Reversal Of Membrane Potential Research Articles

Relationship between loss of desiccation tolerance and programmed cell death (PCD) in mung bean (Vigna radiata) seeds.

Mung bean (Vigna radiata), an important legume crop, has the property of desiccation tolerance (DT), which is lost in the final stage of germination (preimbibition, 18 h-24 h). We compared parameters related to the programmed cell death (PCD) of mung bean seeds before and after dehydration at different imbibition stages through various detection methods. The results of Evans blue and TTC staining methods showed that the dehydration process could lead to cell death. The results of optical and subcellular morphology showed that PCD occurred after dehydration. The destruction of DNA integrity and the activity changes in caspase and total nuclease in mung bean seeds after dehydration treatment indicated that the loss of desiccation tolerance was related to PCD. Dehydration resulted in the destruction of the mitochondrial structure, reversal of the membrane potential, and the entrance of cytochrome C into the cytoplasm. These processes all indicate that the mitochondrial apoptosis pathway was the main form of dehydration-induced PCD. The results of cytoplasmic Ca2+ concentration showed that Ca2+ signaling also played a role in inducing PCD, with the upstream signal being dehydration-induced changes in water potential and the downstream signal being the ROS and mitochondrial PT channel, according to the order in which these signals happened. The mitochondrial apoptosis pathway can be considered the main mechanism of dehydration-induced PCD based on our analysis of the sequence of major events in PCD. The main processes include dehydration induction, changes in Ca2+ and mitochondrial respiratory electron transport, the reversal of mitochondrial membrane potential induced by ROS and Ca2+, and the transmission and execution of PCD downstream signals induced by cytochrome C release.

Synchronous GABA-receptor-dependent potentials in limbic areas of the in-vitro isolated adult guinea pig brain.

Epileptiform discharges are known to reflect the hypersynchronous glutamatergic activation of cortical neurons. However, experimental evidence has revealed that epileptiform synchronization is also contributed to by population events mediated by GABA(A) receptors. Here, we analysed the spatial distribution of GABA(A)-receptor-dependent interictal events in the hippocampal/parahippocampal region of the adult guinea pig brain isolated in vitro. We found that arterial perfusion of this preparation with 4-aminopyridine caused the appearance of glutamatergic-independent interictal potentials that were reversibly abolished by GABA(A) receptor antagonism. Laminar profiles and current source density analysis performed in different limbic areas demonstrated that these GABA(A)-receptor-mediated events were independently generated in different areas of the hippocampal/parahippocampal formation (most often in the medial entorhinal cortex) and propagated between interconnected limbic structures of both hemispheres. Finally, intracellular recordings from principal neurons of the medial entorhinal cortex demonstrated that the GABAergic field potential correlated to inhibitory postsynaptic potentials (membrane potential reversal, -68.12 +/- 8.01 mV, n = 5) that were interrupted by ectopic spiking. Our findings demonstrate that, in an acute seizure model developed in the adult guinea pig brain, hypersynchronous GABA(A)-receptor-mediated interictal events are generated from independent sources and propagate within limbic cortices in the absence of excitatory synaptic transmission. As spared or enhanced inhibition was reported in models of epilepsy, our data may support a role of GABA-mediated signaling in ictogenesis and epileptogenesis.

Inotropic effects of electric currents. I. Positive and negative effects of constant electric currents or current pulses applied during cardiac action potentials. II. Hypotheses: calcium movements, excitation-contraction coupling and inotropic effects.

I. Tension was recorded from a segment of a thin bundle of calf or sheep ventricular fibers in which the membrane potential could be changed electrically, using a sucrose-gap technique. After reversal of membrane potential and consequent contracture for 2 to 3 seconds, a positive inotropic effect 2 to 12 times that which occurred during paired-pulse stimulation was obtained. The decay of this potentiated state was slow in resting preparations (average half-life, 95 seconds), but was determined by the number of beats in driven preparations (about eight beats to return to control amplitude). Lengthening consecutive action potentials by subthreshold constant depolarizing currents or 5-mv shifts in the membrane potential by 50-msec current pulses during the plateau phase of successive action potentials caused positive inotropic effects, attaining a steady state in about eight beats. Reversing the polarity of such currents (hyperpolarization) caused negative inotropic effects with the same phase dependency as for depolarizing currents. These negative inotropic effects attained a steady state in six to eight beats. The first action potential after cessation of these currents was close to normal, whereas return to control tension required six to eight beats. The half-life of decay of these negative inotropic states at rest was similar to that of positive inotropic states. Decay of inotropic states in contracting preparations was phase dependent, occurring only during the rapid depolarization and plateau phases of the action potential. II. These and other strength-interval effects on cardiac contractility in a constant chemical and physical environment are consistent with two hypotheses. (1) The presystolic level of intracellular calcium bound to specific (rapid release) sites on the sarcoplasmic reticulum, the inner surfaces of the sarcolemma and T tubules, is a major determinant of the level of [Ca 2+ ] 1 attained during the initial phase of an action potential and hence the tension developed by mammalian cardiac muscle (that is, its inotropic state) at that particular instant. (2) The amount of intracellular calcium present at these sites at any instant after an action potential (that is, the moment-to-moment inotropic state of the muscle) is predetermined in the inverse direction of the intersystolic intervals of prior systoles and in the positive direction by the duration and magnitude of the plateau phases of prior action potentials and (for rest periods as long as 10 seconds) the elapsed time, in an exponentially decreasing fashion, since termination of the last absolute refractory period.

Ca-abh�ngige Membranpotential�nderungen am Herzen und ihre Bedeutung f�r die elektro-mechanische Kopplung. Versuche mit Tetrodotoxin in Na-haltigen L�sungen

1. Tetrodotoxin (TTX), at a concentration of less than 10−6 g/ml, had no effect on membrane potential and contraction of isolated, thin ventricular trabeculae of sheep and calf hearts. 10−6 to 2 × 10−5 g/ml TTX decreased the rate of rise, over-shoot, and duration (phase of 90% repolarisation) of the action potential and the amplitude of contraction, without change in the resting potential and the plateau (20% repolarisation phase) of the action potential. Excitation block regularly occurred only with 10−5 to 2×10−5 g/ml TTX. 2. In a solution containing Na and TTX (5×10−6-2×10−5 g/ml) graded depolarisation was possible if the preparations were stimulated by square wave pulses of 500 msec duration across a sucrose bridge. In Ca-containing solutions the time-course of the electrotonic potentials showed two steps. The second step of depolarisation (SSD) began when the membrane potential reached−60 to−50 mV (threshold), and tension was initiated at the same level. Contractions reached their steady-state values only after about 5 depolarisations of the same size. There was no difference in the amplitude of contractions (steady-state level) elicited in Tyrode and in Tyrode + TTX. 3. SSD and contraction were dependent on the [Ca]e. Rate of rise (V/sec) and amplitude (mV) of SSD and tension increased with increasing [Ca]e. In Ca-free solutions the electrotonic potentials reached their steady-state levels in one step. No SSD and tension were observed in the absence of Ca, even with reversal of membrane potential. 4. In Tyrode + TTX the SSD was identical with the changes in membrane potential which could be observed in Na-free solution without TTX, but in Na-free solutions, contractions already reached their maximum during the first SSD. 5. SSD in Na-free solution was not blocked by TTX.

The hyperpolarizing effect of hydrogen ions on transmembrane potential in frog skeletal muscle.

The transmembrane potential of frog sartorius fibers in normal Ringer's solution is increased by a few millivolts following a fall in pH of the external medium. The same result is observed in either a bicarbonate–CO2 or a phosphate buffer system. The hyperpolarizing response is still observed when sodium in the Ringer solution is replaced by choline. Immediately after increasing the external potassium from 2.5 to 10 meq/l this effect is reversed. On changing the external potassium back to 2.5 meq/l the effect returns and is considerably enhanced. Chloride loading by presoaking muscles in isotonic KCl and transferring to sucrose causes a reversal of membrane potential and leads to an exaggerated pH effect. These observations, together with the fact that transmembrane conductance falls at a lower pH, suggest that hydrogen ions act mainly by lowering PCl. When ECl > Em, hydrogen ions will increase transmembrane potential. This effect may form the basis of a local control mechanism by which cell activity and local metabolic conditions regulate transmembrane potential.

Membrane potentials in Amoeba proteus.

ABSTRACT Amoebae can be penetrated by microelectrodes at either end. One records voltage and the other supplies alternating current. Step-like increases in alternating voltage superimposed on potentials recorded by the voltage electrode when in either the pseudopod or rear region demonstrate that low potentials recorded from a pseudopod and high ones from the rear region exist across a discrete impedance barrier. The only structure so far shown to fulfil this function is the plasma membrane. A resistance inserted in the earth path monitors current flowing through the system and confirms observations made when recording with single electrodes that there is a reduction of electrode resistance when the cell is entered. Pronounced depolarization in the rear region is shown when the currentcarrying electrode penetrates the pseudopod, but not vice versa. Morphological changes associated with membrane potential reversal are illustrated. Consideration is given to the role of step-like potential changes in movement.