External Ca Concentration Research Articles

Calcium action potentials in innervated and denervated rat muscle fibres.

We studied the generation of calcium action potentials (Ca APs) in innervated and denervated fibres of the extensor digitorum longus of the rat in a tetraethylammonium (TEA) sulphate solution plus 3-4 diaminopiridine (3-4 DAP). The main results are the following: (1) more than 90% of the innervated fibres were capable of developing well-sustained Ca APs that were blocked by Cd or nifedipine; (2) the incidence of Ca APs obtained from the denervated fibres was substantially lower than in the control preparations; (3) no relation was found between the appearance of Ca APs in the denervated fibres and the resting membrane potential (Vm), stimulus duration (500-2000 ms) or holding potential (-80, -100 mV); (4) The percentage of denervated fibres that exhibited Ca APs was increased significantly with the following procedures. First, by raising the external Ca concentration to 14 mM; second, by depleting the intracellular K concentration by overnight exposure of the muscles to a free K-Cs solution; (c) and third, by incubating the muscles in 500 nM apamin, a venom that inhibits the K conductance activated by Ca. Several factors may be involved in the lower incidence of Ca APs obtained in denervated fibres: (1) a diminished Ca current due to a reduction in the driving force as a result of an increment in the intracellular Ca concentration; (2) a persistence of a shunting K conductance that is not inhibited by TEA and 3-4 DAP; (3) a shift in the voltage dependence of the activation and inactivation parameters of the Ca current or the appearance of a new type of Ca channel with a different kinetics.

Ca-sensitive slow inactivation and lidocaine-induced block of sodium channels in rat cardiac cells

Macroscopic and single Na channel currents were studied using the patch-clamp technique in rat cardiac cells. Slow inactivation (SI) characterized by 100 ms order kinetics was investigated. This inactivation was incomplete, as no more than 50% of Na channels were able to enter the SI state during 1–2s of membrane depolarization. The maximal fraction of Na channels in the SI state decreased with increasing external Ca concentration. Single-channel analysis led us to conclude that Na channels undergo transition from the resting (R) state to the SI state, bypassing both open (O) and ordinary fast inactivation (I) states. A new fast inactivation state (Y) was postulated between the R and SI states. Exposure to the local anesthetic, lidocaine, decreased the probability of Na channel opening and dramatically slowed reactivation. The latter effect was due to lidocaine interaction with the Y state rather than with the I state. Increasing external Ca concentration in the presence of lidocaine diminished the fraction of Na channels capable of making the transition to a slowly reactivatable state (lidocaine-bound Y and/or SI states).

Weak agonists and platelet cytoplasmatic alkalynization. Effect of aggregation, thromboxane A2 (TxA2) and concentration of external Ca2+[Ca2+o

Weak agonists and platelet cytoplasmatic alkalynization. Effect of aggregation, thromboxane A2 (TxA2) and concentration of external Ca2+[Ca2+o

Translocation mechanism of Na-Ca exchange in single cardiac cells of guinea pig.

We have studied in single cardiac ventricular cells of guinea pig the ionic translocation mechanism of the electrogenic Na-Ca exchange, i.e., whether Na and Ca ions countercross the membrane simultaneously or consecutively with "ping pong" kinetics. The dose-response relation between the external Ca concentrations [( Ca]o) and the current density of the outward Na-Ca exchange current were measured at three different intracellular Na concentrations [( Na]i) in the absence of external Na. Nonlinear regression curves of the dose-response relation obtained by computer revealed Michaelis-Menten type hyperbola from which the [Ca]o giving a half-maximal response (apparent KmCao or K'mCao) and the apparent maximum current magnitude (I'max) were estimated at each [Na]i. As [Na]i increased, the K'mCao increased progressively and the value of K'mCao/I'max tended to decrease. These results are consistent with the simultaneous mechanism. The K'mCao/I'max values, however, were small and close to each other, so it was not possible to completely preclude a consecutive mechanism.

Entrainment and Phase-Shifting of the Circadian Rhythm of Cell Division by Calcium in Synchronous Cultures of the Wild-Type Z Strain and of the ZC Achlorophyllous Mutant of Euglena gracilis

Cell division in exponentially increasing populations of the wild-type, photosynthetic Z strain of Euglena gracilis Klebs cultured autotrophically on an aerated, magnetically stirred, minimal mineral medium (pH 7.0) in constant light (LL) or in a light-dark 1 hour:1 hour cycle (LD:1,1) at 25 degrees C could be synchronized by a 10-hour:10-hour low (2 micromolar):normal (200 micromolar) cycle in the concentration of external calcium. Similar results were obtained with the photosynthesis-deficient, achlorophyllous ZC mutant cultured in darkness at 16 degrees C on mineral medium supplemented with 0.1% ethanol as a carbon source; even a single low-Ca(2+) (2 micromolar) pulse was effective in eliciting synchrony. In contrast, whereas the 20-hour entrained rhythm of cell division in ZC then free-ran with a circadian period (tau = 26 hours) for many cycles after the imposed calcium regimen was discontinued, division rhythmicity did not persist in the Z strain in LL. The rhythm in wild-type cultures (free-running in LD:1,1) could be phase-shifted by a single 2-hour increase (from 200 micromolar to 10 millimolar; HiCa) or decrease (from 200-2 micromolar; LoCa) in external Ca(2+) concentration (varied by the addition of CaCl(2) or EDTA, respectively, to the medium). Pulses were terminated by returning the cells to medium containing 200 micromolar Ca(2+) (the normal concentration), and the steady-state phase-shifts engendered (if any) after transients had subsided were calculated with reference to an unperturbed culture. For both HiCa and LoCa pulses given at different circadian times, strong (type 0) phase-response curves (PRCs) were obtained, but although the LoCa PRC was the same as that obtained for light signals, the HiCa PRC was the opposite (a mirror image). These results implicate calcium in clock function, although it is likely that only a small portion of the total intracellular Ca(2+) ion is playing a role since the period of the division rhythm in cultures grown in the continuous presence of excess Ca(2+) or under LoCa was not altered significantly.

Marcroscopic and single-channel studies of two Ca2+ channel types in oocytes of the ascidianCiona intestinalis

Whole-cell and single-channel patch-clamp experiments were performed on unfertilized oocytes of the ascidian Ciona intestinalis to investigate the properties of two voltage-dependent Ca2+ currents found in this cell. The peak of the low threshold current (channel I) occurred at -20 mV, the peak of the high-threshold current (channel II) at +20 mV. The two currents could be distinguished by voltage dependence, kinetics of inactivation and ion selectivity. During large depolarizing voltage pulses, a transient outward current was recorded which appeared to be due to potassium efflux through channel II. When the external concentrations of Ca2+ and Mg2+ were reduced sufficiently, large inward Na currents flowed through both channels I and II. Using divalent-free solutions in cell-attached patch recordings, single-channel currents representing Na influx through channels I and II were recorded. The two types of unitary events could be distinguished on the basis of open time (channel I longer) and conductance (channel I smaller). Blocking events during channel I openings were recorded when micromolar concentrations of Ca2+ or Mg2+ were added to the patch pipette solutions. Slopes of the blocking rate constant vs. concentration gave binding constants of 6.4 X 10(6) M-1 sec-1 for Mg2+ and 4.5 X 10(8) M-1 sec-1 for Ca2+. The Ca2+ block was somewhat relieved at negative potentials, whereas the Mg2+ block was not, suggesting that Ca2+, but not Mg2+, can exit from the binding site toward the cell interior.

Cytoplasmic calcium affects the gating of potassium channels in the plasma membrane ofChara corallina: a whole-cell study using calcium-channel effectors

The action of a wide range of drugs effective on Ca(2+) channels in animal tissues has been measured on Ca(2+) channels open during the action potential of the giant-celled green alga,Chara corallina. Of the organic effectors used, only the 1,4-dihydropyridines were found to inhibit reversibly Ca(2+) influx, including, unexpectedly, Bay K 8644 and both isomers of 202-791. Methoxyverapamil (D-600), diltiazem, and the diphenylbutylpiperidines, fluspirilene and pimozide were found not to affect the Ca(2+) influx. Conversely, bepridil greatly and irreversibly stimulated Ca(2+) influx, and with time, stopped cytoplasmic streaming (which is sensitive to increases in cytoplasmic Ca(2+)). By apparently altering the cytoplasmic Ca(2+) levels with various drugs, it was found that (with the exception of the inorganic cation, La(3+)) treatments likely to lead to an increase in cytoplasmic Ca(2+) levels caused an increase in the rate of closure of the K(+) channels. Similarly, treatments likely to lead to a decrease in cytoplasmic Ca(2+) decreased the rate of K(+) channel closure. The main effect of bepridil on the K(+) channels was to increase the rate of voltage-dependent channel closure. The same effect was obtained upon increasing the external concentration of Ca(2+), but it is likely that this was due to effects on the external face of the K(+) channel. Addition of any of the 1,4-dihydropyridines had the opposite effect on the K(+) channels, slowing the rate of channel closure. They sometimes also reduced K(+) conductance, but this could well be a direct effect on the K(+) channel; high concentrations (50 to 100 μM) of bepridil also reduced K(+) conductance. No effect of photon irradiance or of abscisic acid could be consistently shown on the K(+) channels. These results indicate a control of the gating of K(+) channels by cytoplasmic Ca(2+), with increased free Ca(2+) levels leading to an increased rate of K(+)-channel closure. As well as inhibiting Ca(2+) channels, it is suggested that La(3+) acts on a Ca(2+)-binding site of the K(+) channel, mimicking the effect of Ca(2+) and increasing the rate of channel closure.

Action potentials and membrane currents of isolated single smooth muscle cells of cat and rabbit colon

Membrane potentials, action potentials and macroscopic currents in enzymatically dispersed, single smooth muscle cells of the circular layer of cat and rabbit colon were investigated. The cells did not exhibit spontaneous depolarizations and repolarizations (slow waves) or spontaneous action potentials. Single action potentials of smooth muscle cells were evoked by depolarizing current pulses of 5 ms to 3 s duration. A repetitive action potential discharge and an increase in the duration of the action potential was observed in cells during long depolarizing current pulses by superfusion with tetraethylammonium (TEA) or 4-aminopyridine (4-AP). Tetrodotoxin (TTX) did not alter the configuration of the action potential. Voltage-clamp experiments revealed two major outward macroscopic currents: a quasi-instantaneous (time-independent) and a time-dependent outward current. Both currents were identified as potassium (K) currents due to their pharmacological sensitivity to K antagonists [TEA, 4-AP and cesium (Cs)] and due to the reversal potential of outward tail currents. Barium selectively blocked the time-independent current. A time-dependent outward K current in colon cells was observed which appeared to be dependent upon entry of calcium ions (Ca2+) through voltage-dependent Ca-channels, since it was blocked by cadmium and low concentrations of nifedipine. The majority of cells did not exhibit transient outward currents. Inward currents were exposed in some of the cells when the K currents were blocked by external TEA and by replacement of K by Cs and TEA in the recording pipette. Inward currents were presumably carried by Ca2+, since they were not altered by TTX, were sensitive to external Ca concentrations and were abolished by the Ca channel antagonist, nifedipine. Carbachol augmented the amplitude of the inward Ca current.

Regulation of Ca2+ fluxes in rat liver mitochondria by Ca2+. Effects on Ca2+ distribution

Rat liver mitochondria are able to temporarily lower the steady-state concentration of external Ca2+ after having accumulated a pulse of added Ca2+. This has been attributed to inhibition of a putative delta psi-modulated efflux pathway [Bernardi, P. (1984) Biochim. Biophys. Acta 766, 277-282]. On the other hand, the rebounding could be due to stimulation of the uniporter by Ca2+ [Kröner, H. (1987) Biol. Chem. Hoppe-Seyler 369, 149-155]. By measuring unidirectional Ca2+ fluxes, it was found that the uniporter was stimulated during the rebounding peak both under Bernardi's and Kröner's conditions, while no effects on the efflux could be demonstrated. The rate of unidirectional efflux of Ca2+ was not affected by inhibition of the uniporter. It appears likely that the rebounding is due to stimulation of the uniporter rather than inhibition of efflux.

Effect of temperature on the inhibition of the GABA-gated response by intracellular calcium

The effects of the voltage-dependent Ca 2+inward current (I Ca) on γ-aminobutyric acid (GABA)-induced Cl − current (I Cl) in isolated sensory neurons were examined at different temperatures and with different concentrations of external Ca 2+ using the ‘concentration-clamp’ technique. The total amount of inhibition of GABA-induced I Cl by a preceding I Ca increased in a hyperbolic manner with increasing Ca 2+ influx. The time course of recovery of the GABA response after inhibition by Ca 2+influx followed a single exponential and was facilitated by warming but slowed dramatically by a slight cooling from 20 to 15 °C in spite of a decrease in Ca 2+ influx. It is discussed that the energy-dependent, temperature-sensitive ionic pump and exchange systems at the surface membrane and intracellular organelles regulate the cytoplasmic free Ca 2+, thus explaining the quantitative effects of Ca 2+ influx and temperature on the inhibition of the GABA-gated Cl − channel.

The uptake of manganese induced by agonists in the isolated vas deferens of the guinea pig.

The uptake of manganese (Mn) induced by 100 mM potassium (K), 10 microM norepinephrine (NE) or 10 microM acetylcholine (ACh) was measured by atomic absorption spectroscopy in isolated vas deferens of the guinea pig. The agonists at these concentrations caused the maximal contraction of the vas deferens. The contents of Mn were increased with the repetitive treatments of Mn with K and were not significantly decreased after 1 h washing. The uptake of Mn was also stimulated by NE and ACh. The stimulation of uptake of Mn by K was the most potent while that by ACh was the smallest. The uptake of Mn was enhanced by elimination of Ca from the medium, while inhibited by the higher concentration of external Ca and by 2.1 microM diltiazem. The time course of K-induced Mn uptake was biphasic: an initial faster phase and a following slower phase of accumulation. The extents of increments of the Mn contents were dependent on the order of the applications of K and Mn: the increments became smaller in the following order, 1) when Mn was applied prior to K, 2) Mn was applied simultaneously with K, 3) Mn was applied after K. These results suggested that superficially bound Mn penetrates into the smooth muscle cells of vas deferens during the stimulation by agonists through the voltage-dependent calcium-channel (VDC) and that intracellular Mn was hardly extruded. It was also suggested that the degree of activation of VDC, through which Mn can enter the cells, was in the following order, K greater than NE greater than ACh. These results were consistent with our previous report about the dual effects of Mn: the inhibition and potentiation of contractions. It was also suggested that Mn may be a useful tool as a Ca analogue because Mn can penetrate into cells through VDC and, once taken up into the cells, Mn is not readily extruded and remains in the cells even after extracellular Mn is washed away.

ATP-activated channels in rat and bullfrog sensory neurons: concentration dependence and kinetics

The concentration dependence and kinetics of ionic currents activated by extracellular adenosine 5'-triphosphate (ATP) were studied in voltage-clamped dorsal root ganglion neurons from rats and bullfrogs. About 40% of neurons of both species responded to ATP with an increase in membrane conductance. The ATP-activated currents were similar in the 2 species, except that currents in rat neurons desensitized faster. In bullfrog neurons, the conductance was half-maximally activated by about 3 microM ATP; at low concentrations, the conductance increased 3- to 7-fold for a doubling in [ATP], suggesting that several ATP molecules must bind in order to activate the current. A steeper concentration-response relationship than expected from 1:1 binding was also seen in rat neurons. The current activated quickly upon application of ATP and decayed quickly when ATP was removed. Activation kinetics were faster at higher [ATP], with time constants decreasing from about 200 msec at 0.3 microM ATP to about 10 msec at 100 microM ATP. Deactivation kinetics (tau approximately 100-200 msec) were independent of the ATP concentration. The rapid activation and deactivation make it seem likely that the ATP-activated current is mediated by direct ligand binding rather than by a second-messenger system. The experimental observations can be mimicked by a simple model in which ATP must bind to 3 identical, noninteracting sites in order to activate a channel. The potency and kinetics of ATP action were voltage-dependent, with hyperpolarization slowing deactivation and increasing ATP's potency. Deactivation kinetics were also sensitive to the concentration of external Ca, becoming faster in higher Ca.

Response of lupin cultivars to concentration of calcium and activity of aluminum in dilute nutrient solutions

Abstract Although lupins (Lupinus angustifolius L.) are usually grown in slightly acidic soils covering the pH range 6.0 to 6.5, the response of lupin cultivars to a range of acid soil infertility factors has not been investigated. Effects of external Ca concentrations of 150 or 1000 μM and S activities of monomeric Al species (Al reactive with aluminon in 30 min, without acidification and heating; SaA1 mono) up to 80 μM on early growth and mineral composition in tops of three lupin cultivars (Unicrop, Uniharvest, and Uniwhite) were investigated in continuously flowing nutrient solution at pH 4.3 ± 0.1. The dry weight of tops of the three cultivars was generally similar in Uniwhite and Unicrop, and considerably lover in Uniharvest. The weight of tops increased by 24 to 39% while that of roots increased by 11 to 14% with an increase in Ca concentration from 150 to 1000 μM in Al‐free solutions. At each Ca concentration, an increase in activity of monomeric Al resulted in a linear decrease in dry weight of t...

Acclimation of Sodium Regulation to Low pH and the Role of Calcium in the Acid-Tolerant SunfishEnneacanthus obesus

Aspects of Na regulation of Enneacanthus obesus were examined to elucidate the adaptations that allow them to inhabit low-pH environments. We examined Na uptake kinetics in this native of extremely soft waters, and the mechanisms for the prevention of Na loss upon exposure to low pH. Active Na uptake (Jin) at pH 5.8 demonstrated saturation kinetics typical of a soft-water organism with a low Vmax (128 μmol/kg . h). However the relatively high Km (125 μM) compared to that of other, less acid-tolerant fish, indicated a surprisingly low afinity of the uptake mechanism for Na. Jin, which is fully inhibited upon exposure to pH 4.0, showed no recovery after up to 1 wk at this pH. In contrast to Jin, Na eflux (Jout) showed a great resistance to disruption by low pH, and the ability to acclimate. Prior exposure to pH 4. 0 significantly reduced Jout at pH 3.25. It is suggested that an already low Na permeability is reduced further during the acclimation period, and that this change may be hormonally regulated. The external Ca concentration had a large effect on Jout at pH 3.25, but only over a narrow concentration range of 0-50 μM, which indicates an extremely high affinity of the gills for Ca. The Ca concentration of the medium had no effect on Jout at pH 3.0. In conjunction with the literature these results suggest that there are two components to stimulation of Jout by H⁺: (1) leaching of Ca from the gills, and (2) structural damage of gill epithelium. It appears that, as pH declines, Ca leaching becomes less important and gill epithelial damagepredominates as the cause of high Jout.

Retinomotor movements in the frog retinal pigment epithelium: Dependence of pigment migration on Na + and Ca 2+

The ionic dependence of the screening-pigment migrations in the frog retinal epithelium (RPE) was quantitatively studied with eyecups incubated in media of different compositions. Typical migrations in response to light and darkness, equivalent to those observed in the intact animal, were fully accomplished and maintained for up to 6 hr by the isolated organ bathed in Ringer solution rich with O 2. Pigment migration in either direction was completed under the appropriate illumination conditions at any time during the day, indicating that circadian influences, if present in the intact animal, can be overridden in the isolated organ by light or darkness alone. Pigment aggregation toward the dark-adapted position was inhibited by: (a) low external Ca 2+, (b) high external Na +, and (c) drugs expected to increase the cytoplasmic levels of either Na +, or Ca 2+, like ouabain, caffeine and the ionophore A23187. However, the inhibition caused by low Ca 2+ did not occur if Na + was also reduced in the incubation medium. On the other hand, an increase in the concentration of external Ca 2+ or the addition of Co 2+ to the normal Ringer facilitated pigment aggregation in the dark. Pigment dispersion to the light-adapted position was unaffected by any of the above conditions. This is the first report of full and stable pigment responses in the RPE of vertebrate eyes incubated under simple physiological conditions. The results seem to conciliate a discrepancy of previous reports on the Ca 2+ dependence of RPE movements, and are compatible with current views on ionic mechanisms in analogous systems of intracellular transport.

Ca2+-dependent facilitated shortening in isotonic contraction of trachealis muscle.

We compared isotonic shortening with isometric force generation as a function of external Ca2+ in 166 tracheal smooth muscle (TSM) strips from 27 mongrel dogs in vitro. Concentration-response curves were generated with muscarinic stimulation (acetylcholine, ACh), alpha-adrenergic receptor activation (norepinephrine after beta-adrenoceptor blockade, NE), serotonin (5-HT), and KCl-substituted Krebs-Henseleit solution. The concentrations of 5-HT causing half-maximal shortening (ECS50, 1.54 +/- 0.14 X 10(-7) M) and half-maximal active isometric tension (ECT50, 1.72 +/- 0.30 X 10(-7) M) were similar (P = NS). Likewise, ECS50 (21.9 +/- 0.7 mM) and ECT50, (22.0 +/- 0.9 mM) were similar for KCl. In contrast, facilitated isotonic shortening (i.e., greater isotonic shortening for comparable degrees of force generation) was elicited with ACh and NE for all levels of force generation between 15 and 85% of maximum and for all concentrations of ACh from 3 X 10(-8) to 3 X 10(-5) M (P less than 0.05 for all points). Facilitated isotonic shortening also was elicited for all concentrations of NE from 10(-8) to 10(-6) M (P less than 0.05 for all points). Removal of Ca2+ from the perfusate substantially reduced the potency of ACh (P less than 0.001) and abolished differences between ECS50 (2.23 +/- 0.28 X 10(-5) M) and ECT50 (2.50 +/- 0.46 X 10(-5) M, P = NS). We demonstrate that for comparable degrees of force generation, muscarinic and alpha-adrenergic receptor activation cause greater isotonic shortening than KCl or 5-HT and that this facilitated shortening is associated with the concentration of external Ca2+.

Regulation of Vacuolar pH of Plant Cells

The vacuolar pH and the trans-tonoplast DeltapH modifications induced by the activity of the two proton pumps H(+)-ATPase and H(+)-PPase and by the proton exchanges catalyzed by the Na(+)/H(+) and Ca(2+)/H(+) antiports at the tonoplast of isolated intact vacuoles prepared from Catharanthus roseus cells enriched in inorganic phosphate (Y Mathieu et al 1988 Plant Physiol [in press]) were measured using the (31)P NMR technique. The H(+)-ATPase induced an intravacuolar acidification as large as 0.8 pH unit, building a trans-tonoplast DeltapH up to 2.2 pH units. The hydrolysis of the phosphorylated substrate and the vacuolar acidification were monitored simultaneously to estimate kinetically the apparent stoichiometry between the vectorial proton pumping and the hydrolytic activity of the H(+)-ATPase. A ratio of H(+) translocated/ATP hydrolyzed of 1.97 +/- 0.06 (mean +/- standard error) was calculated. Pyrophosphate-treated vacuoles were also acidified to a significant extent. The H(+)-PPase at 2 millimolar PPi displayed hydrolytic and vectorial activities comparable to those of the H(+)-ATPase, building a steady state DeltapH of 2.1 pH units. Vacuoles incubated in the presence of 10 millimolar Na(+) were alkalinized by 0.4 to 0.8 pH unit. It has been shown by using (23)Na NMR that sodium uptake was coupled to the H(+) efflux and occurred against rather large concentration gradients. For the first time, the activity of the Ca(2+)/H(+) antiport has been measured on isolated intact vacuoles. Ca(2+) uptake was strongly inhibited by NH(4)Cl or gramicidin. Vacuoles incubated with 1 millimolar Ca(2+) were alkalinized by about 0.6 pH unit and this H(+) efflux was associated to a Ca(2+) uptake as demonstrated by measuring the external Ca(2+) concentration with a calcium specific electrode. Steady state accumulation ratios of Ca(2+) as high as 100 were reached for steady state external concentrations about 200 micromolar. The rate of Ca(2+) uptake appeared markedly amplified in intact vacuoles when compared to tonoplast vesicles but the antiport displayed a much lower affinity for calcium. The different behavior of intact vacuoles compared to vesicles appears mainly to be due to differences in the surface to volume ratio and in the rates of dissipation of the pH gradient. Despite its low affinity, the Ca(2+)/H(+) antiport has a high potential capacity to regulate cytoplasmic concentration of calcium.

The effects of external calcium on long-term potentiation in the rat hippocampal slice

Extracellular excitatory postsynaptic potentials (EPSPs) were recorded from the stratum radiatum of CA1 of the rat hippocampal slice. The effect of altering the external Ca concentration was studied on the amplitude of the low frequency evoked EPSP preceeding and following the production of long-term potentiation (LTP). The double logarithmic plot of the relationship between the EPSP amplitude and external Ca had a maximum slope of 2.1, implying Ca cooperativity in transmitter release. The production of LTP did not alter the slope. The amplitude of LTP was found to be highly dependent on the external Ca concentration, with LTP increasing from a 5% increase in the EPSP amplitude in 0.8 mM Ca, to a 65% increase in the EPSP amplitude in 2.0 mM Ca.

The action potential of Dionaea muscipula Ellis

The intention of this investigation was to acquire more concise information about the nature of the action potential of Dionaea muscipula Ellis and the different types of cells generating and conducting it. It is shown by microelectrode measurements that, besides the sensory cells, all the major tissues of the trap lobes are excitable, firing action potentials with pronounced after-hyperpolarizations. The action potentials are strictly dependent on Ca(2+). Their peak depolarizations are shifted 25-27 mV in a positive direction after a tenfold increase in external Ca(2+) concentration. Perfusions with 1 mM ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or 1 mM LaCl3 completely inhibit excitability. Magnesium ions only slightly affect the peak depolarizations but considerably prolong action potentials. Sodium azide and 2,4-dinitrophenol also abolish excitation, probably by reducing the intracellular ATP concentration. Furthermore, it is tested whether the sensory cells can be distinguished from the other cells of the trap by their electrical behaviour. The resting potentials of sensory cells (-161±7 mV) and mesophyll cells (-155±8 mV) are of the same magnitude. Changes in external ion concentrations affect resting and action potentials in both cell types in a similar way. Additional freeze-fracture studies of both cell types reveal similar numbers and distributions of intramembrane particles on the fracture faces of the plasma membrane, which is most likely the mechanosensor. These findings stress the view that the high mechanosensitivity of the sensory hair results from its anatomy and not from a specialized perception mechanism. It is proposed that trap closure is triggered by a rise in the cytoplasmic concentration of Ca(2+) or a Ca(2+)-activated regulatory complex, which must exceed a threshold concentration. Since the Ca(2+) influx during a single action potential does not suffice to reach this threshold, at least two stimulations of the trap are necessary to elicit movement.

Involvement of calcium in calcium-current inactivation in smooth muscle cells from rat vas deferens

Ca-channel currents were recorded in Cs-loaded single smooth muscle cells from rat vas deferens to define the dependence of the inactivation time course on Ca concentration. The decay of Ca-channel current obtained in a Ba2+- or Sr2+-containing external solution during long voltage-clamp pulses was much slower than that in a Ca-containing solution. The difference was not due to a change in the surface potential of the membrane as judged from the steady-state activation and inactivation curves. When Ca was the charge carrier, increasing external Ca concentration slightly accelerated the rate of inactivation. In addition, the rate of inactivation of Ca-channel current in 10.8 mM Ba was also accelerated by adding Ca to the external solution in a concentration-dependent manner. The time course of Ca-current inactivation was slowed when the cells were dialyzed with a high concentration of citrate, a Ca-chelating agent. From these results, we concluded that a mechanism regulated by intracellular Ca activity plays a role in the inactivation of Ca channels in smooth muscle. The Ca-dependent process may protect against Ca overload by regulating Ca entry in smooth muscle cells.