Functional characterization of the recombinant type 3 Ca2+ release channel (ryanodine receptor) expressed in HEK293 cells

Changes in the expression of myometrial ryanodine receptor mRNAs during human pregnancy

Uterine contraction is triggered by a rise in intracellular free Ca(2+) concentration ([Ca2+]i), and although ryanodine-sensitive Ca(2+) release channels (RyRs) play a key role in the regulation of [Ca(2+)](i) in skeletal and cardiac muscle, much less is known about their role in smooth muscle. In this study, we investigated the expression of RyR mRNAs (ryr1-3) during human pregnancy by examining myometrial samples (n=18) taken, with informed consent and ethical approval, from non-pregnant patients undergoing hysterectomy, and patients undergoing elective caesarean section (at term, prior to or following the onset of labour). Ca(2+) release channel expression was determined both qualitatively and quantitatively, using reverse transcription-polymerase chain reaction (RT-PCR) analysis, RNase protection assays, and in situ mRNA hybridisation. RT-PCR analysis demonstrated that all three ryr genes, as well as the gene encoding the type I inositol 1,4,5-trisphosphate receptor (InsP(3)RI), are expressed in human myometrium. Quantitation by RNase protection assays showed that ryr3 and InsP(3)RI mRNAs are the most abundant, while ryr2 mRNA is barely detectable. In situ mRNA hybridisation confirmed that ryr3 and InsP(3)RI mRNAs are both localised to myometrial smooth muscle cells. The expression of ryr2 and ryr3 mRNA is down-regulated at the end of pregnancy compared to non-pregnant myometrium, indicating that ryanodine-sensitive Ca(2+) release channels are differentially expressed. The relative conservation of ryr1 expression is consistent with a role for Ca(2+) release from ryanodine-sensitive stores in the mechanism of uterine contractility during labour.

Molecular identification of the ryanodine receptor pore-forming segment

A sequence motif, GXRXGGGXGD, located in the putative channel-forming domain, is conserved in all known ryanodine receptors and inositol 1,4,5-trisphosphate receptors. The functional significance of this conserved region was investigated by using site-directed mutagenesis together with functional assays consisting of Ca(2+) release measurements, [(3)H]ryanodine binding, and single channel recordings in planar lipid bilayers. We report here that single point mutations introduced into this region of the mouse cardiac ryanodine receptor reduce or abolish high affinity [(3)H]ryanodine binding. Single channel analysis revealed that a single substitution of alanine for glycine 4824 within this region reduced the single channel conductance by 97%, from 798 picosiemens (pS) for the wild type channel to 22 pS. The G4824A mutant channel was modulated by Ca(2+), Mg(2+), ATP, caffeine, ruthenium red, and ryanodine. Co-expression of the wild type and G4824A mutant proteins produced single channels that have intermediate unitary conductances of 516, 256, 176, and 60 pS. These data suggest that this conserved region constitutes an essential part of the ryanodine binding site and the channel conduction pathway of the ryanodine receptor.

Contribution of ryanodine receptor type 3 to Ca(2+) sparks in embryonic mouse skeletal muscle

The kinetic behavior of Ca(2+) sparks in knockout mice lacking a specific ryanodine receptor (RyR) isoform should provide molecular information on function and assembly of clusters of RyRs. We examined resting Ca(2+) sparks in RyR type 3-null intercostal myotubes from embryonic day 18 (E18) mice and compared them to Ca(2+) sparks in wild-type (wt) mice of the same age and to Ca(2+) sparks in fast-twitch muscle cells from the foot of wt adult mice. Sparks from RyR type 3-null embryonic cells (368 events) were significantly smaller, briefer, and had a faster time to peak than sparks from wt cells (280 events) of the same age. Sparks in adult cells (220 events) were infrequent, yet they were highly reproducible with population means smaller than those in embryonic RyR type 3-null cells but similar to those reported in adult amphibian skeletal muscle fibers. Three-dimensional representations of the spark peak intensity (DeltaF/Fo) vs. full width at half-maximal intensity (FWHM) vs. full duration at half-maximal intensity (FTHM) showed that wt embryonic sparks were considerably more variable in size and kinetics than sparks in adult muscle. In all cases, tetracaine (0.2 mM) abolished Ca(2+) spark activity, whereas caffeine (0.1 mM) lengthened the spark duration in wt embryonic and adult cells but not in RyR type 3-null cells. These results confirmed that sparks arose from RyRs. The low caffeine sensitivity of RyR type 3-null cells is entirely consistent with observations by other investigators. There are three conclusions from this study: i) RyR type-1 engages in Ca(2+) spark activity in the absence of other RyR isoforms in RyR type 3-null myotubes; ii) Ca(2+) sparks with parameters similar to those reported in adult amphibian skeletal muscle can be detected, albeit at a low frequency, in adult mammalian skeletal muscle cells; and iii) a major contributor to the unusually large Ca(2+) sparks observed in normal (wt) embryonic muscle is RyR type 3. To explain the reduction in the size of sparks in adult compared to embryonic skeletal muscle, we suggest that in embryonic muscle, RyR type 1 and RyR type 3 channels co-contribute to Ca(2+) release during the same spark and that Ca(2+) sparks undergo a maturation process which involves a decrease in RyR type 3.

Type 3 and type 1 ryanodine receptors are localized in triads of the same mammalian skeletal muscle fibers

The type 3 ryanodine receptor (RyR3) is a ubiquitous calcium release channel that has recently been found in mammalian skeletal muscles. However, in contrast to the skeletal muscle isoform (RyR1), neither the subcellular distribution nor the physiological role of RyR3 are known. Here, we used isoform-specific antibodies to localize RyR3 in muscles of normal and RyR knockout mice. In normal hind limb and diaphragm muscles of young mice, RyR3 was expressed in all fibers where it was codistributed with RyR1 and with the skeletal muscle dihydropyridine receptor. This distribution pattern indicates that RyR3 is localized in the triadic junctions between the transverse tubules and the sarcoplasmic reticulum. During development, RyR3 expression declined rapidly in some fibers whereas other fibers maintained expression of RyR3 into adulthood. Comparing the distribution of RyR3-containing fibers with that of known fiber types did not show a direct correlation. Targeted deletion of the RyR1 or RyR3 gene resulted in the expected loss of the targeted isoform, but had no adverse effects on the expression and localization of the respective other RyR isoform. The localization of RyR3 in skeletal muscle triads, together with RyR1, is consistent with an accessory function of RyR3 in skeletal muscle excitation-contraction coupling.

Pregnant rat myometrial cells show heterogeneous ryanodine- and caffeine-sensitive calcium stores

Intracellular Ca(2+) release channels such as ryanodine receptors play crucial roles in the Ca(2+)-mediated signaling that triggers excitation-contraction coupling in muscles. Although the existence and the role of these channels are well characterized in skeletal and cardiac muscles, their existence in smooth muscles, and more particularly in the myometrium, is very controversial. We have now clearly demonstrated the expression of ryanodine receptor Ca(2+) release channels in rat myometrial smooth muscle, and for the first time, intracellular Ca(2+) concentration experiments with indo 1 on single myometrial cells have revealed the existence of a functional ryanodine- and caffeine-sensitive Ca(2+) release mechanism in 30% of rat myometrial cells. RT-PCR and RNase protection assay on whole myometrial smooth muscle demonstrate the existence of all three ryr mRNAs in the myometrium: ryr3 mRNA is the predominant subtype, with much lower levels of expression for ryr1 and ryr2 mRNAs, suggesting that the ryanodine Ca(2+) release mechanism in rat myometrium is largely encoded by ryr3. Moreover, using intracellular Ca(2+) concentration measurements and RNase protection assays, we have demonstrated that the expression, the percentage of cells responding to ryanodine, and the function of these channels are not modified during pregnancy.

Ca2+ waves during triggered propagated contractions in intact trabeculae. Determinants of the velocity of propagation

During triggered propagated contractions, Ca2+ waves travel along cardiac trabeculae with a constant velocity (Vprop) ranging from 0. 34 to 5.47 mm/s. To explore the determinants of Vprop, we studied (1) the relationship between [Ca2+]i and Vprop and (2) the effect of low concentrations of caffeine on Vprop. Trabeculae were dissected from the right ventricle of rat hearts. [Ca2+]i was measured using electrophoretically injected fura-2 and an image-intensified CCD camera. Force was measured using a silicon strain gauge, and sarcomere length was measured using laser diffraction techniques. After induction of reproducible Ca2+ waves by trains of electrical stimuli (2.5 Hz) at 21.9+/-0.2 degrees C, the number of stimuli or [Ca2+]o was varied in 9 trabeculae. In 5 trabeculae, the effects of caffeine (0.1 to 1.0 mmol/L) at [Ca2+]o of 2.2+/-0.3 mmol/L were determined. All images were recorded under stable conditions of wave propagation. The increment in [Ca2+]i during the last electrically stimulated transient (DeltaCaT) and [Ca2+]i just before onset of the Ca2+ waves (CaD) were used to estimate the Ca2+ loading of the sarcoplasmic reticulum (SR) and the myoplasm, respectively. The ratio (DeltaCaW/DeltaCaT) of the [Ca2+]i increment during the waves (DeltaCaW) to DeltaCaT was used to estimate the probability of opening of the SR-Ca2+ release channel during wave propagation. As a result of an increase of the number of stimuli or [Ca2+]o, Vprop increased in proportion to (1) DeltaCaT (r=0.82); (2) CaD (r=0.88); (3) DeltaCaW (r=0.85); and (4) DeltaCaW/DeltaCaT (r=0.74). The addition of caffeine (</=0.3 mmol/L) increased Vprop for any DeltaCaT and any DeltaCaW, revealing an increased sensitivity of Vprop to DeltaCaT and DeltaCaW. In contrast, caffeine had little effect on the relationship between Vprop and CaD and no effect on that between Vprop and DeltaCaW/DeltaCaT. These results suggest that both the cellular Ca2+ loading and open probability of the SR-Ca2+ release channels determine the velocity of propagation of Ca2+ waves.

Further characterization of the type 3 ryanodine receptor (RyR3) purified from rabbit diaphragm

We characterized type 3 ryanodine receptor (RyR3) purified from rabbit diaphragm by immunoaffinity chromatography using a specific antibody. The purified receptor was free from 12-kDa FK506-binding protein, although it retained the ability to bind 12-kDa FK506-binding protein. Negatively stained images of RyR3 show a characteristic rectangular structure that was indistinguishable from RyR1. The location of the D2 segment, which exists uniquely in the RyR1 isoform, was determined as the region around domain 9 close to the corner of the square-shaped assembly, with use of D2-directed antibody as a probe. The RyR3 homotetramer had a single class of high affinity [3H]ryanodine-binding sites with a stoichiometry of 1 mol/mol. In planar lipid bilayers, RyR3 displayed cation channel activity that was modulated by several ligands including Ca2+, Mg2+, caffeine, and ATP, which is consistent with [3H]ryanodine binding activity. RyR3 showed a slightly larger unit conductance and a longer mean open time than RyR1. Whereas RyR1 showed two classes of channel activity with distinct open probabilities (Po), RyR3 displayed a homogeneous and steeply Ca2+-dependent activity with Po approximately 1. RyR3 was more steeply affected in the channel activity by sulfhydryl-oxidizing and -reducing reagents than RyR1, suggesting that the channel activity of RyR3 may be transformed more precipitously by the redox state. This is also a likely explanation for the difference in the Ca2+ dependence of RyR3 between [3H]ryanodine binding and channel activity.

Ca2+ sensitization of smooth muscle contractility induced by ruthenium red

The effects of ruthenium red (RuR) on contractility were examined in skinned fibers of guinea pig smooth muscles, where sarcoplasmic reticulum function was destroyed by treatment with A-23187. Contractions of skinned fibers of the urinary bladder were enhanced by RuR in a concentration-dependent manner (EC50 = 60 microM at pCa 6.0). The magnitude of contraction at pCa 6.0 was increased to 320% of control by 100 microM RuR. Qualitatively, the same results were obtained in skinned fibers prepared from the ileal longitudinal smooth muscle layer and mesenteric artery. The maximal contraction induced by pCa 4.5 was not affected significantly by RuR. The enhanced contraction by RuR was not reversed by the addition of guanosine 5'-O-(2-thiodiphosphate) or a peptide inhibitor of protein kinase C [PKC-(19-31)]. The application of microcystin, a potent protein phosphatase inhibitor, induced a tonic contraction of skinned smooth muscle at low Ca2+ concentration ([Ca2+]; pCa > 8.0). RuR had a dual effect on the microcystin-induced contraction-to- enhancement ratio at low concentrations and suppression at high concentrations. The relaxation following the decrease in [Ca2+] from pCa 5.0 to >8.0 was significantly slowed down by an addition of RuR. Phosphorylation of the myosin light chain at pCa 6.3 was significantly increased by RuR in skinned fibers of the guinea pig ileum. These results indicate that RuR markedly increases the Ca2+ sensitivity of the contractile system, at least in part via inhibition of myosin light chain phosphatase.

Evidence for novel caffeine and Ca2+ binding sites on the lobster skeletal ryanodine receptor

1. The effects of Ca2+, ATP and caffeine on the gating of lobster skeletal muscle ryanodine receptors (RyR) was investigated after reconstitution of the channels into planar phospholipid bilayers and by using [3H]-ryanodine binding studies. 2. The single channel studies reveal that the EC50 (60 microM) for activation of the lobster skeletal RyR by Ca2+ as the sole ligand is higher than for any other isoform of RyR studied. 3. Inactivation of the channel by Ca2+ (EC50 = 1 mM) occurs at concentrations slightly higher than those required to inactivate mammalian skeletal RyR (RyR1) but lower than those required to inactivate mammalian cardiac RyR (RyR2). 4. Lifetime analysis demonstrates that cytosolic Ca2+, as the sole activating ligand, cannot fully open the lobster skeletal RyR (maximum Po approximately 0.2). The mechanism for the increase in open probability (Po) is an increase in both the frequency and the duration of the open events. 5. ATP is a very effective activator of the lobster RyR and can almost fully open the channel in the presence of activating cytosolic [Ca2+]. In the presence of 700 microM Ca2+, 1 mM ATP increased Po to approximately 0.8. 6. Caffeine, often used as a tool to identify the presence of RyR channels, is relatively ineffective and cannot increase Po above the level that can be attained with Ca2+ alone. 7. The results reveal that caffeine increases Po by a different mechanism to that of cytosolic Ca2+ demonstrating that the mechanism for channel activation by caffeine is not 'sensitization' to cytosolic Ca2+. 8. By studying the mechanisms involved in the activation of the lobster RyR we have demonstrated that the channel responds in a unique manner to Ca2+ and to caffeine. The results strongly indicate that these ligand binding sites on the channel are different to those on mammalian isoforms of RyR.

Voltage-controlled Ca2+ release in normal and ryanodine receptor type 3 (RyR3)-deficient mouse myotubes

1. Primary cultured myotubes were derived from satellite cells of the diaphragm obtained from both normal mice (RyR3+/+) and mice with a targeted mutation eliminating expression of the type 3 isoform of the ryanodine receptor (RyR3-/-). Using the whole-cell patch clamp technique, L-type Ca2+ currents were measured during step depolarizations. Simultaneously, intracellular Ca2+ transients were recorded with the fluorescent indicator dye fura-2. 2. After correction for non-instantaneous binding of Ca2+ to the indicator dye and taking into account the dynamics of Ca2+ binding to intracellular constituents, an estimate of the time course of the Ca2+ release rate from the sarcoplasmic reticulum (SR) was obtained. 3. The calculated SR Ca2+ release flux exhibited a marked peak within less than 12 ms after the onset of the voltage-clamp depolarization and fell rapidly thereafter to a five times lower, almost steady level. It declined rapidly after termination of the depolarization. 4. Signals in normal and RyR3-deficient myotubes showed no significant difference in the activation of Ca2+ conductance and in amplitude, time course and voltage dependence of the Ca2+ efflux from the SR. 5. In conclusion, the characteristics of voltage-controlled Ca2+ release reported here are similar to those of mature mammalian muscle fibres. In contrast to differences observed in the contractile properties of RyR3-deficient muscle fibres, a contribution of RyR3 to excitation-contraction coupling could not be detected in myotubes.

Potential for pharmacology of ryanodine receptor/calcium release channels

Calcium release channels, known also as ryanodine receptors (RyRs), play an important role in Ca2+ signaling in muscle and nonmuscle cells by releasing Ca2+ from intracellular stores. Mammalian tissues express three different RyR isoforms comprising four 560-kDa (RyR polypeptide) and four 12-kDa (FK506 binding protein) subunits. The large protein complexes conduct monovalent and divalent cations and are capable of multiple interactions with other molecules. The latter include small diffusible endogenous effector molecules including Ca2+, Mg2+, adenine nucleotides, sufhydryl modifying reagents (glutathione, NO, and NO adducts) and lipid intermediates, and proteins such as protein kinases and phosphatases, calmodulin, immunophilins (FK506 binding proteins), and in skeletal muscle the dihydropyridine receptor. Because of their role in regulating intracellular Ca2+ levels and their multiple ligand interactions, RyRs constitute an important, potentially rich pharmacological target for controlling cellular functions. Exogenous effectors found to affect RyR function include ryanoids, toxins, xanthines, anthraquinones, phenol derivatives, adenosine and purinergic agonists and antagonists, NO donors, oxidizing reagents, dantrolene, local anesthetics, and polycationic reagents.

Characterization of brain-type ryanodine receptor permanently expressed in Chinese hamster ovary cells

To clarify a function of brain-type ryanodine receptor (RyR3) and its regulation, we established a stable cell line expressing rabbit RyR3 by transfection of Chinese hamster ovary cells (CHO cells) with the cDNA and investigated characteristics of the RyR3. Scatchard analysis of [3H]-ryanodine binding to the membrane from CHO cells expressing RyR3 showed two distinct binding sites. The Kd values of high and low affinity binding sites were 1.92 and 25.9 nM, respectively. [3H]-ryanodine binding to the membrane from CHO cells expressing RyR3 was dependent on pCa. Extracellular Ca2+ (2-10 mM) and high concentration (more than 30 mM) of caffeine activated the RyR3 in CHO cells and increased its intracellular Ca2+ concentration. The enhancement of [3H]-ryanodine binding to the membrane from CHO cells expressing RyR3 was observed by bromoeudistomin D (BED), a caffeine-like powerful Ca2+ releaser, at pCa 5.5. Stably expressed RyR3 in CHO is useful for characterization of its function.

Contribution of sarcoplasmic reticular calcium to smooth muscle contractile activation: gestational dependence in isolated rat uterus

1. The contribution of Ca2+ released from the sarcoplasmic reticulum (SR) to smooth muscle contractile activation remains poorly understood. By simultaneously monitoring cytosolic [Ca2+] ([Ca2+]i) and force in isolated rat uterine smooth muscle, we report the influence of SR Ca2+ release on contractility during conditions (a) of altered SR Ca2+ homeostasis and (b) where the only source of activating Ca2+ was derived from the SR. 2. In myometria of non-pregnant rats, ryanodine (1-50 microM), a modulator of calcium-induced calcium release (CICR), had no effect on the spontaneous [Ca2+]i or force transients. However, depletion of SR Ca2+ by inhibiting the SR Ca2+-ATPase (with cyclopiazonic acid (CPA), 20 microM) resulted in an enhancement of spontaneous [Ca2+]i and force transients. 3. In myometria of pregnant rats, although ryanodine had no effect in 40% of tissues studied it produced a small but significant enhancement of the integrated spontaneous [Ca2+]i and force transient in 60% of cases. The potentiating effects of CPA were enhanced in myometria of pregnant rats compared with non-pregnant rats, often resulting in maintained [Ca2+]i increases and contraction. 4. In zero external Ca2+, agonist-induced SR Ca2+ release resulted in transient increases in [Ca2+]i and force. The magnitude of these agonist-induced [Ca2+]i and force changes were significantly enhanced in myometria of pregnant rats. No evidence for agonist-induced Ca2+-independent force production was observed. 5. These results indicate that CICR plays little role in SR Ca2+ release from the myometrium, and that there are gestational-dependent alterations in the ability of SR Ca2+ mobilization to contribute to contractile activation. The implications of these findings for the co-ordination of myometrial [Ca2+]i signalling and contractility are discussed.

Purification and characterization of ryanodine receptor 3 from mammalian tissue

The ryanodine receptors are intracellular Ca2+ release channels that play a key role in cell signaling via Ca2+. There are three isoforms. Isoform 1 from skeletal muscle and isoform 2 from heart have been characterized. Isoform 3 is widely distributed in many mammalian tissues although in minuscule amounts. Its low abundance has hampered its study. We now describe methodology to isolate mammalian isoform 3 in amounts sufficient for biochemical and biophysical characterization. Bovine diaphragm sarcoplasmic reticulum fractions enriched in terminal cisternae containing both isoforms 1 (>95%) and 3 (<5% of the ryanodine binding) served as starting source. Isoform 3 was selectively immunoprecipitated from the 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonic acid (CHAPS)-solubilized fraction and eluted with peptide epitope. Isoform 3 thus prepared is highly purified as characterized by SDS-polyacryamide gel electrophoresis, Coomassie Blue staining, and by high affinity ryanodine binding. The purified isoform 3 was incorporated into planar lipid bilayers, and its channel properties were studied. Channel characteristics in common with the other two isoforms are slope conductance, higher selectivity to Ca2+ versus K+ (PCa/K approximately 6), and response to drugs and ligands. In its response to Ca2+ and ATP, it more closely resembles isoform 2. The first two-dimensional structure of isoform 3 was obtained by cryoelectron microscopy and image enhancement techniques.

Molecular identification of the ryanodine receptor Ca2+ sensor

We have investigated the molecular basis for ryanodine receptor (RyR) activation by Ca2+ by using site-directed mutagenesis together with functional assays consisting of Ca2+ release measurements and single channel recordings in planar lipid bilayers. We report here that a single substitution of alanine for glutamate at position 3885 (located in the putative transmembrane sequence M2 of the type 3 RyR) reduces the Ca2+ sensitivity, as measured by single channel activation, by more than 10,000-fold, without apparent changes in channel conductance and in modulation by other ligands (e.g. ATP and ryanodine). Co-expression of the wild type and mutant RyR proteins results in the synthesis of single channels that have intermediate Ca2+ sensitivities. These results suggest that the glutamates at position 3885 of each monomer may act in a coordinated way to form the Ca2+ sensor in the tetrameric structure corresponding to RyR.

Functional properties of the ryanodine receptor type 3 (RyR3) Ca2+ release channel

Single-channel analysis of sarcoplasmic reticulum vesicles prepared from diaphragm muscle, which contains both RyR1 and RyR3 isoforms, revealed the presence of two functionally distinct ryanodine receptor calcium release channels. In addition to channels with properties typical of RyR1 channels, a second population of ryanodine-sensitive channels with properties distinct from those of RyR1 channels was observed. The novel channels displayed close-to-zero open-probability at nanomolar Ca2+ concentrations in the presence of 1 mM ATP, but were shifted to the open conformation by increasing Ca2+ to micromolar levels and were not inhibited at higher Ca2+ concentrations. These novel channels were sensitive to the stimulatory effects of cyclic adenosine 5'-diphosphoribose (cADPR). Detection of this second population of RyR channels in lipid bilayers was always associated with the presence of the RyR3 isoform in muscle preparations used for single-channel measurements and was abrogated by the knockout of the RyR3 gene in mice. Based on the above, we associated the novel population of channels with the RyR3 isoform of Ca2+ release channels. The functional properties of the RyR3 channels are in agreement with a potential qualitative contribution of this channel to Ca2+ release in skeletal muscle and in other tissues.

cDNA cloning and sequencing of the human ryanodine receptor type 3 (RYR3) reveals a novel alternative splice site in the RYR3 gene

The human ryanodine receptor type 3 (RYR3) was cloned from a fetal brain cDNA library and its complete sequence was determined (EMBL accession number AJ001515). The sequenced cDNA spanned 15,564 bp and contained an open reading frame of 14,613 bp. The corresponding protein consisted of 4870 amino acids with a calculated molecular mass of 552 kDa. Amino acid sequence identities to the RYR3 proteins from rabbit, mink, and chicken were 96%, 95%, and 83% respectively. A previously unidentified alternative splice site was detected generating a transcript that lacked bases 11,569-11,650 and encoded a truncated protein.

Contractile impairment and structural alterations of skeletal muscles from knockout mice lacking type 1 and type 3 ryanodine receptors

Skeletal muscle contraction is triggered by the release of Ca2+ from the sarcoplasmic reticulum through the type 1 ryanodine receptor (RyR1). Recently it has been shown that also the type 3 isoform of ryanodine receptor (RyR3), which is expressed in some mammalian skeletal muscles, may participate in the regulation of skeletal muscle contraction. Here we report the generation and the characterization of double mutant mice carrying a targeted disruption of both the RyR1 and the RyR3 genes (RyR1-/-;RyR3-/-). Skeletal muscles from mice homozygous for both mutations are unable to contract in response to caffeine and to ryanodine. In addition, they show a very poor capability to develop tension when directly activated with micromolar [Ca2+]i after membrane permeabilization which indicates either poor development or degeneration of the myofibrils. This was confirmed by biochemical analysis of contractile proteins. Electron microscopy confirms small size of myofibrils and shows complete absence of feet (RyRs) in the junctional SR.