaM, suggesting that CaM activated the skeletal RyR1 but not the cardiac RyR2 channel isoform. The activation of RyR1 by CaM was associated with an approximately sixfold increase in the Ca(2+) sensitivity of [(3)H]ryanodine binding to skeletal muscle SR, whereas the Ca(2+) sensitivity of cardiac SR [(3)H]ryanodine binding was similar in the absence and presence of CaM. Cross-linking experiments identified both RyR1 and RyR2 as predominant CaM binding proteins in skeletal and cardiac SR, respectively, and [(35)S]CaM binding determinations further indicated comparable CaM binding to the two isoforms in the presence of micromolar Ca(2+). In nanomolar Ca(2+), however, the affinity and stoichiometry of RyR2 [(35)S]CaM binding was reduced compared with that of RyR1. Together, our results indicate that CaM activates RyR1 by increasing the Ca(2+) sensitivity of the channel, and further suggest differences in CaM's functional interactions with the RyR1 and RyR2 isoforms that may potentially contribute to differences in the Ca(2+) dependence of channel activation in skeletal and cardiac muscle.

Mechanisms of P(i) regulation of the skeletal muscle SR Ca(2+) release channel

Inorganic phosphate (P(i)) accumulates in the fibers of actively working muscle where it acts at various sites to modulate contraction. To characterize the role of P(i) as a regulator of the sarcoplasmic reticulum (SR) calcium (Ca(2+)) release channel, we examined the action of P(i) on purified SR Ca(2+) release channels, isolated SR vesicles, and skinned skeletal muscle fibers. In single channel studies, addition of P(i) to the cis chamber increased single channel open probability (P(o); 0.079 +/- 0.020 in 0 P(i), 0. 157 +/- 0.034 in 20 mM P(i)) by decreasing mean channel closed time; mean channel open times were unaffected. In contrast, the ATP analog, beta,gamma-methyleneadenosine 5'-triphosphate (AMP-PCP), enhanced P(o) by increasing single channel open time and decreasing channel closed time. P(i) stimulation of [(3)H]ryanodine binding by SR vesicles was similar at all concentrations of AMP-PCP, suggesting P(i) and adenine nucleotides act via independent sites. In skinned muscle fibers, 40 mM P(i) enhanced Ca(2+)-induced Ca(2+) release, suggesting an in situ stimulation of the release channel by high concentrations of P(i). Our results support the hypothesis that P(i) may be an important endogenous modulator of the skeletal muscle SR Ca(2+) release channel under fatiguing conditions in vivo, acting via a mechanism distinct from adenine nucleotides.

Regulation of cardiac muscle Ca2+ release channel by sarcoplasmic reticulum lumenal Ca2+

The cardiac muscle sarcoplasmic reticulum Ca2+ release channel (ryanodine receptor) is a ligand-gated channel that is activated by micromolar cytoplasmic Ca2+ concentrations and inactivated by millimolar cytoplasmic Ca2+ concentrations. The effects of sarcoplasmic reticulum lumenal Ca2+ on the purified release channel were examined in single channel measurements using the planar lipid bilayer method. In the presence of caffeine and nanomolar cytosolic Ca2+ concentrations, lumenal-to-cytosolic Ca2+ fluxes >/=0.25 pA activated the channel. At the maximally activating cytosolic Ca2+ concentration of 4 microM, lumenal Ca2+ fluxes of 8 pA and greater caused a decline in channel activity. Lumenal Ca2+ fluxes primarily increased channel activity by increasing the duration of mean open times. Addition of the fast Ca2+-complexing buffer 1,2-bis(2-aminophenoxy)ethanetetraacetic acid (BAPTA) to the cytosolic side of the bilayer increased lumenal Ca2+-activated channel activities, suggesting that it lowered Ca2+ concentrations at cytosolic Ca2+-inactivating sites. Regulation of channel activities by lumenal Ca2+ could be also observed in the absence of caffeine and in the presence of 5 mM MgATP. These results suggest that lumenal Ca2+ can regulate cardiac Ca2+ release channel activity by passing through the open channel and binding to the channel's cytosolic Ca2+ activation and inactivation sites.

The Ca(2+)-ATPase of the sarcoplasmic reticulum in skeletal and cardiac muscle. An overview from the very beginning to more recent prospects

The discovery of the ATP-driven calcium pump in the sarcoplasmic reticulum membranes reaches back to the postwar (World War II) years and would not be possible without the generous support by the American scientific community. It was this community that in pre- and postwar years gave shelter to many European scientists, which in return stimulated scientific development in the United States. These pre- and postwar relations helped to establish the calcium pump as a physiologically relevant mechanism in all kinds of cells. The pump and its counterpart, the calcium release channel, proved to be controlled by various intrinsic mechanisms. Rising hydrogen concentrations as occurring in ischemic muscles switch off pump activity and counteract allosterically caffeine-induced calcium release (CICR). Rising phosphate or the presence of other calcium-precipitating anions, on the other hand, prevents pump inhibition by intraluminal calcium precipitation, which, simultaneously, can increase the quantity of releasable calcium. The inactivation of CICR by removing medium chloride must be considered as a hint of additional mechanisms by which calcium-dependent activity regulation can be modified.

Glycolytic pathway intermediates activate cardiac ryanodine receptors

During myocardial ischaemia and reperfusion, enhancement of glycolytic activity occurs and this may lead to fluctuating levels of glycolytic intermediates. We demonstrate that sugar phosphate intermediates of glycolysis, particularly fructose-1,6-diphosphate (FDP; 100 microM-10 mM), can activate sheep cardiac ryanodine receptor (RyR) channels incorporated into bilayers (open probability (Po) increases up to approximately 0.6) and stimulate [3H]ryanodine binding (> 200%) to isolated cardiac sarcoplasmic reticulum (SR) membrane vesicles. The relative effectiveness of the sugar phosphates in stimulating [3H]ryanodine binding and increasing the Po of the channels was FDP > glucose-1-phosphate (G-1-P) > fructose-6-phosphate (F-6-P) > glucose-6-phosphate (G-6-P). These novel properties of the sugar phosphate compounds indicate that changes in glycolytic flux may influence the release of SR Ca2+ by modulating RyR channel gating.

Voltage change-induced gating transitions of the rabbit skeletal muscle Ca2+ release channel

1. We used the planar lipid bilayer method to study single ryanodine receptor Ca2+ release channels (RyRCs) from fast skeletal muscle of the rabbit. We found that changes in membrane voltage directly induced gating transitions of the RyRC: (i) in the steady state, even at activating Ca2+ concentrations (20 microM), at a constant membrane potential the channels resided in a low open probability (Po) state (inactivated-, I-mode), and (ii) upon abrupt changes of voltage, the apparent inactivation of the RyRCs was relieved, resulting in a rapid and transient increase in Po. 2. The magnitude of the Po increase was a function of both the duration and the amplitude of the applied prepulse, but was independent of the channel activity during the prepulse. 3. The voltage-induced Po increase probably involved major conformational changes of the channel, as it resulted in substantial alterations in the gating pattern of the channels: the voltage change-induced increase in Po was accompanied by the rapid appearance of two types of channel activity (high (H) and low (L) open probability modes). 4. The response of the RyRC to voltage changes raises the interesting possibility that the activation of RyRC in situ might involve electrical events, i.e. a possible dipole-dipole coupling between the release channel and the voltage sensor.

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.

The interactions of ATP, ADP, and inorganic phosphate with the sheep cardiac ryanodine receptor

The effects of ATP, ADP, and inorganic phosphate (Pi) on the gating of native sheep cardiac ryanodine receptor channels incorporated into planar phospholipid bilayers were investigated. We demonstrate that ATP and ADP can activate the channel by Ca2+-dependent and Ca2+-independent mechanisms. ATP and ADP appear to compete for the same site/s on the cardiac ryanodine receptor, and in the presence of cytosolic Ca2+ both agents tend to inactivate the channel at supramaximal concentrations. Our results reveal that ATP not only has a greater affinity for the adenine nucleotide site/s than ADP, but also has a greater efficacy. The EC50 value for channel activation is approximately 0.2 mM for ATP compared to 1.2 mM for ADP. Most interesting is the fact that, even in the presence of cytosolic Ca2+, ADP cannot activate the channel much above an open probability (Po) of 0.5, and therefore acts as a partial agonist at the adenine nucleotide binding site on the channel. We demonstrate that Pi also increases Po in a concentration and Ca2+-dependent manner, but unlike ATP and ADP, has no effect in the absence of activating cytosolic [Ca2+]. We demonstrate that Pi does not interact with the adenine nucleotide site/s but binds to a distinct domain on the channel to produce an increase in Po.

Investigation of the effect of inositol trisphosphate in skinned skeletal muscle fibres with functional excitation-contraction coupling

The effect of inositol trisphosphate (IP3) was investigated in mechanically skinned fibres which had the endogenous level of sarcoplasmic reticulum (SR) Ca2+ and in which the normal excitation-contraction (E-C) coupling mechanism was still functional. Application of 50 or 100 microM IP3 failed to induce a detectable force response in any such skinned fibre from either the extensor digitorum longus muscle of the rat or iliofibularis muscle of the toad, irrespective of whether the fibre was: (a) in its normally polarized, resting state; (b) chronically depolarized to inactivate the voltage sensors; (c) paralysed with D600; or (d) depolarized to threshold for force activation. Furthermore, the size of the response to subsequent depolarization or exposure to caffeine (2mM) or reduced myoplasmic [Mg2+] indicated that little if any Ca2+ had been lost from the SR during the period of IP3 exposure (> or = 1 min). Also, IP3 did not induce a detectable force response when SR Ca2+ uptake was potently inhibited with 20 microM TBQ. Exposure to IP3 (50 microM) slightly potentiated the peak force response to depolarization in toad fibres, and this was probably because of an accompanying small increase in Ca2+ sensitivity of the contractile apparatus. These results appear inconsistent with the proposal that IP3 acts as the second messenger in E-C coupling in skeletal muscle.

Modulation of Ca(2+)-gated cardiac muscle Ca(2+)-release channel (ryanodine receptor) by mono- and divalent ions

The effects of mono- and divalent ions on Ca(2+)-gated cardiac muscle Ca(2+)-release channel (ryanodine receptor) activity were examined in [3H]ryanodine-binding measurements. Ca2+ bound with the highest apparent affinity to Ca2+ activation sites in choline chloride medium, followed by KCl, CsCl, NaCl, and LiCl media. The apparent Ca2+ binding affinities of Ca2+ inactivation sites were lower in choline chloride and CsCl media than in LiCl, NaCl, and KCl media. Sr2+ activated the ryanodine receptor with a lower efficacy than Ca2+. Competition studies indicated that Li+, K+, Mg2+, and Ba2+ compete with Ca2+ for Ca2+ activation sites. In 0.125 M KCl medium, the Ca2+ dependence of [3H]ryanodine binding was modified by 5 mM Mg2+ and 5 mM beta,gamma-methyleneadenosine 5'-triphosphate (a nonhydrolyzable ATP analog). The addition of 5 mM glutathione was without appreciable effect. Substitution of Cl- by 2-(N-morpholino)ethanesulfonic acid ion caused an increase in the apparent Ca2+ affinity of the Ca2+ inactivation sites, whereas an increase in KCl concentration had the opposite effect. These results suggest that cardiac muscle ryanodine receptor activity may be regulated by 1) competitive binding of mono- and divalent cations to Ca2+ activation sites, 2) binding of monovalent cations to Ca2+ inactivation sites, and 3) binding of anions to anion regulatory sites.

Dantrolene inhibition of sarcoplasmic reticulum Ca2+ release by direct and specific action at skeletal muscle ryanodine receptors

The skeletal muscle relaxant dantrolene inhibits the release of Ca2+ from the sarcoplasmic reticulum during excitation-contraction coupling and suppresses the uncontrolled Ca2+ release that underlies the skeletal muscle pharmacogenetic disorder malignant hyperthermia; however, the molecular mechanism by which dantrolene selectively affects skeletal muscle Ca2+ regulation remains to be defined. Here we provide evidence of a high-affinity, monophasic inhibition by dantrolene of ryanodine receptor Ca2+ channel function in isolated sarcoplasmic reticulum vesicles prepared from malignant hyperthermia-susceptible and normal pig skeletal muscle. In media simulating resting myoplasm, dantrolene increased the half-time for 45Ca2+ release from both malignant hyperthermia and normal vesicles approximately 3.5-fold and inhibited sarcoplasmic reticulum vesicle [3H]ryanodine binding (Ki approximately 150 nM for both malignant hyperthermia and normal). Inhibition of vesicle [3H]ryanodine binding by dantrolene was associated with a decrease in the extent of activation by both calmodulin and Ca2+. Dantrolene also inhibited [3H]ryanodine binding to purified skeletal muscle ryanodine receptor protein reconstituted into liposomes. In contrast, cardiac sarcoplasmic reticulum vesicle 45Ca2+ release and [3H]ryanodine binding were unaffected by dantrolene. Together, these results demonstrate selective effects of dantrolene on skeletal muscle ryanodine receptors that are consistent with the actions of dantrolene in vivo and suggest a mechanism of action in which dantrolene may act directly at the skeletal muscle ryanodine receptor complex to limit its activation by calmodulin and Ca2+. The potential implications of these results for understanding how dantrolene and malignant hyperthermia mutations may affect the voltage-dependent activation of Ca2+ release in intact skeletal muscle are discussed.

Inositol 1,4,5-trisphosphate and basal Ca2+ release is affected by the cytoplasmic concentration of Cl- in endothelial cells

The effects of varying Cl- concentration in the intracellular bathing medium, on IP3-induced 45Ca2+ release from internal stores, were examined in saponin-permeabilised bovine aortic endothelial (BAE) cells. Results from this study show that the release of Ca2+ from the internal stores is affected by the cytoplasmic concentration of Cl- ions. Complete replacement of Cl- with gluconate augmented IP3 (3 microM)-induced 45Ca2+ release by 33 +/- 8%. Replacement of both Cl- and K+ with gluconate and NMG, respectively, had no significant effect on 45Ca2+ release. However, resting levels of internal 45Ca2+ were found to be affected by Cl- removal. These data suggest that in BAE cells, IP3 and also basal 45Ca2+ release may be regulated by the physiological intracellular Cl- concentration.

Functional behaviour of the ryanodine receptor/Ca(2+)-release channel in vesiculated derivatives of the junctional membrane of terminal cisternae of rabbit fast muscle sarcoplasmic reticulum

We have devised a novel procedure, employing Chaps rather than Triton [Costello B., Chadwick C., Saito A., Chu A., Maurer A., Fleischer S. J Cell Biol 1986; 103: 741-753], for obtaining vesiculated derivatives of the junctional face membrane (JFM) domain of isolated terminal cisternae (TC) from fast skeletal muscle of the rabbit. Enriched JFM is minimally contaminated with junctional transverse tubules. The characteristic ultrastructural features and the most essential features of TC function relating to this membrane domain-i.e. both the Ca(2+)-release system and the Ca2+ and calmodulin (CaM)-dependent protein kinase (CaM I PK) system-appear to be retained in enriched JFM. We show that our isolation procedure, yielding up to a 2.5-fold enrichment in ryanodine receptor (RyR) protein and in the maximum number of high affinity [3H]-ryanodine binding sites, does not alter the assembly for integral proteins associated with the receptor in its native membrane environment, i.e. FKBP-12, triadin and the structurally related protein junction [Jones L.R., Zhang L., Sanborn K., Jorgensen A., Kelley J. J Biol Chem 1995; 270: 30787-30796] having, in common, the property to bind calsequestrin (CS) in overlays in the presence of EGTA. The substrate specificity of endogenous CaM I PK is also the same as that of parent TC vesicles. Phosphorylation of mainly triadin and of a high M(r) polypeptide, and not of the RyR, is the most remarkable common property. Retention of peripheral proteins, like CS and histidine-rich Ca(2+)-binding protein, although not that endogenous CaM, and of a unique set of CaM-binding proteins, unlike that of junctional SR-specific integral proteins, is shown to be influenced by the concentration of Ca2+ during incubation of TC vesicles with Chaps. Characterization of RyR functional behaviour with [3H]-ryanodine has indicated extensive similarities between the enriched JFM and parent TC vessicles, as far as the characteristic bell shaped Ca(2+)-dependence of [3H]-ryanodine binding and the dose-dependent sensitization to Ca2+ by caffeine, reflecting the inherent properties of SR Ca(2+)-release channel, as well as concerning the stimulation of [3H]-ryanodine binding by increasing concentrations of KCl. Stabilizing the RyR in a maximally active state by optimizing concentrations of KCl (1 M), at also optimal concentrations of Ca2+ (pCa 4), rendered the receptor less sensitive to inhibition by 1 microM CaM, to a greater extent in the case of enriched JFM. That was not accounted for by any significant difference in the IC50 concentrations of CaM varying between 40 nM to approximately 80 nM, at low-intermediate and at high KCl concentrations, respectively. Additional results with enriched JFM using doxorubicin, a pharmacological Ca2+ channel allosteric modifier, strengthen the hypothesis that the conformational state at which RyR is stabilized, according to the experimental assay conditions for [3H]-ryanodine binding, directly influences CaM-sensitivity.

Functional and morphological features of skeletal muscle from mutant mice lacking both type 1 and type 3 ryanodine receptors

1. We generated mice with targeted disruptions in the genes for both ryanodine receptor type 1 (RyR-1) and type 3 (RyR-3) to study the functional roles of RyR subtypes in skeletal muscle. 2. In permeabilized myocytes lacking both the RyRs, the Ca(2+)-induced Ca2+ release (CICR) mechanism was completely lost, and caffeine failed to induce Ca2+ release. 3. Replacement of potassium methanesulphonate in an experimental intracellular solution with choline chloride resulted in Ca2+ release in the wild-type muscle but not in the mutant muscle lacking RyR-1. 4. The double-mutant mice exhibited more severe muscular degeneration than RyR-1-deficient mice with formation of large vacuoles and swollen mitochondria while structural coupling between T-tubules and the sarcoplasmic reticulum was retained. 5. These results demonstrate that CICR is mediated solely by RyR-1 and RyR-3 in skeletal muscle cells, and suggest that RyR-1 is involved in Cl(-)-induced Ca2+ release. The results also suggest the presence of molecular components other than RyRs responsible for the triad formation. RyR-3 may have a role in the normal morphogenesis of skeletal muscle cells, although functionally it can be replaced by RyR-1.