Characteristics of action potentials in Helianthus annuus

The action potentials induced by nondamaging electrical stimuli in 16- to 22-day-old plants of Helianthus annuus were examined. Typical recordings are presented. Mean values of their amplitudes and conduction velocities in the stem, the strength-duration relation, the 'all-or-none' law and the refractory periods have been determined. The amplitude and velocity of propagation were essentially identical in the upward and downward direction, but greater in the upper than in the lower half. In 'electrically active' plants, the rheobase value is 2 V, the minimum period for stimulation is 1.8 s. and the chronaxie 2.3 s. It is noted that the excitability level between similar plants on the same day and in the same plant on different days is highly variable and undergoes periodic changes.     

Light-triggered action potentials in the liverwort Conocephalum conicum

The response to light of a liverwort, Conocephalum conicum L., measured as a change in the resting potential, consists of two stages. The first stage is a slight depolarization dependent on light intensity. This plays the role of a generator potential (GP) which induces the second stage - an action potential of the all-or-none character. Action potentials induced by light and by electrical stimuli have the same properties, i.e. identical time course, propagation velocity, and refractory periods. A summation occurs of sub threshold light stimuli and of light and electrical stimuli. The presence of 5⋅10-⁻⁶ M DCMU cancelled the light response and blocked - by inhibition of the electron transport chain - the mechanism leading to GP generation. However, this effect did not produce any change in the response to electrical stimuli.     

Cold‐ and menthol‐evoked membrane potential changes in the moss Physcomitrella patens: influence of ion channel inhibitors and phytohormones

Microelectrode measurements carried out on leaf cells from Physcomitrella patens revealed that a sudden temperature drop and application of menthol evoked two types of different‐shaped membrane potential changes. Cold stimulation evoked spike‐type responses. Menthol depolarized the cell membrane with different rates. When it reached above 1 mV s^?1, the full response was recorded. Characteristic for the full responses was also a few‐minute plateau of the membrane potential recorded after depolarization. The influence of inhibitors of calcium channels (5 mM Gd³⁺), potassium channels (5 mM Ba²⁺), chloride channels (200 μM Zn²⁺, 50 μM niflumic acid) and proton pumps (10 μM DES), an activator of calcium release from intracellular stores (Sr²⁺), calcium chelation (by 400 μM EGTA) and phytohormones (50 μM auxin, 50 μM abscisic acid (ABA), 500 μM salicylic acid) on cold‐ and menthol‐evoked responses was tested. Both responses are different in respect to the ion mechanism: cold‐evoked depolarizations were influenced by Ba²⁺ and DES; in turn, menthol‐evoked potential changes were most effectively blocked by Zn²⁺. Moreover, the effectiveness of menthol in generation of full responses was reduced after administration of auxin or ABA, i.e. phytohormones known for their participation in responses to cold and regulation of proton pumps. The effects of DES indicated that one of the main conditions for generation of menthol‐evoked responses is inhibition of the proton pump activity. Our results indicate that perception of cold and menthol by plants proceeds in different ways due to the differences in ionic mechanism and hormone dependence of cold‐ and menthol‐evoked responses.     

The influence of glutamic and aminoacetic acids on the excitability of the liverwort Conocephalum conicum

Intracellular microelectrode measurements revealed that a resting potential (RP), an action potential (AP) and a calcium component of AP (named voltage transient, VT) can be influenced by glutamic acid (Glu) and aminoacetic acid (glycine, Gly) in the liverwort Conocephalum conicum. In the continuous presence of 5mM Glu or 5mM Gly, the RP hyperpolarized constantly and the plants became desensitized to the excitatory amino acids (Glu or Gly). Under such circumstances, the amplitudes of APs evoked by stimuli other than Glu or Gly grew, as did their calcium components (VTs). The sudden application of 1-15 mM Glu or Gly to a thallus not yet desensitized resulted in an excitation, i.e. a single AP or AP series. Aspartate (Asp) could not substitute for Glu in any way. Simultaneous action of both amino acids acted synergically to trigger APs. The same phenomenon was observed when glycine solution was enriched with N-methyl-D-aspartic acid (NMDA). Gly-induced APs were totally hindered by 1mM D-amino-5-phosphonopentanoic acid (AP5)--an inhibitor of ionotropic glutamate receptors of the NMDA kind. Glu-induced APs could be totally suppressed by 1mM AP5 as well as by 1mM 6,7-dinitroquinoxaline-2,3-dione (DNQX)--an inhibitor of AMPA/KA receptors. DNQX also completely blocked the calcium component of Glu-evoked APs. After DNQX treatment, the only response to Glu was a membrane potential hyperpolarization (like the Glu response in a desensitized plant). It was concluded that the Glu-induced depolarization and hyperpolarization are separate phenomena. The stimulatory effects of both Glu and Gly on liverwort excitability may be the consequences of an activation of a variety of ionotropic Glu receptor subtypes.     

Slow vacuolar channels in vacuoles from winter and spring varieties of rape (Brassica napus)

Currents passing through slowly activating vacuolar channels (SV) in isolated vacuoles from winter (Górczański) and spring (M?ochowski) varieties of rape (Brassica napus) were examined using the patch-clamp technique. Eight-week-long vernalization at 5/2 degrees C (day/night) was applied to obtain the generative stage of winter rape. SV channels of vacuoles isolated from vegetative (rosette) and generative leaves of both varieties were examined in order to investigate a possible role of these ion channels in rape flowering. Single SV channel conductance measured in a vacuole-attached configuration (natural cell sap) ranged from 60 to 83 pS. Lower values were observed in the generative leaves of both varieties. Unitary conductance measured in excised cytoplasm-out membrane patches did not differ significantly among the experimental variants, with the exception of spring generative vacuoles, where it was significantly lower. There was also no difference in SV current densities measured in the whole-vacuole configuration. Gibberellic acid (GA(3)) (2mg/l) caused lowering of macroscopic SV currents by 20%, and had no significant effect on the single channel conductance. We conclude that SV channels play a role in rape vernalization and flowering owing to their multifactor regulation abilities rather than structural changes.     

Effects of ion channel inhibitors on cold- and electrically-induced action potentials in Dionaea muscipula

Glass microelectrodes were inserted into Dionaea muscipula (Venus flytrap) lobes and the action potentials (APs) were recorded in response to a sudden temperature drop or a direct current (DC) application. The effect of potassium channel inhibitor, tetraethylammonium ion, was the lengthening of the depolarization phase of AP. APs were also affected by the anion channel inhibitor, anthracene-9-carboxylic acid, that made them slower and smaller. Neomycin, which disturbs inositol triphosphate-dependent Ca²⁺ release, caused the visible inhibition of AP, too. Ruthenium red, which blocks cyclic ADP-ribose-dependent Ca²⁺ release, totally inhibited DC-triggered APs and induced the decrease in the amplitudes of cold-evoked APs. Lanthanum ions significantly inhibited both cold- and DC-induced membrane potential changes. It was concluded that during excitation Dionaea muscipula relied upon the calcium influxes from both the extra- and intracellular compartments.     

Variation and action potentials evoked by thermal stimuli accompany enhancement of ethylene emission in distant non-stimulated leaves of Vicia faba minor seedlings

Electrical activity (action and variation potentials) and ethylene emission were measured in thermally stimulated Vicia faba minor seedlings. It was determined that variation potential with or without super-imposed action potentials was generated and propagated basipetally in response to scorching of the upper leaf. In stimulated plants the level of ethylene production measured in lower, non stimulated leaf was significantly higher than that in the control plants and the difference correlated with the amplitude of the electrical response. Neither variation nor action potential was recorded when ethylene was injected to the chamber covering the experimental leaf. The level of ethylene emission showed clear circadian rhythm when measured at photoperiod 16:8 (LD) or at constant light (LL). It is concluded that the sequence of ion fluxes registered as an electrical response of a plant to the thermal stimulus is a signal evoking an enhancement of ethylene emission.     

Age-dependent effect of limb immobilization and remobilization on rat bone

The effect of unilateral hindlimb immobilization and subsequent free remobilization on bone tissue in rats was examined. Right hindlimb of intensively growing (G), young adult (Y) and adult (A) male rats was immobilized by taping for two weeks. Bone tissue was investigated post mortem in experimental and age-matched control rats, either directly after immobilization (Imm) or after two or four weeks of remobilization (Re2, Re4). Apparent density (d(app)) and mineralization (Min) were estimated in femora and pelvis. The mechanical state of bone tissue in femora was evaluated using an ultrasonic method. Additionally, activity of serum alkaline phosphatase, and serum calcium and phosphorus were measured in each group. Min and d(app) in Imm bones were changed in G rats, while in Y and A only d(app) in Imm femora was affected. Velocity of ultrasound was significantly lower in immobilized femora in each age group, indicating decreased elasticity of bone tissue. The differences between immobilized and control limbs were still significant in Re2 and Re4 groups in G rats. In Y rats the differences between experimental and control bones increased during remobilization. It is concluded that deterioration of bone initiated during two weeks of unloading, last within at least four weeks of free remobilization, despite restoration of normal activity.     

Action Potentials Evoked by Light in Traps of Dionaea muscipula Ellis

At low light intensities (less than 50 µmol m^–2s^–1) illumination evokes transient depolarization of membranepotential in mesophyll cells of the leaf-trap of Dionaea muscipulaEllis. Darkening causes hyperpolarization approximately symmetricto the response to illumination. The amplitude as well as therate of potential changes depend on light intensity. After exceedinga definite threshold (usually between 50 and 80 µmol m^–2s^–1)the depolarization plays the role of a generator potential andan all-or-none action potential (AP) is released. Switchinglight off in a depolarization phase of an AP does not changeits shape and the amplitude. When the light intensity is increasedto 80–150 µmol m^–2 s^–1 a single lightstimulus triggers two successive APs. The time interval betweenthe two APs decreases with increasing stimulus strength andreaches the minimum between 300 and 400 µmol m^–2s^–1. At higher light intensities the interval increasesagain, and finally only a single AP is triggered. It was shownthat the effect was evoked by light but not by temperature changeaccompanying illumination. An inhibitor of the photosyn-theticelectron transport chain, DCMU, blocked the generator potentialsmediating between light absorption and APs. Residual responsesto light stimuli in plants treated with DCMU had reverse polarityand strongly reduced the amplitudes. (Received September 16, 1997; Accepted January 16, 1998)     

Thermal stabilization of collagen molecules in bone tissue

Differential thermal calorimetry (DSC) analysis of partially dehydrated bovine bone, demineralized bone and bovine tendon collagen was performed up to 300 °C to determine factors influencing stability of mineralized collagen in bone tissue. Two endothermal regions were recognized. The first, attributed to denaturation of collagen triple helix, was hydration dependent and had a peak at 155–165 °C in bone, 118–137 °C in tendon and 131–136 °C in demineralized bone. The second region extended from 245 to 290 °C in bone and from 200 to 280 °C in tendon and was connected with melting and decomposition of collagen. Differences in thermodynamic parameters between cortical and trabecular bone tissue were stated. Activation energy of collagen unfolding in native bone tissue increased with dehydration of the bone. From the results of the present study we conclude that dehydrated bone collagen is thermally very stable both in native and in demineralized bone. Presence of mineral additionally stabilizes bone tissue.