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I Jóna

Publications and source records attributed to I Jóna.

12 recordsLinked to original sources
PubMed2001

Altered inhibition of the rat skeletal ryanodine receptor/calcium release channel by magnesium in the presence of ATP.

Magnesium-induced inhibition of the skeletal ryanodine receptor/calcium-release channel (RyR) was studied in the presence and absence of ATP under isolated conditions and in situ, by examining the RyR incorporated into a planar lipid bilayer and the calcium release flux (Rrel) in isolated single fibres mounted in the double Vaseline gap system. When the incorporated RyR had been activated by calcium (50 microM) in the absence of ATP, the magnesium-induced inhibition showed co-operativity with a Hill coefficient (N) of 1.83 and a half-inhibitory concentration (IC50) of 635 microM. When the open probability was measured in the presence of 5 mM ATP and at a low calcium concentration, the magnesium-induced inhibition was non-cooperative (N=1.1, IC50= 860 microM). In isolated muscle fibres, in the presence of ATP, lowering the intracellular magnesium concentration ([Mg2+]i) increased the maximal Rrel and shifted its voltage dependence to more negative membrane potentials. Increasing [Mg2+]i had the opposite effect. The concentration dependence was described with an IC50 of 174 microM, N=1, under depolarized conditions and showed a tenfold increase in affinity in polarized fibres. At the concentration required for the measurements from isolated fibres, ATP had a full activatory effect on the isolated channel. At a low calcium concentration, the RyR had two ATP-binding sites with half-activatory concentrations of 19 and 350 microM, respectively.

Adenosine Triphosphate↗
PubMed2000

Regulation of the rat sarcoplasmic reticulum calcium release channel by calcium.

The regulation by calcium of the ryanodine receptor/SR calcium release channel (RyR) from rat skeletal muscle was studied under isolated conditions and in situ. RyRs were either solubilized and incorporated into lipid bilayers or single fibres were mounted into a Vaseline gap voltage clamp. Single channel data were compared to parameters determined from the calculated calcium release flux. With K+ (250 mM) being the charge carrier the single channel conductance was 529 pS at 50 microM Ca2+ cis and trans, and decreased with increasing cis [Ca2+]. Open probability showed a bell shaped calcium dependence revealing an activatory and an inhibitory Ca2+ binding site (Hill coefficients of 1.18 and 1.28, respectively) with half activatory and inhibitory concentrations of 9.4 and 298 microM. The parameters of the inhibitory site agreed with the calcium dependence of channel inactivation deduced from the decline in SR calcium release in isolated fibres. Mean open time showed slight [Ca2+] dependence following a single exponential at every Ca2+ concentration tested. Closed time histograms, at high [Ca2+], were fitted with three exponentials, from which the longest was calcium independent, and resembled the recovery time constant of SR inactivation (115+/-15 ms) obtained in isolated fibres. The data are in agreement with a model where calcium binding to the inhibitory site on RyR would be responsible for the calcium dependent inactivation in situ.

Animals↗
PubMed2000

Biphasic effect of bimoclomol on calcium handling in mammalian ventricular myocardium.

1. Concentration-dependent effects of bimoclomol, the novel heat shock protein coinducer, on intracellular calcium transients and contractility were studied in Langendorff-perfused guinea-pig hearts loaded with the fluorescent calcium indicator dye Fura-2. Bimoclomol had a biphasic effect on contractility: both peak left ventricular pressure and the rate of force development significantly increased at a concentration of 10 nM or higher. The maximal effect was observed between 0.1 and 1 microM, and the positive inotropic action disappeared by further increasing the concentration of bimoclomol. The drug increased systolic calcium concentration with a similar concentration-dependence. In contrast, diastolic calcium concentration increased monotonically in the presence of bimoclomol. Thus low concentrations of the drug (10 - 100 nM) increased, whereas high concentrations (10 microM) decreased the amplitude of intracellular calcium transients. 2. Effects of bimoclomol on action potential configuration was studied in isolated canine ventricular myocytes. Action potential duration was increased at low (10 nM), unaffected at intermediate (0.1 - 1 microM) and decreased at high (10 - 100 microM) concentrations of the drug. 3. In single canine sarcoplasmic calcium release channels (ryanodine receptor), incorporated into artificial lipid bilayer, bimoclomol significantly increased the open probability of the channel in the concentration range of 1 - 10 microM. The increased open probability was associated with increased mean open time. The effect of bimoclomol was again biphasic: the open probability decreased below the control level in the presence of 1 mM bimoclomol. 4. Bimoclomol (10 microM - 1 mM) had no significant effect on the rate of calcium uptake into sarcoplasmic reticulum vesicles of the dog, indicating that in vivo calcium reuptake might not substantially be affected by the drug. 5. In conclusion, the positive inotropic action of bimoclomol is likely due to the activation of the sarcoplasmic reticulum calcium release channel in mammalian ventricular myocardium.

Action Potentials↗
PubMed1999

Calsequestrin: more than 'only' a luminal Ca2+ buffer inside the sarcoplasmic reticulum.

In striated muscle, the sarcoplasmic reticulum (SR) Ca2+ release/ryanodine receptor (RyR) channel provides the pathway through which stored Ca2+ is released into the myoplasm during excitation-contraction coupling. Various luminal Ca2+-binding proteins are responsible for maintaining the free [Ca2+] at 10(-3)-10(-4) M in the SR lumen; in skeletal-muscle SR, it is mainly calsequestrin. Here we show that, depending on its phosphorylation state, calsequestrin selectively controls the RyR channel activity at 1 mM free luminal [Ca2+]. Calsequestrin exclusively in the dephosphorylated state enhanced the open probability by approx. 5-fold with a Hill coefficient (h) of 3.3, and increased the mean open time by about 2-fold, i.e. solely dephosphorylated calsequestrin regulates Ca2+ release from the SR. Because calsequestrin has been found to occur mainly in the phosphorylated state in the skeletal-muscle SR for the regulation of RyR channel activity, the dephosphorylation of calsequestrin would appear to be a quintessential physiological event.

Animals↗
PubMed1994

Enzymatic properties, metal composition and SH-group reactivity of the light and heavy sarcoplasmic reticulum vesicles.

Light (LT) and heavy (TTC) microsomes were isolated from the fragmented sarcoplasmic reticulum (FSR) of rabbit skeletal muscle by sucrose gradient centrifugation. The amount of the protein components (ATPase, "feet proteins", calsequestrine) showed substantial differences between the light and heavy fractions. The amounts of calcium, magnesium and zinc were about 2-4 times higher in the TTC fraction, then those of the LT fraction. The activities of the Ca2+ + Mg2+ activated para-nitrophenyl-phosphatase and acetylcholin-esterase were (about 1.5 times) also higher in the TTC fraction compared to the LT fraction. The ratio of the Ca-transport vs. ATP was 2.1 in case of the LT and 0.6 in case of the TTC fraction. The number of titerable SH-groups of the LT fraction, measured in EGTA-medium containing Ca2+, was higher than those measured in the absence of Ca2+, while for the TTC fraction this number was higher when determined in the absence of Ca2+. We suppose that due to the higher amount of Ca2+ and Zn2+ in the TTC fraction those SH-groups which were present as Ca- or Zn-thiolates became titerable in EGTA-medium in the absence of Ca2+.

4-Nitrophenylphosphatase↗
PubMed1989

Interaction between the Ca2(+)-ATPase and the proteolipid in artificial membranes.

The interaction between the Ca2+ transport ATPase and the proteolipid of rabbit sarcoplasmic reticulum was analyzed by fluorescence energy transfer, using the following donor: acceptor combinations: Ca2(+)-ATPase tryptophan----IAEDANS-proteolipid; IAEDANS-ATPase----IAF-proteolipid; IAEDANS-proteolipid----IAF-ATPase. The observed energy transfer may indicate weak interaction between the Ca2(+)-ATPase and proteolipid, but collisional energy transfer definitely contributes. The energy transfer was abolished by deoxycholate or sodium dodecylsulfate at concentrations sufficient to solubilize the membrane. In view of the low proteolipid content of sarcoplasmic reticulum and the weak interaction suggested by the energy transfer, at best only a small fraction of ATPase molecules could exist in the form of ATPase-proteolipid complexes.

Amino Acid Sequence↗
PubMed1988

Changes of calcium compartmentalization of skeletal muscle under ionic and drug influence.

Compartmentalization of calcium was studied in frog semitendinosus using 45Ca radioisotope under exchange diffusion condition. Computer based deconvolution was employed to determine the "volume" and the apparent binding constant of the resolved six components. Assuming kinetically parallel and independent compartments nickel treatment slows down the calcium movement through the sarcoplasmic reticulum membrane and for the calcium ions causes competition of the membrane of transverse tubuli and of the sarcoplasmic reticulum. Veratrine treatment caused different effect in the presence and in the absence of sodium ions, demonstrating the essential role of Na+ in equilibrium with Ca2+ exchange processes/ATPases in the Ca2+ uptake and release by sarcoplasmic reticulum. Effect of caffeine and Dantrolene basically opposite on the compartment corresponds to the sarcoplasmic reticulum and similar on the transverse tubuli in term of speed of exchange diffusion.

Animals↗
PubMed1985

Vesicle release from erythrocytes during storage and failure of rejuvenation to restore cell morphology.

During storage, discocyte-echinocyte transformation of erythrocytes occurs progressively and microvesicles release from the spicules in the form of membrane-encapsulated red cell fragments. After rejuvenation part of the membrane-deficient cells, mainly spheroechinocytes, transform into spherostomatocytes because of their inability of restoring the surface area to volume ratios of normal discocytes. The proportion of spherostomatocytes formed on rejuvenation increases with storage time. Numerical indices for characterizing the "level of reversion" and the "quality of reversion" were developed. Osmotic fragility curves and light scattering histograms also reflected a partially irreversible increase of inhomogeneity of stored cell populations.

2,3-Diphosphoglycerate↗
PubMed1984

Experimental calcipenic osteopathy.

Calcipenic osteopathy was used as a model, in order to reveal the diagnostic value and capacity of the noninvasive radiological methods (microradioscopy, densitometry, and Roentgen-morphometry), in comparison with the results of histological, histomorphometrical, and atomic absorption methods. The study proved that calcipenic diet causes an osteoporosis in the experimental animals. The alterations observed meet both the histological and radiological criteria.

Animals↗