[A study of the nature of the resting potential of Nitella cells. 2. The effect of the pH of the medium on the resting potential of Nitella cells].
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Hydrogen-Ion Concentration↗
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Some unusual high speed responses to electrical stimulation in Nitella with microscopic observations of electromagnetically induced changes in the cytoplasm.
Nitella flexilis living in Surrey ponds shows an unusual response to external electrical stimulation. This response is shorter (250 milliseconds) than the normal response (5 seconds), nor does it seem to obey the all-or-none law; decreasing the level of stimulus introduces a delay in its appearance. Microelectrode studies suggest that this response though propagated down the length of the cell is a surface phenomenon as compared to the normal response which is generated across the membrane. Normasky interference microscopic observation of the cytoplasm of those cells exhibiting this response shows large vesicles whose contents rotate, as opposed to those cells showing a normal response which do not show these vesicles. This response is associated with local extinction of groups of Nitella flexilis from ponds in Surrey and a temporary reduction in numbers in certain areas of the New Forest. Electrical excitation itself induces vesicle formation and its associated response in Nitella flexilis, and to a far lesser degree in Nitella translucens. In Nitella translucens alternating electromagnetic fields of low strength induce the formation of vesicles whose contents actively rotate in some individual cells. These vesicles are always accompanied by distortions in electrical response to stimulation. In its response to electromagnetic fields, Nitella translucens is very variable. Nitella opaca exhibits the same response as Nitella flexilis including the same fragility of survival. Electrical studies may throw light on the contentious problem as to whether Nitella opaca is in reality a separate species from Nitella flexilis.
Action Potentials↗
Identification of myosin in Nitella flexilis.
A myosin B-like protein was extracted from the alga Nitella flexilis. SDS-polyacrylamide gel electrophoresis revealed the presence of myosin heavy chain and actin as the main components. At high ionic strength, its ATPase [EC 3.6.1.3] reaction was activated by EDTA or Ca2+ and inhibited by Mg2+. At low ionic strength, superprecipitation was induced by the addition of ATP. Myosin was purified from Nitella myosin B. The molecular weight of the heavy chain of Nitella myosin, estimated by SDS-gel electrophoresis, was slightly higher than that of skeletal muscle myosin. At low ionic strength, Nitella myosin aggregated to form bipolar filaments about 0.2 micron long. At high ionic strength, its ATPase reaction was activated by EDTA or Ca2+, and inhibited by Mg2+. The Mg2+-ATPase reaction of Nitella myosin was activated by skeletal muscle F-actin.
Adenosine Triphosphatases↗
Intracellular potassium compartments in Nitella axillaris.
Three intracellular compartments for potassium exchange have been observed in intact cells of the giant-celled alga, Nitella axillaris. These compartments have been compared with the exchange properties of isolated subcellular structures. The smallest and fastest compartment (apparent half-time, 23 seconds) appears to involve passive absorption on the cell wall. The next largest (apparent half-time, 5 hours) may represent exchange with the cytoplasmic layer through the plasma membrane, the chloroplasts being in rapid equilibrium with the surrounding cytoplasm. The largest and slowest compartment (apparent half-time, 40 days) has been identified with the central vacuole. The vacuolar membrane and the plasma membrane have similar properties with respect to K permeability. Thus, the experimental data from the whole cell can be accounted for by a structural model of the compartments. Cyanide in concentrations up to 10(-3)M causes no net loss of K. The fastest compartment in Nitella and in higher plants is compared, and the ecological significance of the slow rate of potassium transport in Nitella is discussed.
Cell Membrane↗
Protein kinase C is involved in regulation of Ca2+ channels in plasmalemma of Nitella syncarpa.
Ca2+ current recordings have been made on Nitella syncarpa cells using the intracellular perfusion and the voltage-clamp technique. TPA (12-O-tetradecanoylphorbol-13-acetate), a substance capable of activating protein kinase C from plasmalemma of Nitella cells, modulates voltage-dependent Ca2+ channels. Polymixin B, inhibitor of protein kinase C, blocks the Nitella plasmalemma Ca2+ channels; the rate of channel blockage depends on the concentration and exposure time of the substance.
Calcium↗
Photosynthesis by protoplasm extruded from Chara and Nitella.
(a) Photosynthesis with protoplasm isolated from Chara or Nitella as measured by C(14) fixation has been obtained at a rate 12 to 15 per cent of that of the whole cells. (b) Photosynthesis by cut cells of Chara or Nitella with the vacuolar sap removed was at a rate comparable to that of the whole cells. (c) Both the protoplasm and the cut cells reduced CO(2) in the light to sucrose and hexose phosphates. Other products formed were also detected by paper chromatography. In contrast, dark controls fixed the C(14) into products associated with plant respiration. (d) An important difference in the products from the extruded protoplasm was the absence of C(14)-labelled pentoses or sedoheptulose which were formed, however, by the whole or cut cells. This suggests that the most sensitive site affected by disruption of the cells may be the steps involved in the regeneration of the "C-2 acceptor" for CO(2) fixation in photosynthesis.
Chara↗
Electroosmosis in Nitella.
The role of electroosmosis was studied directly in Nitella. The cells were mounted in a water-tight barrier between two chambers containing reversible electrodes for the application of potentials, and fitted with calibrated capillaries to measure water movement. No water movement was found when small existing bioelectric potentials were short-circuited through an external connection, nor when external potentials up to 1 or 2 volts were applied (producing currents up to 5 microa). Higher potentials (up to 10 volts) caused small movements of water, toward the negative pole. Larger and often irreversible water movements were produced by potentials up to 20 volts-sometimes persisting after current flow. A variety of evidence suggests that the effects are caused by injury at the cathodal end of the cell, allowing water to be attracted osmotically at the intact end and forced out at the injured end (transosmosis). This injury is reversible under small applied potentials, irreversible after large ones (100 to 200 times the natural bioelectric values). Such water flows persist in low salt concentrations (up to 0.09 M NaCl) but almost completely vanish in isotonic (0.26 M) mannitol. This confirms the osmotic, rather than the electroosmotic nature of the water movement. It is estimated that electroosmosis cannot account for more than 1 per cent of the water movement (or turgor) in Nitella cells. The dead cellulose walls display a small electroosmotic water flow at very high current densities (under 20 volts applied potential).
Electroosmosis↗
Protoplasmic streaming of an internodal cell of Nitella flexilis; its correlation with electric stimulus.
The sudden cessation or sudden decrease in velocity of the protoplasmic streaming of Nitella flexilis is observed whenever an action potential is elicited. The action potential can be generated by an electric stimulus after its refractory period, whether the flow is at a complete standstill or on the way to recovery. The membrane potential is generally decreased more or less when the rate of flow is decreased on application of salts or other agents. There is, however, no parallelism between these two. The membrane potential decreases proportionally with applied voltage of subthreshold intensity, while the rate of flow does not change appreciably. Only on application of a superthreshold voltage does the flow stop suddenly. In one case the rate of flow decreased to half without appreciable decrease in membrane potential. In another case it continued flowing at about one-half rate, although the membrane potential was almost zero. The Q(10) of the rate of flow is about 2, while it is 1.1 to 1.5 for the membrane potential. The sudden cessation of the protoplasmic streaming is supposed to be caused by the temporary formation of certain interlinkages among contractile protein networks in the endoplasm during excitation at the cathodal half of Nitella.
Action Potentials↗
[Estimation of efficiency of complex-method intestinal therapy using Nitella flexilis test-object for acute intestinal obstruction of various etiology].
The aim of our investigation was to define the presence of correlation between toxicity of intestinal contents and clinical and laboratory indices in the cases of ileus and peritonitis with the use of complex method of enteral therapy. Experiments have been carried out on internodal cells of Nitella Flexilis water plant. Efficiency of the given method (investigation of toxicity of the intestinal content with the use of test-object Nitella Flexilis) has been studied in 25 patients, who underwent correction of morpho-functional status of small intestine. Results of investigation have shown that changes in toxicity of intestinal content reliably reflect the nature and dynamics of the pathobiologic processes proceeding in GIS in treated patients suffering from acute intestinal impassability. Using the method of correction of morpho-functional status of small intestine decreases the role of "Intestinal factor" in formation of endogenous intoxication, which in turn results in early normalization of clinical and laboratory indices, favored process of early post-operational period, decreased rate of lethality and shortened rehabilitation period.
Acute Disease↗
Electrical responses of isolated protoplasm from Nitella.
Isolated protoplasmic droplets of the alga Nitella were investigated with microelectrodes in artificial vacuolar sap. The following observations were made: 1. Two types of preparations could be distinguished differing in size and in time of adaptation to artificial pond water but only slightly in their electrical behaviour. 2. The droplets proved to be electrically excitable in the sense that short current pulses elicited electrical responses which outlasted the stimuli. 3. The responses resembled nerve action potentials in shape and duration but they were graded and could be elicited as well in positive as in negative direction. Moreover, spontaneous changes of the normal resting potential (a few millivolts inside negative) did not influence their amplitudes. 4. In most cases the amplitudes of the responses grew with time and saturated after about 90 min. Before saturation the relation between stimulus strength and amplitude of responses was almost linear but became slightly S-shaped after saturation. The saturation value of the responses caused by 100 mus pulses of 1 muA/mm2 was taken as a standard response. In 32 experiments the standard response varied considerable between 2 and 90 mV and was 13 mV on the average. The observations suggest that quite different mechanisms are responsible for the transients of the Nitella droplets and the all-or-none responses of nerve fibres.
Chlorophyta↗
Response of Nitella internodal cell to chemical stimulation. A model for olfactory receptor system.
Electrical response to excitable internodal cell of Nitella was studied by applying various kinds of odorants to the cell. Changes in membrane potential and resistance during responses induced by odorants were measured intracellularly under a variety of ionic environments in the media. Results were: 1) Some odorants (coumarin, isoamylacetate, methylacetate, 1-octanol, 1-butanol, 1-propanol) produced an all-or-nothing type action potential when the concentration of odorant exceeded a certain threshold. The action potential was followed by a gradual depolarization of the potential whose amplitude depended on the odorant concentration, C. Other odorants (heptanoic acid, beta-ionon) induced gradual depolarization of the membrane potential without evoking an action potential. 2) Membrane resistance Rm changed in various ways during depolarization: some odorants led to a temporal or gradual decrease in Rm, and others caused an increase in Rm when the membrane potential was depolarized by the application of odorants. 3) Magnitude of response to odorants OR was found to be represented by the following equation: OR =(alpha + beta square root I) log (C/Cth) for C greater than or equal to Cth where alpha and beta are constants for a given odorant, I the ionic strength in the medium, and Cth the threshold concentration of the odorant. 4) Plots of olfactory threshold of human and of internodal cell of Nitella gave a straight line having slope unity. 5) Local application of odorants on the internodal cell induced impulses which transmitted from the part treated by odorants to the other portion. Physico-chemical and physiological implications of the results obtained were discussed.
1-Propanol↗
Tonoplast action potential in Nitella in relation to vacuolar chloride concentration.
The action potential of Nitella internode was studied in relation to K+ and C1- concentrations in the vacuole. When the vacuole of Nitella pulchella was filled with an artificial solution with extremely low C1- concentration, a diphasic action potential (DAP) was observed. T he first phase consists of a rapid depolarization followed by a relatively rapid repolarization, and the second one consists of a strong hyperpolarization followed by a gradual return to the resting potential. When the cell was stimulated immediately after the generation of DAP, a monophasic action potential which resembles an action potential of the natural cell was observed, indicating that the DAP consists of two components with different refractory periods. The refractory period of the component responsible for the depolarizing is shorter than that of a component responsible for the hyperpolarizing phase. Measuring the plasmalemma potential and vascuolar potential separately, it was demonstrated that the hyperpolarizing component of DAP originates from the tonoplast. The action potential of the tonoplast, in contrast with that of the plasmalemma, could be generated independently of concentration of K+ in the vasuole. Since the maximum amplitude of hyperpolarization decreased significantly by increasing C1- concentration of the vacuole, it is concluded that the tonoplast is very sensitive to C1- during excitation.
Action Potentials↗
Nitella fluctuation and instability in the membrane potential near threshold.
Excitation of Nitella internodal cell was investigated as an example of the phase transition in an open system far more thermal equilibrium. The power density spectrum of the membrane potential fluctuation had a bulge in a frequency range lower than 1 Hz at the resting state and a peak at approximately 0.03 Hz at a depolarized state near the threshold. A critical oscillation in the membrane potential was observed when threshold was gradually approached from the resting state. Repetitive firing was observed under a step-current of the superthreshold value. The frequency of spectral peaking, critical oscillation, and repetitive firing agree well with each other. The result suggests that the hard-mode instability occurs in the Nitella internodal cell. The membrane impedance had no peak in the same frequency region as the peak of the voltage spectrum. The spectral peak may be ascribed to be electrogenic pump modulated by the metabolic feedback system in photosynthesis.
Biophysical Phenomena↗
Cell wall extension in Nitella as influenced by acids and ions.
The giant internode cells of Nitella axillaris exhibit acid-induced growth similar to that found in higher plants. The threshold pH is 4.5, with a maximum at 3.5. The acid growth effect is transient, lasting no more than 32 min. Extensibility measurements of isolated cell walls showed a similar pattern of acid enhancement. Prolonged boiling in water (12 hr) only partially inhibited the acid-induced wall extensibility and actually increased the extensibility at pH 6. It was concluded that physical, rather than enzymatic, processes were responsible for acid-enhanced continuous extension ("creep") in Nitella walls. A complex cation-sensitive mechanism that affects extensibility was also characterized. Among the stimulatory (wall-softening) cations, divalents were generally more effective than monovalents, with magnesium being the most stimulatory. The inhibitory (wall-hardening) cations included divalents and trivalents, aluminum being the most inhibitory. Ionic effects on extensibility were even less sensitive to prolonged boiling in water than acid effects.
Cations, Divalent↗
The effects of colchicine on spermatogenesis in Nitella.
Treatment of Nitella antheridia with colchicine results in various sperm abnormalities, depending upon duration of exposure and subsequent recovery. Early effects of treatment include disappearance of spindle fibers and a cessation of ordered cell wall formation in dividing cells. Sperm released from antheridia treated for 24 hr and allowed to recover for 4-5 days possess branched flagella. After a recovery period of 6-10 days the sperm appear normal; however, following longer recovery periods, the sperm exhibit variations in size and number of flagella. Branched flagella contain a variety of microtubule patterns ranging from branches containing a single microtubule to flagella with an excess of microtubules. Spermatids which differentiate in the presence of colchicine lack flagella and a microtubular sheath. Nuclear contents undergo condensation stages; however, the nucleus as a whole does not undergo the orderly elongation and coiling characteristic of untreated Nitella spermatids. Long-term colchicine treatment followed by a recovery period produces atypical microtubules and microtubular aggregations in the spermatid. The results indicate that colchicine affects not only polymerization of microtubule subunits but also factors responsible for their ordered spatial relationships in the cell. The presence of microtubules is a prerequisite for normal morphological changes during spermiogenesis.
Chromosomes↗
Identification of actin in situ at the ectoplasm-endoplasm interface of Nitella. Microfilament-chloroplast association.
Using a glycerination procedure designed to avoid excessive plasmolysis or disruption of the ectoplasm, microfilaments in bundles at the ectoplasm-endoplasm interface of Nitella internode cell segments were found to bind rabbit heavy meromyosin (HMM) in situ. All HMM arrowheads in a bundle seem to have the same polarity and many lie in register as judged from the electron micrographs; the arrowhead periodicity is approximately 380 . The decorated microfilaments are thus similar to those seen in negatively stained cytoplasmic suspensions of internode cells. In glycerinated material, as well as in suspensions, the microfilaments are closely associated with chloroplasts. The microfilaments lie adjacent to or are attached to the chloroplast envelope. The results provide further evidence that the microfilaments thought to play a role in cytoplasmic streaming in vivo in Nitella consist of actin and suggest that they may be anchored to the chloroplasts.
Actins↗
The influence of H+ on the membrane potential and ion fluxes of Nitella.
The resting membrane potential of the Nitella cell is relatively insensitive to [K](o), but behaves like a hydrogen electrode. K(+) and Cl(-) effluxes from the cell were measured continuously, while the membrane potential was changed either by means of a negative feedback circuit or by external pH changes. The experiments indicate that P(K) and P(Cl) are independent of pH but are a function of membrane potential. Slope ion conductances, G(K), G(Cl), and G(Na) were calculated from efflux measurements, and their sum was found to be negligible compared to membrane conductance. The possibility that a boundary potential change might be responsible for the membrane potential change was considered but was ruled out by the fact that the peak of the action potential remained at a constant level regardless of pH changes in the external solution. The conductance for H(+) was estimated by measuring the membrane current change during an external pH change while the membrane potential was clamped at K(+) equilibrium potential. In the range of external pH 5 to 6, H(+) chord conductance was substantially equal to the membrane conductance. However, the [H](i) measured by various methods was not such as would be predicted from the [H](o) and the membrane potential using the Nernst equation. In artificial pond water containing DNP, the resting membrane potential decreased; this suggested that some energy-consuming mechanism maintains the membrane potential at the resting level. It is probable that there is a H(+) extrusion mechanism in the Nitella cell, because the potential difference between the resting potential and the H(+) equilibrium potential is always maintained notwithstanding a continuous H(+) inward current which should result from the potential difference.
Action Potentials↗
Metabolic and physical control of cell elongation rate: in vivo studies in nitella.
Several levels of control of elongation rate are revealed through the detailed study of responses of the Nitella internode to abrupt shifts in turgor. The immediate response, which apparently reflects the physical state of the cell, is approximately described by the equation r = (P - Y)m where r is rate, P is pressure, Y is the wall's yielding threshold, and m is related to the wall's apparent fluidity (reciprocal viscosity). Because P and Y are in the range 5 to 6 atmospheres, and (P - Y) is roughly 0.2 atmosphere, elongation rate is initially extremely sensitive to changes in P. A small step-down in turgor (0.7 atmosphere) stops growth, and a similar rise greatly accelerates it. These initial responses are, however, soon (15 minutes) compensated by changes in Y. An apparent metabolism-dependent reaction (azide-sensitive) lowers Y; strain hardening (azide-insensitive) raises it. These two opposing processes, acting on Y, serve as a governor on (P - Y), tending to maintain it at a given value despite changes in P. This ability to compensate is itself a function of turgor. Turgor step-downs are less and less well compensated, leading to lower rate, as turgor falls from 5 atmospheres to about 2 atmospheres where growth appears not to resume. This is the lowest attainable yield value, Y(1). The turgor dependency of compensation reflects a turgor requirement of the Y-lowering ("wall-softening") process. Thus the relation between steady state, r(s), and turgor is an indirect one, derived from time-dependent alterations of the cell wall. This relationship superficially resembles the instantaneously valid one in that, roughly, r(s) = (P - Y(1))m(s). Y(1) and m(s), however, have much lower values than Y and m. The duality of the elongation rate versus turgor relation and the prominent role of Y in regulating rate are the major features of growth control in Nitella.
Journal Article↗