Failure of inositol 1,4,5-trisphosphate to elicit or potentiate Ca2+ release from isolated skeletal muscle sarcoplasmic reticulum

Eugenol-induced contractions of saponin-skinned fibers are inhibited by heparin or by a ryanodine receptor blocker

The effects of eugenol on the sarcoplasmic reticulum (SR) and contractile apparatus of chemically skinned skeletal muscle fibers of the frog Rana catesbeiana were investigated. In saponin-skinned fibers, eugenol (5 mmol/L) induced muscle contractions, probably by releasing Ca(2+) from the SR. The Ca(2+)-induced Ca(2+) release blocker ruthenium red (10 micromol/L) inhibited both caffeine- and eugenol-induced muscle contractions. Ryanodine (200 micromol/L), a specific ryanodine receptor/Ca(2+) release channel blocker, promoted complete inhibition of the contractions induced by caffeine, but only partially blocked the contractions induced by eugenol. Heparin (2.5 mg/mL), an inositol 1,4,5-trisphosphate (InsP3) receptor blocker, strongly inhibited the contractions induced by eugenol but had only a small effect on the caffeine-induced contractions. Eugenol neither altered the Ca(2+) sensitivity nor the maximal force in Triton X-100 skinned muscle fibers. These data suggest that muscle contraction induced by eugenol involves at least 2 mechanisms of Ca(2+) release from the SR: one related to the activation of the ryanodine receptors and another through a heparin-sensitive pathway.

Curcumin, a molecule that inhibits the Ca2+-ATPase of sarcoplasmic reticulum but increases the rate of accumulation of Ca2+

Curcumin, an important inhibitor of carcinogenesis, is an inhibitor of the ATPase activity of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR). Inhibition by curcumin is structurally specific, requiring the presence of a pair of -OH groups at the 4-position of the rings. Inhibition is not competitive with ATP. Unexpectedly, addition of curcumin to SR vesicles leads to an increase in the rate of accumulation of Ca(2+), unlike other inhibitors of the Ca(2+)-ATPase that result in a reduced rate of accumulation. An increase in the rate of accumulation of Ca(2+) is seen in the presence of phosphate ion, which lowers the concentration of free Ca(2+) within the lumen of the SR, showing that the effect is not passive leak across the SR membrane. Rather, simulations suggest that the effect is to reduce the rate of slippage on the ATPase, a process in which a Ca(2+)-bound, phosphorylated intermediate releases its bound Ca(2+) on the cytoplasmic rather than on the lumenal side of the membrane. The structural specificity of the effects of curcumin on ATPase activity and on Ca(2+) accumulation is the same, and the apparent dissociation constants for the two effects are similar, suggesting that the two effects of curcumin could follow from binding to a single site on the ATPase.

Negative inotropic effect of heparin on tension development in rat skinned skeletal muscle fibres

Heparin inhibits inositol trisphosphate receptors, particularly in smooth muscle, but its effect on skeletal muscle is controversial. Our study showed that heparin induced a decrease in the amplitude of 10 mM caffeine-induced contracture in slow and fast saponin-skinned fibres. Moreover, measurements on Triton X-100-skinned fibres in soleus muscle showed that heparin alone decreased maximal Ca2(+)-activated tension and Ca2+ sensitivity of contractile proteins, whereas no significant effect was observed in extensor digitorum longus muscle. However, in the presence of caffeine, heparin decreased maximal Ca2(+)-activated tension in both muscles. It would appear that the heparin-induced decrease in the amplitude of caffeine contracture in rat skeletal muscle was not related to a direct inhibition of Ca2+ release from sarcoplasmic reticulum but to a desensitising effect of heparin and caffeine on myofilaments.

The role of phosphoinositide metabolism in Ca2+ signalling of skeletal muscle cells

1. The mobilization of Ca2+ from intracellular stores by D-myo-inositol 1,4,5-triphosphate[Ins(1,4,5)P3] is now widely accepted as the primary link between plasma membrane receptors that stimulate phospholipase C and the subsequent increase in intracellular free Ca2+ that occurs when such receptors are activated (Berridge, 1993). Since the observations of Volpe et al. (1985) which showed that Ins(1,4,5)P3 could induce Ca2+ release from isolated terminal cisternae membranes and elicit contracture of chemically skinned muscle fibres, research has focused on the role of Ins(1,4,5)P3 in the generation of SR Ca2+ transients and in the mechanism of excitation-contraction coupling (EC-coupling). 2. The mechanism of signal transduction at the triadic junction during EC-coupling is unknown. Asymmetric charge movement and mechanical coupling between highly specialized triadic proteins has been proposed as the primary mechanism for voltage-activated generation of SR Ca2+ signals and subsequent contraction. Ins(1,4,5)P3 has also been proposed as the major signal transduction molecule for the generation of the primary Ca2+ transient produced during EC-coupling. 3. Investigations on the generation of Ca2+ transients by Ins(1,4,5)P3 have been conducted on ion channels incorporated into lipid bilayers, skinned and intact fibres and isolated membrane vesicles. Ins(1,4,5)P3 induces SR Ca2+ release and the enzymes responsible for its synthesis and degradation are present in muscle tissue. However, the sensitivity of the Ca2+ release mechanism to Ins(1,4,5)P3 is highly dependent on experimental conditions and on membrane potential. 4. While Ins(1,4,5)P3 may not be the major signal transduction molecule for the generation of the primary Ca2+ signal produced during voltage-activated contraction, this inositol polyphosphate may play a functional role as a modulator of EC-coupling and/or of the processes of myoplasmic Ca2+ regulation occurring on a time scale of seconds, during the events of contraction.

Role of ryanodine receptors

Recent findings on the ryanodine receptor of vertebrates, a Ca-release channel protein for the caffeine- and ryanodine-sensitive Ca pools, are reviewed in this article. Three distinct genes, i.e., ryr1, ryr2, and ryr3, express different isoforms in specific locations: Ryr1 in skeletal muscle and Purkinje cells of cerebellum; Ryr2 in cardiac muscle and brain, especially cerebellum; Ryr3 in skeletal muscle of nonmammalian vertebrates, the corpus striatum, and limbic cortex of brain, smooth muscles, and the other cells in vertebrates. While only one isoform (Ryr1) is expressed in mammalian skeletal muscles, two isoforms (alpha- and beta-isoforms expressed by ryr1 and ryr3, respectively) are found in nonmammalian vertebrate skeletal muscles. Although the coexistence of two isoforms may merely be related to differentiation and specialization, the biological significance remains to be clarified. Ryanodine receptors in vertebrate skeletal muscles are believed to mediate two different modes of Ca release: Ca(2+)-induced Ca release and action potential-induced Ca release. All results obtained so far with any isoform of ryanodine receptor are related to Ca(2+)-induced Ca release and show very similar characteristics. Ca(2+)-induced Ca release, however, cannot be the underlying mechanism of Ca release on skeletal muscle activation. Susceptibility of the ryanodine receptor's ryanodine-binding activity to modification by physical factors, such as osmolality of the medium, might be related to action potential-induced Ca release. A hypothesis of molecular interaction in view of the plunger model of action potential-induced Ca release is discussed, suggesting that the model could be compatible with Ryr1 and Ryr3, but incompatible with Ryr2. The functional relevance of ryanodine receptor isoforms, especially Ryr3, in brain also remains to be clarified. Among ryr1 gene-related diseases, malignant hyperthermia was the first to be identified; however, there is still the possibility of involvement of the other genes. Central core disease has been added to the list recently. A molecular approach for the diagnosis and treatment of diseases is now in progress.

Inositol trisphosphate (InsP3) causes contraction in skeletal muscle only under artificial conditions: evidence that Ca2+ release can result from depolarization of T-tubules

It has been proposed that in striated muscle inositol 1,4,5-trisphosphate (InsP3) may serve as a chemical transmitter linking membrane depolarization to Ca(2+)-release from the sarcoplasmic reticulum. Key to that hypothesis of excitation-concentration (EC) coupling was the observation that skinned muscle fibres contract on the application of InsP3. Yet skinned fibres do not always respond in this way, and in our hands intact fibres do not contract when InsP3 (1 microM-1 mM) is microinjected into them. Glycerol-shocked fibres do contract, however, and so do intact fibres that have been depolarized to about -50 mV by increasing [K+]0. These observations and related pharmacological evidence support the hypothesis that InsP3 causes a low-level depolarizing current to cross the T-tubular membrane. This current is sufficient to depolarize the T-tubules to the threshold for contraction only when the tubules are sealed over or when they are already close to the threshold. The InsP3-induced Ca2+ release sometimes observed in skinned muscle fibres and in vesicles derived from junctional sarcoplasmic reticulum probably often results from an action on sealed-over transverse tubules; in such situations it is an artifact of cell disruption. The fact that high concentrations of InsP3 do not cause contraction in normal muscle fibres is strong evidence against the hypothesis that InsP3 plays a central role in EC coupling in skeletal muscle.

Lack of effect on the sodium efflux of the microinjection of D-Ins(1,4,5)P3 into ouabain-poisoned barnacle muscle-fibers

A study has been carried out using relatively intact mature muscle fibers from the barnacle Balanus nubilus to see whether D-Ins(1,4,5)P3 stimulates the ouabain-insensitive Na efflux following its microinjection and whether this is accompanied by a contraction of the fiber. Part of the impetus for a study of this type came from the on-going debate between Vergara, Rojas and co-workers and Lea and co-workers, the former group holding the view that skinned barnacle fibers and skeletal fibers in general are responsive to this isomer. The evidence brought forward indicates that the injection of D-Ins(1,4,5)P3 in concentrations in the range of 10(-2) M to 10(-6) M into cannulated unskinned fibers pretreated with ouabain fails to increase the residual efflux, and additionally fails to elicit a contraction. A similar picture emerges with the use of non-hydrolyzable DL-Ins(1,4,5)P3[S]3, analog following its injection in a concentration of 0.5 microM. Higher concentrations of the analog were unavailable for use. This work also involved verification of the idea that an effect might be obtainable in depolarized fibers. However, doubling or tripling K0+ and injection of the isomer or the analog simultaneously failed to support this idea. Since nothing is known as to whether D-Ins(1,4,5)P3 influences the behavior of the Na(+)-Ca2+ exchanger when operating in the reverse mode, experiments were done to check this possibility. ATPNa2 which is though to activate Na(+)-Ca2+ exchange was injected prior to the isomer or the analog but no significant results were obtained. A similar line of reasoning was followed, that of activating the L-type Ca2+ channel by injecting GTPNa2 which in addition is known to activate adenylate cyclase. Again, neither the isomer nor the analog were effective. Thus, the only conclusion possible is that in relatively intact, mature barnacle fibers there is no coupling between the phosphoinositide signalling pathway and two other key systems, viz. the Na(+)-Ca2+ exchanger when operating in the reverse mode and the L-type Ca2+ channel. Equally clear is that for some unknown reason the ouabain-insensitive Na efflux and the contractile apparatus are insensitive to D-Ins(1,4,5)P3[S]3.

Fast release of 45Ca2+ induced by inositol 1,4,5-trisphosphate and Ca2+ in the sarcoplasmic reticulum of rabbit skeletal muscle: evidence for two types of Ca2+ release channels

The kinetics of Ca2+ release induced by the second messenger D-myoinositol 1,4,5 trisphosphate (IP3), by the hydrolysis-resistant analogue D-myoinositol 1,4,5 trisphosphorothioate (IPS3), and by micromolar Ca2+ were resolved on a millisecond time scale in the junctional sarcoplasmic reticulum (SR) of rabbit skeletal muscle. The total Ca2+ mobilized by IP3 and IPS3 varied with concentration and with time of exposure. Approximately 5% of the 45Ca2+ passively loaded into the SR was released by 2 microM IPS3 in 150 ms, 10% was released by 10 microM IPS3 in 100 ms, and 20% was released by 50 microM IPS3 in 20 ms. Released 45Ca2+ reached a limiting value of approximately 30% of the original load at a concentration of 10 microM IP3 or 25-50 microM IPS3. Ca(2+)-induced Ca2+ release (CICR) was studied by elevating the extravesicular Ca2+ while maintaining a constant 5-mM intravesicular 45Ca2+. An increase in extravesicular Ca2+ from 7 nM to 10 microM resulted in a release of 55 +/- 7% of the passively loaded 45Ca2+ in 150 ms. CICR was blocked by 5 mM Mg2+ or by 10 microM ruthenium red, but was not blocked by heparin at concentrations as high as 2.5 mg/ml. In contrast, the release produced by IPS3 was not affected by Mg2+ or ruthenium red but was totally inhibited by heparin at concentrations of 2.5 mg/ml or lower. The release produced by 10 microM Ca2+ plus 25 microM IPS3 was similar to that produced by 10 microM Ca2+ alone and suggested that IP3-sensitive channels were present in SR vesicles also containing ruthenium red-sensitive Ca2+ release channels. The junctional SR of rabbit skeletal muscle may thus have two types of intracellular Ca2+ releasing channels displaying fast activation kinetics, namely, IP3-sensitive and Ca(2+)-sensitive channels.

The significance of Na+ in E-C coupling in muscle

Recent studies have revealed arguments in favour of the possible triggering role of Na+ ions in E-C coupling in skeletal muscle fibres of vertebrates: (i) Na+ is one of the four major inorganic cations widespreaded in the biosphere ununiformly (with gradient) distributed across plasmic membranes of all muscle fibres; (ii) there is correlation between contractile parameters and a pattern of transsarcolemmal Na+ distribution in skeletal muscles; (iii) "Na+ current-induced Ca2+ release" mechanism is corresponded to the criterions for intracellular mediators: a) excitation of plasmic membrane increases [Na+] in junctional space; b) increase of [Na+] in surroundings in vitro induces efflux of Ca2+ from SR; c) estimated rate of [Na+] increase in junctional space in vivo is exceeded the threshold that induces Ca2+ from the SR in vitro; d) there is endogenic system (Na+, K(+)-ATPase) of quick removal of Na+ from junctional gap of triads; e) pharmacological modification of Na+ current through T-tubule membrane leads to correlated changing in twitch response. A definite order of Na(+)- and Ca2+ transmembrane triggering fluxes involved in E-C coupling in fast skeletal muscle fibers provide a very protective intracellular functional system of Ca2+ regulation, coordinated in time and space, and garantees the most complete dependence of voluntary muscle contractions on the CNS control.

Inositol 1,4,5-trisphosphate increases myoplasmic [Ca2+] in isolated muscle fibers. Depolarization enhances its effects

Inositol 1,4,5-trisphosphate (InsP3) has been proposed as an intracellular messenger which mobilizes calcium from the sarcoplasmic reticulum, during excitation-contraction coupling in skeletal muscle. We have measured the myoplasmic free calcium concentration ([Ca2+]i) by means of calcium selective microelectrodes in intact fibers isolated from Leptodactylus insularis microinjected with InsP3. In muscle fibers bathed in normal Ringer, the mean resting [Ca2+]i was 0.11 +/- 0.01 microM (M +/- SEM, n = 30). The microinjection of 0.3, 0.5 and 1 microM InsP3 induced transient increments in the [Ca2+]i to 0.35 +/- 0.02 microM (n = 9), to 0.53 +/- 0.03 microM (n = 11) and 0.94 +/- 0.06 microM (n = 10) respectively. Microinjection of 0.3, 0.5 and 1 microM InsP3 in muscle fibers incubated in low Ca2+ solution induced increments in [Ca2+]i similar to those observed in fibers bathed with normal Ringer. The microinjection of 0.3, 0.5 and 1 microM InsP3 in muscle fibers partially depolarized with 10 mM [K+]o induced transient enhancements of the resting [Ca2+]i that were greater than the transients observed in the normally polarized muscle. In partially depolarized fibers microinjected with 0.3, 0.5 and 1 microM InsP3, the [Ca2+]i was changed to 1.45 +/- 0.14 microM (n = 20), to 3.37 +/- 0.34 microM (n = 7) and to 7.43 +/- 0.70 microM (n = 6) respectively. In all partially depolarized fibers these increments in [Ca2+]i were associated with local contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ release by inositol-trisphosphorothioate in isolated triads of rabbit skeletal muscle

The effectiveness of the nonmetabolizable second messenger analogue DL-myo-inositol 1,4,5-trisphosphorothioate (IPS3) described by Cooke, A. M., R. Gigg, and B. V. L. Potter, (1987b. Jour. Chem. Soc. Chem. Commun. 1525-1526.) was examined in triads purified from rabbit skeletal muscle. A Ca2+ electrode uptake-release assay was used to determine the size and sensitivity of the IPS3-releasable pool of Ca2+ in isolated triads. Uptake was initiated by 1 mM MgATP, pCa 5.8, pH 7.5 Release was initiated when the free Ca2+ had lowered to pCa approximately 7. We found that 5-25 microM myo-inositol 1,4,5-trisphosphate (IP3), and separately IPS3, consistently released 5-20% of the Ca2+ pool actively loaded into triads. Single channel recording was used to determine if ryanodine receptor Ca2+ release channels were affected by IPS3 at the same myoplasmic Ca2+ and IPS3 concentrations. Open probability of ryanodine receptor Ca2+ release channels was monitored in triads fused to bilayers over long periods (200 s) in the absence and following addition of 30 microM IPS3 to the same channel. At myoplasmic pCa approximately 7, IPS3 had no effect in the absence of MgATP (Po = 0.0094 +/- 0.001 in control and Po = 0.01 +/- 0.006 after IPS3) and slightly increased activity in the presence of 1 mM MgATP (Po = 0.024 +/- 0.03 in control and Po = 0.05 +/- 0.03 after IPS3). Equally small effects were observed at higher myoplasmic Ca2+. The onset of channel activation by IPS3 or IP3 was slow, on the time scale 20-60 s. We suggest that in isolated triads of rabbit skeletal muscle, IP3-induced release of stored Ca2+ is probably not mediated by the opening of Ca2+ release channels.

Calcium release modulated by inositol trisphosphate in ruptured fibers from frog skeletal muscle

To investigate the effect of inositol 1,4,5-trisphosphate on calcium release, we used fiber bundles of frog sartorius muscle mechanically permeabilized by a scratching procedure, and we detected increments in calcium concentration by measuring aqueorin light signals. Submicromolar concentrations of inositol 1,4,5-trisphosphate induced fast calcium-release signals, with a half time to peak of 60 ms or less. Similar responses were elicited by caffeine. The calcium-release signal induced by inositol 1,4,5-trisphosphate occurred at pCa values of 7 or lower, and the dose-response curve depended on the ionic composition of the incubation solution. Lower inositol 1,4,5-trisphosphate concentrations were needed to induce release when incubation solutions of ionic composition expected to depolarize the transverse tubule membrane were used. Inositol 1,4,5-trisphosphate was more effective than inositol 1,3,4-trisphosphate, inositol 1,4,5,6-tetrakisphosphate, and inositol 1,4-bisphosphate. The effect of inositol 1,4,5-trisphosphate was synergistic with that of caffeine, and was not inhibited by heparin. These results, by showing directly that at resting calcium levels inositol 1,4,5-trisphosphate elicited calcium release, are consistent with a role for inositol 1,4,5-trisphosphate as a chemical modulator in excitation/contraction coupling in skeletal muscle.

Inositol trisphosphate and excitation-contraction coupling in skeletal muscle

The role of inositol trisphosphate as a chemical messenger in excitation-contraction coupling is discussed, both in terms of positive and negative results. The evidence presented includes experiments on the effect of inositol trisphosphate in intact and skinned fibers, in calcium release from isolated sarcoplasmic reticulum vesicles, in activation of single calcium release channels incorporated in planar bilayers, and biochemical experiments that have established the presence of all the intermediate steps involved in the metabolism of phosphoinositides, both in intact muscle and in isolated membranes. From these results, it is clear that a role for inositol triphosphate in skeletal muscle function is highly likely; whether this molecule is the physiological messenger in excitation-contraction coupling remains to be established.

Alteration of calcium sensitivity of skinned frog skeletal muscle fibres by inositol triphosphate and calmodulin antagonists

We investigated the influence of inositol triphosphate (IP3), trifluoperazine (TFP), and perhexiline on the calcium sensitivity of freeze-dried frog semitendinosus muscle fibres. Further, the effect of IP3 on calcium release from the sarcoplasmic reticulum (SR) of frog semitendinosus fibres skinned by saponin was studied. IP3 decreased the calcium sensitivity of freeze-dried frog skeletal muscle fibres and failed to induce a calcium release from SR of saponin-skinned fibres. Freeze-dried frog skeletal muscle fibres were strongly sensitized for calcium by TFP and perhexiline.

Calcium modulation of phosphoinositide kinases in transverse tubule vesicles from frog skeletal muscle

Highly purified transverse tubule membranes isolated from frog skeletal muscle phosphorylate phosphatidylinositol to phosphatidylinositol 4-phosphate and phosphatidylinositol (4,5)-bisphosphate. The two phosphorylation reactions have different calcium requirements. Phosphorylation of phosphatidylinositol to phosphatidylinositol 4-phosphate, which takes place in both isolated transverse tubules and sarcoplasmic reticulum membrane, is independent of calcium in a range of concentrations from 10(-9) to 10(-6) M, and is progressively inhibited to 10% of the maximal values by increasing calcium to 10(-4) M or higher (K0.5 = 5 X 10(-6) M). In contrast, phosphorylation of phosphatidylinositol 4-phosphate to phosphatidylinositol (4,5)-bisphosphate, a reaction exclusively present in transverse tubule membranes, is maximal at calcium concentrations higher than 2 X 10(-6) M and decreases to 30% of maximal values at calcium concentrations of 2 X 10(-7) M or lower (K0.5 = 10(-6) M). Unlike frog membranes, transverse tubules from rabbit muscle need exogenous phosphatidylinositol 4-phosphate in order to produce the bisphosphate derivative in the same range of calcium concentrations. Inositol (1,4,5)-trisphosphate has been proposed recently as a chemical messenger in excitation-contraction coupling in skeletal muscle. Calcium regulation of the synthesis of phosphatidylinositol (4,5)-bisphosphate, the membrane-bound precursor of inositol (1,4,5)-trisphosphate, might have physiological implications regarding modulation of excitation-contraction coupling by intracellular calcium levels.