SUMMARY1. Post-ganglionic neurones of the isolated rat superior cervical ganglion were voltage clamped at 37 0C using separate intracellular voltage and current micro-electrodes.2. Control experiments in current clamp suggested that the neurone is electrotonically compact, the soma and the proximal dendritic membranes being under good spatial voltage uniformity.3. Depolarizing voltage steps from membrane potentials near -50 mV evoked: (i) a voltage-dependent inward Na+ current, (ii) an inward Ca2+ current, (iii) a voltage-dependent outward K+ current, (iv) a Ca2+-activated K+ outward current.4. Depolarizations from holding potentials more negative than -60 mV elicited, besides the currents mentioned above, a fast transient outward current IA which peaked in 1-2 5 ms and then decayed to zero following an exponential time course. 5. The IA current was shown to be primarily, if not exclusively, carried by K+. It was unaffected by removal of external Ca2+ or addition of Cd2+ and was weakly blocked by tetraethylammonium ions and partially by 4-aminopyridine.6. The IA current showed a linear instantaneous current-voltage relationship. Its activation ranged from -60 to 0 mV with a mid-point at -30 mV. The A conductance could be described in terms of a simple Boltzmann distribution for a single gating particle with a valency of + 3.7. Both the development and removal of inactivation followed a single exponential time course with a voltage-dependent time constant which was large near the resting potential (42 ms at -70 mV) and small (11 ms) near -100 and -40 mV.8. Steady-state inactivation h. ranged from -100 to -50 mV, with a mid-point at -78 mV, suggesting that approximately 50 % of the IA channels are available at the physiological resting potential.9. Action potentials elicited from various holding potentials showed maximal repolarization rates dependent on the holding potential itself. This voltage dependence was found to be in reasonably good agreement with that of h. curve.
Post-ganglionic neurones of the isolated rat superior cervical ganglion were studied at 37 degrees C under two-electrode voltage-clamp conditions. Membrane depolarization beyond -40 mV from holding levels between -50 and -100 mV produced a delayed outward current which exhibited no inactivation within this voltage range. The current is carried primarily by K+ ions and its instantaneous I-V relation is linear. The total outward current could be separated into two distinct components on the basis of ion-substitution experiments. A voltage-dependent component of the delayed current, termed IK(V), is activated by membrane depolarization beyond -40 mV when Ca2+ fluxes are selectively blocked by Cd2+ or in Ca2+-free solution. IK(V) develops following first-order kinetics and rises to a peak with a voltage-dependent delay (239 ms at -30 mV and 23 ms at +10 mV). GK(V) attains a saturating value of the order of 17 mS/cm2 at about +20 mV and can be described in terms of a simple Boltzmann distribution for a single gating particle with a valency equal to +2.5. A second component of the delayed outward current, termed IK(Ca), depends on Ca2+ entry for its activation and was isolated as difference current before and after block of Ca2+ movements across the membrane. IK(Ca) is larger and faster than IK(V): it is strictly related to Ca2+ influx and also depends on membrane potential depolarization. A distinct Ca2+ current, ICa, was recorded from the neurone exposed to Na+-free or tetrodotoxin solution. ICa was activated by membrane depolarization beyond -30 mV and reached a maximum value near 0 mV. Its activation agrees with fourth-order kinetics and becomes faster with increasing depolarization. The Ca2+ current developed with a voltage-dependent time to peak of 2.9-1.8 ms and thereafter completely inactivated. The relationship between ICa and IK(Ca) is discussed. The Ca2+-k+ repolarizing system is expected to be mainly associated with action potentials arising from a depolarized neurone, whereas the IA current (Belluzzi, Sacchi & Wanke, 1985) dominates the repolarization mechanism at the normal membrane potential. The effect of muscarine was examined. Muscarine (10-50 microM) produced a fall in conductance with a voltage dependence similar to that exhibited by GK(Ca) and was ineffective when removing extracellular Ca2+ or adding Cd2+. A partial suppression of ICa by muscarine is demonstrated. It is suggested that the decrease of the outward current magnitude in the presence of muscarine may be accounted for qualitatively by the reduction in ICa.
The presence of a novel voltage-dependent chloride current, active in the subthreshold range of membrane potential, was detected in the mature and intact rat sympathetic neuron in vitro by using the two-microelectrode voltage-clamp technique. Hyperpolarizing voltage steps applied to a neuron held at -40/-50 mV elicited inward currents, whose initial magnitude displayed a linear instantaneous current-voltage (I-V) relationship; afterward, the currents decayed exponentially with a single voltage-dependent time constant (63.5 s at -40 mV; 10.8 s at -130 mV). The cell input conductance decreased during the command step with the same time course as the current. On returning to the holding potential, the ensuing outward currents were accompanied by a slow increase in input conductance toward the initial values; the inward charge movement during the transient ON response (a mean of 76 nC in 8 neurons stepped from -50 to -90 mV) was completely balanced by outward charge displacement during the OFF response. The chloride movements accompanying voltage modifications were studied by estimating the chloride equilibrium potential (E(Cl)) at different holding potentials from the reversal of GABA evoked currents. [Cl(-)](i) was strongly affected by membrane potential, and at steady state it was systematically higher than expected from passive ion distribution. The transient current was blocked by substitution of isethionate for chloride and by Cl(-) channel blockers (9AC and DIDS). It proved insensitive to K(+) channel blockers, external Cd(2+), intracellular Ca(2+) chelators [bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)] and reduction of [Na(+)](e). It is concluded that membrane potential shifts elicit a chloride current that reflects readjustment of [Cl(-)](i). The cell input conductance was measured over the -40/-120-mV voltage range, in control medium, and under conditions in which either the chloride or the potassium current was blocked. A mix of chloride, potassium, and leakage conductances was detected at all potentials. The leakage component was voltage independent and constant at approximately 14 nS. Conversely, gCl decreased with hyperpolarization (80 nS at -40 mV, undetectable below -110 mV), whereas gK displayed a maximum at -80 mV (55.3 nS). Thus the ratio gCl/gK continuously varied with membrane polarization (2.72 at -50 mV; 0.33 at -110 mV). These data were forced in a model of the three current components here described, which accurately simulates the behavior observed in the "resting" neuron during membrane migrations in the subthreshold potential range, thereby confirming that active K and Cl conductances contribute to the genesis of membrane potential and possibly to the control of neuronal excitability.
Fe and Có rdoba provinces, respectively (Galich and Galich, 1996). Assays in Pergamino (Buenos Aires prov-In Argentina, head blight (caused by Fusarium graminearum ince) showed that kernel number rather than kernel size Schwabe), is a highly risky disease of wheat (Triticum aestivum L.was seriously affected as a consequence of pathogen emm. Thell), although its occurrence is sporadic, depending on prevalent environmental variables. This unpredictability has stimulated the action (Annone and Frutos, 1988). development of predictive models of head blight occurrence that, ifInfection begins when spores are windblown onto successful, would help growers in the selection of control strategies. exposed anthers of wheat heads. The probability of in-As a result of our earlier work, empirical equations for predicting fection is greater when the anthers are exposed for a head blight incidence were developed at Pergamino INTA Experilonger period of time. This period with exposed anthers ment Center (Lat., 33؇ 56S), associating temperature and moisture lasts approximately 30 d in a commercial wheat field variables with mean disease observations from many wheat cultivars. (Reis, 1987). The critical period for infection extends In the current study our objective was to validate two of these meteofrom the beginning till the end of flowering, the crop rological based equations developed for Pergamino to predict wheat being less susceptible when grain filling starts. The anhead blight incidence and severity at Zavalla (Lat., 33؇ 1S) and Olithers constitute an important nutritional substrate for veros (Lat., 32؇ 33S) in moderately susceptible to susceptible cultivars the fungus, enhancing its rapid growth towards the ovafor the years 1993 to 1995. Even though the t-test determined nonsignificant differences between mean observed disease values versus ries and developed grains. Under proper environmental predicted values, a graphic method and a deviation examination conditions, the fungus grows into the kernels, glumes, or showed an underestimation at high disease levels. Simple analyses of other head parts (McMullen et al., 1997), thus producing sensitivity were able to detect the improvement in incidence and flower abortion early in the season and shriveled grain severity goodness of fit estimations that resulted from increasing the later. The pathogen overwinters in cereal residues and maximum temperature threshold and the heat accumulation defining secondary hosts until it is hydrated the next growing the length of the wheat critical period for infection. This study showed season, causing it to produce perithecia which release that meteorological based empirical equations developed for Pergamspores to infect another crop.ino can be useful for predicting disease intensity at more northern According to Galich and Galich (1996), Argentine locations in the Pampas region, making only a few changes in temperawheat cultivars have demonstrated two degrees of sensiture thresholds.
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