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M Inui

Publications and source records attributed to M Inui.

At least 37 records · Page 2Linked to original sources
PubMed1989

Nature and site of phospholamban regulation of the Ca2+ pump of sarcoplasmic reticulum.

The rapid removal of Ca2+ ions from the cytosol, necessary for the efficient relaxation of cardiac muscle cells, is performed by the Ca2+-pumping ATPase of the sarcoplasmic reticulum. The calcium pump is activated by cyclic AMP- and calmodulin-dependent phosphorylation of phospholamban, an integral membrane protein of the sarcoplasmic reticulum. Using a heterobifunctional crosslinking agent which can be cleaved and photoactivated, we provide evidence for a direct interaction between the two proteins. Only the non-phosphorylated form of phospholamban interacts with the ATPase, demonstrating that phospholamban is an endogenous inhibitor that is removed from the ATPase by phosphorylation. Non-phosphorylated phospholamban interacts only with the calcium-free conformation of the ATPase and is released when it is converted to the calcium-bound state. We localized the site of interaction to a single peptide isolated after cyanogen bromide cleavage of the ATPase. The peptide derives from a domain just C-terminal to the aspartyl phosphate of the active site. This domain is unique to ATPases of the sarcoplasmic reticulum in that it has no homology with any other phosphorylation-type ion pump. The domain occurs in both slow- and fast-twitch isoforms of the ATPase, even though phospholamban is not expressed in fast-twitch muscles.

Adenosine Triphosphatases↗
PubMed1989

Three-dimensional architecture of the calcium channel/foot structure of sarcoplasmic reticulum.

The calcium channel responsible for the release of Ca2+ from the sarcoplasmic reticulum of skeletal muscle during excitation-contraction coupling has recently been identified and purified. The isolated calcium channel has been identified morphologically with the 'foot' structures which are associated with the junctional face membrane of the terminal cisternae of sarcoplasmic reticulum. In situ, the foot structure extends across the gap of the triad junction from the terminal cisternae of the reticulum to the transverse tubule. We describe here the three-dimensional architecture (3.7 nm resolution) of the calcium channel/foot structure from fast-twitch rabbit skeletal muscle, which we determined from electron micrographs of isolated, non-crystalline structures that had been tilted in the electron microscope. The reconstruction reveals two different faces and an internal structure in which stain accumulates at several interconnected locations, which could empty into the junctional gap of the triad junction. The detailed architecture of the channel complex is relevant to understanding both the physical path followed by calcium ions during excitation-contraction coupling and the association of the terminal cisternae and the transverse tubules in the triad junction.

Animals↗
PubMed1989

Positive cooperativity of ryanodine binding to the calcium release channel of sarcoplasmic reticulum from heart and skeletal muscle.

Ryanodine is a specific ligand for the calcium release channel which mediates calcium release in excitation-contraction coupling in muscle. In this study, ryanodine binding in sarcoplasmic reticulum from heart muscle and skeletal muscle is further compared and correlated with function. The new findings include the following: (1) Two types of binding, high affinity (KD1 approximately 5-10 nM) and low affinity (KD2 approximately 3 microM), can now be discerned for the skeletal muscle receptor. KD1 is approximately the same as and KD2 of similar magnitude to that previously reported for heart. (2) The dissociation rates for the high-affinity binding have been directly measured for both heart and skeletal muscle (t1/2 approximately 30-40 min). These rates are more rapid than previously reported (t1/2 approximately 14 h). (3) KD1's obtained from the ratio of the dissociation and association rate constants agree with the dissociation constant measured by equilibrium binding Scatchard analysis. (4) Ryanodine binding to the low-affinity site can be correlated with a decrease in the dissociation rate constant (k-1) of the high-affinity site, and thereby in the apparent dissociation constant (KD1). The inhibition constant (KI) for inhibiting the high-affinity off rate obtained from a double-reciprocal plot of the change in off rate vs [ryanodine] is practically the same in heart (0.66 microM) and skeletal muscle (0.64 microM) and in the range of the KD2. The binding of cold ryanodine to the low-affinity site appears to lock the bound [3H]ryanodine onto the high-affinity site rather than to exchange with it. Thus, in this sense, the ryanodine receptor exhibits "positive cooperativity".(ABSTRACT TRUNCATED AT 250 WORDS)

Alkaloids↗
PubMed1989

Comparison of the calcium release channel of cardiac and skeletal muscle sarcoplasmic reticulum by target inactivation analysis.

The calcium release channel of sarcoplasmic reticulum which triggers muscle contraction in excitation-contraction coupling has recently been isolated. The channel has been found to be morphologically identical with the feet structures of the junctional face membrane of terminal cisternae and consists of an oligomer of a unique high molecular weight polypeptide. In this study, we compare the target size of the calcium release channel from heart and skeletal muscle using target inactivation analysis. The target molecular weights of the calcium release channel estimated by measuring ryanodine binding after irradiation are similar for heart (139,000) and skeletal muscle (143,000) and are smaller than the monomeric unit (estimated to be about 360,000). The target size, estimated by measuring polypeptide remaining after irradiation, was essentially the same for heart and skeletal muscle, 1,061,000 and 1,070,000, respectively, indicating an oligomeric association of protomers. Thus, the calcium release channel of both cardiac and skeletal muscle reacts uniquely with regard to target inactivation analysis in that (1) the size by ryanodine binding is smaller than the monomeric unit and (2) a single hit leads to destruction of more than one polypeptide, by measuring polypeptide remaining. Our target inactivation analysis studies indicate that heart and skeletal muscle receptors are structurally very similar.

Alkaloids↗
PubMed1989

Molecular cloning and characterization of the ryanodine receptor/junctional channel complex cDNA from skeletal muscle sarcoplasmic reticulum.

Major progress has been made in elucidating the calcium release mechanism involved in excitation-contraction coupling. The ryanodine receptor of sarcoplasmic reticulum has been isolated and found to be morphologically identical to the foot structure, which is involved in the junctional association of terminal cisternae with the transverse tubule. The foot structure also contains the calcium release channel itself. For this reason, we refer to the foot structure as the junctional channel complex (JCC). The JCC consists of an oligomer of a single high molecular weight protein. Although progress has been made in characterizing important aspects of the structure and function of the JCC, further understanding of the JCC protein subunit awaits the molecular cloning of the JCC. We report on the isolation of cDNA clones encoding portions of the JCC from rabbit fast-twitch skeletal muscle and its tissue distribution and expression. The large size and lack of solubility of the JCC protein posed particular challenges to cloning this molecule. Among these was the necessity to develop techniques for partially digesting the JCC protein subunit with endoproteases in the presence of detergent. With this approach we obtained partial amino acid sequences from regions of the JCC and designed oligonucleotide primers and probes to synthesize and screen cDNA libraries. The rabbit skeletal muscle JCC mRNA encodes an approximately 16-kilobase mRNA present in skeletal, heart, and aortic smooth muscle, as determined by RNA blot analysis with a 700-base-pair cDNA probe. Whereas the JCC mRNA appears to be relatively abundant in adult rabbit fast-twitch skeletal muscle, it is much less abundant in heart and smooth muscle. The JCC mRNA in BC3H1 (a myoblast cell line) is reversibly regulated by growth factors in a manner similar to muscle-specific contractile protein genes.

Amino Acid Sequence↗
PubMed1989

Acute aortic dissection due to giant cell arteritis. Report of two autopsy cases.

Two autopsy cases of giant cell arteritis with aortic dissection are reported. One patient was an 87-year-old man, and the other was an 80-year-old woman. In both cases the patient died suddenly, and fatal pericardial bleeding due to the aortic dissection was revealed at autopsy. Histologically, marked inflammation with giant cells was observed in the wall of the aorta and in other arteries including the cranial arteries. The authors therefore consider that giant cell arteritis was one of the preceding lesions that culminated in aortic dissection.

Aged↗
PubMed1988

Characterization of junctional and longitudinal sarcoplasmic reticulum from heart muscle.

Longitudinal tubules and junctional sarcoplasmic reticulum (SR) were prepared from heart muscle microsomes by Ca2+-phosphate loading followed by sucrose density gradient centrifugation. The longitudinal SR had a high Ca2+ loading rate (0.93 +/- 0.08 mumol.mg-1.min) which was unchanged by addition of ruthenium red. Junctional SR had a low Ca2+ loading rate (0.16 +/- 0.02 mumol.mg-1.min) which was enhanced about 5-fold by ruthenium red. Junctional SR had feet structures observed by electron microscopy and a high molecular weight protein with Mr of 340,000, whereas longitudinal SR was essentially devoid of both. Thus, these subfractions have similar characteristics to longitudinal and junctional terminal cisternae of SR from fast twitch skeletal muscle. Ryanodine binding was localized to junctional cardiac SR as determined by [3H]ryanodine binding. Scatchard analysis of the binding data showed two types of binding (high affinity, Kd approximately 7.9 nM; low affinity, Kd approximately 1 microM), contrasting with skeletal junctional terminal cisternae where only one site with Kd of approximately 50 nM was observed. The ruthenium red enhancement of Ca2+ loading rate in junctional cardiac SR was blocked by pretreatment with low concentrations of ryanodine as reported for junctional terminal cisternae of skeletal muscle SR. The Ca2+ loading rate of junctional cardiac SR was enhanced by preincubation with high concentrations of ryanodine. The apparent inhibition constant (Ki approximately 7 nM) and stimulation constant (Km approximately 1.1 microM) for ryanodine on junctional SR corresponded to the Kd for high affinity binding (Kd approximately 7.9 nM) and low affinity binding (Kd approximately 1.1 microM), respectively. These results suggest that high affinity ryanodine binding locks the Ca2+ release channels in the open state and that low affinity binding closes the Ca2+ release channels of the junctional cardiac SR. The characteristics of the Ca2+ release channels of junctional cardiac SR appear to be similar to that of skeletal muscle SR, but the Ca2+ release channels of cardiac SR are more sensitive to ryanodine.

Animals↗
PubMed1988

Identification and purification of a transverse tubule coupling protein which binds to the ryanodine receptor of terminal cisternae at the triad junction in skeletal muscle.

In fast twitch skeletal muscle, the signal for excitation-contraction coupling is transferred from transverse tubule across the triad junction; calcium is thereby released from the terminal cisternae of sarcoplasmic reticulum triggering muscle contraction. Recently, the feet structures of terminal cisternae, which bridge the gap at the triad junction, have been identified as the ryanodine receptor and in turn with the calcium release channels of sarcoplasmic reticulum. The latter consists of an oligomer of a single high molecular weight polypeptide (Mr 360,000). This study attempts to identify the component in the transverse tubule which ligands with the foot structure to form the triad junction. The purified ryanodine receptor, derivatized with sulfosuccinimidyl-2-(p-azidosalicylimido)-1,3'-dithiopropionate (SASD), a thiol-cleavable, 125I-iodinatable, and photoactive probe, was shown to selectively cross-link to a protein with Mr of 71,000 in isolated transverse tubules. This coupling protein was purified from transverse tubule by solubilization with the detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS) and then purified by sequential column chromatography. In the absence of sulfhydryl agents, the purified polypeptide has an Mr of 61,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A complementary approach using SASD was employed to confirm association of the coupling protein with the ryanodine receptor of terminal cisternae. We conclude that the transverse tubule coupling protein together with the ryanodine receptor (foot structure) is involved in the liganding between transverse tubule and terminal cisternae of sacroplasmic reticulum.

Amino Acids↗
PubMed1988

Reconstitution of purified cardiac muscle calcium release channel (ryanodine receptor) in planar bilayers.

The purified ryanodine receptor of heart sarcoplasmic reticulum (SR) has been reconstituted into planar phospholipid bilayers and found to form Ca2+-specific channels. The channels are strongly activated by Ca2+ (10 nM) in the presence of ATP (1 mM) and ryanodine, and inactivated by Mg2+ (3 mM) or ruthenium red (30 microM). These characteristics are diagnostic of calcium release from heart SR. The cardiac ryanodine receptor, which has previously been identified as the foot structure, is now identified as the calcium release channel. A similar identity of the calcium release channel has recently been reported for skeletal muscle. The characteristics of the calcium release channel from skeletal muscle and heart are similar in that they: 1) consist of an oligomer of a single high molecular weight polypeptide (Mr 360,000 for skeletal muscle and 340,000 for heart); 2) exist morphologically as the foot structure; 3) are activated (ATP, Ca2+, ryanodine) and inhibited (ruthenium red and Mg2+) by a number of the same ligands. Important differences include: 1) Ca2+ activation at lower concentration of Ca2+ for the heart; 2) more dramatic stabilization by ryanodine of the open state for the skeletal muscle channel; and 3) different relative permeabilities (PCa/PK).

Adenosine Triphosphate↗
PubMed1988

Purified ryanodine receptor of skeletal muscle sarcoplasmic reticulum forms Ca2+-activated oligomeric Ca2+ channels in planar bilayers.

The ryanodine receptor of sarcoplasmic reticulum (SR) from fast-twitch skeletal muscle has been purified and found by electron microscopy to be equivalent to the feet structures that are involved in situ in the junctional association of transverse tubules with terminal cisternae of SR. We now find that when the purified receptor is incorporated into vesicle-derived planar bilayers, it forms Ca2+-specific channels, which are dependent on submicromolar Ca2+ for activity. In the presence of 1 mM ATP, the channel shows essentially no activity at 10 nM Ca2+ but becomes highly activated at 50 nM Ca2+. At suboptimal Ca2+ levels (100 nM), the channel is strongly activated by 1 mM ATP and can be blocked by ruthenium red, both effects being prevented by higher Ca2+ levels (1 microM). Mg2+, added from the cis side at millimolar concentrations, blocks Ca2+ flux through the channel from trans to cis (equivalent to flux from luminal to myoplasmic compartment). Ryanodine stabilizes the open state of the channel and blocks the action of ruthenium red to close the channel. Thus, the purified ryanodine receptor incorporated into a bilayer has the Ca2+-channel characteristics consistent with the calcium release observed in isolated terminal cisternae vesicles. Furthermore, ryanodine induced the appearance of a sublevel gating mode characterized by long open conductance states, which were integral multiples of the smallest observed conductance, 3.8 pS in 50 mM Ca2+. The purified receptor consists essentially of a single-sized high molecular weight polypeptide (Mr. approximately equal to 360,000), which on reconstitution forms the square rectangles diagnostic of the feet structures. We conclude that the identity of the Ca2+-release channel of SR is the foot structure, which consists of an oligomer of the high molecular weight polypeptide.

Adenosine Triphosphate↗
PubMed1988

Ultrastructure of the calcium release channel of sarcoplasmic reticulum.

This study is concerned with the characterization of the morphology of the calcium release channel of sarcoplasmic reticulum (SR) from fast-twitch skeletal muscle, which is involved in excitation-contraction coupling. We have previously purified the ryanodine receptor and found it to be equivalent to the feet structures, which are involved, in situ, in the junctional association of transverse tubules with terminal cisternae of SR. The receptor is an oligomer of a single high molecular weight polypeptide and when incorporated into phospholipid bilayers, has channel conductance which is characteristic of calcium release in terminal cisternae of SR. The purified channel can be observed by electron microscopy using different methods of sample preparation, with complementary views being observed by negative staining, double staining, thin section and rotary shadowing electron microscopy. Three views can be observed and interpreted: (a) a square face which, in situ, is junctionally associated with the transverse tubule or junctional face membrane; (b) a rectangle equivalent to the side view; and (c) a diamond shape equivalent to the side view, of which the base portion appears to be equivalent to the transmembrane segment. Negative staining reveals detailed substructure of the channel. A computer averaged view of the receptor displays fourfold symmetry and ultrastructural detail. The dense central mass is divided into four domains with a 2-nm hole in the center, and is enclosed within an outer frame which has a pinwheel appearance. Double staining shows substructure of the square face in the form of parallel linear arrays (six/face). The features of the isolated receptor can be correlated with the structure observed in terminal cisternae vesicles. Sections tangential to the junctional face membrane reveal that the feet structures (23-nm squares) overlap so as to enclose smaller square spaces of approximately 14 nm/side. We suggest that this is equivalent to the transverse tubule face and that the terminal cisternae face is smaller (approximately 17 nm/face) and has larger alternating spaces as a consequence of the tapered sides of the foot structures. Image reconstruction analysis appears to be feasible and should provide the three-dimensional structure of the channel.

Animals↗
PubMed1988

Positive influence of dietary deoxycholic acid on development of pre-neoplastic lesions initiated by N-methyl-N-nitrosourea in rat liver.

The effect of deoxycholic acid (DCA) treatment subsequent to initiation of F344 male rats with N-methyl-N-nitrosourea (MNU), a wide spectrum carcinogen inducing tumors in many organs, was investigated. Rats were initially given four doses of MNU (50 mg/kg) i.p. within a 2-week period combined with a two-thirds partial hepatectomy performed at day 7 and then placed on basal diet containing DCA at concentrations of 0.313, 0.125, 0.050 and 0.020% for 21 weeks prior to final sacrifice. All organs studied were carefully examined histologically and histochemically for development of neoplastic and pre-neoplastic lesions. DCA enhanced the induction of glutathione S-transferase positive (GST-P+) liver cell foci in a dose-related manner. Furthermore groups of rats given DCA without prior MNU administration also developed dose-dependent numbers of pre-neoplastic liver lesions. In addition, increased numbers of small intestine tumors were apparent in DCA-treated animals although the difference was not significant. Induction of tumors in the thyroids, Zymbal glands, skin and peripheral nerves was not affected. The results indicate that DCA is a strong promoter of hepatocarcinogenesis with possible complete carcinogenicity in the liver and promotion potential for tumor development in the small intestine.

Animals↗