Biomedical subjects

R Albitz

Publications and source records attributed to R Albitz.

12 recordsLinked to original sources

PubMed1992

Thrombin stimulates L-type calcium channels of guinea pig cardiomyocytes in cell-attached patches but not after intracellular dialysis.

The action of the blood clotting enzyme thrombin on single channel and whole cell Ca(2+)-currents was studied in isolated mammalian cardiac myocytes. Thrombin, at a concentration of 10(-8) mol/l, increased the Ca(2+)-channel activity in cell-attached patches. The mean open probability of the channel was enhanced, while the number of sweeps without openings, which reflects the availability of the channel, was significantly reduced. Neither the single channel conductance nor the activation curve were affected by thrombin. Thrombin was added to the bath solution, and its effect is therefore indirect and probably mediated via a second messenger. However, thrombin did not affect whole-cell Ca(2+)-currents, whereas a beta-adrenergic stimulation in the same cell increased the Ca(2+)-current. It is concluded that thrombin affects an intracellular mechanism for Ca2+ channel current regulation, which is still unknown and which is rapidly lost during conventional whole-cell Ca2+ current measurements.

Animals↗

PubMed1991

The conductance of single cardiac sodium channels from guinea pig depends on the intracellular sodium concentration.

Currents through DPI 201-106 modified single sodium channels have been measured in cell-free inside-out patches from guinea-pig ventricular myocytes. Single-channel conductance and reversal potential of the sodium channel have been calculated at different intracellular sodium concentrations [( Na+]i) from microscopic I-V curves, which were obtained by application of linear voltage ramps. The relation between the reversal potential and [Na+]i could be fitted with a modified Goldman-Hodgkin-Katz equation with a relative permeability for K+ over Na+ ions of 0.054. The zero-current conductance of the Na channel as a function of [Na+]i shows a plateau value at low Na concentrations, and increases in a sigmoidal manner at higher concentrations. It is concluded that the Na channel can carry outward currents and that its conductance depends on [Na+]i.

Animals↗

PubMed1991

Substates of cardiac sodium channels are due to both decrease in conductance and changes in selectivity.

Currents through DPI 201-106 modified single cardiac sodium channels in guinea pig ventricular cells were measured using the patch clamp technique in the cell-free configuration to control the sodium concentrations on both sides of the patch membrane. Current-voltage relationships of the single channels were obtained by application of linear voltage ramps from -140 to 100 mV. With 10 mmol/l Na+ at the inner surface of the patch, openings of sodium channels with conductances of 17 pS (selectivity ratios PK/PNa = 0.083 and PK/PNa = 0.58) and 12 pS (selectivity ratios PK/PNa = 0.084 and PK/PNa = 1.832) were obtained. With 30 mmol/l internal sodium, conductances of 20, 10, and 7 pS and selectivity ratios of 0.084, 0.386, and 0.543, respectively, could be measured. It is concluded that substates of sodium channel currents are due to changes in single channel conductance as well as in selectivity, or to changes of both independently of each other which accounts for the variability of conductance levels of cardiac Na channels.

Animals↗

PubMed1990

Block of single cardiac sodium channels by intracellular magnesium.

Currents through single cardiac sodium channels have been measured in inside-out patches from guinea pig ventricular cells. To abolish the fast inactivation, Na channels were modified by DPI 201-106. In symmetrical Na solutions, a diminution of outward sodium currents can be observed that depends on the intracellular magnesium concentration and the membrane potential. Inward currents were not altered by the concentrations of magnesium used (between 0 and 22.5 mmol/l). In Mg free solutions a linear current-voltage relation can also be measured in the range of outward Na currents. At +60 mV (symmetrical Na solutions, single channel conductance 24 pS) a half maximal block of cardiac Na channels by intracellular magnesium was found at 2.1 mmol/l. From the analysis of single channel current-voltage relationships the concentration and voltage-dependent block by intracellular magnesium of cardiac sodium channels could be described as binding of Mg at one site with a Kd value of 5.1 mmol/l at 0 mV. The site is located at an electrical distance of 0.18 from the inside.

Animals↗

PubMed1990

Effects of thrombin on single calcium channels in frog ventricular cells.

Single calcium channel (Ca channel) currents were measured using the patch-clamp technique in isolated ventricular myocytes of the frog (Rana esculenta). Sodium was used as the charge carrier. After formation of cell-attached patches, the proteolytic enzyme thrombin was added to the bath solution, where it increased the amplitude of the averaged currents more than twofold, by decreasing the number of empty sweeps and reducing the time constant of the slow exponential term of the shut-time histogram. Single channel conductance was not changed by thrombin. If the activation kinetics of the Ca channels are described by the commonly used C1-C2-O model, where C1 and C2 indicate closed states 1 and 2 respectively and O denotes the open state, thrombin increases the open-state probability in the non-empty sweeps by increasing the rate constant (k1) for the transition from C1 to C2. It is shown that thrombin acts via an H-7 blockable pathway.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗

PubMed1990

Free energy of ATP-hydrolysis fails to affect ATP-dependent potassium channels in isolated mouse ventricular cells.

The single channel recording technique has been used to study the adenosine-5'-triphosphate (ATP)-dependent K+ channel in isolated mouse ventricular cells. The aim of this work was to determine if the activity of the K+ channel depends on the free energy of ATP-hydrolytic reaction (phosphorylation potential) in addition to the well-studied direct block by ATP. When the phosphorylation potential was changed from 60 to 50, to 40 and back to 60 kJ/mol at a constant ATP-concentration of 10(-4) mol/l, the ATP-channel activity showed a run down that is best described by a linear function. Changing the ATP-concentration of the bath solution from 0.01 to 1 and back to 0.01 mmol/l by a factor of 10 at a constant phosphorylation potential of 50 kJ/mol, resulted in a run down of the mean average current by a reduction of the open state probability in a concentration dependent manner. A dissociation constant of about 0.14 mmol/l could be estimated. The amplitude of the single channel current was not affected by ATP-concentrations in contrast to changes in phosphorylation potential. A significant increase in the single channel current accompanying a decrease in the phosphorylation potential at constant ATP concentrations was observed. This effect might be due to a decrease of free Mg2(+)-concentration by an increase of the ADP concentration in solutions with lower phosphorylation potentials. An allosteric regulation of the ATP-dependent K+ channel dependent on the free ATP concentration together with the Mg2(+)-ADP concentration seems to be a more likely explanation than regulation by phosphorylation/dephosphorylation secondary to a breakdown of ATP.

Action Potentials↗

PubMed1989

Modulation of cardiac Na channels by angiotensin II.

The modulation of Na channels by the vasoactive peptide angiotensin II (AT II) has been studied in isolated ventricular cells of guinea pigs using the patch clamp technique. In cell-attached patches the maximal probability of the channel being open was increased in a concentration range between 0.05 and 1 microM, but decreased at higher concentrations. A maximal increased of 2.5 +/- 0.86 was found at 1 microM AT II. The increase in the probability of the channel being open was due to a decrease in the number of nulls. In all affected cells (n = 17) we observed a delayed inactivation after application of AT II at concentrations between 0.05 and 10 microM. At -30 mV, the time constant of inactivation increased from 1.1 +/- 0.1 ms (controls) to 5.6 +/- 1.6 ms (10 microM AT II). This effect was due to an increased number of openings per sweeps. No significant effect on the mean open time and the first latency were observed. However, due to pronounced bursting, the averaged closed time was significantly increased from 0.8 +/- 0.1 ms to 1.3 +/- 0.1 ms in the presence of 1 microM AT II at -30 mV. An effect of AT II on cardiac Na channels via protein kinase C is discussed.

Angiotensin II↗

PubMed1989

Ca-channel currents in isolated frog ventricular cells are increased by thrombin.

Effects of the proteolytic enzyme thrombin in the modulation of cardiac Ca-channel currents were examined in single ventricular cells from frog myocardium, using the whole-cell voltage clamp technique (1). Application of 3.8 . 10(-9) M thrombin to the bath increased the peak of the Ca-channel current by 84 +/- 35% (8 cells). Hirudin (31.10(-9)M), a specific thrombin inhibitor, blocked the thrombin-induced increase of this current. The increase in the current can be made responsible for the measured positive inotropic effects on frog heart of thrombin.

Animals↗

PubMed1988

Thrombin stimulates Ca-channel currents in isolated frog ventricular cells.

Effects of the proteolytic enzyme thrombin in the modulation of cardiac Ca-channel currents were examined in single ventricular cells from frog myocardium, using the whole-cell voltage clamp technique (Hamill et al. 1981). Application of 3.8.10(-9) M thrombin to the bath increased the peak of the Ca-channel current by 84 +/- 35% (8 cells). Hirudin (31.10(-9) M), a specific thrombin inhibitor, blocked the thrombin-induced increase of this current. The increase in the current can be made responsible for the measured positive inotropic effects of thrombin as well as for the shift of the plateau voltage of the action potentials towards more positive values.

Action Potentials↗

PubMed1988

Pharmacological effects on oscillatory afterpotentials in partially depolarized ventricular myocardium.

Oscillatory afterpotentials induced by toxic concentrations of ouabain were studied in isolated papillary muscles from the right ventricular myocardium of guinea pigs. In partially depolarized muscles (22 mM extracellular K+) oscillatory afterpotentials (OAP) that follow action potentials can be described as nearly harmonic, periodical damped oscillation using a five parameter model. We measured the time course of the appearance and disappearance of ouabain-induced OAP under control conditions and modified by different pharmacological tools. Changes were quantified by the parameters of the model used. Adriamycin (50 microM) significantly delayed the time to the first OAP after administration of ouabain but also shortened the time necessary for recovery after a 30 min application of ouabain. Adriamycin also reduced the damping ratio of OAP but unlike caffeine (2 mM) it did not completely suppress OAP. The reduction of the extracellular Na+ concentration from 126 to 63 mM strikingly depressed OAP both in damping ratio and amplitude. Dexamethasone (2 microM) transiently depressed already developed OAP. The effects of adriamycin as well as Na+ reduction point to a contribution of a Na/Ca exchange in the generation of OAP. The effect of dexamethasone may refer to an involvement of eicosanoides in the generation of arrhythmogenic OAP.

Animals↗

PubMed1988

Mechanisms involved in generation of oscillatory afterpotentials in myocardium.

We used ouabain-treated papillary muscles from guinea pig to study oscillatory afterpotentials (OAP) that follow an action potential. Two effects were investigated in more detail. 1. The membrane Na gradient was changed by isoomotical substitution of NaCl with sucrose. 50% Na substitution induced a nearly complete suppression of OAP. 2. Different K+ concentrations were used to compare OAP at high and low resting potentials. Depolarization of the ventricular preparations from about-85 mV (4mM KCl) to -55 mV (22 mM KCl) reversibly reduced OAP. Both effects can be predicted by a model of OAP induced by a transient inward current. This inward current is generated by an electrogenic Na/Ca exchanger. A supposed oscillatory changing of intracellular Ca concentration modifies the exchange current in agreement with the experimental data.

Animals↗

PubMed1988

Contribution of a Ca-dependent component to the transient outward current in rabbit ventricular fibres.

To evaluate a Ca-sensitive component of the transient outward current, we studied action potentials, isometric tension, and membrane currents (single sucrose gap voltage clamp) of rabbit ventricular heart muscle in the presence of Ryanodine that selectively blocks the intracellular Ca release. Ryanodine (1 microM) prolonged the post rest (10 s) action potential, abolished the "notch" preceding the plateau phase, and depressed the isometric tension. With the same protocol a diminished transient outward current was measured. Application of 1 mM 4-aminopyridine in the presence of Ryanodine further prolonged the duration of the action potential, decreased the transient outward current but increased the maximal tension. It is concluded that in rabbit ventricular myocardium both a Ca-dependent component and a 4-aminopyridine sensitive component compose the transient outward current; however, the 4-aminopyridine sensitive component is dominating.

Action Potentials↗