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Determinanti strutturali della sensibilita al pH nella famiglia di cotrasportatori di amino acidi Na+/Cl- -dipendenti
No full textResting membrane potential and inward current properties of mouse ovarian oocytes and eggs.
No full textThe electrical properties of the membrane of the ovarian oocyte at the germinal vesicle (GV) stage and of the ovulated egg of the mouse have been studied using a two-microelectrode voltage-clamp technique. The stable resting potential measured with a single electrode was -38.2 +/- 2.8 mV SE (18 oocytes, 5 animals) and -27.8 +/- 1.4 mV SE (28 eggs, 8 animals) in a solution containing 20 mM [Ca2+]0. The lower values appear to be strongly affected by damage due to electrode insertion. However, there was no evidence of the resting potential being more negative than -40 to -50 mV. Voltage-dependent inward current could not be activated from a holding potential (Vh) close to the resting potential. When Vh was set at -90 mV, depolarizing pulses activated a transient inward current in both oocytes and eggs. The threshold voltage, peak voltage and inactivation vs potential curve were very similar in oocytes and eggs. On the other hand, the current amplitude appeared reduced in ovulated eggs, whilst times to peak and inactivation time constants in eggs were significantly longer than in oocytes. In oocytes the inward current was blocked by 10 mM Co2+ and decreased by lowering [Ca2+]0 to 5 mM similarly to the results reported for eggs. It therefore appears that GV ovarian oocytes possess Ca2+ channels which differ from those present in eggs mainly with respect to their kinetic properties. The physiological role of this inward current remains obscure in both preparations since they are almost completely inactivated at the resting potential
Calcium current in mouse eggs recorded with the tight-seal, whole-cell voltage-clamp technique.
No full textThe tight-seal, whole-cell recording technique has been used to voltage-clamp unfertilized eggs of the mouse. Good seal formation was obtained in a solution containing 20 mM Ca2+ and the transient inward Ca2+ current recorded. This technique offers several advantages over the conventional, two-microelectrodes voltage-clamp: improved signal to noise ratio; larger membrane-pipette seal resistance; possibility of very stable, long duration, recordings and possibility of controlling the intracellular medium. The main disadvantage of the technique, namely the rundown of channels due to loss of intracellular components, was not encountered in this preparation
The calcium current of mouse egg measured in physiological calcium and temperature conditions.
No full text1. Voltage-clamp experiments have been performed on ovulated mouse eggs using the whole-cell recording technique. 2. Whole-cell recording offers improved signal-to-noise ratio and excellent stability over time. This allowed the study of the Ca2+ current of these eggs under physiological conditions (i.e. 1.7 mM-external Ca2+ and 37 degrees C). 3. In these conditions a negative shift of the reversal potential of the current (about -25 mV) and also of the activation and inactivation parameters (about -10 mV) compared with those recorded in 20 mM-external Ca2+ is found. 4. No significant diminution of the inward current was detected when external Na+ was substituted with impermeant cations, indicating no relevant participation of Na+ to the current in physiological conditions. 5. In a medium free of divalent cations a large inward current appeared, together with a large decrease in membrane resistance. 6. In Ca2+-free medium containing 1.2 mM-Mg2+ the inward current was largely suppressed, while an outward transient current appeared for depolarizations greater than +10 mV. 7. The 'outward surge current' previously described in this preparation appears to possess the same inactivation time constant and the same steady-state inactivation curve as the inward Ca2+ current. This suggests that the two currents flow through the same channels. 8. The time constant of inactivation was the same for both inward and outward currents and was independent of the current amplitude. These observations exclude a Ca2+-induced type of inactivation. 9. The channel which physiologically carries the Ca2+ current in mouse eggs belongs then to the class of Ca2+ channels that owe their selectivity to high-affinity Ca2+ binding sites
InsP3- and Ca2(+)-induced Ca2+ release in single mouse oocytes.
No full textTo better understand the mechanism of intracellular Ca2+ mobilization, mouse oocytes were micro-injected with 'caged'-inositol-1,4,5 triphosphate caged-InsP3) together with the Ca2+ indicator Fluo-3 to directly induce and monitor Ca2+ redistribution. Photo-released InsP3 elicits [Ca2+]i changes exhibiting several kinetic phases and threshold behaviour. Often Ca2+ oscillations were induced after a single InsP3 pulse. Autoregenerative Ca2+ transients could also be induced by injections of Ca2+ itself, demonstrating unequivocally the presence of a Ca2(+)-induced Ca2(+)-release mechanism in these cells
Computer reconstruction of the spread of excitation in nerve terminals with inhomogeneous channel distribution.
No full textA direct numerical integration method, as modified by Du Fort and Frankel (1953), has been used to solve the partial differential equation system which describes the spread of action potential in a mammalian nerve terminal. Branching of the terminal as well as inhomogeneous distributions of Na+ and K+ voltage-dependent channels (Brigant and Mallart 1982) have been incorporated in the model. Using the channel densities and the kinetic parameters measured in the node of Ranvier, the depolarization in the terminal branches has an amplitude of only 60% of the action potential in the node. Furthermore, the time courses of the calculated membrane currents differ considerably from the ones measured by Brigant and Mallart (1982) and by Konishi and Sears (1984). Increasing the Na+ and K+ channel densities may considerably increase the terminal depolarization and also reproduce qualitatively the current wave-forms observed experimentally. The model can also reproduce some of the effects of pharmacological channel blocks. The simulation allows a new interpretation of the different components of membrane current along the terminal
Charge movement and membrane capacity in frog muscle
No full text1. The transient current required to impose a step charge of potential has a complex time course especially in the region of internal potential between -50 and -40 mV. 2. Examination of non-linear transient current in this voltage range suggests two components of charge movement: (a) an initial more-or-less exponential movement, and (b) a slower component with a complex time course. 3. Measurements of membrane capacity support such a division and confirm the steeper voltage dependence of the slower charge movement. 4. Permanent depolarization to 40 mV appears to immobilize the slowly moving charge. Depolarization to -20 mV immobilizes both charge movements, and uncovers the presence of a third charge which seems to correspond to Charge 2 (cf. Adrian & Almers, 1976b; Adrian, Chandler & Rakowski, 1976)
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