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Biomedical subjects

G Isenberg

Publications and source records attributed to G Isenberg.

At least 199 records · Page 11Linked to original sources
PubMed1977

Cardiac Purkinje fibres: [Ca2+]i controls steady state potassium conductance.

The influence of intracellular calcium concentration [Ca2+]i on the steady state membrane currents i infinity was studied in a range of clamp potentials between -20 and -100 mV. Injection of CaCl2 or Ca-EGTA (pCa less than or equal to 6) increased i infinity whereas injection of K-EGTA diminished it. The changes delta i infinity were attributed to a change in steady state potassium conductance, gK infinity, by four arguments: delta i infinity was restricted to potentials negative to -20 mV and depended on clamp potential in an inward rectifying manner. delta i infinity displayed a reversal potential, Erev' which followed log [K+]o with 60 mV for a tenfold change. Since Erev obtained during Ca injection agreed with Erev observed during EGTA injection the potassium driving force had to be constant. When gK was blocked by superfusion with 20 mM Cesium neither CaCl2 nor K-EGTA injection modified i infinity.

Animals↗
PubMed1977

Cardiac Purkinje fibres: [Ca2+]i controls the potassium permeability via the conductance components gK1 and gK2.

The pacemaker tail current of sheep Purkinje fibres became larger when CaCl2 was injected intracellularly by pressure, whereas it was reduced or even abolished by injection of K-EGTA. The analysis of the [Ca2+]k-induced changes in the steady state activation (s infinity) curve suggested that the effect could not be explained on the basis of internal screening of surface charges. The position of the s infinity-curve on the potential axis remained almost unchanged after the injection of CaCl2 or K-EGTA. On the other hand, the vertical amplitude of the s infinity-curve on the current axis, proportional to gK2, was enlarged during CaCl2 injections and strongly diminished by K-EGTA injections. Ca injection displaced the fully deactivated part of the s infinity-curve to larger outward current which corresponds to an increase in gK1. EGTA injection reduced gI1. The results correspond to a [Ca2+]i-sensitive potassium permeability. A possible interpretation is that [Ca2+]i controls Ca-binding near the inner mouth of a potassium channel and that the binding is followed by a change in the channel's configuration from a closed to an open state.

Animals↗
PubMed1977

Depletion and accumulation of potassium in the extracellular clefts of cardiac Purkinje fibers during voltage clamp hyperpolarization and depolarization.

1. Voltage clamp hyperpolarization and depolarization elicited current records consistent with depletion and accumulation, respectively, of potassium in the extracellular clefts of cardiac Purkinje fibers. Hyperpolarization was shown to shift the reversal potential for the pacemaker current, ik2, a measure of Ek, to more negative potentials. Upon depolarization, a slowly increasing outward current was observed. Analysis of the tail currents elicited by hyperpolarization revealed that a time-dependent change in gx could not explain the time-dependent outward current. However, the tail currents were consistent with a shift of Ek to more positive potentials during the depolarization. 2. Alteration in potassium driving force over time results in a time-dependent ik1 even though the underlying conductance is time-independent [29]. This time-dependent current may contribute to the currents usually identified as ik2 and ix. 3. The potential at which ik2 reverses direction is altered by the clamp program used to elicit it and is obscured by the superimposition of a time-dependent current due to depletion. 4. Records consistent with the extracellular cleft potassium concentration being less than that of the bulk phase in the quiescent fiber were obtained. However, an unequivocal interpretation of these current reocrds could not be made. 5. These results suggest that conclusions based on the assumption that potassium driving force remains constant during a voltage clamp pulse may be in error. Thus, time-dependent currents cannot be assumed to result solely from time-dependent conductance changes.

Animals↗
PubMed1977

Depletion and accumulation of potassium in the extracellular clefts of cardiac Purkinje fibers during voltage clamp hyperpolarization and depolarization: experiments in sodium-free bathing media.

Voltage clamp hyperpolarization and depolarization result in currents consistent with depletion and accumulation of potassium in the extracellular clefts o cardiac Purkinje fibers exposed to sodium-free solutions. Upon hyperpolarization, an inward current that decreased with time (id) was observed. The time course of tail currents could not be explained by a conductance exhibiting voltage-dependent kinetics. The effect of exposure to cesium, changes in bathing media potassium concentration and osmolarity, and the behavior of membrane potential after hyperpolarizing pulses are all consistent with depletion of potassium upon hyperpolarization. A declining outward current was observed upon depolarization. Increasing the bathing media potassium concentration reduced the magnitude of this current. After voltage clamp depolarizations, membrane potential transiently became more positive. These findings suggest that accumulation of potassium occurs upon depolarization. The results indicate that changes in ionic driving force may be easily and rapidly induced. Consequently, conclusions based on the assumption that driving force remains constant during the course of a voltage step may be in error.

Animals↗
PubMed1976

Transformation of cytoplasmic actin. Importance for the organization of the contractile gel reticulum and the contraction--relasation cycle of cytoplasmic actomyosin.

(1) Within the low viscous flowing endoplasm of Physarum polycephalum a considerable amount of actin is in the non-filamentous state. This can be demonstrated by applying poly-L-lysin to surface spreads of native protoplasm. (2) It has been shown that in protoplasmic drops the endoplasm-ectoplasm transformation is accompanied by an actin polymerization from the non-filamentous state to F-actin. (3) The actual state of the labile G-F-actin equilibrium determines the varying consistency (viscosity) of the cytoplasm. (4) Increasing viscosity can be interpreted as being brought about by a) shifting of the G-F-actin equilibrium to the filamentous side, and (b) increased myosin-mediated binding sites between actin filaments. (5) Polymerization and depolymerization processes are involved in the rhythmically occurring contraction-relaxation cycle of cytoplasmic actomyosin in Physarum. (6) Cytoplasmic actin and myosin represent the architectural proteins of the contractile gel reticulum in eukaryotic cells. (7) The importance of the regulation of actin polymerization as a basic control mechanism of the eukaryotic cell is discussed.

Actins↗
PubMed1976

Cardiac Purkinje fibers: cesium as a tool to block inward rectifying potassium currents.

When a cardiac Purkinje fiber is exposed to 20 mM Cs the membrane potential falls to about -60 mV within 1 min. In voltage clamp experiments, exposure to Cs blocks both the pacemaker current iK2 and the instantaneous outward current iK1, while the delayed outward rectifying potassium current ix is not affected. In the presence of 20 mM Cs, the steady state currents are related linearly to the clamp potential and are insensitive to alterations in [K]0. The Cs sensitive current was defined as the difference between control and membrane currents measured in the presence of 20 mM Cs. This current displays inward-going rectification and its reversal potential follows log E1K]0 with a slope of 60 mV per decade.

Animals↗
PubMed1976

Cytoplasmic actomyosin fibrils in tissue culture cells: direct proof of contractility by visualization of ATP-induced contraction in fibrils isolated by laser micro-beam dissection.

A special cell line derived from a rat mammary adenocarcinoma (RMCD cells) displays a distinct pattern of actomyosin fibrils (AM fibrils) visible with phase contrast, Nomarski interference and polarized light optics. It was shown that the cytoplasmic AM fibrils are arranged as bundles of highly parallel F-actin filaments. The chimical nature of the filaments was identified by incubation with heavy meromyosin from rabbit skeletal muscle. These cytoplasmic actomyosin fibrils actively contract under isotonic conditions. This was shown by contraction experiments under polarized light optics, by cinematographic analysis and by direct proof of the contractility of AM fibrils isolated by laser micro-dissection. Thus, cytoplasmic AM fibrils can be assumed to represent structures essential for motive force generation in contraction processes in non-muscle cells.

Actomyosin↗
PubMed1976

Cell surgery by laser micro-dissection: a preparative method.

A new UV-laser device is introduced as a micro-manipulator and micro-dissection apparatus for experimental cell research. High energy irradiation is produced by a quasi-continuous N2-laser. The laser micro-beam can be focused to a minimal diameter of 0-75 mum. The micro-laser can be used for cutting and, hence, as a micro-surgical instrument at the level of cellular organelles. Cytoplasmic actomyosin fibrils (AM fibrils) from tissue culture cells (RMCD-cells) are isolated from the surrounding ground cytoplasm. It is shown that the contractile proteins are not damaged by the laser dissection method to an extent that would prevent their normal function, e.g. contraction.

Actomyosin↗
PubMed1976

The gingival autograft and gingivectomy.

A rationale and technique for the utilization of the combined procedures of the gingival autograft and the gingivectomy technics for the purpose of pocket elimination and creation of an adequate zone of attached gingiva has been presented. It allows for the predictable and relatively atraumatic treatment of gingival problems in which there has been no deformity of the underlying osseous structures.

Connective Tissue↗
PubMed1975

Temperature sensitivity of outward current in cardiac Purkinje fibers. Evidence of electrogenicity of active transport.

1. In cardiac Purkinje fibers the temperature sensitivity of the membrane current flowing after 2 sec in response to depolarizing clamp steps was recorded. When the temperature was quickly lowered (30 sec) from 37 degrees C to 20 degrees C for a period of 2 min the outward current was markedly reduced. The effect was immediately reversed upon rewarming. The reduction in outward current on cooling was most pronounced between 30 degrees C and 20 degrees C. 2. In the range of anomalous rectification cooling to 20 degrees C shifted the i.v. relation in a negative direction by a constant amount of 20 nA. Outside this potential range (negative to -80 mV and positive to -45 mV) the slope conductance was reduced with a Q10 of about 1.3. 3. In the presence of dihydroouabain cooling did not further reduce the outward current in the potential range of anomalous rectification. However, negative to -80 mV and positive to -40 mV the slope conductance was reduced. The results support the view that part of the outward current is generated by an electrogenic sodium pump which is inhibited by cooling.

Animals↗
PubMed1975

Outward current and electrogenic sodium pump in Purkinje fibers.

The effects of metabolic inhibitors (ouabain, dihydroouabain, and 2,4-dinitrophenol (2,4-DNP) and of cooling on the membrane current of Purkinje fibers were studied by means of a voltage clamp. Within seconds after cooling or application of the drugs the outward current was found to be reduced. On longer cooling or poisoning, the potassium equilibrium potential was shifted in positive direction, the time-dependent membrane currents were depressed, and possibly the potassium conductance was altered. It is suggested that the early reduction in outward current is caused by inhibition of an electrogenic sodium pump.

Action Potentials↗
PubMed1975

Demonstration of cytoplasmic actomyosin fibrils by the freeze-etching technique.

Cytoplasmic actomyosin fibrils of Physarum polycephalum were demonstrated by the freeze-etching technique. Within the fibrils the electron micrographs reveal 40-80 A thick filaments presumably representing F-actin. Thicker filaments (myosin) could not be detected. To avoid freezing artefacts, a pretreatment of the living material with 50% sucrose was necessary. The influence of freeze-protecting agents upon the fine structure of cytoplasmic actomyosins and the resulting difficulties in the investigation of constituents of the groundplasm with the freeze-etching technique are discussed.

Cytoplasm↗