Eric Rousseau

Activation of the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum by caffeine and related compounds

The caffeine-sensitive Ca2+ release pathway in skeletal muscle was identified and characterized by studying the release of 45Ca2+ from heavy sarcoplasmic reticulum (SR) vesicles and by incorporating the vesicles or the purified Ca2+ release channel protein complex into planar lipid bilayers. First-order rate constants for 45Ca2+ efflux of 1 s-1 were obtained in the presence of 1-10 microM free Ca2+ or 2 X 10(-9) M free Ca2+ plus 20 mM caffeine. Caffeine- and Ca2+-induced 45Ca2+ release were potentiated by ATP and Mg.ATP, and were both inhibited by Mg2+. Dimethylxanthines were similarly (3,9-dimethylxanthine) or more (1,7-, 1,3-, and 3,7-dimethylxanthine) effective than caffeine in increasing the 45Ca2+ efflux rate. 1,9-Dimethylxanthine and 1,3-dimethyluracil (which lacks the imidazole ring) did not appreciably stimulate 45Ca2+ efflux. Recordings of calcium ion currents through single channels showed that the Ca2+- and ATP-gated SR Ca2+ release channel is activated by addition of caffeine to the cis (cytoplasmic) and not the trans (lumenal) side of the channel in the bilayer. The single channel measurements further revealed that caffeine activated Ca2+ release by increasing the number and duration of open channel events without a change of unit conductance (107 pS in 50 mM Ca2+ trans). These results suggest that caffeine exerts its Ca2+ releasing effects in muscle by activating the high-conductance, ligand-gated Ca2+ release channel of sarcoplasmic reticulum.

Single channel and 45Ca2+ flux measurements of the cardiac sarcoplasmic reticulum calcium channel

Purified canine cardiac sarcoplasmic reticulum vesicles were passively loaded with 45CaCl2 and assayed for Ca2+ releasing activity according to a rapid quench protocol. Ca2+ release from a subpopulation of vesicles was found to be activated by micromolar Ca2+ and millimolar adenine nucleotides, and inhibited by millimolar Mg2+ and micromolar ruthenium red. 45Ca2+ release in the presence of 10 microM free Ca2+ gave a half-time for efflux of 20 ms. Addition of 5 mM ATP to 10 microM free Ca2+ increased efflux twofold (t1/2 = 10 ms). A high-conductance calcium-conducting channel was incorporated into planar lipid bilayers from the purified cardiac sarcoplasmic reticulum fractions. The channel displayed a unitary conductance of 75 +/- 3 pS in 53 mM trans Ca2+ and was selective for Ca2+ vs. Tris+ by a ratio of 8.74. The channel was dependent on cis Ca2+ for activity and was also stimulated by millimolar ATP. Micromolar ruthenium red and millimolar Mg2+ were inhibitory, and reduced open probability in single-channel recordings. These studies suggest that cardiac sarcoplasmic reticulum contains a high-conductance Ca2+ channel that releases Ca2+ with rates significant to excitation-contraction coupling.

Calmodulin modulation of single sarcoplasmic reticulum Ca2+-release channels from cardiac and skeletal muscle.

Sarcoplasmic reticulum (SR) contains a Ca2+-conducting channel that is believed to play a central role in excitation-contraction coupling by releasing the Ca2+ necessary for muscle contraction. The effects of calmodulin on single cardiac and skeletal muscle SR Ca2+-release channels were studied using the planar lipid bilayer-vesicle fusion technique. Calmodulin inhibited Ca2+-release channel opening by reducing the mean duration of single-channel open events without having an effect on single-channel conductance. Inhibition by calmodulin was dependent on Ca2+ concentration and occurred in the absence of ATP. The effects of calmodulin were reversed by mastoparan, a calmodulin-binding peptide. Two other calmodulin antagonists [calmidazolium and N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide] modified the gating behavior of the channel in the absence of exogenous calmodulin in a concentration- and Ca2+-dependent manner. Our results suggest that calmodulin can modulate excitation-contraction coupling by directly interacting with the SR Ca2+-release channel of cardiac and skeletal muscle.

Multiple conductance states of the purified calcium release channel complex from skeletal sarcoplasmic reticulum.

The CHAPS-solubilized and purified 30S ryanodine receptor protein complex from skeletal sarcoplasmic reticulum (SR) was incorporated into planar lipid bilayers. The resulting electrical activity displayed similar responses to agents such as Ca2+, ATP, ryanodine, or caffeine as the native Ca2+ release channel, confirming the identification of the 30S complex as the Ca2+ release channel. The purified channel was permeable to monovalent ions such as Na+, with the permeability ratio PCa/PNa approximately 5, and was highly selective for cations over anions. The purified channel also showed at least four distinct conductance levels for both Na+ and Ca2+ conducting ions, with the major subconducting level in NaCl buffers possessing half the conductance value of the main conductance state. These levels may be produced by intrinsic subconductances present within the channel oligomer. Several of these conductances may be cooperatively coupled to produce the characteristic 100 +/- 10 pS unitary Ca2+ conductance of the native channel.

Evidence for a Ca2+ channel within the ryanodine receptor complex from cardiac sarcoplasmic reticulum

The solubilized [3H]ryanodine receptor from cardiac sarcoplasmic reticulum was centrifuged through linear sucrose gradients. A single peak of radioactivity with apparent sedimentation coefficient of approximately 30S specifically comigrated with a high molecular weight protein of apparent relative molecular mass approximately 400,000. Incorporation of the ryanodine receptor into lipid bilayers induced single Ca2+ channel currents with conductance and kinetic behavior almost identical to that of native cardiac Ca2+ release channels. These results suggest that the cardiac ryanodine receptor comprises the Ca2+ release channel involved in excitation-contraction coupling in cardiac muscle.

Biochemical characterization of the Ca2+ release channel of skeletal and cardiac sarcoplasmic reticulum

SummaryRapid mixing-vesicle ion flux and planar lipid bilayer-single channel measurements have shown that a high-conductance, ligand-gated Ca2+ release channel is present in ‘heavy’, junctional-derived membrane fractions of skeletal and cardiac muscle sarcoplasmic reticulum. Using the release channel-specific probe, ryanodine, a 30S protein complex composed of polypeptides of Mr ∼ 400 000 has been isolated from cardiac and skeletal muscle. Reconstitution of the complex into planar lipid bilayers has revealed a Ca2+ conductance with properties characteristic of the native Ca2+ release channel.

Single chloride-selective channel from cardiac sarcoplasmic reticulum studied in planar lipid bilayers

SummaryThe behavior of single Cl− channel was studied by fusing isolated canine cardiac sarcoplasmic reticulum (SR) vesicles into planar lipid bilayers. The channel exhibited unitary conductance of 55 pS (in 260mm Cl−) and steady-state activation. Subconductance states were observed. Open probability was dependent on holding potentials (−60 to +60 mV) and displayed a bell-shaped relationship, with probability values ranging from 0.2 to 0.8 with a maximum at −10 mV. Channel activity was irreversibly inhibited by DIDS, a stilbene derivative. Time analysis revealed the presence of one time constant for the full open state and three time constants for the closed states. The open and the longer closed time constants were found to be voltage dependent. The behavior of the channel was not affected by changing Ca2+ and Mg2+ concentrations in both chambers, nor by adding millimolar adenosine triphosphate, or by changing the pH from 7.4 to 6.8. The presence of sulfate anions decreased the unit current amplitude, but did not affect the open probability. These results reveal that at the unitary level the cardiac SR anion-selective channel has distinctive as well as similar electrical properties characteristic of other types of Cl− channels.

Comparison of CHAPS-induced current fluctuations with sarcoplasmic reticulum Ca2+ release channel activity

The process of purifying and reconstituting transport membrane proteins generally involves the use of detergents, which often cannot be completely separated from the proteins. The effects of the zwitterionic detergent CHAPS on planar lipid bilayers have been measured, and it is demonstrated that CHAPS can induce microscopic electrical activity in the bilayers. Typical CHAPS-induced activity consists of large current bursts, often separated by intervals of quiescent activity, with no definable conductance levels. The size of the current bursts is generally increased by higher CHAPS concentration or by millimolar ATP and usually reduced by millimolar Mg2+ and micromolar ruthenium red. The response of the CHAPS-induced currents to these agents is compared to that of the ligand-gated Ca2+ release channel of muscle sarcoplasmic reticulum.