Muscle contraction is a phenomena which fascinated already the ancient Greeks. People have long sought to understand the mechanism of muscle contraction. Today we know that in order for muscle to contract, an action potential propagates from the nerve cell to the muscle cell. Upon arriving at the muscle cell, via a mechanism called Excitation- Contraction (E-C) coupling, Ca2 + is released from an intracellular membrane system, the sarcoplasmic reticulum (SR), into the intracellular fluid. The increase of intracellular Ca2 + initiates the interaction between the contractile units which results in force development and tension. The least well understood step in the contractile process is mechanism of E-C coupling. During the last 15-20 years various theories have been proposed to describe this process. Our laboratory came up with a theory several years ago, that critical sultbydryl groups on a protein, the ryanodine receptor(RyR)/Ca2 + release channel, are oxidized and subsequently reduced during the process of contraction and relaxation. In this thesis a reagent, o-Phthalaldehyde (OPA), was used to better understand the gating mechanism of the RyR/Ca2 + release channel. This reagent has the ability to form an isoindole derivative with the amino acids cysteine and lysine, if they are separated by not more than 3 A .In this study, it was shown that OP A interacts directly with the Ca2 + release channel by forming a covalent derivative with a critical thiol and a nearby lysine. High affinity [3H]Ryanodine binding to the RyR\Ca2 + release channel is activated by < 130μM OP A, but is inhibited by OPA at concentrations ranging from 200-300 μM OPA. This biphasic behavior indicates that at least two sets of cysteine-lysine pairs regulate Ca2 + channel activity. Moreover, the binding of OP A results in increasing the affinity of the receptor for the binding of ryanodine, in a Ca2 + independent manner, which may indicate that there are two different sets of RyR\Ca2+ release channels present in the SR.

The sarcoplasmic reticulum is the intramuscular organelle responsible for the regulation of cytoplasmic calcium levels in muscle. This thesis investigates the effects of the cardioprotective drug, dexrazoxane, and its metabolite ADR-925 on doxorubicin induced calcium release from skeletal sarcoplasmic reticulum. Doxorubicin is a widely used antineoplastic agent. One of the major side effects of doxorubicin usage is chronic cardiotoxicity. Doxorubicin is a potent activator of the calcium release mechanism from the SR. The interaction between doxorubicin and the calcium release channel has been proposed as the possible underlying mechanism behind cardiotoxicity. A short overview of different hypotheses describing doxorubicin induced cardiotoxicity and proposed mechanisms of cardioprotection by dexrazoxane are presented. While dexrazoxane did not appear to affect the calcium permeability of the SR, its metabolite, ADR-925, modulates the ryanodine receptor complex. ADR-925 inhibits high affinity ryanodine binding to the ryanodine receptor/calcium release channel complex by decreasing the sensitivity of the receptor for stimulation by calcium. ADR-925's ability to inhibit doxorubicin stimulated ryanodine binding is independent of the doxorubicin concentration. These results demonstrate that ADR-925 directly affects the ryanodine receptor complex of the SR by desensitizing the receptor to activation by calcium. Furthermore, ADR-925 reduces the inhibitory effect of hydrogen peroxide on the ryanodine receptor/ calcium release channel. This suggests that ADR-925 may protect the SR from oxidative effects of free radicals. It has been somewhat controversial whether doxorubicin induced cardiotoxicity is due to a specific interaction with the calcium release mechanism of SR. The findings presented in this thesis which demonstrate that the cardioprotectant ADR-925 interacts directly with the ryanodine receptor from SR, further support the hypothesis that the ryanodine receptor is a primary target of doxorubicin's action.

published_or_final_version / Physiology / Doctoral / Doctor of Philosophy

国立情報学研究所で電子化したコンテンツを使用している。

Thesis (Ph. D.)--University of Virginia, 1998. / Spine title: Cryo-EM of Ca²⁺-ATPase from SR. Includes bibliographical references (p. 149-159). Also available online through Digital Dissertations.

Thesis (Ph. D.)--Ohio State University, 2008.

Thesis (Ph.D.)--McMaster University, 2003. / Advisor: A. K. Grover. Also available via World Wide Web.

Abstract Skeletal muscles are composed of long, multinucleated cells called myofibers, which are highly differentiated cells and therefore unique in structure. In the present study the organization of the endocytic and exocytic pathways in isolated rat skeletal myofibers was defined with confocal and electron microscopic methods. In isolated myofibers the I band areas were shown to be active in endocytosis. The sorting endosomes were distributed in a cross-striated fashion while the recycling and late endosomal compartments were located to perinuclear areas and interfibrillar spaces, where they followed the course of microtubules. Protein trafficking in the different stages of muscle cell differentation was also analyzed. The studies with L6 myoblasts and myotubes showed that during myogenesis varying fractions of different viral glycoproteins were sorted from the endoplasmic reticulum (ER) into a specific compartment that did not recycle with the Golgi apparatus. This compartment is suggested to be the sarcoplasmic reticulum (SR). The studies with living muscle cells showed further changes in vesicle trafficking taking place during myogenesis. With GFP-tagged tsO45G protein, transport containers were detected in 20% of the infected myofibers, while all infected L6 myoblasts or myotubes showed intense movement of corresponding structures. We also detected significant differences between the pre-and post-Golgi traffickings in myofibers. When the distribution of the ER in adult myofibers was studied, the confocal microscopic data showed that the labeling patterns of the rough endoplasmic reticulum (RER) and the SR markers were different. Blocking of different cargo proteins in the RER revealed two discrete distribution patterns, neither of them identical with the SR. The collected electron microscopic data supported the idea that in mature myofibers there are two separate RER compartments. We suggest that the RER compartment capable of export function located around the myonuclei and on the Z lines, while the non-exporting RER compartment localized to terminal cisternae and probably took care of the synthesis of the SR proteins.

The objective of these studies was to investigate mechanisms whereby cardiac sarcoplasmic reticulum (SR) calcium transport activity may be influenced by changes in the lipid environment of the SR membrane in the experimental endocrine disease states hypothyroidism and insulin-dependent diabetes mellitus. These endocrine disease states were studied to determine, respectively, if SR function is influenced by endogenous acylcarnitine associated with the SR membrane and if SR phospholipid acyl composition plays a role in diabetes-induced cardiomyopathy. The effects of endogenous acylcarnitines on SR calcium transport in hypothyroidism were of interest since it has previously been implicated that acyl carnitines play a regulatory role in SR function. SR calcium transport was not affected at two weeks, but was significantly reduced at four, six and eight weeks following thyroidectomy. Endogenous acyl carnitines were detectable in the SR membrane fraction isolated from both euthyroid control and thyroidectomized animals. The level of acyl carnitine associated with the SR did not correlate with calcium transport activity. Since acylcarnitine did not appear to play a role in the reduced SR calcium transport, SR calcium pump protein was quantified. The reduced SR calcium transport of thyroidectomized animals, manifest at four weeks, was shown to correlate with a reduction in SR acylphosphoprotein level. Therefore the reduced SR calcium transport activity of hypothyroidism is not related to the level of SR acyl carnitine, but rather a hypothyroid-induced reduction in SR calcium pump sites. Since omega-3 fatty acids affect parameters relevant to diabetes-induced cardiomyopathy, it was of interest to determine the cardiac effects of omega-3 fatty acid treatment of streptozocin (STZ)-induced diabetic animals. Omega-3 fatty acid treatment significantly reduced the development of diabetic cardiomyopathy and improved isolated cardiac SR calcium transport activity of STZ-induced diabetic animals. To determine if the cardiac and SR changes were related to membrane changes induced by omega-3 fatty acids, the fatty acyl composition of phospholipid was determined. Phospholipid analysis of cardiac phosphatidylcholine and phosphatidylethanolamine, and total SR phospholipid indicated modest changes in the omega-3 fatty acid component. Omega-3 fatty acid treatment produced slight (statistically insignificant) changes in SR cholesterol levels. Therefore a change in membrane phospholipid acyl composition may not account for the observed cardiac and SR functional changes. / Pharmaceutical Sciences, Faculty of / Graduate

Properties of the regulatory nucleotide binding site of the Ca²⁺-ATPase of skeletal muscle sarcoplasmic reticulum have been investigated. Previously, several lines of evidence have indicated the existence of both catalytic and regulatory nucleotide binding sites on the same polypeptide species. The present study concentrates on the interaction of the ATP analogue, 2'-3'-0-(2,4,6-trinitrocyclohexadienylidine) adenosine 5'-triphosphate, (TNP-ATP), with sites on the non-phosphorylated and phosphorylated enzyme. In particular those conformational transitions linking TNP-ATP fluorescence to the phosphoenzyme subspecies have been sought. Previous studies have demonstrated a close relationship between TNP-ATP fluorescence and phosphoenzyme formed from ATP plus Ca²⁺, or from inorganic phosphate (Pi) in the absence of Ca²⁺, in the reverse direction of the cycle. However, the precise relationship of TNP-ATP fluorescence to the energy transducing conformations of the ATPase is controversial. TNP-ATP binding was investigated by spectrophotometric methods and by the synthesis of [ ¹⁴C] TNP-ATP. [ ¹⁴C] TNP-ATP bound to the ATPase site with high affinity ([TNP-ATP] 0. 5 = 0.12 uM), and · a stoichiometry of 5.4 nmol/mg. [ ¹⁴C] ATP binding stoichiometry was 6.1 nmol/mg, demonstrating that TNP-ATP binds to a single family of sites. The nature of the phosphoenzyme intermediate species that results in enhanced TNP-ATP fluorescence was investigated. NEM derivitization, Sr²⁺-transport and Ca²⁺-oxalate uptake have previously been found to alter the distribution or relative levels of phosphoenzyme intermediates. Modification of thiol groups responsible for phosphoenzyme decomposition (SHd), using N-ethylmaleimide (NEM) (0.4 mM) with 50 uM Ca²⁺, 1 mM AMP-PNP at pH 7.0, resulted in a 50% decrease in Ca²⁺-uptake, Ca²⁺-ATPase activity and ADP-insensitive E-P (E₂-P), while total EP (E₁-P + E₂-P = 3.2 nmol/mg), remained unaltered. ATP-dependent TNP-ATP enhanced fluorescence decreased by 50% under these conditions. Ca²⁺-oxalate induced turnover has previously been shown to decrease steady-state E₂-P levels by prevention of Ca²⁺ gradient formation. Oxalate (5 mM) caused a 40% decrease in ATP-induced TNP-ATP fluorescence levels while total EP levels remained relatively unaltered. Previous studies have shown that Sr²⁺-induced turnover favours higher levels of E₂-P by inhibiting the reverse reaction from E₂-P to E₁-P. Strontium-induced turnover increased TNP-ATP fluorescence by 10% as compared to that of Ca²⁺, without affecting steady-state E-P levels, consistent with an E₂-P conformation relationship to enhanced TNP-ATP fluorescence. The binding site for TNP-ATP on the enzyme was investigated by chase studies using millimolar concentrations of nucleotides. ATP and ADP diminished TNP-ATP fluorescence competitively, with apparent Km values of 1.25 and 0.54 mM respectively, consistent with their affinities of binding to the regulatory site. The rates of decrease of fluorescence (25 and 34 sec⁻¹ at 5 ᵒC, respectively), were of the same order of magnitude as the derived "off" rate of TNP-ATP from the site of enhanced fluorescence (33 sec⁻¹), consistent with TNP-ATP being bound to the regulatory site of the enzyme. Enhanced TNP-ATP fluorescence has previously been related to decreased water activity of the probe site. Alteration of water activity by structure- forming (Deuterium oxide) and structure-breaking solutes (KSCN) in relation to fluorescence were explored. Replacement of H₂O by D₂O altered the fluorescence of unbound TNP-ATP. The apparent for TNP-ATP binding to the E₂-P conformation of the regulatory site. The regulatory site appears to be a modified form of the phosphorylated catalytic site. It is proposed that TNP-ATP fluorescence monitors an enzyme conformation related to Ca²⁺ binding to an inward oriented site of low affinity. The mechanism of K⁺ fluorescence quenching appears to be via an acceleration of dephosphorylation, as opposed to a change in affinity of the enzyme for TNP-ATP, as previously suggested. The K⁺ sensitivity of TNP-ATP fluorescence has proved useful in demonstrating a direct interaction of valinomycin with the enzyme through the monovalent cation binding site. Valinomycin appears to bind directly to the enzyme and to selectively accelerate the "off" rate of K⁺ from this site.