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  • 1
    Electronic Resource
    Electronic Resource
    Calcium transients which accompany the activation of sodium current in rat ventricular myocytes at 37°C: a trigger role for reverse Na-Ca exchange activated by membrane potential? (1995)
    Springer
    Pflügers Archiv 430 (1995), S. 887-893 
    Details
    ISSN: 1432-2013
    Keywords: Sodium current ; Na-Ca exchange ; Excitation-contraction coupling ; Cardiac myocyte ; Calcium transient
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract We investigated the role of the fast sodium current (I Na) in triggering Ca release from the sarcoplasmic reticulum (SR), using adult rat left ventricular myocytes, loaded with Fura-2 to measure intracellular Ca (Cai), which were whole-cell patch-clamped at 35–37°C. Before each test pulse, a series of 400-ms conditioning pulses to +10 mV were applied to establish a constant level of SR Ca load. Pulses were applied every 15 s. A test pulse from −80 mV to −50 mV elicited a rapidI Na and a phasic Cai transient. When the solution perfusing a myocyte was rapidly switched for 15 s before a test pulse to one containing the L-type Ca channel blocker nifedipine (20 μM), the test pulse still activatedI Na and a phasic Cai transient, the amplitude of which was not significantly different from control (P〉0.05;t-test). When a rapid switch to 20 μM nifedipine plus 30 μM tetrodotoxin (TTX) was made 15 s before a test pulse, bothI Na and the Cai transient were completely abolished (n=6). When a switch was made to Na-free (Li) solution, which contained 20 μM nifedipine to block L-type Ca current,I Ca,L, there was no significant difference in the Cai transient amplitude from that of control (P〉0.05;n=6). Brief depolarising test pulses (−80 mV to +20 mV, 10 ms duration) to simulate membrane potential escape also elicited a Cai transient which attained 90.0% (±2.8%;n=7) of the Cai transient activated by a conditioning pulse to +10 mV. The Cai transient with a brief pulse was not significantly affected by application of 20 μM nifedipine (P〉0.05), but adding TTX with nifedipine reduced the Cai transient amplitude to 76.9% (±6.8%;P〈0.02;n=8). In four cells, the Cai transient remaining in the presence of nifedipine plus TTX was abolished by adding 5 mM Ni. These data are consistent with “voltage escape” during activation ofI Na leading to a trigger Ca entry via a mechanism other than L-type Ca channels or subsarcolemmal Na accumulation with reverse Na-Ca exchange. The block by Ni of the Cai transient suggests that a brief membrane potential escape might directly activate reverse mode Na-Ca exchange to trigger SR release, and this mechanism would seem to account largely for the Cai transient which accompaniesI Na in rat myocytes, under these experimental recording conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01837401
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  • 2
    Electronic Resource
    Electronic Resource
    Characteristics of the delayed rectifier K current compared in myocytes isolated from the atrioventricular node and ventricle of the rabbit heart (1996)
    Springer
    Pflügers Archiv 431 (1996), S. 713-722 
    Details
    ISSN: 1432-2013
    Keywords: Atrioventricular node ; Myocyte ; Ventricle ; Delayed rectifier potassium current (I k) ; E-4031
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The delayed rectifier potassium current (I K) is known to be important in action potential repolarisation and may contribute to the diastolic pacemaker depolarisation in pacemaker cells from the heart. In this study, using whole-cell patch clamp, we investigated the characteristics ofI K in morphologically normal cells from the atrioventricular node (AVN) and ventricle of the rabbit heart. Cells were held at −40 mV and 5 μM external nifedipine was used to block L-type calcium current (I Ca,L). SignificantI K was observed with pulses to potentials more positive than −30 mV. The steady-state activation curve in both cell types showed maximal activation at between + 10 and + 20 mV. Half-maximal activation ofI K occurred at −4.9 and −4.1 mV with slope factors of 8.3 and 12.4 mV in ventricular and AVN cells, respectively. Using pulses of increasing duration, significantI K tails after repolarisation from + 40 mV were observed with pulses of 20 ms and increased with pulses up to 100–120 ms in both cell types. Pulses of longer duration did not activate furtherI K and this suggested that only the rapid component ofI K, calledI Kr, was present in either cell type. Moreover,I K tails after pulses to all potentials were blocked completely by E-4031, a selective blocker ofI Kr. The reversal potential ofI K varied with the concentration of external K. Superfusion of AVN cells with medium containing 4, 15 and 40 mM [K+]o resulted in reversal potentials of −81, −56 and −32 mV respectively, which are close to values predicted if theI K channel were highly selective for K. The time constants for deactivation ofI K in ventricle and AVN on return to −40 mV after a 500-ms activating pulse to + 60 mV were 480 ms and 230 ms, respectively. The faster deactivation ofI K in AVN cells was a distinguishing feature and suggests that there may be differences in theI Kr channel protein between ventricular and AVN cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02253834
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  • 3
    Electronic Resource
    Electronic Resource
    Actions of myristyl-FRCRCFa, a cell-permeant blocker of the cardiac sarcolemmal Na-Ca exchanger, tested in rabbit ventricular myocytes (1998)
    Springer
    Pflügers Archiv 436 (1998), S. 581-590 
    Details
    ISSN: 1432-2013
    Keywords: Key words Delayed rectifier ; FRCRCFa ; Inward rectifier ; L-type calcium current ; Myocyte ; Myristyl-FRCRCFa ; Na-Ca exchange ; Rabbit ventricle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  In cardiac muscle, the electrogenic Na-Ca exchanger plays important roles in determining action potential shape and in the beat-to-beat homeostasis of intracellular calcium. In this study we tested the actions of a putative cell-permeant blocker of the cardiac sarcolemmal Na-Ca exchange, ”Myristyl- (Myr-) FRCRCFa”. Experiments were performed using isolated rabbit right ventricular myocytes and whole-cell patch-clamp at 35–37°C. The Na-Ca exchange current (I Na-Ca), L-type calcium current (I Ca,L), inward rectifier potassium current (I K1) and delayed rectifier potassium current (I K) were compared in untreated cells and cells incubated in a solution containing N-myristylated FRCRCFa. With other major currents blocked, I Na-Ca was measured as the Ni-sensitive component of current during a voltage ramp applied from the holding potential of –40 mV, between +80 and –120 mV (ramp velocity 0.1 V s–1). In untreated cells, I Na-Ca at +60 mV was 7.1±0.6 pA/pF and at –100 mV was –2.7±0.3 pA/pF (n=9). After a 15-min pre-incubation with 20 µM Myr-FRCRCFa, I Na-Ca was reduced to 4.2±0.3 pA/pF at +60 mV and –1.5±0.2 pA/pF at –100 mV (P〈0.02; n=7). After incubation with 20 µM Myr-FRCRCFa for 1 h, I Na-Ca at both potentials was further reduced (2.3±0.8 pA/pF at +60 mV; –0.9±0.3 pA/pF at –100 mV; P〈0.008 compared with control; n=4). Under selective recording conditions for I Ca,L, there was little difference in I Ca,L density between untreated and cells incubated with Myr-FRCRCFa. A Boltzmann fit to the I Ca,L/V relation showed no significant alteration of half-maximal activation potential or slope factor of activation. I K1 was also largely unaffected by pre-incubation of cells with Myr-FRCRCFa. I K, measured as deactivating tail current following 1-s test depolarisations to a range of test potentials, was also not significantly altered by Myr-FRCRCFa. The suppression of I Na-Ca in cells incubated in Myr-FRCRCFa suggests that addition of the myristyl group to FRCRCFa peptide conveys cell permeancy to the peptide and that Myr-FRCRCFa applied externally to rabbit ventricular myocytes is moderately effective as an I Na-Ca blocker. I Ca,L, I K1 and I K were largely unaffected by Myr-FRCRCFa. N-Myristylation of such conformationally constrained hexapeptides may, therefore, provide a means of producing cell-permeant inhibitors of the cardiac Na-Ca exchanger.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050675
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  • 4
    Electronic Resource
    Electronic Resource
    The Fura-2 transient can show two types of voltage dependence at 36°C in ventricular myocytes isolated from the rat heart (1996)
    Springer
    Pflügers Archiv 432 (1996), S. 215-224 
    Details
    ISSN: 1432-2013
    Keywords: Key words Excitation-contraction coupling ; Na-Ca exchange ; Calcium transient ; cardiac myocyte
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract We used the whole-cell patch-clamp method to investigate the voltage dependence of the L-type Ca current (I Ca,L) and intracellular Ca (Cai) transient in ventricular myocytes isolated from the rat heart. Intracellular Ca was monitored using Fura-2 and the experiments were carried out at 36° C. We measured I Ca,L by using a caesium-based internal dialysis solution to eliminate interfering K currents. The voltage dependence of peak I Ca,L amplitude was bell-shaped: I Ca,L was maximal at +10 mV and declined at more positive potentials. When I Ca,L was integrated over the first 25 ms to estimate the magnitude of Ca entry, this had a very similar voltage dependence to peak I Ca,L. In all cells, phasic Fura-2 transients were abolished by 5 μM ryanodine (a blocker of the sarcoplasmic reticulum, SR) showing that the Fura-2 transient provided an index of the magnitude of SR Ca release. For experiments measuring the Cai transient, we used a K-based internal dialysis solution to preserve normal excitation-contraction coupling. In 30–40% of cells, we found that the Fura-2 transient had a bell-shaped voltage dependence. This suggests that, in these cells, the primary trigger mechanism for Ca-induced Ca-release might have been Ca entry via I Ca,L. In the remaining 60–70% of cells, the voltage dependence of the Fura-2 transient was not bell-shaped. The Fura-2 transient reached a maximum with a pulse to +10 mV, and the amplitude of the transient did not decline significantly at more positive potentials to this. In cells with a non-bell-shaped voltage dependence of the Fura-2 transient, pulses to potentials as far positive as +140 mV elicited phasic Fura-2 transients. Since this potential exceeded the Nernst potential for Ca, it was unlikely there was any tigger Ca entry via I Ca,L at this potential. This would suggest that, in these cells, another trigger for SR Ca release (in addition to I Ca,L) might be present. We conclude that rat ventricular myocytes, produced using a standard isolation technique and under standard recording conditions, can show either a bell-shaped or a sigmoidal voltage dependence of the Fura-2 transient.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050127
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  • 5
    Electronic Resource
    Electronic Resource
    Voltage dependence of the Fura-2 transient in rabbit left atrial myocytes at 37°C (1997)
    Springer
    Pflügers Archiv 433 (1997), S. 817-826 
    Details
    ISSN: 1432-2013
    Keywords: Key words Excitation ; contraction coupling ; Na-Ca exchange ; Calcium transient ; Atrial myocyte ; Cardiac myocyte
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  We used the whole-cell patch-clamp technique and monitoring of Fura-2 fluorescence to investigate the voltage dependence of the L-type Ca current (I Ca,L) and intracellular Ca (Cai) transient in rabbit atrial myocytes at 37°C. Imaging the atrial cell membrane with Di-4-ANNEPS showed (in contrast to ventricular cells) that atrial cells had very few transverse tubules. We measured I Ca,L using a Cs-based internal dialysis solution to eliminate interfering K currents. The voltage dependence of peak I Ca,L amplitude was bell-shaped: I Ca,L was maximal at +10 mV and declined at more negative and positive potentials. For measuring the Fura-2 (Cai) transient, we used a K-based internal dialysis solution to preserve normal excitation–contraction coupling. Ryanodine (20 μM) plus thapsigargin (2 μM) (blockers of the sarcoplasmic reticulum, SR) abolished the phasic component of the Fura-2 transient (n = 5), demonstrating that the phasic Fura-2 transient provided an index of the magnitude of SR release. The Fura-2 transient also showed bell-shaped voltage dependence, but this was different from that for I Ca,L. The Fura-2 transient peaked at +30 mV and partially declined at more positive potentials; but at potentials where inward I Ca,L was small (if not absent), the phasic Fura-2 transient still attained a significant amplitude. We used a rapid application of nifedipine (32 μM), and of nifedipine plus 5 mM Ni, to assess the ability of I Ca,L and reverse-mode Na-Ca exchange to trigger SR Ca release. With test pulses to +10 mV and +60 mV, a rapid switch to nifedipine (which blocked I Ca,L) produced no significant reduction in phasic Fura-2 transient amplitude. This suggests that in the absence of I Ca,L, another mechanism was able to trigger SR release. With pulses to +10 and +60 mV, a single beat switch to nifedipine plus 5 mM Ni almost completely abolished the phasic transient. Since 5 mM Ni inhibits Na-Ca exchange, this suggests that, in the absence of I Ca,L, trigger Ca entry via reverse Na-Ca exchange was able to activate SR Ca release in atrial cells at 37°C. The mechanisms underlying the Fura-2 transient in atrial cells, and differences with pre-existing data from rabbit ventricular cells, are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050350
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Characteristics of the delayed rectifier K current compared in myocytes isolated from the atrioventricular node and ventricle of the rabbit heart (1996)
    Springer
    Pflügers Archiv 431 (1996), S. 713-722 
    Details
    ISSN: 1432-2013
    Keywords: Key words Atrioventricular node ; Myocyte ; Ventricle ; Delayed rectifier potassium current (Ik) ; E-4031
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The delayed rectifier potassium current (I K) is known to be important in action potential repolarisation and may contribute to the diastolic pacemaker depolarisation in pacemaker cells from the heart. In this study, using whole-cell patch clamp, we investigated the characteristics of I K in morphologically normal cells from the atrioventricular node (AVN) and ventricle of the rabbit heart. Cells were held at −40 mV and 5 μM external nifedipine was used to block L-type calcium current (I Ca,L). Significant I K was observed with pulses to potentials more positive than −30 mV. The steady-state activation curve in both cell types showed maximal activation at between + 10 and + 20 mV. Half-maximal activation of I K occurred at −4.9 and −4.1 mV with slope factors of 8.3 and 12.4 mV in ventricular and AVN cells, respectively. Using pulses of increasing duration, significant I K tails after repolarisation from + 40 mV were observed with pulses of 20 ms and increased with pulses up to 100–120 ms in both cell types. Pulses of longer duration did not activate further I K and this suggested that only the rapid component of I K, called I Kr, was present in either cell type. Moreover, I K tails after pulses to all potentials were blocked completely by E-4031, a selective blocker of I Kr. The reversal potential of I K varied with the concentration of external K. Superfusion of AVN cells with medium containing 4, 15 and 40 mM [K+]o resulted in reversal potentials of −81, −56 and −32 mV, respectively, which are close to values predicted if the I K channel were highly selective for K. The time constants for deactivation of I K in ventricle and AVN on return to −40 mV after a 500-ms activating pulse to + 60 mV were 480 ms and 230 ms, respectively. The faster deactivation of I K in AVN cells was a distinguishing feature and suggests that there may be differences in the I Kr channel protein between ventricular and AVN cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050056
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    Paper (German National Licenses)
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