Calcium release from the sarcoplasmic reticulum

Requirement of ryanodine receptors for pacemaker Ca2+ activity in ICC and HEK293 cells

Intracellular Ca(2+) ([Ca(2+)](i)) oscillations seen in interstitial cells of Cajal (ICCs) are considered to be the primary pacemaker activity in the gut. Here, we show evidence that periodic Ca(2+) release from intracellular Ca(2+) stores produces [Ca(2+)](i) oscillations in ICCs, using cell cluster preparations isolated from mouse ileum. The pacemaker [Ca(2+)](i) oscillations in ICCs are preserved in the presence of dihydropyridine Ca(2+) antagonists, which suppress Ca(2+) activity in smooth muscle cells. However, applications of drugs affecting either ryanodine receptors or inositol 1,4,5-trisphosphate receptors terminated [Ca(2+)](i) oscillations at relatively low concentrations. RT-PCR analyses revealed a predominant expression of type 3 RyR (RyR3) in isolated c-Kit-immunopositive cells (ICCs). Furthermore, we demonstrate that pacemaker-like global [Ca(2+)](i) oscillation activity is endowed by introducing RyR3 into HEK293 cells, which originally express only IP(3)Rs. The reconstituted [Ca(2+)](i) oscillations in HEK293 cells possess essentially the same pharmacological characteristics as seen in ICCs. The results support the functional role of RyR3 in ICCs.

Catchlike property of skeletal muscle: Recent findings and clinical implications

The catchlike property of skeletal muscle is the force augmentation produced by the inclusion of an initial, brief, high-frequency burst of two to four pulses at the start of a subtetanic low-frequency stimulation train. Catchlike-inducing trains take advantage of the catchlike property of skeletal muscle and augment muscle performance compared with constant-frequency trains, especially in the fatigued state. Literature spanning more than 30 years has provided comprehensive information about the catchlike property of skeletal muscle. The pattern of the catchlike-inducing train that maximizes muscle performance is fairly similar across different muscles of different species and under various stimulation conditions. This review summarizes the mechanisms of the catchlike property, factors affecting force augmentation, techniques used to identify patterns of catchlike-inducing trains that maximize muscle performance, and potential clinical applications to provide a historical and current perspective of our understanding of the catchlike property.

A proton-led model of fast calcium waves

Fast (10-30 microm/s) calcium waves can be propagated through all nucleated eukaryotic cells that have been tested as well as certain cell-free extracts. In a widely used model, they are propagated by a reaction-diffusion cycle in which calcium ions diffuse along the outside of endoplasmic reticula and induce their own release from calsequestrin or calreticulin molecules stored within the reticulum's lumen. Here we propose a new tandem wave model in which they are also propagated by a reaction-diffusion cycle within a reticulum's lumen. In this cycle, increases in luminal [H(+)] induce proton release from luminal calsequestrin or calreticulin. The released protons diffuse ahead to where they release more protons from these luminal storage proteins. What might be called proton induced proton release. They also raise luminal electropositivity. The resultant luminal waves are coordinated with extrareticular ones by movements of calcium and hydrogen ions through the reticular membrane. This model makes five testable predictions which include the autorelease of protons in solutions of calsequestrins or calreticulins as well as waves of increased [H(+)], of increased [Ca(2+)] and of more positive voltage within the reticula of whole cells. Moreover, under some conditions, such luminal waves should cross regions without cytosolic ones.

Calcium dyshomeostasis in beta-amyloid and tau-bearing skeletal myotubes

The relative scarcity of inclusion-affected muscle cells or markers of cell death in inclusion body myositis (IBM) is in distinction to the specific and early intracellular deposition of several Alzheimer's Disease (AD)-related proteins. The current study examined the possible correlation between myotube beta-amyloid and/or Tau accumulations and a widespread mishandling of intracellular muscle calcium concentration that could potentially account for the unrelenting weakness in affected patients. Cultured myogenic cells (C(2)C(12)) expressed beta-amyloid-42 (Abeta(42)) and fetal Tau peptides, as human transgenes encoded by herpes simplex virus, either individually or concurrently. Co-expression of Abeta(42) in C(2)C(12) myotubes resulted in hyperphosphorylation of Tau protein that was not observed when Tau was expressed alone. Resting calcium concentration and agonist-induced RyR-mediated Ca(2+) release were examined using calcium-specific microelectrodes and Fluo-4 epifluorescence, respectively. Co-expression of Abeta(42) and Tau cooperatively elevated basal levels of myoplasmic-free calcium, an effect that was accompanied by depolarization of the plasma membrane. Sarcoplasmic reticulum (SR) calcium release, induced by KCl depolarization, was not affected by Abeta(42) or Tau. In contrast, expression of Abeta(42), Tau, or Abeta(42) together with Tau resulted in enhanced sensitivity of ryanodine receptors to activation by caffeine. Notably, expression of beta-amyloid, alone, was sufficient to result in an increased sensitivity to direct activation by caffeine. Current results indicate that amyloid proteins cooperate to raise resting calcium levels and that these effects are associated with a passive SR Ca(2+) leak and Tau hyperphosphorylation in skeletal muscle.

Thymol: a classical small-molecule compound that has a dual effect (potentiating and inhibitory) on myosin

The effect of thymol on the ATPase activity of myosin subfragment-1 (S1) and on the contractile properties of skinned skeletal muscle fibers was studied. At concentrations of 1.5-2 mM, thymol activated the S1 ATPase substantially and the actin-activated S1 ATPase modestly. At the same concentrations, the isometric force of skinned skeletal muscle fibers was modestly suppressed (11% at 2 mM). However, the kinetic parameters of contraction were suppressed more: the velocity of shortening and the rate of force redevelopment after shortening were suppressed by 43% and 31% at 2 mM, respectively. Thus, among other small-molecule inhibitors, thymol is unique in that it has opposite effects on the enzymatic activity and kinetic parameters of contraction. Thymol may serve as a potent tool for studying the mechanism of coupling between the ATPase reaction and contraction in muscle.

Changes in skeletal muscle SR Ca2+ pump in congestive heart failure due to myocardial infarction are prevented by angiotensin II blockade

In order to understand the mechanisms of exercise intolerance and muscle fatigue, which are commonly observed in congestive heart failure, we studied sarcoplasmic reticulum (SR) Ca2+-transport in the hind-leg skeletal muscle of rats subjected to myocardial infarction (MI). Sham-operated animals were used for comparison. On one hand, the maximal velocities (Vmax) for both SR Ca2+-uptake and Ca2+-stimulated ATPase activities in skeletal muscle of rats at 8 weeks of MI were higher than those of controls. On the other hand, the Vmax values for both SR Ca2+-uptake and Ca2+-stimulated ATPase activities were decreased significantly at 16 weeks of MI when compared with controls. These alterations in Ca2+-transport activities were not associated with any change in the affinity (1/Ka) of the SR Ca2+-pump for Ca2+. Furthermore, the stimulation of SR Ca2+-stimulated ATPase activity by cyclic AMP-dependent protein kinase was not altered at 8 or 16 weeks of MI when compared with the respective control values. Treatment of 3-week infarcted animals with angiotensin-converting enzyme (ACE) inhibitors such as captopril, imidapril, and enalapril or an angiotensin receptor (AT1R) antagonist, losartan, for a period of 13 weeks not only attenuated changes in left ventricular function but also prevented defects in SR Ca2+-pump in skeletal muscle. These results indicate that the skeletal muscle SR Ca2+-transport is altered in a biphasic manner in heart failure due to MI. It is suggested that the initial increase in SR Ca2+-pump activity in skeletal muscle may be compensatory whereas the depression at late stages of MI may play a role in exercise intolerance and muscle fatigue in congestive heart failure. Furthermore, the improvements in the skeletal muscle SR Ca2+-transport by ACE inhibitors may be due to the decreased activity of renin-angiotensin system in congestive heart failure.Key words: skeletal muscle, sarcoplasmic reticulum, Ca2+-transport, SR Ca2+-pump, congestive heart failure, renin-angiotensin system.

RyR1 exhibits lower gain of CICR activity than RyR3 in the SR: evidence for selective stabilization of RyR1 channel

We showed that frog alpha-ryanodine receptor (alpha-RyR) had a lower gain of Ca(2+)-induced Ca(2+) release (CICR) activity than beta-RyR in sarcoplasmic reticulum (SR) vesicles, indicating selective "stabilization" of the former isoform (Murayama T and Ogawa Y. J Biol Chem 276: 2953-2960, 2001). To know whether this is also the case with mammalian RyR1, we determined [(3)H]ryanodine binding of RyR1 and RyR3 in bovine diaphragm SR vesicles. The value of [(3)H]ryanodine binding (B) was normalized by the number of maximal binding sites (B(max)), whereby the specific activity of each isoform was expressed. This B/B(max) expression demonstrated that ryanodine binding of individual channels for RyR1 was <15% that for RyR3. Responses to Ca(2+), Mg(2+), adenine nucleotides, and caffeine were not substantially different between in situ and purified isoforms. These results suggest that the gain of CICR activity of RyR1 is markedly lower than that of RyR3 in mammalian skeletal muscle, indicating selective stabilization of RyR1 as is true of frog alpha-RyR. The stabilization was partly eliminated by FK506 and partly by solubilization of the vesicles with CHAPS, each of which was additive to the other. In contrast, high salt, which greatly enhances [(3)H]ryanodine binding, caused only a minor effect on the stabilization of RyR1. None of the T-tubule components, coexisting RyR3, or calmodulin was the cause. The CHAPS-sensitive intra- and intermolecular interactions that are common between mammalian and frog skeletal muscles and the isoform-specific inhibition by FKBP12, which is characteristic of mammals, are likely to be the underlying mechanisms.

Ion-channel currents of smooth muscle cells isolated from the prostate of guinea-pig

OBJECTIVE

To characterize the voltage-activated ion-channel currents in guinea-pig prostate smooth muscle cells (GPSMCs).

MATERIALS AND METHODS

GPSMCs were isolated using collagenase, and used in a whole-cell patch clamp study.

RESULTS

When GPSMCs were dialysed with a CsCl solution all the outward K+ currents were blocked and the step-like depolarization (holding voltage -70 mV) of the cell membrane evoked inward currents that were completely blocked by nifedipine (1 micromol/L). With KCl solution, step depolarizations showed outward K+ currents composed of fast, transient outward current (Ito) and outward currents that did not inactivate. Ito was resistant to a high concentration of tetraethylammonium (TEA, 5 mmol/L) but was blocked by 4-aminopyridine (5 mmol/L). The half-activation and half-inactivation voltages of Ito were 6 mV and -58 mV, respectively. With low Ca2+ buffer (0.1 mmol/L EGTA) in the solution, there were spontaneous transient outward currents (STOCs) at depolarized membrane voltages (0 mV). STOCs were blocked by TEA (1 mmol/L) or iberiotoxin (10 nmol/L) but were insensitive to apamin (100 nmol/L).

CONCLUSION

This voltage-clamp study showed that GPSMCs have l-type Ca2+ channels and more than two types of K+ channels. The voltage- and time-dependent changes of these ion channels and their interactions might be important in forming action potentials and regulating contractility.

The Predicted TM10 Transmembrane Sequence of the Cardiac Ca2+ Release Channel (Ryanodine Receptor) Is Crucial for Channel Activation and Gating

The predicted TM10 transmembrane sequence, (4844)IIFDITFFFFVIVILLAIIQGLII(4867), has been proposed to be the pore inner helix of the ryanodine receptor (RyR) and to play a crucial role in channel activation and gating, as with the inner helix of bacterial potassium channels. However, experimental evidence for the involvement of the TM10 sequence in RyR channel activation and gating is lacking. In the present study, we have systematically investigated the effects of mutations of each residue within the 24-amino acid TM10 sequence of the mouse cardiac ryanodine receptor (RyR2) on channel activation by caffeine and Ca(2+). Intracellular Ca(2+) release measurements in human embryonic kidney 293 cells expressing the RyR2 wild type and TM10 mutants revealed that several mutations in the TM10 sequence either abolished caffeine response or markedly reduced the sensitivity of the RyR2 channel to activation by caffeine. By assessing the Ca(2+) dependence of [(3)H]ryanodine binding to RyR2 wild type and TM10 mutants we also found that mutations in the TM10 sequence altered the sensitivity of the channel to activation by Ca(2+) and enhanced the basal activity of [(3)H]ryanodine binding. Furthermore, single I4862A mutant channels exhibited considerable channel openings and altered gating at very low concentrations of Ca(2+). Our data indicate that the TM10 sequence constitutes an essential determinant for channel activation and gating, in keeping with the proposed role of TM10 as an inner helix of RyR. Our results also shed insight into the orientation of the TM10 helix within the RyR channel pore.

Effect of caffeine on response of rabbit isolated corpus cavernosum to high K+ solution, noradrenaline and transmural electrical stimulation

1. Caffeine has wide-ranging activities on smooth muscles, including contractile and relaxant effects. The aim of the present study was to examine the activity of caffeine on rabbit corpus cavernosum (RCC). 2. The effects of caffeine (0.5-4.0 mmol/L) on the response of RCC to high K+ solution, noradrenaline (NA) and transmural electrical stimulation (EFS) were studied in a tissue bath system. 3. Caffeine did not contract the RCC. However, 0.5-4.0 mmol/L caffeine caused concentration-dependent relaxation of tension development in high-K+ (120 mmol/L) solution in contrast with the solvent control. At 4.0 mmol/L caffeine, high-K+ solution-induced tone of the RCC was reduced by 73.4 +/- 7.3%. Caffeine (0.5-4.0 mmol/L) also concentration-dependently relaxed NA (12.5 micro mol/L)-induced tonic contraction of the RCC. At 4.0 mmol/L caffeine, NA-induced tone of the RCC was reduced by 41.1 +/- 7.0%. Incubation of RCC in 2.0 mmol/L caffeine for 30 min prior to EFS (1-40 Hz) caused a marked rightward shift in the frequency-response curve. 4. The results of the present study suggest that caffeine exhibits relaxant activity on rabbit cavernosal smooth muscle and the mechanism of this activity possibly involves inhibition of Ca2+ signalling.

All three ryanodine receptor isoforms generate rapid cooling responses in muscle cells

The rapid cooling (RC) response in muscle is an increase in cytoplasmic Ca2+ concentration ([Ca2+]i) that is probably caused by Ca2+ release from the sarcoplasmic reticulum (SR). However, the molecular bases of this response have not been completely elucidated. Three different isoforms of the SR Ca2+ release channels, or ryanodine receptors (RyRs), have been isolated (RyR1, RyR2, and RyR3). In the current investigation, the RC response was studied in RyR-null muscle cells (1B5) before and after transduction with HSV-1 virions containing the cDNAs encoding for RyR1, RyR2, or RyR3. Cells were loaded with fluo 4-AM to monitor changes in [Ca2+]i and perfused with either cold ( approximately 0 degrees C), room temperature (RT), or RT buffer containing 40 mM caffeine. Control cells showed no significant response to cold or caffeine, whereas robust Ca2+ transients were recorded in response to both RC and caffeine in transduced cells expressing any one of the three RyR isoforms. Our data demonstrate directly that RyRs are responsible for the RC response and that all three isoforms respond in a similar manner. Ca2+ release from RyRs is likely caused by a RC-induced conformational change of the channel from the closed to the open state.

Abnormal junctional membrane structures in cardiac myocytes expressing ectopic junctophilin type 1

Recent studies indicate that junctophilin (JP) subtypes contribute to the formation of the junctional membrane complexes between the plasma membrane and the endoplasmic/sarcoplasmic reticulum (ER/SR) in excitable cells. Cardiac muscle contains the diad, in which the transverse (T) tubule of the invaginated cell membrane is closely associated with the SR membrane, and skeletal muscle bears the triad, in which the T-tubule is associated with two SR membranes on the both sides. Among defined JP subtypes, JP-2 is specifically expressed in cardiac muscle, while skeletal muscle cells contain both JP-1 and JP-2. These observations, together with other findings, suggest that the triad might be constructed in a JP-1-dependent manner after the achievement of JP-2-mediated diad formation during skeletal muscle maturation. In this study using transgenic mice, we examined whether the triad can be formed when JP-1 is additionally expressed in cardiac muscle. Immunochemical analysis demonstrated co-expression of JP-1 and JP-2 in cardiac myocytes from the transgenic mice. In cardiac muscle expressing JP-1, abnormal junctional membranes were frequently observed under the electron microscope, in which the T-tubules were rolled up with the SR membranes at several turns, but authentic triad formation could not be detected. Therefore, ectopic JP-1 expression cannot convert the diad to the triad in cardiac myocytes. The present results suggest that triad formation requires an as yet unknown skeletal muscle-specific mechanism, in addition to the JP subtypes.

Mastoparan binds to glycogen phosphorylase to regulate sarcoplasmic reticular Ca2+ release in skeletal muscle

The ryanodine receptor, a Ca(2+)-releasing channel in sarcoplasmic reticulum (SR), plays an important role in the excitation-contraction coupling of skeletal muscle. In a previous study [Hirata, Nakahata and Ohizumi (2000) Mol. Pharmacol. 57, 1235-1242], we reported that mastoparan caused Ca(2+) release through ryanodine receptor from the heavy fraction of SR (HSR) isolated from rabbit skeletal muscle, and that it specifically bound to a 97 kDa protein which was distinct from Ca(2+)-pump or triadin. The present study was undertaken to identify and characterize the 97 kDa mastoparan-binding protein. The 97 kDa protein was purified from solubilized HSR by DEAE-Sepharose column chromatography and preparative SDS/PAGE. The partial amino acid sequence of the purified 97 kDa protein was matched with that of glycogen phosphorylase (GP). The proteolytic cleavage pattern of the 97 kDa protein was identical with that of GP. Furthermore, [(125)I-Tyr(3)]mastoparan specifically bound to GP. Interestingly, mastoparan-induced Ca(2+) release was inhibited by exogenous addition of GP-a, and mastoparan dissociated GP from HSR. These results indicate that the 97 kDa mastoparan-binding protein is GP, which negatively regulates Ca(2+) release from HSR. There may be a functional cross-talk between Ca(2+) release from HSR and glycogenolysis for energy supply mediated through GP in skeletal muscles.

Reduced effect of caffeine on twitch contraction of oesophageal striated muscle from stroke-prone spontaneously hypertensive rats

1. There are known differences in the sensitivity to caffeine between skeletal muscle (soleus) of normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). The present study was performed in order to examine differences in the effects of caffeine on twitch contraction between visceral striated muscle using the outer layer of the oesophagus from WKY rats and stroke-prone SHR (SHRSP). 2. Caffeine, at concentrations ranging from 0.3 to 10 mmol/L, exhibited potentiating effects on twitch contraction in preparations from both WKY rats and SHRSP. The potentiating effect of caffeine was markedly less prominent in preparations from SHRSP compared with preparations from WKY rats. 3. The rate of contraction and relaxation, the time to peak tension and 80% relaxation time were not significantly altered by caffeine at concentrations lower than 3 mmol/L in preparations from either strain. 4. With 10 mmol/L caffeine, the rate of relaxation was markedly reduced and the 80% relaxation time was prolonged, with no significant changes in the rate of contraction, in preparations from WKY rats. These changes were significantly smaller in preparations from SHRSP. 5. The duration of the action potential was greater in preparations from SHRSP than in preparations from WKY rats, although the membrane potential and the amplitude of the action potential were not significantly different between preparations from WKY rats and SHRSP. 6. Caffeine, at 10 mmol/L, prolonged the duration of the action potential in preparations from both strains. The effect of caffeine was not different between preparations from WKY rats and SHRSP. 7. The results of the present study suggest that caffeine augments release of Ca2+ from the sarcoplasmic reticulum (SR) at low concentrations and attenuates Ca2+ re-uptake at 10 mmol/L. Decreased reactivity of SR to caffeine may be a cause of the lesser potentiation of twitch contraction by caffeine in preparations from SHRSP.

Caffeine-induced contracture in oesophageal striated muscle of normotensive and hypertensive rats

To elucidate whether properties of the sarcoplasmic reticulum are altered, not only in vascular smooth muscle, but also in visceral striated muscle of spontaneously hypertensive rats (SHR), caffeine-induced contractures in oesophageal striated muscle of Wistar Kyoto rats (WKY) and stroke-prone SHR (SHRSP) were compared. In both preparations, 30 mM caffeine induced a contracture with two components. The second component, which was diminished by extracellular Ca(2+) removal or Ni(2+) but not by verapamil, was much smaller in SHRSP. Both components and differences between WKY and SHRSP coincided with changes in intracellular Ca(2+). Although membrane potential was identical between these preparations, caffeine induced slight depolarization only in WKY preparations. Similar depolarization was observed with 10 mM K(+), which induced no contraction. It is suggested that the first and the second components of caffeine-induced contracture were induced by Ca(2+) released from sarcoplasmic reticulum and by Ca(2+) that entered through channels activated by sarcoplasmic reticulum Ca(2+) depletion, respectively. In SHRSP preparations, Ca(2+) from the latter pathway was clearly decreased, although this change is thought not to be related to the initiation of hypertension. These results suggest that Ca(2+) handling properties of cell membrane and sarcoplasmic reticulum are generally altered in muscles of SHRSP.

Human phospholamban null results in lethal dilated cardiomyopathy revealing a critical difference between mouse and human

In human disease and experimental animal models, depressed Ca(2+) handling in failing cardiomyocytes is widely attributed to impaired sarcoplasmic reticulum (SR) function. In mice, disruption of the PLN gene encoding phospholamban (PLN) or expression of dominant-negative PLN mutants enhances SR and cardiac function, but effects of PLN mutations in humans are unknown. Here, a T116G point mutation, substituting a termination codon for Leu-39 (L39stop), was identified in two families with hereditary heart failure. The heterozygous individuals exhibited hypertrophy without diminished contractile performance. Strikingly, both individuals homozygous for L39stop developed dilated cardiomyopathy and heart failure, requiring cardiac transplantation at ages 16 and 27. An over 50% reduction in PLN mRNA and no detectable PLN protein were noted in one explanted heart. The expression of recombinant PLN-L39stop in human embryonic kidney (HEK) 293 cells and adult rat cardiomyocytes showed no PLN inhibition of SR Ca(2+)-ATPase and the virtual absence of stable PLN expression; where PLN was expressed, it was misrouted to the cytosol or plasma membrane. These findings describe a naturally-occurring loss-of-function human PLN mutation (PLN null). In contrast to reported benefits of PLN ablation in mouse heart failure, humans lacking PLN develop lethal dilated cardiomyopathy.

RyR1/SERCA1 cross-talk regulation of calcium transport in heavy sarcoplasmic reticulum vesicles

We investigated the functional interdependence of sarco-endoplasmic reticulum Ca2+ ATPase isoform 1 and ryanodine receptor isoform 1 in heavy sarcoplasmic reticulum membranes by synchronous fluorescence determination of extravesicular Ca2+ transients and catalytic activity. Under conditions of dynamic Ca2+ exchange ATPase catalytic activity was well coordinated to ryanodine receptor activation/inactivation states. Ryanodine-induced activation of Ca2+ release channel leaks also produced marked ATPase activation in the absence of measurable increases in bulk free extravesicular Ca2+. This suggested that Ca2+ pumps are highly sensitive to Ca2+ release channel leak status and potently buffer Ca2+ ions exiting cytoplasmic openings of ryanodine receptors. Conversely, ryanodine receptor activation was dependent on Ca2+-ATPase pump activity. Ryanodine receptor activation by cytosolic Ca2+ was (i) inversely proportional to luminal Ca2+ load and (ii) dependent upon the rate of presentation of cytosolic Ca2+. Progressive Ca2+ filling coincided with progressive loss of Ca2+ sequestration rates and at a threshold loading, ryanodine-induced Ca2+ release produced small transient reversals of catalytic activity. These data indicate that attainment of threshold luminal Ca2+ loads coordinates sensitization of Ca2+ release channels with autogenic inhibition of Ca2+ pumping. This suggests that Ca2+-dependent control of Ca2+ release in intact heavy sarcoplasmic reticulum membranes involves a Ca2+-mediated "cross-talk" between sarco-endoplasmic reticulum Ca2+ ATPase isoform 1 and ryanodine receptor isoform 1.

Ca2+-Releasing Effect of Cerivastatin on the Sarcoplasmic Reticulum of Mouse and Rat Skeletal Muscle Fibers

We analyzed the effect of HMG-CoA reductase inhibitors on Ca(2+) release from the sarcoplasmic reticulum (SR) using chemically skinned skeletal muscle fibers from the mouse and the rat. Cerivastatin (>20 microM) released Ca(2+) from the SR, while pravastatin showed only a little effect. The rates of Ca(2+) release were increased by cerivastatin at all Ca(2+) concentrations tested. Cerivastatin-induced Ca(2+) release in the presence of Ca(2+) was affected by adenosine monophosphate, Mg(2+), and procaine in essentially the same way as for caffeine-induced Ca(2+) release. The Ca(2+)-uptake capacity of the SR was reduced after co-treatment with ryanodine and cerivastatin at pCa 6.0 to a much greater extent than with ryanodine alone. Thus, cerivastatin-induced Ca(2+) release in the presence of Ca(2+) must be a result of the activation of the Ca(2+)-induced Ca(2+) release (CICR) mechanism of the ryanodine receptor. However, even when CICR was maximally inhibited by Mg(2+) and procaine, or in the practical absence of Ca(2+) (pCa >8), cerivastatin still caused Ca(2+) release. These results indicate that cerivastatin causes Ca(2+) release also by activating some other mechanism(s) in addition to the activation of CICR. Either or both of these effects might be related to its adverse effect, rhabdomyolysis.

Molecular dissection, tissue localization and Ca2+ binding of the ryanodine receptor of Caenorhabditis elegans

The ryanodine receptor of Caenorhabditis elegans (CeRyR) which contains 5,071 amino acid residues, is encoded by a single gene, ryr-1/unc-68. The unc-68(kh30) mutation, isolated in an animal showing abnormal response to the anesthetic ketamine, has the substitution Ser1444Asn in CeRyR, predicted to be a phosphorylation site. To elucidate the function of the region of CeRyR, and to determine the localization of CeRyR in this animal, ten region-peptides were produced in Escherichia coli by using expression plasmids and eight antisera were raised against these fusion peptides. One antibody against the region corresponding to the kh30 mutation site enabled detection of CeRyR from mutant animals both in Western analysis and in situ. Specificity of this antiserum was demonstrated using Western analysis, which showed the full size and the partial size bands in wild-type and in the Tc1-induced deletion mutant animals, respectively, but no corresponding bands in unc-68 null mutant animals. CeRyR was detected in I-bands of muscle sarcomeres by double immunostaining. CeRyR was found in the body wall, pharyngeal, vulval, anal and sex muscles of adult worms and also found to be present in embryonic muscle, but not in non-muscle cells. Two EF-hand motifs and the C terminus were demonstrated to be Ca(2+) binding regions. On the basis of these results, we propose a model for the functional domains of CeRyR, which agrees well with the model of mammalian skeletal RyR, which is based on proteolysis and cross-linking analysis. We discuss the usefulness and limitations of the molecular dissection approach, which uses peptides and peptide-specific antibodies to determine the local structure and function of individual domains within a large molecule.

The endoplasmic reticulum and neuronal calcium signalling

The endoplasmic reticulum (ER) is a multifunctional signalling organelle regulating a wide range of neuronal functional responses. The ER is intimately involved in intracellular Ca(2+) signalling, producing local or global cytosolic calcium fluctuations via Ca(2+)-induced Ca(2+) release (CICR) or inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The CICR and IICR are controlled by two subsets of Ca(2+) release channels residing in the ER membrane, the Ca(2+)-gated Ca(2+) release channels, generally known as ryanodine receptors (RyRs) and InsP(3)-gated Ca(2+) release channels, referred to as InsP(3)-receptors (InsP(3)Rs). Both types of Ca(2+) release channels are expressed abundantly in nerve cells and their activation triggers cytoplasmic Ca(2+) signals important for synaptic transmission and plasticity. The RyRs and InsP(3)Rs show heterogeneous localisation in distinct cellular sub-compartments, conferring thus specificity in local Ca(2+) signals. At the same time, the ER Ca(2+) store emerges as a single interconnected pool fenced by the endomembrane. The continuity of the ER Ca(2+) store could play an important role in various aspects of neuronal signalling. For example, Ca(2+) ions may diffuse within the ER lumen with comparative ease, endowing this organelle with the capacity for "Ca(2+) tunnelling". Thus, continuous intra-ER Ca(2+) highways may be very important for the rapid replenishment of parts of the pool subjected to excessive stimulation (e.g. in small compartments within dendritic spines), the facilitated removal of localised Ca(2+) loads, and finally in conveying Ca(2+) signals from the site of entry towards the cell interior and nucleus.

Ryanodine receptor calcium release channels

The ryanodine receptors (RyRs) are a family of Ca2+ release channels found on intracellular Ca2+ storage/release organelles. The RyR channels are ubiquitously expressed in many types of cells and participate in a variety of important Ca2+ signaling phenomena (neurotransmission, secretion, etc.). In striated muscle, the RyR channels represent the primary pathway for Ca2+ release during the excitation-contraction coupling process. In general, the signals that activate the RyR channels are known (e.g., sarcolemmal Ca2+ influx or depolarization), but the specific mechanisms involved are still being debated. The signals that modulate and/or turn off the RyR channels remain ambiguous and the mechanisms involved unclear. Over the last decade, studies of RyR-mediated Ca2+ release have taken many forms and have steadily advanced our knowledge. This robust field, however, is not without controversial ideas and contradictory results. Controversies surrounding the complex Ca2+ regulation of single RyR channels receive particular attention here. In addition, a large body of information is synthesized into a focused perspective of single RyR channel function. The present status of the single RyR channel field and its likely future directions are also discussed.

Roles of two ryanodine receptor isoforms coexisting in skeletal muscle

Ryanodine receptor (RyR) is a Ca(2)(+) release channel in the sarcoplasmic reticulum and plays an important role in excitation-contraction coupling in skeletal muscle. The Ca(2)(+) release through the RyR channel can be gated by two distinct modes: depolarization-induced Ca(2)(+) release (DICR) and Ca(2)(+)-induced Ca(2)(+) release (CICR). Two different RyR isoforms, RyR1 (or alpha-RyR) and RyR3 (or beta-RyR), have been found to be expressed in skeletal muscle. Most adult mammalian muscles express primarily RyR1, whereas almost equal amounts of the two RyR isoforms exist in many nonmammalian vertebrate muscles. RyR1 is believed to be responsible for both DICR and CICR, whereas RyR3 may function as the CICR channel. Recent findings demonstrate that alpha-RyR is selectively and markedly suppressed in CICR activity in frog skeletal muscle. This selective suppression of RyR1, although to a lesser extent, also was found to occur in mammalian skeletal muscle. This short review describes the biological meanings of this selective suppression and discusses physiological roles and significance of the two RyR isoforms in vertebrate skeletal muscle.

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.

Effects of quercetin on single Ca(2+) release channel behavior of skeletal muscle

Quercetin, a bioflavonoid, is known to affect Ca(2+) fluxes in sarcoplasmic reticulum, although its direct effect on Ca(2+) release channel (CRC) in sarcoplasmic reticulum has remained to be elucidated. The present study examined the effect of quercetin on the behavior of single skeletal CRC in planar lipid bilayer. The effect of caffeine was also studied for comparison. At very low [Ca(2+)](cis) (80 pM), quercetin activated CRC marginally, whereas at elevated [Ca(2+)](cis) (10 microM), both open probability (P(o)) and sensitivity to the drug increased markedly. Caffeine showed a similar tendency. Analysis of lifetimes for single CRC showed that quercetin and caffeine led to different mean open-time and closed-time constants and their proportions. Addition of 10 microM ryanodine to CRC activated by quercetin or caffeine led to the typical subconductance state (approximately 54%) and a subsequent addition of 5 microM ruthenium red completely blocked CRC activity. When 6 microM quercetin and 3 mM caffeine were added together to the cis side of CRC, a time-dependent increase of P(o) was observed (from mode 1 (0.376 +/- 0.043, n = 5) to mode 2 (0.854 +/- 0.062, n = 5)). On the other hand, no further activation was observed when quercetin was added after caffeine. Quercetin affected only the ascending phase of the bell-shaped Ca(2+) activation/inactivation curve, whereas caffeine affected both ascending and descending phases. [(3)H]ryanodine binding to sarcoplasmic reticulum showed that channel activity increased more by both quercetin and caffeine than by caffeine alone. These characteristic differences in the modes of activation of CRC by quercetin and caffeine suggest that the channel activation mechanisms and presumably the binding sites on CRC are different for the two drugs.

Mercuric chloride alters the membrane potential and intracellular calcium level in mouse pancreatic islet cells

In this study, mercuric chloride was applied to the primary cultures of mouse pancreatic islet cells for studying its effects on resting membrane potential and the intracellular free calcium ion concentration ([Ca 2+), using the techniques of electrophysiology and fluorometry. It was observed that mercuric chloride (1-100 microM) caused a rapid and sustained depolarization, and induced a rapid first phase and a large sustained second phase of elevation in fura-2 fluorescence ratio in islet cells. The depolarization and increased lCa2+]i induced by mercuric chloride could be inhibited by dithiothreitol (a sulfhydryl-containing reducing agent). Removing Ca2+ from the external medium inhibited the mercuric chloride-induced elevation of [Ca2+]i. The increased [Ca2+]i may also originate from the endoplasmic reticulum of pancreatic islet cells, since caffeine (an activator of Ca2+ release from endoplasmic reticulum) and thapsigargin (an inhibitor of endoplasmic reticulum Ca2+-ATPase) could antagonize the effect of mercuric chloride. Moreover, in the absence of glucose in the medium, the response of islet cells to mercuric chloride was a rapid first phase of increased [Ca2+]i followed by a small sustained second phase. Readministration of 5 mM glucose was sufficient but transient to restore sustained phase of increased [Ca2+]i. The increase of [Ca2+]i in islet cells induced by a lower concentration of mercuric chloride (5 microM) was potentiated in higher glucose (7.5 mM) medium. Tolbutamide, an inhibitor of the ATP-sensitive K+-channel, could also inhibit the effect of mercuric chloride. These findings suggest that mercuric chloride initially interacts with the sulfhydryl groups of membrane-bound proteins, which may be an ATP-sensitive K+ channel, to cause depolarization of the islet cells. This depolarization triggers Ca2+ influx and then the release of Ca2+ from the endoplasmic reticulum.

Cyclic ADPR and calcium signaling in sea bream (Sparus aurata) egg fertilization

The cell egg is in a state of quiescence and only after its fusion with the sperm, a series of pre-programmed metabolic processes will be activated, culminating with embryonic development. The egg/sperm fusion induces a transitory increase of Ca(2+) in the cytoplasm, which is responsible for the activation of both precocious and late reactions. The release of Ca(2+) occurs by stimulation of the ionic specific channels. In addition to IP(3), a new Ca-release inducer was recently evidenced, cyclic ADP ribose (cADPR), in some invertebrates and mammals. Here, we report the first evidence of the cADPR presence in fish. Our data also demonstrate that in the sea bream egg, cADPR is involved in the fertilization process; in fact, its level increases after the entrance of the sperm. By in vitro experiments, it was shown that cADPR induces a release of Ca(2+) in the egg homogenate, indicating that in sea bream, the increase of cADPR can induce an intracellular Ca(2+) release. Since cADPR is a product of NAD(+) metabolism, the activity of several enzymes involved in the NAD(+) metabolism was investigated. Sea bream eggs are pelagic and only floating eggs after insemination develop into viable embryos. In the present work, NAD(+) metabolism was studied in both types of egg. All the tested enzymes showed similar specific activity in both floating and sinking eggs. In the latter, cADPR was not detectable and the nucleotides content was significantly lower, evidencing a scarce energetic charge in sinking eggs.

Apocalmodulin and Ca2+-calmodulin bind to neighboring locations on the ryanodine receptor

Calmodulin (CaM) binds to the ryanodine receptor/calcium release channel of skeletal muscle (RyR1), both in the absence and presence of Ca(2+), and regulates the activity of the channel activity by activating and inhibiting it, respectively. Using cryo-electron microscopy and three-dimensional reconstruction, we found that one apoCaM binds per RyR1 subunit along the sides of the cytoplasmic assembly of the receptor. This location is distinct from but close to the location found for Ca(2+)-CaM, providing a structural basis for efficient switching of CaM between these two positions with the oscillating intracellular Ca(2+) concentration that generates muscle relaxation/contraction cycles. The locations of apoCaM and Ca(2+)-CaM at a critical region for RYR1-dihydropyridine receptor interaction are suggestive of a direct role for CaM in the mechanism of excitation-contraction coupling.

Effect of taurine on sarcoplasmic reticulum function and force in skinned fast-twitch skeletal muscle fibres of the rat

We examined the effect of taurine on depolarisation-induced force responses and sarcoplasmic reticulum (SR) function in mechanically skinned skeletal muscle fibres from the extensor digitorum longus (EDL) of the rat. Taurine (20 mM) produced a small but significant (P < 0.01) decrease in the sensitivity of the contractile apparatus to Ca(2+) (increase in the [Ca(2+)] corresponding to 50 % of maximum force of about 7 %; n = 10) and in maximum force (92.0 +/- 1.0 % of controls) in the skinned fibres. Taurine had no statistically significant effect on the slope of the force-pCa curve. Depolarisation-induced force responses in the skinned fibres were markedly increased in peak value by 20 mM taurine, to 120.8 +/- 5.3 % of control measurements (P = 0.0006, n = 27). Taurine (20 mM) significantly increased the SR Ca(2+) accumulation in the skinned fibres by 34.6 +/- 9.3 % compared to control conditions (measured by comparing the integral of caffeine contractures in fibres previously loaded with Ca(2+) in the absence or presence of taurine; P = 0.0014, n = 10). Taurine (20 mM) also increased both the peak and rate of rise of caffeine-induced force responses in the fibres by 29.2 +/- 9.7 % (P = 0.0298, n = 6) and 27.6 +/- 8.9 % (P = 0.037), respectively, compared with controls. This study shows that taurine is a modulator of contractile function in mammalian skeletal muscle. Taurine may increase the size of depolarisation-induced force responses by augmenting SR Ca(2+) accumulation and release.

Sarcoplasmic reticulum Ca(2+) content affects 4-CmC and caffeine contractures of rat skinned skeletal muscle fibers

This study investigated whether the sarcoplasmic reticulum Ca(2+) content of rat skeletal muscle fibers affected contractile responses obtained by an application of 4-chloro-m-cresol (4-CmC) and caffeine. Contractures were elicited on saponin-skinned fibers under different Ca(2+) loading conditions. The amplitude of 4-CmC and caffeine contractures of fast-twitch muscle fibers (edl, extensor digitorum longus) differed between the different loading conditions, and this is associated with a greater change in sensitivity to 4-CmC. When the sarcoplasmic reticulum was loaded with a low Ca(2+) concentration for a short period, the 4-CmC concentration providing half-maximal response was tenfold higher than with a larger sarcoplasmic reticulum Ca(2+) loading for a longer period, whereas for caffeine this concentration was only twofold higher in the same conditions. These findings indicate that 4-CmC contractile responses of edl muscle fibers are more dependent on luminal Ca(2+) activity than those of caffeine are. Thus 4-CmC would appear to be of greater interest than caffeine for the study of muscle contractile responses where variations in intracellular Ca(2+) activity exist.

Calcium currents from jellyfish striated muscle cells: preservation of phenotype, characterisation of currents and channel localisation

When striated muscle cells of the jellyfish Polyorchis penicillatus were dissociated at 30°C they retained their in vivo morphology and the integrity of ionic currents. This contrasted with cells dissociated at room temperature that rarely expressed any inward currents. Whole-cell, patch-clamp recordings from dissociated muscle cells revealed that the inward component of the total ionic current consisted of only one calcium current. This calcium current activated at –70 mV, peaked at –30 mV, and inactivated within 5 ms. In comparison with barium and strontium ions, calcium ions were the preferred current carriers. Calcium channels can be blocked by dihydropyridines and nickel ions at micromolar levels. Several properties of this current are reminiscent of T-type calcium currents. Localisation of this channel using the fluorescent channel blocker fDHP and the fluorescent dye RH414 indicated that myofibres had a higher density of these channels than the somata.

Properties of Ca(2+) release induced by clofibric acid from the sarcoplasmic reticulum of mouse skeletal muscle fibres

1. To characterize the effect of clofibric acid (Clof) on the Ca(2+) release mechanism in the sarcoplasmic reticulum (SR) of skeletal muscle, we analysed the properties of Clof-induced Ca(2+) release under various conditions using chemically skinned skeletal muscle fibres of the mouse. 2. Clof (>0.5 mM) released Ca(2+) from the SR under Ca(2+)-free conditions buffered with 10 mM EGTA (pCa >8). 3. Co-application of ryanodine and Clof at pCa >8 but not ryanodine alone reduced the Ca(2+) uptake capacity of the SR. Thus, Ca(2+) release induced by Clof at pCa >8 must be a result of the activation of the ryanodine receptor (RyR). 4. At pCa >8, (i) Clof-induced Ca(2+) release was inhibited by adenosine monophosphate (AMP), (ii) the inhibitory effect of Mg(2+) on the Clof-induced Ca(2+) release was saturated at about 1 mM, and (iii) Clof-induced Ca(2+) release was not inhibited by procaine (10 mM). These results indicate that Clof may activate the RyR-Ca(2+) release channels in a manner different from Ca(2+)-induced Ca(2+) release (CICR). 5. In addition to this unique mode of opening, Clof also enhanced the CICR mode of opening of RyR-Ca(2+) release channels. 6. Apart from CICR, a high concentration of Ca(2+) might also enhance the unique mode of opening by Clof. 7. These results suggest that some features of Ca(2+) release activated by Clof are similar to those of physiological Ca(2+) release (PCR) in living muscle cells and raise the possibility that Clof may be useful in elucidating the mechanism of PCR in skeletal muscle.

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.

Invited review: mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca(2+) regulates the contractile state of smooth muscle cells and tissues. Elevations in global cytoplasmic Ca(2+) resulting in contraction are accomplished by Ca(2+) entry and release from intracellular stores. Pathways for Ca(2+) entry include dihydropyridine-sensitive and -insensitive Ca(2+) channels and receptor and store-operated nonselective channels permeable to Ca(2+). Intracellular release from the sarcoplasmic reticulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors. The impact of Ca(2+) entry and release on cytoplasmic concentration is modulated by Ca(2+) reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca(2+) transients (i.e., sparks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca(2+) entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca(2+) handling in smooth muscles.

Local Ca(2+) transients and distribution of BK channels and ryanodine receptors in smooth muscle cells of guinea-pig vas deferens and urinary bladder

1. The relationship between Ca(2+) sparks spontaneously occurring at rest and local Ca(2+) transients elicited by depolarization was analysed using two-dimensional confocal Ca(2+) images of single smooth muscle cells isolated from guinea-pig vas deferens and urinary bladder. The current activation by these Ca(2+) events was also recorded simultaneously under whole-cell voltage clamp. 2. Spontaneous transient outward currents (STOCs) and Ca(2+) sparks were simultaneously detected at -40 mV in approximately 50 % of myocytes of either type. Ca(2+) sparks and corresponding STOCs occurred repetitively in several discrete sites in the subplasmalemmal area. Large conductance Ca(2+)-dependent K(+) (BK) channel density in the plasmalemma near the Ca(2+) spark sites generating STOCs was calculated to be 21 channels microm(-2). 3. When myocytes were depolarized from -60 to 0 mV, several local Ca(2+) transients were elicited within 20 ms in exactly the same peripheral sites where sparks occurred at rest. The local Ca(2+) transients often lasted over 300 ms and spread into other areas. The appearance of local Ca(2+) transients occurred synchronously with the activation of Ca(2+)-dependent K(+) current (I(K,Ca)). 4. Immunofluorescence staining of the BK channel alpha-subunit (BKalpha) revealed a spot-like pattern on the plasmalemma, in contrast to the uniform staining of voltage-dependent Ca(2+) channel alpha1C subunits along the plasmalemma. Ryanodine receptor (RyR) immunostaining also suggested punctate localization predominantly in the periphery. Double staining of BKalpha and RyRs revealed spot-like co-localization on/beneath the plasmalemma. 5. Using pipettes of relatively low resistance, inside-out patches that included both clustered BK channels at a density of over 20 channels microm(-2) and functional Ca(2+) storage sites were obtained at a low probability of approximately 5%. The averaged BK channel density was 3-4 channels microm(-2) in both types of myocyte. 6. These results support the idea that a limited number of discrete sarcoplasmic reticulum (SR) fragments in the subplasmalemmal area play key roles in the control of BK channel activity in two ways: (i) by generating Ca(2+) sparks at rest to activate STOCs and (ii) by generating Ca(2+) transients presumably triggered by sparks during an action potential to activate a large I(K,Ca) and also induce a contraction. BK channels and RyRs may co-localize densely at the junctional areas of plasmalemma and SR fragments, where Ca(2+) sparks occur to elicit STOCs.

Effects of membrane cholesterol manipulation on excitation-contraction coupling in skeletal muscle of the toad

1. Single mechanically skinned fibres and intact bundles of fibres from the twitch region of the iliofibularis muscle of cane toads were used to investigate the effects of membrane cholesterol manipulation on excitation-contraction (E-C) coupling. The cholesterol content of membranes was manipulated with methyl-beta-cyclodextrin (MbetaCD). 2. In mechanically skinned fibres, depletion of membrane cholesterol with MbetaCD caused a dose- and time-dependent decrease in transverse tubular (t)-system depolarization-induced force responses (TSDIFRs). TSDIFRs were completely abolished within 2 min in the presence of 10 mM MbetaCD but were not affected after 2 min in the presence of a 10 mM MbetaCD-1 mM cholesterol complex. There was a very steep dependence between the change in TSDIFRs and the MbetaCD : cholesterol ratio at 10 mM MbetaCD, indicating that the inhibitory effect of MbetaCD was due to membrane cholesterol depletion and not to a pharmacological effect of the agent. Tetanic responses in bundles of intact fibres were abolished after 3-4 h in the presence of 10 mM MbetaCD. 3. The duration of TSDIFRs increased markedly soon (< 2 min) after application of 10 mM MbetaCD and 10 mM MbetaCD-cholesterol complexes, but the Ca(2+) activation properties of the contractile apparatus were minimally affected by 10 mM MbetaCD. The Ca(2+) handling abilities of the sarcoplasmic reticulum appeared to be modified after 10 min exposure to 10 mM MbetaCD. 4. Confocal laser scanning microscopy revealed that the integrity of the t-system was not compromised by either intra- or extracellular application of 10 mM MbetaCD and that a large [Ca(2+)] gradient was maintained across the t-system. 5. Membrane cholesterol depletion caused rapid depolarization of the polarized t-system as shown independently by spontaneous TSDIFRs induced by MbetaCD and by changes in the fluorescence intensity of an anionic potentiometric dye (DiBAC(4)(3)) in the presence of MbetaCD. This rapid depolarization of the t-system by cholesterol depletion was not prevented by blocking the Na(+) channels with TTX (10 microM) or the L-type Ca(2+) channels with Co(2+) (5 mM). 6. The results demonstrate that cholesterol is important for maintaining the functional integrity of the t-system and sarcoplasmic reticulum, probably by having specific effects on different membrane proteins that may be directly or indirectly involved in E-C coupling.

Molecular basis of Ca(2)+ activation of the mouse cardiac Ca(2)+ release channel (ryanodine receptor)

Activation of the cardiac ryanodine receptor (RyR2) by Ca(2)+ is an essential step in excitation-contraction coupling in heart muscle. However, little is known about the molecular basis of activation of RyR2 by Ca(2)+. In this study, we investigated the role in Ca(2)+ sensing of the conserved glutamate 3987 located in the predicted transmembrane segment M2 of the mouse RyR2. Single point mutation of this conserved glutamate to alanine (E3987A) reduced markedly the sensitivity of the channel to activation by Ca(2)+, as measured by using single-channel recordings in planar lipid bilayers and by [(3)H]ryanodine binding assay. However, this mutation did not alter the affinity of [(3)H]ryanodine binding and the single-channel conductance. In addition, the E3987A mutant channel was activated by caffeine and ATP, was inhibited by Mg(2)+, and was modified by ryanodine in a fashion similar to that of the wild-type channel. Coexpression of the wild-type and mutant E3987A RyR2 proteins in HEK293 cells produced individual single channels with intermediate sensitivities to activating Ca(2)+. These results are consistent with the view that glutamate 3987 is a major determinant of Ca(2)+ sensitivity to activation of the mouse RyR2 channel, and that Ca(2)+ sensing by RyR2 involves the cooperative action between ryanodine receptor monomers. The results of this study also provide initial insights into the structural and functional properties of the mouse RyR2, which should be useful for studying RyR2 function and regulation in genetically modified mouse models.

Morphological and physiological restorations of hereditary form of dilated cardiomyopathy by somatic gene therapy

TO-2 strain hamsters with dilated cardiomyopathy, gene deletion of delta-sarcoglycan (SG) and no expression of alpha-, beta-, gamma-, and delta-SG proteins are useful for developing the potential gene therapy of intractable heart failure. We prepared recombinant adeno-associated virus vector including normal delta-SG gene driven by CMV promoter and intramurally administered in vivo. The transfected myocardium induced robust expression of both transcript and transgene for 2/3 period of the animal's life expectancy. Immunostaining demonstrated reexpression of not only delta-SG but also other three SGs in 40% cells in the transfected region and normalization of the diameter of transduced cardiomyocytes. Hemodynamic study revealed preferential amelioration of the diastolic indices (LVEDP, the dP/dt(min) and CVP). These results provide the first evidence that supplementation of a specific gene with efficient and sustained transfection capability restores the genetic, morphological, and functional deteriorations.

Cyclic ADP-ribose as a second messenger revisited from a new aspect of signal transduction from receptors to ADP-ribosyl cyclase

Cyclic ADP-ribose (cADPR), an endogenous modulator of ryanodine receptor Ca(2+)-releasing channels, is found in various tissues. Cytosolic injection of cADPR induces an elevation of intracellular Ca(2+) concentrations or potentiates Ca(2+) increases. cADPR facilitates neurotransmitter or insulin release and modifies ionic currents. cADPR is synthesized by ADP-ribosyl cyclase and is metabolized by cADPR hydrolase. ADP-ribosyl cyclase activity is up-regulated by nitric oxide/cyclic GMP-dependent phosphorylation or receptor stimulation via G-proteins within membranes. These findings suggest that cADPR is a second messenger in cellular Ca(2+) signaling. However, many intriguing issues remain to be addressed before this identity is confirmed.

Postsynaptic Modulation of AMPA Receptor-Mediated Synaptic Responses and LTP by the Type 3 Ryanodine Receptor

The precise function of ryanodine receptors (RyRs) in synaptic transmission is unknown, but three of their subtypes are expressed in the brain. We examined the roleof RyRs in excitatory synaptic transmission in hippocampal slices, using type 3 RyR (RyR3)-deficient mice. The alpha-amino-3-hydroxy-5-methyl-4-isoxozolepropionic acid (AMPA) receptor-mediated basal synaptic responses in the CA1 region of mutant mice were smaller than those of wild-type mice, while there was no difference in N-methyl-d-aspartate receptor-mediated responses, suggesting selective postsynaptic modification of AMPA receptors by RyR3. The expression of synaptic AMPA receptor subunits examined by Western blotting or immunohistochemistry was indistinguishable, suggesting that the smaller AMPA synaptic responses in mutant mice were not due to the reduced number of synaptic AMPA receptors. Although the initial potentiation following tetanic stimulation of afferent fibers was similar, long-term potentiation (LTP) was smaller in mutant mice. There were no differences in presynaptic electrophysiological properties. We thus conclude that RyR3 postsynaptically regulates the properties of AMPA receptors and LTP.

Ca(2+) signaling in porcine duodenal glands by muscarinic receptor activation

The duodenal glands have been thought to play an important role in defense against proximal duodenal ulcer; however, the secretory mechanisms of these glands remain to be determined. In isolated duodenal acinar cells of the pig, we investigated the effects of ACh on intracellular Ca(2+) concentration ([Ca(2+)](i)) and on membrane currents with fura 2 fluorometry and the patch clamp technique. ACh caused a transient increase in [Ca(2+)](i), and the increase was markedly inhibited by atropine or 4-diphenylacetoxy-N-methylpiperidine methiodide but not by hexamethonium, pirenzepine, or methoctramine. The expression of mRNA for the M(3) subtype far exceeded that for either M(1) or M(2) as revealed by real-time quantitative PCR and in situ hybridization. The rise in [Ca(2+)](i) evoked by ACh was largely inhibited by thapsigargin but slightly affected by extracellular Ca(2+) deprivation. Caffeine had no effect on [Ca(2+)](i). ACh elicited Ca(2+)-dependent K(+) currents, a finding similar to the response to inositol 1,4,5,-trisphosphate applied intracellularly. These results suggest the presence of M(3) receptors linked to Ca(2+) release in porcine duodenal glands.

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.

Caffeine stores and dopamine differentially require Ca(2+) channels in goldfish somatotropes

The regulation of growth hormone (GH) secretion by intracellular Ca(2+) stores was studied in dissociated goldfish somatotropes. We characterized a caffeine-activated intracellular store that had been shown to mediate GH release in response to gonadotropin-releasing hormone. The peak response of caffeine stimulation was reduced by approximately 28% by 100 microM ryanodine in a use-dependent manner suggesting that the first 10 min of GH release is partially mediated by a caffeine-activated ryanodine receptor. The temporal sensitivities of caffeine- and dopamine-evoked GH release to blockade of Cd(2+)-sensitive Ca(2+) channels were compared. We demonstrated that the initial phase of dopamine-evoked release was dependent on Ca(2+) channels, whereas the initial phase of caffeine-evoked release was sensitive only to pretreatment blockade. This would suggest that the maintenance of one class of caffeine-activated intracellular stores requires entry of Ca(2+) through Cd(2+)-sensitive Ca(2+) channels. This differential temporal requirement for Ca(2+) channels in Ca(2+) signaling may be a mechanism to segregate intracellular signaling pathways of multiple neuroendocrine regulators in the teleost pituitary.

The differential effect of propofol on contractility of isolated myocardial trabeculae of rat and guinea-pig

1. The effects of propofol on myocardial contractility were studied in rat, in which the contractile activation mainly depends on calcium derived from the sarcoplasmic reticulum (SR), and guinea-pig, in which transsarcolemmal influx of calcium plays a major role. 2. Intact and chemically skinned trabeculae from the right ventricle were studied. Intact trabeculae were electrically stimulated and force development during steady state and post rest contractions was measured. In saponin skinned trabeculae Ca(2+) uptake and release by the SR was studied. In Triton skinned trabeculae the influence of propofol on calcium sensitivity of the myofilaments was studied. 3. In intact rat trabeculae propofol in concentrations of 28, 112 and 280 microM did not change peak force development nor the pattern of post rest contraction. In guinea-pig trabeculae propofol significantly reduced peak force to respectively 64, 40 and 23% of control values and the post rest contractions were potentiated. In skinned trabeculae propofol did not affect Ca(2+) handling by the SR, nor did it change force production and Ca(2+) sensitivity of the myofilaments. 4. This study shows that, in contrast to rat, in guinea-pig propofol directly depresses myocardial contractility, probably by decreasing transsarcolemmal Ca(2+) influx. There is no significant influence of propofol on Ca(2+) handling by the SR, nor on the contractile proteins.

Signal transduction from bradykinin, angiotensin, adrenergic and muscarinic receptors to effector enzymes, including ADP-ribosyl cyclase

Muscarinic acetylcholine receptors in NG108-15 neuroblastoma x glioma cells, and beta-adrenergic or angiotensin II receptors in cortical astrocytes and/or ventricular myocytes, utilize the direct signaling pathway to ADP-ribosyl cyclase within cell membranes to produce cyclic ADP-ribose (cADPR) from beta-NAD+. This signal cascade is analogous to the previously established transduction pathways from bradykinin receptors to phospholipase Cbeta and beta-adrenoceptors to adenylyl cyclase via G proteins. Upon receptor stimulation, the newly-formed cADPR may coordinately function to upregulate the release of Ca2+ from the type II ryanodine receptors as well as to facilitate Ca2+ influx through voltage-dependent Ca2+ channels. cADPR interacts with FK506, an immunosuppressant, at FKBP12.6, FK506-binding-protein, and calcineurin, or ryanodine receptors. cADPR also functions through activating calcineurin released from A-kinase anchoring protein (AKAP79). Thus, some G(q/11)-coupled receptors can control cADPR-dependent modulation in Ca2+ signaling.

Cyclic ADP-ribose as a potential second messenger for neuronal Ca2+ signaling

Cyclic ADP-ribose (cADPR), a known endogenous modulator of ryanodine receptor Ca2+ releasing channels, is found in the nervous system. Injection of cADPR into neuronal cells primarily induces a transient elevation of intracellular Ca2+ concentration ([Ca2+]i), and/or secondarily potentiates [Ca2+]i increases that are the result of depolarization-induced Ca2+ influx. Acetylcholine release from cholinergic neurons is facilitated by cADPR. cADPR modifies K+ currents or elicits Ca2+-dependent inward currents. cADPR is synthesized by both membrane-bound and cytosolic forms of ADP-ribosyl cyclase in neuronal cells. cADPR hydrolase activity is weak in the membrane fraction, but high in the cytoplasm. Cytosolic ADP-ribosyl cyclase activity is upregulated by nitric oxide/cyclic GMP-dependent phosphorylation. Stimulation of muscarinic and beta-adrenergic receptors activates membrane-bound ADP-ribosyl cyclase via G proteins within membranes of neuronal tumor cells and cortical astrocytes. These findings strongly suggest that cADPR is a second messenger in Ca2+ signaling in the nervous system, although many intriguing issues remain to be addressed before this identity is confirmed.

The asp-rich region at the carboxyl-terminus of calsequestrin binds to Ca(2+) and interacts with triadin

Calsequestrin (CSQ) is a high capacity Ca(2+) binding protein in the junctional sarcoplasmic reticulum of striated muscles, and has been shown to regulate the ryanodine receptor (RyR) through triadin and junctin. In order to identify the functional roles of specific regions on CSQ, several CSQ deletion mutants were prepared by molecular cloning and Escherichia coli expression. 45Ca(2+) overlay assay using a native gel system revealed that the major Ca(2+) binding motif of CSQ resides in the asp-rich region (amino acids 354-367). In an in vitro binding assay using a glutathione-S-transferase affinity column, the interaction between CSQ and triadin was found to be Ca(2+)-dependent, and the site of interaction was confined to the asp-rich region of CSQ. Our results suggest that the asp-rich region of CSQ could participate in the RyR-mediated Ca(2+) release process by offering a direct binding site to luminal Ca(2+) as well as triadin.