PubMed Health⌕ Search

Biomedical subjects

D J Beuckelmann

Publications and source records attributed to D J Beuckelmann.

45 records · Page 3Linked to original sources
PubMed1989

Sodium-calcium exchange in mammalian heart: current-voltage relation and intracellular calcium concentration.

Membrane currents and changes in intracellular calcium ion concentration ([Ca2+]i) have been recorded that can be attributed to the operation of an electrogenic, voltage-dependent sodium-calcium (Na-Ca) exchanger in mammalian heart cells. Single guinea-pig ventricular myocytes under voltage clamp were perfused internally with the fluorescent Ca2+-indicator, fura-2, and changes in [Ca2+]i and membrane current that resulted from Na-Ca exchange were isolated through the use of various organic channel blockers (verapamil, TTX), impermeant ions (Cs+, Ni2+), and inhibitors of sarcoplasmic reticulum (ryanodine). The I-V relation of Na-Ca exchange was obtained from the Ni2+-sensitive current elicited by ramp repolarization from +90 mV to -80 mV. Ramps were sufficiently rapid that little change in [Ca2+]i occurred during the ramp. The (constant) [Ca2+]i during the ramp was varied over the range 100 nM to 1000 nM by varying the amplitude and duration of a pre-pulse to the ramp. The reversal potential of the Ni2+-sensitive ramp current varied linearly with 1n[( Ca2+])i. The I-V relations at different [Ca2+]i over the range -60 mV to +140 mV were in reasonable accord with the predictions of a simple, simultaneous scheme of Na-Ca exchange, on the basis that only [Ca2+]i had changed. The relationship between [Ca2+]i and current at a constant membrane voltage was also in accord with this scheme. We suggest that Ca2+-fluxes through the exchanger during the cardiac action potential can be understood quantitatively by considering the binding of Ca2+ to the exchanger during the [Ca2+]i-transient and the effects of membrane voltage on the exchanger.

Animals↗
PubMed1989

Sodium-calcium exchange in guinea-pig cardiac cells: exchange current and changes in intracellular Ca2+.

1. Membrane currents and changes in [Ca2+]i attributable to the operation of an electrogenic Na-Ca exchange mechanism were recorded in single isolated guinea-pig ventricular myocytes under voltage clamp and internal perfusion with the Ca2+ indicator Fura-2. 2. Ionic currents that interfere with the measurement of Na-Ca exchange current were blocked through the use of caesium (Cs+), verapamil and tetrodotoxin (TTX). Entry of Ca2+ through surface membrane Ca2+ channels and release of Ca2+ from sarcoplasmic reticulum were blocked with verapamil and ryanodine, respectively. 3. In the presence of the blockers listed above, depolarization to positive membrane potentials elicited slow increases in [Ca2+]i and, after an instantaneous increase, a declining outward current. Repolarization elicited a decline in [Ca2+]i and, after an instantaneous increase, a declining inward current. The changes in [Ca2+]i and a major component of the current were abolished by nickel ions (Ni2+; 5 mM). 4. The reversal potential of the current abolished by Ni2+ (Ni2+-sensitive current) was determined at different levels of [Ca2+]i by ramp repolarizations from +80 to -80 mV (1-5 mV/ms). The reversal potential of the current increased linearly with log [Ca2+]i. As a result of the foregoing and other data, the Ni2+-sensitive current was taken to be Na-Ca exchange current (INaCa). 5. The relation between INaCa and [Ca2+]i (less than 1 microM) at constant voltage over the range of -80 to +60 mV was approximately linear. No evidence of saturation could be found; small deviations from linearity at high [Ca2+]i were in the direction expected for a minor contribution from Ca2+-activated non-specific cation current (Ehara, Noma & Ono, 1988). 6. When measured at the same [Ca2+]i, the peak INaCa upon repolarization to -80 to -140 mV seemed to approach a limiting value at very negative potentials. 7. Over the range of +40 to +160 mV INaCa (measured soon after depolarization and thus at the same [Ca2+]i) increased exponentially with clamp-pulse potential. These pulses (to potentials up to +160 mV) elicited a slow rise in [Ca2+]i with the peak at the end of the pulse also increasing exponentially with pulse potential. 8. Inward membrane currents with properties similar to those described above were also recorded in association with physiological [Ca2+]i transients, when Ca2+ channels and the sarcoplasmic reticulum were not blocked. 9. Some of the results are not consistent with certain predictions of a sequential step model, or with those of a simultaneous step model in which the internal binding site for Ca2+ is saturated, or with those of a model based only on thermodynamics.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Mechanism of release of calcium from sarcoplasmic reticulum of guinea-pig cardiac cells.

1. The mechanisms that control release of Ca2+ from the sarcoplasmic reticulum (SR) of guinea-pig ventricular cells were studied by observing intracellular calcium concentration ([Ca2+]i transients) and membrane currents in voltage-clamped guinea-pig ventricular myocytes perfused internally with Fura-2. 2. Sarcolemmal Ca2+ current was identified through the use of tetrodotoxin (TTX) and Ca2+ channel antagonists (verapamil) and agonists (Bay K 8644). 3. Changes in [Ca2+]i attributable to release of Ca2+ from the SR were identified through the use of ryanodine, which abolishes the ability of the SR to release Ca2+. Ryanodine-sensitive increases in [Ca2+]i could be elicited either by depolarization or by repolarization (from depolarizing pulses to relatively positive membrane potentials). 4. At appropriate voltages, it is the initial fast change in [Ca2+]i elicited by either depolarization or repolarization that is abolished by ryanodine, and is defined here as ryanodine sensitive. 5. The amplitude of the ryanodine-sensitive [Ca2+]i transient elicited by depolarization had a bell-shaped dependence on membrane potential with a maximum of about 500 nM at 10 mV, and with the upper minimum between 60 and 70 mV. Verapamil-sensitive current activated over approximately the same potential range as the [Ca2+]i transient, with a peak amplitude at 10 mV, and a reversal potential of 65 mV. 6. When a holding potential of -68 mV and TTX (30 microM) were used, the most negative pulse potential at which activation of an inward current occurred was -49 mV while changes in [Ca2+]i occurred at -43 mV. 7. Ryanodine-sensitive increases in [Ca2+]i elicited by repolarization (tail transients) were maximal for repolarization to 0 mV. Smaller changes in [Ca2+]i than maximal were elicited by repolarization to both more positive and more negative potentials than 0 mV. The peak amplitude of the verapamil-sensitive tail currents elicited by repolarization increased continuously as the membrane was repolarized to potentials more negative than 60 mV. 8. Increasing depolarizing pulse duration beyond 10-20 ms did not increase the amplitude of the [Ca2+]i transient, but prolonged it. 9. The experimental results are compared to the predictions of two theories on the mechanism of excitation-contraction coupling: Ca2+-induced release of Ca2+ (CICR), as it has been formulated from data in skinned cardiac cells, and a charge-coupled release mechanism (CCRM), as it has been formulated to explain excitation-contraction coupling in skeletal muscle. 10. Some of the results are clearly not consistent with certain features of a charge-coupled release mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1988

[Ca2+]i in single isolated cardiac cells: a review of recent results obtained with digital imaging microscopy and fura-2.

The use of fluorescent Ca2+ indicators to observe [Ca2+]i transients in voltage-clamped single cells has many advantages over previous methods, such as the use of aequorin in multicellular preparations, for studying excitation-contraction coupling. In the studies reviewed in this article, [Ca2+]i in single isolated mammalian ventricular myocytes was observed through the use of the fluorescent Ca2+ indicator, fura-2. Individual cells, loaded with fura-2 either by internal perfusion or by exposure to fura-2/AM, were generally studied with the use of inverted microscopes equipped with ultraviolet epifluorescence illumination, intensified silicon intensifier target cameras (ISIT), and (or) a photomultiplier tube. Analysis of subcellular patterns of fura-2 fluorescence was performed by digital analysis of the images obtained with the ISIT camera. Variation of membrane voltage and exposure of cells to ryanodine (which was assumed to selectively block the release of Ca2+ from the sarcoplasmic reticulum) were used to investigate the cellular processes that determine the [Ca2+]i transient. The main results of these studies are the following. (1) In any population of enzymatically isolated heart cells, there are (i) mechanically quiescent cells in which [Ca2+]i is spatially uniform, constant over time, and relatively low; (ii) spontaneously contracting cells, which have a relatively elevated [Ca2+]i, but in which the spatial uniformity of [Ca2+]i is interrupted periodically by spontaneous, propagating waves of high [Ca2+]i; and (iii) cells that are hypercontracted (rounded up) and that have higher levels of [Ca2+]i than the other two types.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Sodium-calcium exchange in heart: membrane currents and changes in [Ca2+]i.

Recordings have been made of changes in intracellular calcium ion concentration ([Ca2+]i) that can be attributed to the operation of an electrogenic, voltage-dependent sodium-calcium (Na-Ca) exchanger in mammalian heart cells. Guinea pig ventricular myocytes under voltage clamp were perfused internally with fura-2, a fluorescent Ca2+-indicator, and changes in [Ca2+]i and membrane current that resulted from Na-Ca exchange were identified through the use of various organic channel blockers and impermeant ions. Depolarization of cells elicited slow increases in [Ca2+]i, with the maximum increase depending on internal [Na+], external [Ca2+], and membrane voltage. Repolarization was associated with net Ca2+ efflux and a decline in the inward current that developed instantaneously upon repolarization. The relation between [Ca2+]i and current was linear, and the slope was made steeper by hyperpolarization.

Animals↗
PubMedDate not supplied

Hyperpolarization-activated inward current in ventricular myocytes from normal and failing human hearts.

BACKGROUND: The hyperpolarization-activated inward current (I[f]) was found to be overexpressed in hypertrophied rat ventricular myocytes, indicating that I(f) might favor arrhythmias in hypertrophied or failing ventricular myocardium. In the present study, we evaluated whether I(f) is expressed in human ventricular myocardium, if it may be increased in human heart failure, and if its autonomic modulation may be altered. METHODS AND RESULTS: The whole-cell patch-clamp technique was used to record I(f) in isolated ventricular myocytes from 34 failing (dilated [DCM] or ischemic [ICM] cardiomyopathy) and 13 donor hearts (NF). I(f) was observed in all myocytes showing typical current properties, ie, time and voltage dependence, block by [Cs+]o, permeability for K+ and Na+, and current increase with raising [K+]o. There was a trend toward larger current densities in myopathic (at -130 mV in [K+]o 25 mmol/L; DCM: -1.37 +/- 0.12 pA/pF, n = 50; ICM: -1.39 +/- 0.24 pA/pF, n = 30) than in nonfailing cells (-1.18 +/- 0.21 pA/pF, n = 24), although this difference did not reach statistical significance (P=.23). Boltzmann distributions yielded an activation threshold of -80 mV and half-maximal activation at -110.96 +/- 0.06 mV in myopathic and normal myocytes. Isoproterenol (10(-5) mol/L) shifted the current activation by 10 mV (31 myopathic, 5 NF). Carbachol and adenosine had no direct effect on I(f) (6 and 12 myopathic, 3 and 3 NF, respectively) but reversibly antagonized beta-adrenergic stimulation (5 and 7 myopathic, 2 and 2 NF, respectively). Autonomic modulation was similar in failing and nonfailing cells. CONCLUSIONS: In end-stage heart failure, no significant change of I(f) could be found, although there was a trend toward increased I(f). Together with an elevated plasma norepinephrine concentration and a previously reported reduction in I(K1) in human heart failure, I(f) might favor diastolic depolarization in individual myopathic cells.

Action Potentials↗