A prolonged,voltage-dependent calcium permeability revealed by tetraethylammonium in the soma and axon of Aplysia giant neuron

The soma but not the axon of the giant neuron, R2, of Aplysia can generate an all-or-none Ca spike in Na-free or TTX-containing medium (Junge and Miller, 1974). Extracellular axonal recordings made at several distances from the soma provide evidence that the transition in ability to fire a spike in Na-free medium occurs within the first 250 μm of the axon. Application of 25 mM TEA-Br to the bathing medium causes a more than tenfold increase in the duration of the somatic action potential. The duration of the axonal action potential in TEA decreases with distance from the soma. At distances greater than 3 mm from the soma this concentration of TEA causes little or no increase in the duration of the axon spike. The effect of 25 mM TEA on both the soma and proximal axon is blocked reversibly by 30 mM CoCl₂ or 1 mM CdCl₂. The duration of the somatic action potential in TEA increases with an increase in Ca concentration of the bath. At a constant concentration of Na, the voltage level of the somatic plateau increases with Ca concentration in the manner predicted for a Ca electrode. In the presence of 11 mM Ca²⁺ the potential of the plateau is relatively insensitive to Na concentration. The TEA plateau in R2 reveals a prolonged voltage-dependent permeability to Ca. The duration of the plateau may indicate the degree of Ca activation during a spike.     

Calcium exchange and contraction strength of guinea pig atrium in normal and hypertonic media

A Krebs-Henseleit (KH) medium made hypertonic by adding nonpermeant molecules substantially increased the isometric peak tension at steady-state contractions below 3 per sec in guinea pig atrium at 27°C. Action potential durations were decreased. KH plus 100 mM raffinose or sucrose resulted in similar and nearly maximal changes which were essentially reversible upon return to normal KH. When one active contracting atrium was used to passively stretch a second atrium, the difference in Ca ion exchange (1 min exchange with the extracellular space) between active and stretched atria significantly increased at 1 per sec and at 2 per sec in going from normal to 100 mM hypertonic KH. The calculated mean Ca ion cellular exchange per beat per 100 g of cells (a) doubled in changing from normal to 100 mM hypertonic KH, and (b) decreased slightly in changing from contractions of 1 per sec to 2 per sec in normal KH. These data are consistent with the hypothesis (a) that Ca ion entry per beat from the extracellular space is proportional to membrane depolarized time with a constant medium and a steady-state condition, and the hypothesis (b) that 100 mM hypertonicity doubles the Ca ion entry rate during depolarization. These data enable rejection of the hypothesis that the peak tension is proportional to the Ca ion entry per beat from the extracellular space under steady-state conditions, and suggest that any additional Ca ion involved in the larger contractions at higher frequencies comes from an increase in Ca ion available from intracellular stores.     

Voltage-activated lonic currents in differentiating rat cerebellar granule neurons cultured from the external germinal layer

The electrical properties of the precursor cells of the external germinal layer of rat cerebellum were assessed during their differentiation in control medium (Dulbecco's modified Eagle's medium) supplemented or not with either basic fibroblast growth factor (bFGF) or 25 mM potassium chloride (KCI). Resting potential was shown to be –10 mV in all three conditions 3 hours after plating [days in vitro (DIV)0]. By DIV 5, it reached -63 mV for cells cultured in 25 mM KCI but only –28 mV in control and bFGF media. The main voltage-sensitive ionic current measured at DIV 0 under all conditions was a composite I_k consisting in a sustained K⁺ current blocked by tetraethylammonium (I_k(TEA)), plus a rapidly activating and inactivating TEA-insensitive I_k(A). Both currents increased with time in all conditions, but after 5 days I_K(A) became dominant in terms of density. I_K(TEA) is likely an I_K(Ca), since it was blocked by 67% in 1 mM TEA. On DIV O, I_Na and I_Ca were absent or small in amplitude. By DIV 3, 80% of the cells had currents able to generate a spike. Interestingly, I_Ca mean amplitude and current density measured at –10 mV in control condition on DIV 1 was singnificantly larger than those recorded in bFGF and 25 mM KCI. The order of appearance of the ionic currents, I_K, I_Ca, and I_Na, leads directly to fast spike activity allowing for poor calcium entry. Firing rate likely depends on I_K(A), which increased during the first 6 days of development but could be differentially regulated by bFGF. © 1995 John Wiley & Sons, Inc.     

The action potential in the smooth muscle of the guinea pig taenia coli and ureter studied by the double sucrose-gap method

The configuration of the electrotonic potential and the action potential observed by the double sucrose-gap method was similar to that observed with a microelectrode inserted into a cell in the center pool between the gaps. In the taenia and the ureter, the evoked spike was larger in low Na or in Na-free (sucrose substitute) solution than in normal solution. However, the plateau component in the ureter was suppressed in the absence of Na. In Ca-free solution containing Mg (3–5 mM) and Na (137 mM), the membrane potential and membrane resistance were normal, but no spike could be elicited in both the taenia and ureter. Replacement of Ca with Sr did not affect the spike in the taenia, nor the spike component of the ureter but prolonged the plateau component. The prolonged plateau disappeared on removal of Na, while repetitive spikes could still be evoked. It was concluded that the spike activity in the taenia and in the ureter of the guinea pig is due to Ca entry, that the plateau component in the ureter is due to an increase in the Na conductance of the membrane, and that both mechanisms, for the spike and for the plateau, are separately controlled by Ca bound in the membrane.     

Generation and amplification of the cytosolic calcium signal during secretory responses to gonadotropin-releasing hormone

Gonadotropin-releasing hormone (GnRH) stimulates characteristic biphasic increases in cytosolic calcium concentration ([Ca2+]i) and in luteinizing hormone (LH) release in cultured gonadotrophs, with an early peak followed by a prolonged plateau in both responses. Analysis of [Ca2+]i by dual-wavelength fluorimetric assay and of LH release at 5-sec intervals in perifused pituitary cells revealed increases in both responses within a few seconds of exposure to GnRH. The maximum elevation of [Ca2+]i occurred within 20 sec, and the peak gonadotropin release in 35 sec; the total duration of the spike phase for both [Ca2+]i and LH release was 2.5 min. Under extracellular Ca2(+)-deficient conditions, the GnRH-induced peak in [Ca2+]i was reduced by about 20% and the plateau phase was abolished. Concomitantly, the magnitude of the acute phase of LH release was reduced by 40% and that of the second phase by about 90%. Recovery of the plateau phase of LH release occurred within 25 sec after addition of 1.25 mM Ca2+ to Ca2(+)-deficient medium. In a dose-dependent manner, the non-selective Ca2+ channel blockers Co2+ and Cd2+ reduced the Ca2+ current measured by whole-cell recording in pituitary gonadotrophs and abolished the extracellular Ca2(+)-dependent component of LH release. The selective calcium channel blocker, nifedipine, decreased the magnitude of the Ca2+ current and reduced the plateau phase of LH release by 50%; conversely, the dihydropyridine agonist methyl, 1,4,dihydro-2,6-dimethyl 3-nitro-4-(2-trifluorome) (Bay K 8644) consistently enhanced the amplitudes of both Ca2+ current and GnRH-induced LH release. These data reveal a close temporal correlation between changes in [Ca2+]i and LH release during GnRH action, with Ca2+ mobilization during the spike phase and Ca2+ influx through dihydropyridine-sensitive and insensitive sets of receptor-operated calcium channels during the spike and plateau phases. In addition, analysis of the magnitudes of the [Ca2+]i and LH responses to a wide range of GnRH concentrations in the presence and absence of extracellular Ca2+ is consistent with amplification of the [Ca2+]i signal in agonist-stimulated gonadotrops.     

Recovery of Ins(1,4,5)-trisphosphate-dependent calcium signaling in neonatal gonadotrophs

Pituitary gonadotrophs express non-desensitizing gonadotropin-releasing hormone (GnRH) receptors and their activations leads to inositol 1,4,5-trisphosphate (InsP3)-dependent Ca2+ mobilization. When added in physiological concentration range GnRH induces baseline Ca2+ oscillations, whereas in higher concentrations it induces a prolonged spike response accompanied with non-oscillatory or oscillatory plateau response. Here, we studied the recovery of calcium signaling during repetitive stimulation with short (10-30 s) GnRH pulses and variable interpulse intervals in neonatal gonadotrophs perfused with Ca2+/Na+ -containing, Ca2+ -deficient/Na+ -containing, and Ca2+ -containing/Na+ -deficient media. In Ca2+/Na+ -containing medium, baseline Ca2+ oscillations recovered without refractory period and with a time constant of approximately 20 s, whereas the recovery of spike response occurred after 25-35 s refractory period and with a time constant of approximately 30 s. During repetitive GnRH stimulation, removal of Ca2+ had only a minor effect on baseline oscillations but abolished spike response, whereas removal of Na+ slightly extended duration of baseline oscillations and considerably prolonged spike response. These results indicate that two calcium handling mechanisms are operative in gonadotrophs: redistribution of calcium within InsP3-sensitive and -insensitive pools and a sodium-dependent calcium efflux followed by calcium influx. Redistribution of Ca2+ within the cell leads to rapid recovery of InsP3-dependent pool, whereas the Na+ -dependent Ca2+ efflux pathway is activated by spike response and limits the time of exposure to elevated cytosolic Ca2+ concentrations.     

Some factors influencing sodium extrusion by internally dialyzed squid axons

Squid giant axons were internally dialyzed by a technique previously described. In an axon exposed to cyanide seawater for 1 hr and dialyzed with an ATP-free medium, the Na efflux had a mean value of 1.3 pmole/cm²sec when [Na]_i was 88 mM, in quantitative agreement with flux ratio calculations for a purely passive Na movement. When ATP at a concentration of 5–10 mM was supplied to the axoplasm by dialysis, Na efflux rose almost 30-fold, while if phosphoarginine, 10 mM, was supplied instead of ATP, the Na efflux rose only about 15-fold. The substitution of Li for Na in the seawater outside did not affect the Na efflux from an axon supplied with ATP, while a change to K-free Na seawater reduced the Na efflux to about one-half. When special means were used to free an axon of virtually all ADP, the response of the Na efflux to dialysis with phosphoarginine (PA) at 10 mM was very small (an increment of ca. 3 pmole/cm²sec) and it can be concluded that more than 96% of the Na efflux from an axon is fueled by ATP rather than PA. Measurements of [ATP] in the fluid flowing out of the dialysis tube when the [ATP] supplied was 5 mM made it possible to have a continuous measurement of ATP consumption by the axon. This averaged 43 pmole/cm²sec. The ATP content of axons was also measured and averaged 4.4 mM. Estimates were made of the activities of the following enzymes in axoplasm: ATPase, adenylate kinase, and arginine phosphokinase. Values are scaled to 13°C.     

Ethylene inhibitors promote in vitro regeneration of cowpea (Vigna unguiculata L.)

Summary Ethylene is a plant growth regulator that is known to influence in vitro morphogenesis. This study investigated the effects of three ethylene inhibitors, silver nitrate (AgNO₃), 2,5-norbornadiene, and cobalt chloride (CoCl₂), on the regeneration of cowpea from cotyledon explants. Significant increases in the percentage of regeneration occurred as a result of adding either 50 μM AgNO₃ or 100 μM 2,5-norbornadiene. The number of shoots produced per explant was enhanced by adding 25 μM CoCl₂ or 100 μM norbornadiene. Maximum shoot elongation was obtained with 25 μM of either CoCl₂ or norbornadiene. The effect of the duration of exposure to AgNO₃ was also determined. The greatest percent regeneration was obtained with the addition of 60 μM AgNO₃ either to both the initiation and regeneration stages, or to only the regeneration stage. The promotive effects on organogenesis in response to ethylene inhibitors suggests an important role for ethylene in the process of in vitro morphogenesis of cowpea and may contribute to its normally low regeneration frequency.     

Monosynaptic pathway responsible for generation of bursting activity in theHelix pomatia RPal neuron

The connection between a visceral ganglia interneuron initiating bursting pacemaker activity in the RPal neuron and the RPal neuron itself was investigated inHelix pomatia. Stimulating the interneuron either initiated or intensified bursting activity in the RPal neuron, depending on initial electrical activity in this cell. Replacing calcium with magnesium ions in the extracellular fluid and adding CdCl₂ to this fluid reversibly inhibited the effect of interneuronal stimulation on the RPal neuron. The latter effect was unaffected by increasing the concentration of extracellular Ca²⁺ 10 to 70 mM. Intracellular injection of both Cs⁺ and TEA into the interneuron produced an increase in the duration of its action potentials and rendered the link connecting the neurons more effective. It is deduced that a monosynaptic chemical connection exists between the interneuron and the RPal neuron for which a peptide compound serves as transmitter.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 1, pp. 20–28, January–February, 1987.     

Calcium-cadmium interaction on sugar absorption across the rabbit jejunum

The element Cd is considered to have no biological function and is highly toxic to humans and animals. Toxic effects of this metal upon cell membrane structure and function have been shown. On the other hand, Ca is an essential element in a wide variety of cellular activities. The present study was initiated to research whether the interaction between Ca and Cd could affect D-galactose absorption across the rabbit jejunum in vitro. In media with Ca²⁺, when CdCl₂ was present at 0.5 or 1 mM, Cd was found to significantly reduce the sugar absorption. In Ca²⁺-free media, where CaCl₂, was omitted and replaced isotonically with choline chloride, the sugar transport was not modified by Cd, but when CaCl₂ was replaced isotonically with MgCl₂, the inhibition is observed. Verapamil at 10⁻⁶ M (blocking mainly Ca²⁺ transport) did not modify the inhibitory effect of cadmium on D-galactose transport. When 10⁻⁶ M of A 23187 (Ca²⁺ specific ionophore) was added in media with/without Ca²⁺; CdCl₂ produced no change in D-galactose transport. These results suggest that Ca and Cd could have affinity for the same chemical groups of enterocyte membrane, which would be related with the intestinal absorption of D-galactose.     

On the mechanism of cesium-induced voltage and current tails in single ventricular myocytes

The mechanisms by which different concentrations of cesium modify membrane potentials and currents were investigated in guinea pig single ventricular myocytes. In a dose-dependent manner, cesium reversibly decreases the resting potential and action potential amplitude and duration, and induces a diastolic decaying voltage tail (V_ex), which increases at more negative and reverses at less negative potentials. In voltage-clamped myocytes, Cs⁺ increases the holding current, increases the outward current at plateau levels while decreasing it at potentials closer to resting potential, induces an inward tail current (I_ex) on return to resting potential and causes a negative shift of the threshold for the inward current. During depolarizing ramps, Cs⁺ decreases the outward current negative to inward rectification range, whereas it increases the current past that range. During repolarizing ramps, Cs⁺ shifts the threshold for removal of inward rectification negative slope to less negative values. Cs⁺-induced voltage and current tails are increased by repetitive activity, caffeine (5 mM) and high [Ca²⁺]_o (8.1 mM), and are reduced by low Ca²⁺ (0.45 mM), Cd²⁺ (0.2 mM) and Ni²⁺ (2 mM). Ni²⁺ also abolishes the tail current that follows steps more positive than E_Ca. We conclude that Cs⁺ (1) decreases the resting potential by decreasing the outward current at more negative potentials, (2) shortens the action potential by increasing the outward current at potentials positive to the negative slope of inward rectification, and (3) induces diastolic tails through a Ca²⁺-dependent mechanism, which apparently is an enhanced electrogenic Na-Ca exchange.     

Effect of Cd on Photosynthesis and Transpiration of Excised Leaves of Corn and Sunflower

Detached corn and sunflower leaves exposed to various concentrations of Cd, supplied as CdCl₂, exhibit reduced photosynthesis and transpiration. The reduction is dependent on the concentration of CdCl₂ solution and generally becomes more pronounced with time. In sunflower, net photosynthesis and transpiration are completely inhibited within 45 min after the introduction of 18 mM Cd. Within two hours net photosynthesis is reduced to 40% and 70% of maximum after the introduction of 9 and 4.5 mM Cd respectively. In corn the trend of photo-synthetic response to Cd is similar to that in sunflower except that the inhibition in corn is more pronounced at all treatment levels. A strong linear relationship between photosynthesis and transpiration inhibition is obtained in both species suggesting that Cd contamination induces stomatal closure.     

Effects of tetraethylammonium and 4-aminopyridine on the plateau potential of circular myometrium from the pregnant rat

In the pregnant rat, spontaneous electrical activity of circular muscle (CM) changes from single, plateau-type action potentials at early and mid-term to repetitive spike trains at term. To examine mechanisms underlying the plateau, we studied the effects of potassium channel blockers tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on membrane potentials in CM from rats on gestation Days 14, 15, 16, 21 (term). Apparent membrane conductance was measured at rest and during the plateau in Day 14 muscles with and without TEA. 4-AP depolarized the resting membrane on all gestation days. Therefore, a direct action of 4-AP on plateau configuration could not be separated from an indirect effect of depolarization. TEA did not affect the resting potential but increased action potential size and depolarization rate on all gestation days. On Day 16, TEA reduced plateau amplitude, unmasking small, repetitive depolarizations. D-600 decreased plateau amplitude and duration and attenuated these effects of TEA. Plateau conductance increased initially then decreased before membrane repolarization. Membrane conductance and outward rectification during the plateau were reduced by TEA. The plateau potential may result from an outwardly rectifying TEA-sensitive current combined with a slow inward current, the plateau magnitude being determined by the relative intensity of each current.     

Calcium Buffering and Free Ca2+ in Rat Brain Synaptosomes

Abstract: The effects of K⁺ depolarization and of stimulation by veratridine on apparent cytosolic free Ca²⁺ ([Ca²⁺]_cyt) and net Ca²⁺ accumulation were measured in isolated rat brain presynaptic nerve terminals (synaptosomes). [Ca²⁺]_cyt was determined with fura-2, and Ca²⁺ accumulation was measured with tracer ⁴⁵Ca. [Ca²⁺]_cyt was ～ 325 nM in synaptosomes incubated in the normal physiological salt solution under resting conditions. When [K⁺]₀, was increased from the normal 5 mM to 30 or 50 mM, ⁴⁵Ca uptake and [Ca²⁺]_cyt both increased within 1 s. Both increases were directly related to [Ca²⁺]₀ for [Ca²⁺]₀= 0.02–1.2 mM; however, the increase in ⁴⁵Ca uptake greatly exceeded the increase in [Ca²⁺]_cyt. With small Ca²⁺ loads ≤100 μmol/L of cell water, equivalent to the Ca²⁺ entry during a train of ≤60 impulses), the ⁴⁵Ca uptake exceeded the increase in [Ca²⁺]_cyt by a factor of nearly 1,000. This indicates that ～99.9% of the entering Ca²⁺ was buffered and/or sequestered within ～ 1 s. With larger Ca²⁺ loads, a larger fraction of the entering Ca²⁺ was buffered; ～99.97% of the load was buffered with loads of 250–425 μmol/L of cell water. The ratio between the total Ca²⁺ entry and the increase in [Ca²⁺]_cyt, the “calcium buffer ratio”β, was therefore ～ 3,500:1. This ratio was somewhat lower than the ratio of total intraterminal calcium: [Ca²⁺]_cyt, which ranged between ～7,300:1 and 12,800:1. When the synaptosomes were activated with 10 μM veratridine ([Ca²⁺]₀= 0.2–0.6 mM), ⁴⁵Ca influx and [Ca²⁺]_cyt increased progressively for ～10 s (β= 2,700:13,050:1) and then leveled off. Application of 10 μM tetrodotoxin before the introduction of veratridine prevented the increases in ⁴⁵Ca influx and [Ca²⁺]_cyt. Application of 10 μM tetrodotoxin after 5–10 s of exposure to veratridine caused both the [Ca²⁺]_cyt and the veratridine-stimulated ⁴⁵Ca within the terminals to decline, thereby demonstrating that the Ca²⁺ loading is reversible in the presence of extracellular Ca²⁺. These data show that synaptosomes are capable of buffering and metabolizing Ca²⁺ in a manner expected for intact neurons.     

An atomic absorption method for cation measurements in Kjeldahl digests of biological materials

Sulfate ion produced little or no interference in absorption by sodium, potassium, and magnesium, but produced a large depression in calcium absorbance in the atomic absorption spectrophotometric measurement of these cations in an acetylene-air flame. Nearly maximal depression of calcium absorbance by 2 mM sulfate was followed by a plateau region of only slight depression from 2 mM to 1 M sulfate concentration. Presence of 25 mM lanthanum in the samples resulted in no depression of calcium absorbance up to 2 mM sulfate, a sharp decrease to about 30 mM sulfate and a plateau from 30 mM up to 1 M sulfate. From these observations, it was determined that the addition of H₂SO₄ to provide approximately 40 mM added sulfate in standards and samples permitted accurate measurement of calcium even though the original sample contained relatively high and variable sulfate.     

Optimization of nutrient levels in the medium increases the efficiency of callus induction and plant regeneration in recalcitrant indian barley (<Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.) <Emphasis Type="Italic">in vitro</Emphasis>

Summary The present study reports that a revised nutrient concentration in the basal medium improved shoot bud induction and subsequent plant regeneration in barley (Hordeum vulgare L. var. BL-2). Cultures were raised from immature embryos on MSB₅ medium supplemented with picloram. Concentrations of five nutrients were varied. The effect of these nutrients was investigated on (1) induction, (2) induction and subculture, and (3) induction, subculture and regeneration stages. The basal MSB₅ medium was not optimal for each phase of barley culture. Decreased ammonium nitrate, increased potassium dihydrogen phosphate, sodium molybdate, cobalt chloride, and addition of glycine enhanced shoot bud induction and plant regeneration. The different media that were optimal for immature embryo culture were: MSB₅ medium supplemented with 20.70 μM picloram, 10.30 mM NH₄NO₃, 6.25 mM KH₂PO₄, 2.06 μM Na₂MoO₄, 0.55 μM CoCl₂, and 26.64 μM glycine (for induction); MSB₅ medium supplemented with 12.47 μM picloram, 10.30 mM NH₄NO₃, and 0.55 μM CoCl₂ (for subculture); and MSB₅ medium supplemented with 0.2 μM picloram and 10.3 mM NH₄NO₃ (for regeneration). Primary cultures required 6wk (without transfer) for morphogenic callus formation. Callus required 4wk of subculture and another 4wk on regeneration medium for optimal plant regeneration. The revised medium could also promote regeneration of the recalcitrant barley genotype RD-2552. Histological analysis showed that the major pathway of differentiation was through shoot bud formation.     

Zinc modulates a-type potassium currents and neuronal excitability in snail neurons

Summary 1. Zinc-induced actions were studied on the A-current and neuronal activity in identified and unidentified nerve cells of the snail,Helix pomatia L., under voltage and current clamp conditions.2. Extracellularly applied Zn²⁺ attenuated the peak amplitude of the A-current in a potential- and dose-dependent way (K _i=1.8 mM at –30 mV,n _H=0.6).3. Attenuation of the A-currents was initiated as Zn²⁺ shifted the potential dependence of both activation and inactivation of the currents toward more positive potential values.4. Zinc concomitantly prolonged the time to peak and decay time constant of the A-currents (K _d=1.7 mM,n _H=1.4) as well.5. Zn²⁺ decreased the resting membrane potential and the spike amplitude and increased the action potential duration and the input resistance of the cells in current clamp experiments.6. A complex action of zinc increased the neuronal excitability, indicating spontaneous and synaptically evoked spike discharges.7. Common and specific zinc binding sites are supposed on vertebrate and invertebrate A-type potassium channel proteins, where binding Zn²⁺ can modulate the gating properties and kinetics of the fast outward potassium currents.     

Cerebral [K+]e increase as an index of the differential susceptibility of brain structures to terminal anoxia and electroconvulsive shock

The time course of the [K⁺]_e increase elicited by terminal anoxia or by electroconvulsive shock (ECS) was compared in various parts of the rat brain. The [K⁺]_e was measured with ion-selective microelectrodes stereotaxically introduced into the target area. Respiration arrest induced in anesthetized rats a slow [K⁺]_e increase to about 6–10 mM followed by an abrupt rise to 30–50 mM (doubling time 5–14 sec) in the neocortex, hippocampus, amygdala, caudate nucleus, and thalamus. In the reticular formation, zona incerta, and lateral hypothalamus the second phase of [K⁺]_e increase was much slower (doubling time 30–50 sec) and lacked the autoregenerative character. Trans-pinnate ECS (50 Hz, 0.5 sec, 80 mA), administered to rats immobilized with gallamine triethiodide, elicited a generalized [K⁺]_e increase of the spreading depression type in neocortex and hippocampus (40 mM) as well as in the caudate nucleus and thalamus (20–30 mM), followed by slow [K⁺]_e decrease (half-time 40–60 sec). Much lower ECS-induced [K⁺]_e increase (to 5–6 mM) was observed in the reticular formation, zona incerta, lateral hypothalamus and, surprisingly, in the amygdala. It is concluded that the autoregenerative [K⁺]_e release of spreading depression type develops in structures with high density of membranes reacting to partial depolarization by increased sodium permeability.     

Factors affecting aluminium sorption by calcium pectate

Extracellular processes, particularly the adsorption of aluminium (Al) by pectate in the cell wall, have been proposed as important in the expression of Al toxicity to plant roots. In vitro studies were conducted on the effects of Al concentration (generally ≤ 32 μM), calcium (Ca) concentration (0.05 to 10 mM) and pH (3.2 to 5.4) on Al sorption by Ca pectate. There was a rapid reaction between Al and Ca pectate, there being no difference in Al remaining in solution after reaction times of 1 to 16 min, and only a slight decrease after 24 h. Increased Al concentration in solution increased linearly the sorption of Al by Ca pectate, with 70 to 84% of the Al originally in solution sorbed with ≤32 μM Al. In contrast, Al sorption decreased with increased Ca concentration in solution, and as pH decreased from 5.4 to 3.2. Only ≤30% of the sorbed Al was desorbed after 1 h by 1 mM CaCl₂, 10 mM CaCl₂ or 1 mM HCl. The amount of Al desorbed increased with a desorption period of 5 h, particularly with 1 mM HCl. These studies suggest that Al sorbed by Ca pectate in root cell walls is in equilibrium with Al in solution, and that Al toxicity is associated with the strong binding between Al and Ca pectate external to the cytoplasm.     

Effect of sodium on cellular calcium transport in rat kidney

Summary The influence of extracellular Na (Na _o ) on cellular Ca transport and distribution was studied in rat kidney slices. Calcium efflux from prelabeled slices was depressed when Na _o was completely replaced by choline or tetraethylammonium (TEA) ions and it was markedly stimulated when Na was reintroduced in a Na-free medium. However, reducing Na _o (with choline or TEA as substituting ions) did not increase the total slice⁴⁰Ca, their total exchangeable Ca pool, or the⁴⁰Ca or⁴⁵Ca of mitochondria isolated from these slices. Kinetic analyses of steady-state⁴⁵Ca desaturation curves showed that reducing Na _o depressed the exchange of Ca across the plasma membrane, slightly decreased the cytosolic Ca pool, but did not significantly affect the mitochondrial Ca pool and Ca cycling. Ouabain (10^–3 m) which should reduce the Na gradient across the plasma membrane had no effect on calcium distribution and transport. These results suggest that in kidney cells low Na _o depresses Ca influx as well as Ca efflux; there may be an interaction between Na and Ca at a possible carrier located in the plasma membrane, but there is no Na/Ca exchange as described in several excitable tissues.