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Query: UMLS:C0432222 (SEM)
 47,337 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

At 36 degrees C and 2 mM [Ca2+]o, single guinea-pig ventricular myocytes were voltage clamped with patch electrodes. When paired pulsing had potentiated the contraction to the maximum, the cells were shock-frozen for electron probe microanalysis (EPMA). Shock-freezing was timed at the end of diastole (-80 mV) or at different times during systole (+5 mV). The same paired-pulse protocol was applied to another group of myocytes from which contraction was recorded and [Ca2+]i was estimated by microfluospectroscopy (50 microMNa-Indo-1). In potentiated cells, during the first pulse, contraction peaked within 128 +/- 25 ms after start of depolarization. [Ca2+]i peaked within 25 ms to 890 /+- 220 nM (mean +/- SEM) and fell within 100 ms to about 450 nM. sigma Camyo, the total calcium concentration in the overlapping myofilaments (A-band), was measured by EPMA in 17 potentiated myocytes. During diastole, sigma Camyo was 2.6 +/- 0.4 mmol/kg dry weight (dw), which can be converted to 0.65 mM (mmoles per liter myofibrillar space). Since [Ca2+]i was 180 nM, we estimate that 99.97% of total calcium is bound. A time-course for systolic sigma Camyo was determined by shock-freezing 13 cells at different times after start of depolarization to +5 mV. sigma Camyo was 5.5 +/- 0.3 mmol/kg dw (1.4 mM) after 15-25 ms, 4.6 +/- 0.5 mmol/kg dw (1.1 mM) after 30-45 ms, and 3.1 mmol/kg dw (0.8 mM) after 60-120 ms. The fast time-course of sigma Camyo suggests that calcium binds to and unbinds from troponin C at a fast rate.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Large and rapid changes of myofibrillar total calcium during the cardiac cycle. Electron probe microanalysis of voltage-clamped guinea-pig ventricular myocytes. 178 70

Intracellular passive Ca2+, buffering was measured in voltage-clamped rat ventricular myocytes. Cells were loaded with indo-1 (K+ salt) to an estimated cytosolic concentration of 44 +/- 5 microM (Mean +/- SEM, n = 5), and accessible cell volume was estimated to be 24.5 +/- 3.6 pl. Ca2+ transport by the sarcoplasmic reticulum (SR) Ca-ATPase and sarcolemmal Na-Ca exchange was inhibited by treatment with thapsigargin and Na-free solutions, respectively. Extracellular [Ca2+] was maintained at 10 mM and, in some experiments, the mitochondrial uncoupler "1799" was used to assess the degree of mitochondrial Ca2+ uptake. To perform single cell titrations, intracellular Ca2+ ([Ca2+]i) was increased progressively by a train of depolarizing voltage clamp pulses from -40 to +10 mV. The total Ca2+ gain with each pulse was calculated by integration of the Ca current and then analyzed as a function of the rapid change in [Ca2+]i during the pulse. In the range of [Ca2+]i from 0.1 to 2 microM, overall cell buffering was well described as a single lumped Michaelis-Menten type species with an apparent dissociation constant, KD, of of 0.63 +/- 0.07 microM (n = 5) and a binding capacity, Bmax, of 162 +/- 15 mumol/l cell H2O. Correction for buffering attributable to cytosolic indo-1 gives intrinsic cytosolic Ca2+ buffering parameters of KD = 0.96 +/- 0.18 microM and Bmax = 123 +/- 18 mumol/l cell H2O. The fast Ca2+ buffering measured in this manner agrees reasonably with the characteristics of known rapid Ca buffers (e.g., troponin C, calmodulin, and SR Ca-ATPase), but is only about half of the total Ca2+ buffering measured at equilibrium. Inclusion of slow Ca buffers such as the Ca/Mg sites on troponin C and myosin can account for the differences between fast Ca2+ buffering in phase with the Ca current measured in the present experiments and equilibrium Ca2+ buffering. The present data indicate that a rapid rise of [Ca2+]i from 0.1 to 1 microM during a contraction requires approximately 50 microM Ca2+ to be added to the cytosol.
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PMID:Intrinsic cytosolic calcium buffering properties of single rat cardiac myocytes. 781 10

We examined the influences of Ca2+ and crossbridge kinetics on the maximum rate of force development during Ca2+ activation of cardiac myofibrils and on the maximum rate of relaxation. Flash photolysis of diazo-2 or nitrophenyl-EGTA was used to produce a sudden decrease or increase, respectively, in [Ca2+] within Triton-skinned trabeculae from rat and guinea pig hearts (22 degrees C). Trabeculae from both species had similar Ca2+ sensitivities, suggesting that the rate of dissociation of Ca2+ from troponin C (k(off)) is similar in the 2 species. However, the rate of relaxation after diazo-2 photolysis was 5 times faster in the rat (16.1 +/- 0.9 s(-1), mean +/- SEM, n = 11) than in the guinea pig (2.99 +/- 0.26 s(-1), n = 7). This indicates that the maximum relaxation rate is limited by crossbridge kinetics rather than by k(off). The maximum rates of rapid activation by Ca2+ after nitrophenyl-EGTA photolysis (k(act)) and of force redevelopment after forcible crossbridge dissociation (k(act)) were similar and were approximately 5-fold faster in rat (k(act)= 14.4 +/- 0.9 s(-1), k(tr)= 13.0 +/- 0.6 s(-1)) than in guinea pig (k(act)= 2.57 +/- 0.14 s(-1), k(tr)= 2.69 +/- 0.30 s(-1)) trabeculae. This too may be mainly due to species differences in crossbridge kinetics. Both k(act) and k(tr) increased as [Ca2+] increased. This Ca2+ dependence of the rates of force development is consistent with current models for the Ca2+ activation of the crossbridge cycle, but these models do not explain the similarity in the maximal rates of activation and relaxation within a given species.
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PMID:Roles of Ca2+ and crossbridge kinetics in determining the maximum rates of Ca2+ activation and relaxation in rat and guinea pig skinned trabeculae. 968 64

The aim of this study was to compare the effects of increased concentrations of MgADP, inorganic phosphate (Pi) and H+ ([MgADP], [Pi] and [H+], respectively) on the rate of relaxation in two different muscle types: skinned muscle fibres from the frog Rana temporaria and myofibrillar bundles from the giant Pacific acorn barnacle Balanus nubilus. Relaxation transients are produced by the photolysis of diazo-2 and are well fitted with a double exponential curve, giving two rate constants: k1 [5.6+/-0.1 s-1 for barnacle, n=30; 26.3+/-0.7 s-1 for frog, n=14 (mean+/-SEM)] and k2 [0.6+/-0.1 s-1 in barnacle, n=30; 10.4+/-1.0 s-1 in frog, n=14 (mean+/-SEM)], at 10 degrees C. Decreasing the pH by 0.5 pH units did not significantly affect k1 for barnacle relaxation [5.6+/-0.1 s-1 (mean+/-SEM), n=15] compared to the decrease in k1 of 40% seen in frog. Use of the Ca2+-sensitive fluorescent label acrylodan on barnacle wild-type troponin C demonstrated that decreasing the pH from 7.0 to 6.6 only alters the pCa50 value by 0.23 in the cuvette, while stopped-flow experiments with acrylodan revealed no significant change in koff from the labelled protein [322+/-32 s-1 at pH 7.0 and 381+/-24 s-1 (mean+/-SEM) at pH 6.6]. Increasing [MgADP] by 20 microM (50 microM added ADP) from control values of 50 microM in frog decreased k1 to 12.3+/-0.4 s-1 (mean+/-SEM, n=8), and at 400 microM MgADP, k1=9.6+/-0.1 s-1 (mean+/-SEM, n=12). In barnacle, 500 microM MgADP had a much smaller effect on k1 (4.0+/-0. 9 s-1, mean+/-SEM, n=8). Increasing the free [Pi] from the contaminant level of 0.36 mM to 1.9 mM slowed k1 by approximately 15% in barnacle [4.8+/-0.8 s-1, mean+/-SEM, n=7], compared to a approximately 30% reduction seen in frog. We conclude that the differences between barnacle and frog seen here are most probably due to different isomers of the contractile proteins, and that events underlying the crossbridge cycle are the same or similar. We interpret our results according to a model of crossbridge transitions during relaxation.
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PMID:A diazo-2 study of relaxation mechanisms in frog and barnacle muscle fibres: effects of pH, MgADP, and inorganic phosphate. 992 60

Changes in skeletal troponin C (sTnC) structure during thin filament activation by Ca2+ and strongly bound cross-bridge states were monitored by measuring the linear dichroism of the 5' isomer of iodoacetamidotetramethylrhodamine (5'IATR), attached to Cys98 (sTnC-5'ATR), in sTnC-5'ATR reconstituted single skinned fibers from rabbit psoas muscle. To isolate the effects of Ca2+ and cross-bridge binding on sTnC structure, maximum Ca2+-activated force was inhibited with 0.5 mM AlF4- or with 30 mM 2,3 butanedione-monoxime (BDM) during measurements of the Ca2+ dependence of force and dichroism. Dichroism was 0.08 +/- 0.01 (+/- SEM, n = 9) in relaxing solution (pCa 9.2) and decreased to 0.004 +/- 0.002 (+/- SEM, n = 9) at pCa 4.0. Force and dichroism had similar Ca2+ sensitivities. Force inhibition with BDM caused no change in the amplitude and Ca2+ sensitivity of dichroism. Similarly, inhibition of force at pCa 4.0 with 0.5 mM AlF4- decreased force to 0.04 +/- 0.01 of maximum (+/- SEM, n = 3), and dichroism was 0.04 +/- 0.03 (+/- SEM, n = 3) of the value at pCa 9.2 and unchanged relative to the corresponding normalized value at pCa 4.0 (0.11 +/- 0.05, +/- SEM; n = 3). Inhibition of force with AlF4- also had no effect when sTnC structure was monitored by labeling with either 5-dimethylamino-1-napthalenylsulfonylaziridine (DANZ) or 4-(N-(iodoacetoxy)ethyl-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole (NBD). Increasing sarcomere length from 2.5 to 3.6 microm caused force (pCa 4.0) to decrease, but had no effect on dichroism. In contrast, rigor cross-bridge attachment caused dichroism at pCa 9.2 to decrease to 0.56 +/- 0.03 (+/- SEM, n = 5) of the value at pCa 9. 2, and force was 0.51 +/- 0.04 (+/- SEM, n = 6) of pCa 4.0 control. At pCa 4.0 in rigor, dichroism decreased further to 0.19 +/- 0.03 (+/- SEM, n = 6), slightly above the pCa 4.0 control level; force was 0.66 +/- 0.04 of pCa 4.0 control. These results indicate that cross-bridge binding in the rigor state alters sTnC structure, whereas cycling cross-bridges have little influence at either submaximum or maximum activating [Ca2+].
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PMID:Ca2+ and cross-bridge-induced changes in troponin C in skinned skeletal muscle fibers: effects of force inhibition. 1004 29

Incubation of mechanically skinned barnacle myofibrillar bundles in 10 mM orthovanadate (pH 6.6) results in the loss of Ca2+-dependent force generation, which reduces to 0.98+/-0.006% (mean +/-SEM, n=25) of control levels. Analysis of myofibrillar bundles by gel electrophoresis showed that tension loss is primarily due to the extraction of troponin C (TnC) (65.4+/-5.04% mean +/-SEM, n=5). This is a novel finding, since treating cardiac fibres with orthovanadate results in the removal of both TnC and troponin I (TnI) (28). Ca2+ dependence was restored to the myofibrillar bundles following reconstitution with either native isoform of barnacle TnC (BTnC1: 78. 72+/-12.8%, n=9, BTnC2: 82.73+/-20.3%, n=3). The reversible loss of Ca2+-dependent tension generation following the removal and replacement of TnC indicates that the regulation of contraction in the barnacle is controlled by thin-filament regulatory proteins.
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PMID:A novel method of extraction of TnC from skeletal muscle myofibrils. 1008 46

Levosimendan is a new myofilament calcium (Ca2+) sensitizer that increases myocardial contractility by stabilizing the Ca2+-bound conformation of troponin C. We tested the hypothesis that levosimendan enhances cardiac performance after cardiopulmonary bypass (CPB). Anesthesia was induced and maintained with midazolam, sufentanil, and vecuronium in 18 patients randomly assigned to receive levosimendan (18 or 36 microg/kg loading dose and 0.2 or 0.3 microg/kg/min infusion, respectively) or placebo 15 min before and continued for 6 h after CPB. Significant (p < 0.05) increases in heart rate (HR) and decreases in systemic vascular resistance (SVR) occurred 15 min after CPB in patients receiving placebo. Later increases in mean arterial pressure (MAP) and cardiac output (CO) and decreases in stroke volume (SV) and pulmonary vascular resistance also were observed. HR was greater in patients receiving high- but not low-dose levosimendan versus placebo immediately after CPB. MAP also was lower in patients treated with either dose of levosimendan compared with placebo after CPB. Levosimendan increased CO and decreased SVR (4.2 +/- 0.4 to 7.9 +/- 0.4 L/min and 1,150 +/- 99 to 512 +/- 42 dyn/s/cm5, respectively, 15 min after CPB; mean +/- SEM). CO and SV were higher and SVR was lower in patients receiving levosimendan versus placebo after CPB. No differences in arterial oxygenation and perioperative arrhythmias (Holter analysis) were observed between groups. The results indicate that levosimendan enhances cardiac performance after CPB in humans.
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PMID:Levosimendan enhances cardiac performance after cardiopulmonary bypass: a prospective, randomized placebo-controlled trial. 1044 73