Calcium buffering and slow recovery kinetics of calcium-dependent outward current in molluscan neurones

Functional characterisation and cell specificity of BvSUT1, the transporter that loads sucrose into the phloem of sugar beet (Beta vulgaris L.) source leaves

Sugar beet (Beta vulgaris L.) is one of the most important sugar-producing plants worldwide and provides about one third of the sugar consumed by humans. Here we report on molecular characterisation of the BvSUT1 gene and on the functional characterisation of the encoded transporter. In contrast to the recently identified tonoplast-localised sucrose transporter BvTST2.1 from sugar beet taproots, which evolved within the monosaccharide transporter (MST) superfamily, BvSUT1 represents a classical sucrose transporter and is a typical member of the disaccharide transporter (DST) superfamily. Transgenic Arabidopsis plants expressing the β-GLUCURONIDASE (GUS) reporter gene under control of the BvSUT1-promoter showed GUS histochemical staining of their phloem; an anti-BvSUT1-antiserum identified the BvSUT1 transporter specifically in phloem companion cells. After expression of BvSUT1 cDNA in bakers' yeasts (Saccharomyces cerevisiae) uptake characteristics of the BvSUT1 protein were studied. Moreover, the sugar beet transporter was characterised as a proton-coupled sucrose symporter in Xenopus laevis oocytes. Our findings indicate that BvSUT1 is the sucrose transporter that is responsible for loading of sucrose into the phloem of sugar beet source leaves delivering sucrose to the storage tissue in sugar beet taproot sinks.

Desformylflustrabromine Modulates α4β2 Neuronal Nicotinic Acetylcholine Receptor High- and Low-Sensitivity Isoforms at Allosteric Clefts Containing the β2 Subunit

Alterations in expression patterns of α4β2 nicotinic acetylcholine receptors have been demonstrated to alter cholinergic neurotransmission and are implicated in neurologic disorders, including autism, nicotine addiction, Alzheimer's disease, and Parkinson's disease. Positive allosteric modulators (PAMs) represent promising new leads in the development of therapeutic agents for the treatment of these disorders. This study investigates the involvement of the β2-containing subunit interfaces of α4β2 receptors in the modulation of acetylcholine (ACh)-induced responses by the PAM desformylflustrabromine (dFBr). Eight amino acids on the principal face of the β2 subunit were mutated to alanine to explore the involvement of this region in the potentiation of ACh-induced currents by dFBr. ACh-induced responses obtained from wild-type and mutant α4β2 receptors expressed in Xenopus laevis oocytes were recorded in the presence and absence of dFBr using two-electrode voltage clamp electrophysiology. Wild-type and mutant receptors were expressed in both high and low ACh sensitivity isoforms by using biased injection ratios of 1:5 or 5:1 α4 to β2 complementary RNA. Mutations were made in the B, C, and A loops of the principal face of the β2 subunit, which are regions not involved in the binding of ACh. Mutant β2(Y120A) significantly eliminated dFBr potency in both isoform preparations. Several other mutations altered dFBr potentiation levels in both preparations. Our findings support the involvement of the principal face of the β2 subunit in dFBr modulation of ACh-induced responses. Findings from this study will aid in the improved design of dFBr-like PAMs for potential therapeutic use.

Neurophysiology of magnocellular neuroendocrine cells: recent advances

Magnocellular neuroendocrine cells of the hypothalamic paraventricular and supraoptic nuclei are responsible for most of the vasopressin and oxytocin in the peripheral blood as well as for central release of these peptides in selected brain areas. As the principal component of the hypothalamo-neurohypophysial system, these neurons have been a subject of continual study for half a century. The wealth of solid information from decades of in vivo studies has provided a firm basis for in vitro, brain slice and explant investigations of neural mechanisms involved in the control and regulation of vasopressin and oxytocin neurons. In vitro methods have revealed the presence and permitted the study of monosynaptic projections to supraoptic neurons from the olfactory bulbs, the tuberomammillary nuclei of the posterior hypothalamus and from the organum vasculosum of the lamina terminalis. Such methods have also facilitated the elucidation of the various ionic currents controlling neurosecretory cell activity as well as the roles of calcium binding proteins and release of calcium from internal stores. This review summarizes recent advances in our understanding of the afferent inputs that impinge upon these two cell types, and the cellular and molecular mechanisms intrinsic to these neurons that determine their activity patterns and, in part, their responses to incoming stimuli.

Mechanisms of neuroendocrine cell excitability

Oxytocin (OT) and vasopressin (VP), two neuronally synthesized nonapeptides, are made in the hypothalamic paraventricular and supraoptic nuclei of mammals and released into their blood, eventually to have profound hormonal actions on peripheral tissues. In the rat both OT and VP neurons fire slowly and irregularly under conditions of low demand for peptide release, but natural or artificial depolarizing stimuli result in differential patterns of activity: either regular continuous firing, strongly associated with OT cells, or phasic bursting, characteristic of VP neurons. Recently published findings offer an explanation for the dominant presence of certain Ca(2+)-dependent membrane potentials that typically lead to phasic firing in VP neurons. Mechanisms of excitability involved in the differential activities of the two cell types, as well as of the same cell type under different physiological conditions, include such factors as Ca2+ binding proteins, voltage- and ligand-gated ion channels, release of Ca2+ from internal stores and gap junctional conductances. The evidence for these factors is reviewed here.

Intrinsic controls of intracellular calcium and intercellular communication in the regulation of neuroendocrine cell activity

1. The magnocellular hypothalamoneurohypophysial system, consisting chiefly of the supraoptic and paraventricular nuclei and their axonal projections to the pituitary neural lobe, has become a model for the study of neuroendocrine cell morphology, function, and plasticity. 2. Decades of research have produced a wealth of knowledge about the physiological conditions that activate this system, the peripheral target tissues affected by its outputs, and its capacity to undergo use-dependent, reversible reorganization. 3. Earlier research on the neural control of this system concentrated largely on the synaptic inputs that influence the activity of these magnocellular neurons and, while that task is still far from completed, methods have now been developed that permit insights to be gained into the control exercised by intrinsic cellular and molecular mechanisms. 4. This article reviews the current state of knowledge of roles played by these intrinsic mechanisms, including influences of intracellular calcium buffering, calcium release from internal stores and intercellular communication through gap junctions, in the control of neuroendocrine cell activity.

Ca2+ release from internal stores: role in generating depolarizing after-potentials in rat supraoptic neurones

1. Influences of Ca2+ release from internal stores on the generation of depolarizing after-potentials (DAPs) were investigated in magnocellular neurones of rat supraoptic nucleus (SON) using whole-cell patch recording techniques in brain slices. 2. DAPs were recorded from more than half of the cells encountered, and following evoked single spikes had an amplitude of 3.00 +/- 0.19 mV (mean +/- S.E.M.) and lasted for 1.02 +/- 0.06 s. Their sizes usually increased with the number of preceding spikes, but could be reduced or eliminated when intervals between consecutive current pulses evoking tens of spikes were short. 3. DAPs were eliminated by removal of external Ca2+, and significantly reduced by bath application of nifedipine or omega-conotoxin. 4. Blockade of Ca2+ release from internal stores by perifusion with ryanodine or dantrolene, or direct diffusion of Ruthenium Red into cells suppressed DAP amplitudes by approximately 50% and shortened their durations. 5. Depletion of internal Ca2+ stores by perifusion with thapsigargin or cyclopiazonic acid also reduced DAP amplitudes by approximately 50% and eliminated phasic patterns of firing. 6. Caffeine, an agent known to enhance intracellular Ca2+ release, amplified DAPs and promoted phasic firing. 7. These results suggest that Ca2+ influx via high-voltage-activated Ca2+ channels in SON cells triggers ryanodine receptor-mediated Ca2+ release from internal stores. This process enhances DAPs and promotes phasic firing in SON cells, and would thus contribute to vasopressin release.

Monitoring calcium in turtle hair cells with a calcium-activated potassium channel

1. An apamin-sensitive Ca(2+)-activated K+ channel was characterized in turtle hair cells and utilized to monitor submembranous intracellular Ca2+ and to evaluate the concentration of the mobile endogenous calcium buffer. 2. Isolated hair cells were voltage clamped with whole-cell patch electrodes filled with a Cs(+)-based intracellular solution to block the large-conductance Ca(2+)-activated K+ (BK) channel. Ca2+ currents evoked by depolarization were followed by inward tail currents lasting several hundred milliseconds. Both the Ca2+ current and slow tail current were abolished by nifedipine. 3. The tail current was carried by K+ and Cs+ (relative permeabilities PCa/PK = 0.22), and was fully blocked by 0.1 microM apamin and half blocked by 5 mM external TEA. These properties suggest the tail current flows through a Ca(2+)-activated K+ channel distinct from the BK channels. 4. Intracellular Ca2+ was imaged with a confocal microscope in hair cells filled with the indicator Calcium Green-5N introduced via the patch pipette. Increases in Ca2+ evoked by depolarization were localized to hotspots on the basolateral surface of the cell. The time course of the tail current closely matched the fast component of the fluorescenece monitored at a hotspot. 5. Ca(2+)-ATPase pump inhibitors thapsigargin, 2,4-di-(t-butyl)hydroquinone (BHQ) and vanadate, which are known to influence calcium regulation in turtle hair cells, prolonged the time course of the tail current, supporting the idea that the channel monitors cytoplasmic Ca2+. 6. The mobile endogenous buffer was estimated by combining perforated-patch and whole-cell recordings on a single cell. After recording tail currents with an amphotericin-perforated patch, the patch was ruptured to obtain the whole-cell mode, thus allowing washout of soluble cytoplasmic proteins and exchange with pipette buffers. By varying the concentration of Ca2+ buffer in the pipette, the mobile endogenous buffer was found to be equivalent to about 1 mM BAPTA.

Relationship between intracellular calcium and its muffling measured by calcium iontophoresis in snail neurones

1. We have measured intracellular free calcium ion concentration ([Ca2+]i) with fura-2, and intracellular chloride with chloride-sensitive microelectrodes, in voltage-clamped snail neurones. By making iontophoretic injections of CaCl2 we have investigated calcium muffling, the sum of the processes which minimize the calcium transient, at different values of [Ca2+]i. 2. By injection of calcium into cell-sized droplets of buffer we measured the calcium transport index. It was stable over the range pCa 6-7.4 (0.48 +/- 0.06 measured at pCa 6.70 +/- 0.12, n = 5). 3. Measurement of intracellular chloride activity during a series of fura-2-KCl pressure injections revealed a nearly linear relationship between fura-2 Ca(2+)-insensitive fluorescence and the sum of the increments in intracellular chloride. This allowed us to calculate the intracellular fura-2 concentration ([fura-2]i). 4. The rate of recovery of [Ca2+]i following a depolarization-induced load was increased by low [fura-2]i (10-20 microM) but decreased by higher [fura-2]i (40-80 microM). These effects are consistent with the addition of a mobile buffer to the cytoplasm. 5. Iontophoresis of Ca2+ at various membrane potentials allowed us to calculate the intracellular calcium muffling power (the amount of calcium required to cause a transient tenfold increase in [Ca2+]i per unit volume) and calcium muffling ratio (number of Ca2+ ions injected divided by the maximum increase in [Ca2+]i per unit volume) at different values of [Ca2+]i. 6. Calcium muffling power at resting [Ca2+]i was approximately 40 microM Ca2+ (pCa unit)-1, (about 250 times less than for hydrogen ions). It increased linearly about fivefold with [Ca2+]i over the range 20-120 nM (10 cells, 153 measurements) and therefore exponentially with decreasing pCa. 7. The calcium muffling ratio appeared to be constant (361 +/- 14, n = 10 cells, 130 measurements) over the range 20-120 nM Ca2+. 8. In three experiments we modelled the additional calcium buffering power produced by multiple pressure injections of fura-2 into voltage-clamped snail neurones. Back-extrapolation of the increases in calcium buffering power allowed us to calculate the calcium muffling power of the neurones. 9. Small increases in [fura-2]i (approximately 10 microM) significantly increased intracellular calcium muffling power in individual experiments. However, the variability among neurones in intracellular calcium muffling power was large enough to obscure the additional buffering produced by fura-2 in pooled experiments.

Encoding with bursting, subthreshold oscillations, and noise in mammalian cold receptors

Mammalian cold thermoreceptors encode steady-state temperatures into characteristic temporal patterns of action potentials. We propose a mechanism for the encoding process. It is based on Plant's ionic model of slow wave bursting, to which stochastic forcing is added. The model reproduces firing patterns from cat lingual cold receptors as the parameters most likely to underlie the thermosensitivity of these receptors varied over a 25 degrees C range. The sequence of firing patterns goes from regular bursting, to simple periodic, to stochastically phase-locked firing or "skipping." The skipping at higher temperatures is shown to necessitate an interaction between noise and a subthreshold endogenous oscillation in the receptor. The basic period of all patterns is robust to noise. Further, noise extends the range of encodable stimuli. An increase in firing irregularity with temperature also results from the loss of stability accompanying the approach by the slow dynamics of a reverse Hopf bifurcation. The results are not dependent on the precise details of the Plant model, but are generic features of models where an autonomous slow wave arises through a Hopf bifurcation. The model also addresses the variability of the firing patterns across fibers. An alternate model of slow-wave bursting (Chay and Fan 1993) in which skipping can occur without noise is also analyzed here in the context of cold thermoreception. Our study quantifies the possible origins and relative contribution of deterministic and stochastic dynamics to the coding scheme. Implications of our findings for sensory coding are discussed.

Action of diphenylamine carboxylate derivatives, a family of non-steroidal anti-inflammatory drugs, on [Ca2+]i and Ca(2+)-activated channels in neurons

Ca(2+)-activated channels, including Ca(2+)-activated non-selective (CAN) channels and Ca(2+)-activated Cl- channels play important roles in regulating the electrical activity of neurons. No blockers of neuronal CAN channels have been previously reported. We used 2-electrode voltage clamping to measure membrane currents and fura-2 fluorescence imaging to measure [Ca2+]i in molluscan neurons. We show that the diphenylamine carboxylate derivative flufenamate (FFA), but not mefenamate or the parent compound, cause a transient increase in ICAN and a slow outward current, and a maintained increase in [Ca2+]i. We interpret this as a FFA-dependent release of Ca2+ from intracellular stores and Ca2+ influx, [Ca2+]i-dependent activation of the CAN and slow outward currents, and slow FFA-dependent channel block.

Cytoplasmic Ca2+ activity regulation as measured by a calcium-activated current

Calcium-activated non-selective cation (CAN) currents were activated by quantitative injections of Ca2+ into voltage clamped bursting neurons of the snails Helix aspersa or Helix pomatia. Membrane potential was held at the potassium equilibrium potential and CAN currents were fit with a rising and falling exponential function. Ca2+ transporters and pumps of the cell membrane, endoplasmic reticulum, and mitochondria were selectively blocked with pharmacological agents. Bath solutions containing 0 Na Ringers, chlorpromazine, Na3VO4, or thapsigargin did not significantly change the CAN current decay constants from those measured in Ringers. External 2,4-dinitrophenol or internal ruthenium red, however, significantly lengthened the CAN current decay constant. It is concluded that mitochondria are the most important sink for sub-membrane Ca2+ activity in the range necessary to effectively activate CAN currents.

Photolytic manipulation of Ca2+ and the time course of slow, Ca(2+)-activated K+ current in rat hippocampal neurones

1. Experiments were performed on hippocampal CA1 pyramidal cells to investigate the time course of a slow, Ca(2+)-activated K+ current that follows a burst of action potentials. At a temperature of 27-30 degrees C, this current rises to a peak 200-400 ms following the cessation of Ca2+ entry before decaying to baseline in 4-8s. 2. Intracellular recordings were made using electrodes containing the photolabile calcium buffers nitr-5 or DM-nitrophen loaded appropriately with Ca2+. Under these conditions, photolysis of the compound using an ultraviolet flashlamp caused an instantanous increase in cytoplasmic Ca2+ throughout the cell. The response to flash photolysis was a membrane hyperpolarization with an onset limited by the membrane time constant. Multiple (up to twenty) flash responses could be generated. 3. The postspike slow after-hyperpolarization (AHP) and flash-induced hyperpolarizations showed a common sensitivity to the beta-adrenergic receptor agonist isoprenaline. 4. Following a burst of spikes, the current underlying an AHP in progress could be terminated or reduced by photolysis-induced production of calcium buffer from diazo-4 within the cell. This action was rapid (within the setting time of the flash artifact, i.e. < 10 ms) despite the fact that the manipulation occurred 400-500 ms following the end of Ca2+ entry. 5. Partial block of the slow AHP by buffer production was accompanied by an increase in the time to peak of the event. 6. The time to peak of the slow AHP could also be manipulated by experiments which altered the spatial distribution of Ca2+ entry, such as production of calcium spikes or dendritic depolarization by glutamate in the presence of tetrodotoxin. 7. The Ca(2+)-dependent K+ current responsible for the slow AHP responds immediately to increase or decreases in cytoplasmic Ca2+. It seems likely, therefore, that the slow AHP is controlled solely by changes in free Ca2+ and that the time course is governed by the redistribution of cytoplasmic Ca2+ following activity-induced entry through voltage- or receptor-operated channels.

Evaluation of cellular mechanisms for modulation of calcium transients using a mathematical model of fura-2 Ca2+ imaging in Aplysia sensory neurons

A theoretical model of [Ca++]i diffusion, buffering, and extrusion was developed for Aplysia sensory neurons, and integrated with the measured optical transfer function of our fura-2 microscopic recording system, in order to fully simulate fura-2 video or photomultiplier tube measurements of [Ca++]i. This allowed an analysis of the spatial and temporal distortions introduced during each step of fura-2 measurements of [Ca++]i in cells. In addition, the model was used to evaluate the plausibility of several possible mechanisms for modulating [Ca++]i transients evoked by action potentials. The results of the model support prior experimental work (Blumenfeld, Spira, Kandel, and Siegelbaum, 1990. Neuron. 5: 487-499), suggesting that 5-HT and FMRFamide modulate action potential-induced [Ca++]i transients in Aplysia sensory neurons through changes in Ca++ influx, and not through changes in [Ca++]i homeostasis or release from internal stores.

An analysis of the long-lasting after-hyperpolarization of guinea-pig vagal motoneurones

1. The long-lasting after-hyperpolarization which characterizes the neurones of the dorsal motor nucleus of the vagus in the guinea-pig was studied in vitro. 2. Following a train of action potentials, vagal motoneurones develop a long-lasting after-hyperpolarization. Two different shapes of long-lasting after-hyperpolarization were encountered: an after-hyperpolarization which slowly (0.6-1.2 s) and monotonically developed to peak value; and a second type of long-lasting after-hyperpolarization where the onset of the slow component appears to be masked by an early, relatively fast component. Both shapes of long-lasting after-hyperpolarization depend on Ca2+ influx and increase as a function of the number of action potentials in the train. 3. A novel procedure was used to analyse the ionic processes which underlie the long-lasting after-hyperpolarization. The neuronal responses to a series of long (7 s) hyperpolarizing current pulses during the long-lasting after-hyperpolarization were recorded and the voltage-current curves at 600 different time points along the long-lasting after-hyperpolarization were plotted. The conductance and the reversal potential at each time point were calculated from the slope and the intersection of these curves, respectively. 4. Using this procedure it was found that the long-lasting after-hyperpolarization consists of two conductances that differ in kinetic properties and reversal potential: an early conductance which peaks shortly after the end of the train and decays in a few tenths of seconds (EAHP), and a late conductance which develops slowly (time to peak about 1 s) and decays in 3-8 s (LAHP). The reversal potential for the early conductance is 10 mV more positive than the reversal potential for the late conductance (-84 mV); the latter reversal potential is in agreement with the K+ equilibrium potential. The different shapes of long-lasting after-hyperpolarization can be explained by different ratios of these two conductances. 5. Noradrenaline (10 microM) selectively blocks the late conductance, without an observable effect on the Ca2+ action potential. 6. The behaviour of the noradrenaline-sensitive late conductance was analysed. The amplitude of the conductance change increased sigmoidally as a function of the number of spikes in the train. A log-log plot suggests that at least two Ca2+ ions participate in the opening of a K+ channel. 7. A model that accounts for the slow kinetics of the late conductance was constructed.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca(2+)-activated K+ currents underlying the afterhyperpolarization in guinea pig vagal neurons: a role for Ca(2+)-activated Ca2+ release

We examined the possibility that Ca2+ released from intracellular stores could activate K+ currents underlying the afterhyperpolarization (AHP) in neurons. In neurons of the dorsal motor nucleus of the vagus, the current underlying the AHP had two components: a rapidly decaying component that was maximal following the action potential (GkCa,1) and a slower component that had a distinct rising phase (GkCa,2). Both components required influx of extracellular Ca2+ for their activation, and neither was blocked by extracellular TEA (10 mM). GkCa,1 was selectively blocked by apamin, whereas GkCa,2 was selectively reduced by noradrenaline. The time course of GkCa,2 was markedly temperature sensitive. GkCa,2 was selectively blocked by application of ryanodine or sodium dantrolene, or by loading cells with ruthenium red. These results suggest that influx of Ca2+ directly gates one class of K+ channels and leads to release of Ca2+ from intracellular stores, which activates a different class of K+ channel.

Subcellular calcium transients visualized by confocal microscopy in a voltage-clamped vertebrate neuron

Confocal laser-scanned microscopy and long-wavelength calcium (Ca2+) indicators were combined to monitor both sustained and rapidly dissipating Ca2+ gradients in voltage-clamped sympathetic neurons isolated from the bullfrog. After a brief activation of voltage-dependent Ca2+ channels, Ca2+ spreads inwardly, and reaches the center of these spherical cells in about 300 milliseconds. Although the Ca2+ redistribution in the bulk of the cytosol could be accounted for with a radial diffusion model, local nonlinearities, suggesting either nonuniform Ca2+ entry or spatial buffering, could be seen. After electrical stimulation, Ca2+ signals in the nucleus were consistently larger and decayed more slowly than those in the cytosol. A similar behavior was observed when release of intracellular Ca2+ was induced by caffeine, suggesting that in both cases large responses originate from Ca2+ release sites near or within the nucleus. These results are consistent with an amplification mechanism involving Ca2(+)-induced Ca2+ release, which could be relevant to activity-dependent, Ca2(+)-regulated nuclear events.

Calcium diffusion modeling in a spherical neuron. Relevance of buffering properties

We have developed a calcium diffusion model for a spherical neuron which incorporates calcium influx and extrusion through the plasma membrane as well as three calcium buffer systems with different capacities, mobilities, and kinetics. The model allows us to calculate the concentration of any of the species involved at all locations in the cell and can be used to account for experimental data obtained with high-speed Ca imaging techniques. The influence of several factors on the Ca2+ transients is studied. The relationship between peak [Ca2+]i and calcium load is shown to be nonlinear and to depend on buffer characteristics. The time course of the Ca2+ signals is also shown to be dependent on buffer properties. In particular, buffer mobility strongly determines the size and time course of Ca2+ signals in the cell interior. The model predicts that the presence of exogenous buffer, such as fura-2, modifies the Ca2+ transients to a variable extent depending on its proportion relative to the natural, intrinsic buffers. The conclusions about natural calcium buffer properties that can be derived from Ca imaging experiments are discussed.

Vasoactive intestinal peptide (VIP) may reduce the removal rate of cytosolic Ca2+ after transient elevations in clonal rat lactotrophs

The prolactin-producing rat anterior pituitary GH4C1 cells possess Ca2+-activated K channels which are activated by physiological elevations of the cytosolic Ca2+ concentration even at membrane potentials more negative than the normal level of about -50 mV. Whole-cell current recordings showed a marked outward tail current following depolarizing voltage steps to 0 mV from a holding potential close to the normal membrane potential. The half-time of this tail current was about 1.3 s after a 4-s depolarization step. The GH4C1 cells also possess voltage-activated Ca channels, and we conclude that this tail current is a Ca2+-activated K+ current for the following reasons: (1) The reversal potential for the tail current was close to the K+ equilibrium potential for a range of transmembrane K+ gradients. (2) The tail current was blocked by a Ca2+ antagonist, and the voltage dependence of this current closely mirrored the voltage dependence of the isolated Ca2+ current. The time-course of the decline of the tail current thus reflects the removal rate of the Ca2+ entering the cytosol through voltage-dependent Ca channels during the depolarizing voltage step. VIP stimulates prolactin secretion from GH4C1 cells, and this peptide prolonged the half-time of the tail current by about 47% in 63% of the cells. This indicates that VIP may prolong the transient cytosolic Ca2+ elevations following the action potentials in these cells. Such a mechanism might be an important factor for the control of the cytosolic Ca2+ level, and hence hormone secretion.

"Caged calcium" in Aplysia pacemaker neurons. Characterization of calcium-activated potassium and nonspecific cation currents

We have studied calcium-activated potassium current, IK(Ca), and calcium-activated nonspecific cation current, INS(Ca), in Aplysia bursting pacemaker neurons, using photolysis of a calcium chelator (nitr-5 or nitr-7) to release "caged calcium" intracellularly. A computer model of nitr photolysis, multiple buffer equilibration, and active calcium extrusion was developed to predict volume-average and front-surface calcium concentration transients. Changes in arsenazo III absorbance were used to measure calcium concentration changes caused by nitr photolysis in microcuvettes. Our model predicted the calcium increments caused by successive flashes, and their dependence on calcium loading, nitr concentration, and light intensity. Flashes also triggered the predicted calcium concentration jumps in neurons filled with nitr-arsenazo III mixtures. In physiological experiments, calcium-activated currents were recorded under voltage clamp in response to flashes of different intensity. Both IK(Ca) and INS(Ca) depended linearly without saturation upon calcium concentration jumps of 0.1-20 microM. Peak membrane currents in neurons exposed to repeated flashes first increased and then declined much like the arsenazo III absorbance changes in vitro, which also indicates a first-order calcium activation. Each flash-evoked current rose rapidly to a peak and decayed to half in 3-12 s. Our model mimicked this behavior when it included diffusion of calcium and nitr perpendicular to the surface of the neuron facing the flashlamp. Na/Ca exchange extruding about 1 pmol of calcium per square centimeter per second per micromolar free calcium appeared to speed the decline of calcium-activated membrane currents. Over a range of different membrane potentials, IK(Ca) and INS(Ca) decayed at similar rates, indicating similar calcium stoichiometries independent of voltage. IK(Ca), but not INS(Ca), relaxes exponentially to a different level when the voltage is suddenly changed. We have estimated voltage-dependent rate constants for a one-step first-order reaction scheme of the activation of IK(Ca) by calcium. After a depolarizing pulse, INS(Ca) decays at a rate that is well predicted by a model of diffusion of calcium away from the inner membrane surface after it has entered the cell, with active extrusion by surface pumps and uptake into organelles. IK(Ca) decays somewhat faster than INS(Ca) after a depolarization, because of its voltage-dependent relaxation combined with the decay of submembrane calcium. The interplay of these two currents accounts for the calcium-dependent outward-inward tail current sequence after a depolarization, and the corresponding afterpotentials after a burst

Calcium-dependent slow outward current in visceral primary afferent neurones of the rabbit

Slow outward currents were recorded from voltage-clamped neurones in nodose ganglia excised from rabbits. In the majority of Type C neurones, a short depolarizing command pulse evoked a slow outward tail current (ISAH) with a decay time constant ranging from 0.5 to 2 s. The ISAH was due to an increase in membrane conductance to K+ because its reversal potential was approximately equal to the Nernst potential for K+. The ISAH was reversibly blocked by removal of external Ca2+ or by Ca2+ antagonists. A Ca2+ ionophore, A23187, produced an outward current which was similar to the ISAH. The ISAH was resistant to tetraethylammonium and depressed by Ba2+, whereas it was not affected by Cs+ and 4-aminopyridine. The ISAH was initially augmented and subsequently depressed by apamin (1-10 nM) and (+)-tubocurarine (100-600 microM). It is concluded that the ISAH in visceral primary neurones may be due to a long-lasting increase in K+ conductance caused by an increase in the concentration of intracellular Ca2+, resulting from Ca2+ entry during the depolarizing command pulse.

Measurement of nonuniform current densities and current kinetics in Aplysia neurons using a large patch method

A large patch electrode was used to measure local currents from the cell bodies of Aplysia neurons that were voltage-clamped by a two-microelectrode method. Patch currents recorded at the soma cap, antipodal to the origin of the axon, and whole-cell currents were recorded simultaneously and normalized to membrane capacitance. The patch electrode could be reused and moved to different locations which allowed currents from adjacent patches on a single cell to be compared. The results show that the current density at the soma cap is smaller than the average current density in the cell body for three components of membrane current: the inward Na current (INa), the delayed outward current (Iout), and the transient outward current (IA). Of these three classes of ionic currents, IA is found to reach the highest relative density at the soma cap. Current density varies between adjacent patches on the same cell, suggesting that ion channels occur in clusters. The kinetics of Iout, and on rare occasions IA, were also found to vary between patches. Possible sources of error inherent to this combination of voltage clamp techniques were identified and the maximum amplitudes of the errors estimated. Procedures necessary to reduce errors to acceptable levels are described in an appendix.

Effects of Na+ and Ca2+ gradients on intracellular free Ca2+ in voltage-clamped Aplysia neurons

Selected neurons of the abdominal ganglion of Aplysia californica were voltage-clamped and intracellular free Ca [( Ca2+]i) and Na [( Na+]i) concentrations were monitored with ion selective microelectrodes. Reducing [Na+]o from 500 mM (normal seawater, NSW) to 5 mM resulted in a decrease of the potential measured by the Ca electrode (VCa). Increasing [Ca2+]o from 10 to 50 mM increased [Ca2+]i two-fold, keeping [Ca2+]o at 50 mM and decreasing [Na+]o to 5 mM still led to a decrease in VCa. With 100 mM [Ca2+]o, which also increased [Ca2+]i, decreasing [Na+]o increased VCa in two of the eight cells tested. This indicates that in normal or moderately high resting [Ca2+]i, Ca2+ extrusion by Na/Ca exchange (forward mode) is not essential for [Ca2+]i buffering. [Na+]i was 12.9 +/- 3.6 mM (S.E.M., n = 7) in NSW; reducing [Na+]o to 5 mM decreased [Na+]i to 2.0 +/- 1.1 mM (S.E.M.). Keeping [Na+]o at 5 mM and increasing [Ca2+]o from 10 to 20 mM further decreased [Na+]i to about 1.0 mM, evidence of Na/Ca exchange operating in the reverse mode. Attempts to increase [Ca2+]i by bath application of the Ca ionophores A23187, X537A, ionomycin or ETH 1001 resulted in no measurable change of the resting [Ca2+]i. Application of Ouabain caused an apparent increase in [Ca2+]i in two of the six cells tested. In cells injected with the metallochromic indicator arsenazo III (AIII), the rate of the falling phase of the AIII absorbance increase, following a voltage-clamp pulse, was significantly slower in 5 mM [Na+]o. This indicates that in its forward mode Na-Ca exchange is active in clearing large submembrane increases in [Ca2+]i.

Kinetic analysis of voltage- and ion-dependent conductances in saccular hair cells of the bull-frog, Rana catesbeiana

1. By the use of whole-cell and excised-patch tight-seal recording techniques, we studied ionic conductances in voltage-clamped solitary hair cells isolated from the bull-frog's sacculus. As a basis for assessing their contributions to hair cell electrical resonance, we developed kinetic models describing voltage-dependent Ca2+ and Ca2+-dependent K+ conductances. 2. A transient K+ current (IA) was activated by steps to potentials positive to -50 mV from holding potentials more negative than -70 mV. In the steady state, the current was fully inactivated at the normal resting potential. Possibly due to the dissipation of a Donnan potential between the pipette's interior and the cell, the voltage dependence of IA inactivation slowly shifted in the negative direction during whole-cell recording. 3. The voltage-gated Ca2+ current (ICa) was isolated by blocking IA with 4-aminopyridine (4-AP) and Ca2+-activated K+ current with tetraethylammonium (TEA). The ICa was activated at potentials more positive than -60 to -50 mV and was maximal at about -10 mV. Its magnitude was highly variable among cells, with an average value of -240 pA at -30 mV. Its activation could be fitted well by a third-order (m3) gating scheme. 4. A Ca2+-activated K+ current (IK(Ca)) was isolated as the component of membrane current blocked by TEA. This current was activated at potentials more positive than -60 to -50 mV and had an average value of 1.5 nA at -30 mV. The Ca2+-activated K+ conductance (gK(Ca)) showed a high apparent voltage dependence, increasing e-fold every 3 mV at potentials between -50 and -40 mV. 5. The Ca2+-activated K+ current displayed rapid activation and deactivation kinetics. The current reached half-maximal activation in 2-4 ms at voltages between -50 and -30 mV, and the tail current decayed exponentially with a time constant of 1.0 ms at -70 mV. The activation rate and magnitude of IK(Ca) were reduced by lowering the extracellular Ca2+ concentration. 6. The open probability of Ca2+-activated K+ channels was estimated by ensemble-fluctuation analysis of whole-cell currents evoked by voltage steps to -30 mV. The average open probability was estimated to be 0.8 at this potential. 7. K+-selective channels with a high conductance (140-200 pS) were examined in excised, inside-out membrane patches. The activity of these channels depended on intracellular Ca2+ and membrane potential. These properties suggest that the channels underlie the whole-cell Ca2+-activated K+ current.(ABSTRACT TRUNCATED AT 400 WORDS)

The Ca2+-sensitive K+-currents underlying the slow afterhyperpolarization of bullfrog sympathetic neurones

Ca2+-sensitive K+ currents involved in the slow afterhyperpolarization (a.h.p.) of an action potential of bullfrog sympathetic neurones were studied with a single-electrode voltage clamp method. The outward tail current (IAH) generated after the end of a depolarizing command pulse (from the holding potential of -60 mV to 0 mV, 5-20 ms in duration), mimicking an action potential, was separated into at least two exponential components (IAHf and IAHs). They were identified as K+ currents, since their reversal potentials were close to the K+ equilibrium potential and they were sensitive to external K+. The time constant of IAHf (tf; 44 ms at -60 mV) was decreased by membrane hyperpolarization from -40 to -80 mV, while that of IAHs (ts; 213 ms) remained constant. Removal of external Ca2+ or addition of Cd2+ significantly decreased the IAHs amplitude (As) and tf without a change in ts and the IAHf amplitude (Af). On the other hand, increasing Ca2+ influx by applying repetitive command pulses enhanced both Af and As with negligible effects on tf and ts, and produced a much slower component. Intracellular injection of EGTA reduced Af with no effect on tf, and increased As with a decreased ts. Both muscarine and (+/-)-tubocurarine, which reduced IAHs, hardly affected IAHf. These results indicate that a.h.p. is induced by the activation of two distinct Ca2+-dependent K+ channels, which differ in voltage sensitivity, Ca2+-dependence and pharmacology.

Potassium currents evoked by brief depolarizations in bull-frog sympathetic ganglion cells

1. Sympathetic neurones of the bull-frog Rana catesbeiana were subjected to a two-electrode voltage-clamp technique in order to investigate the K+ currents which can be elicited by action potentials or similar brief depolarizations. 2. Four separate K+ currents were observed (IC, IK, IAHP and IM). These could be separated on the basis of voltage sensitivity, Ca2+ dependence and deactivation kinetics. 3. Two of these currents, which were clearly activated by an action potential, were Ca2+ dependent. A voltage- and TEA (tetraethylammonium)-sensitive K+ current, IC, was activated within the first 1-2 ms of a depolarizing command. This current decayed on average with a time constant of 2.4 ms at -40 mV. The maximal conductance was outside the range which could be adequately voltage clamped but, as much as 2 muS could be activated by brief (2-3 ms) commands. Activation of IC during an action potential accounts for the Ca2+ dependence of the repolarization. IC did not exhibit a transient component. 4. A second Ca2+-dependent K+ current, IAHP, was also activated after as little as 1 ms depolarization but was not voltage sensitive and was much less sensitive to TEA. The current decayed with a time constant of around 150 ms at -40 mV. The maximal conductance was about 30 nS. 5. The voltage-sensitive delayed rectifying current, IK, made a contribution to the total K+ conductance of the cell similar to IC in magnitude; however, the current is not activated within the normal voltage range or time course of an action potential. The current decayed on average with a time constant of 21 ms at -40 mV. 6. IM, a muscarine- and voltage-sensitive current, is not activated to any significant degree by a single action potential. The data further imply that the rate of opening of the ion channels mediating IM is less voltage sensitive than the rate of closing. 7. Large changes in the K+ reversal potential occur following depolarizing commands which evoke large K+ currents. This is attributed to K+ accumulation within a restricted extracellular space. Extracellular K+ may double or even triple during a single action potential.

Two distinct calcium-activated potassium currents in a rat anterior pituitary cell line

1. The single 'giga-seal' patch-electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) was used to record whole-cell currents in the GH3 rat anterior pituitary cell line. 2. GH3 cells have a rapidly inactivating, voltage-dependent K+ current that is selectively inhibited by 4-aminopyridine (4-AP) but not by tetraethylammonium chloride (TEA). 3. The majority of the Ca2+-activated K+ current in these cells is blocked by TEA with an inhibitory concentration that is half-maximal at 1 mM. An additional Ca2+-activated K+ current is also present that is relatively resistant to TEA and is blocked by the polypeptide apamin. The apamin-sensitive component represents less than 18% of the total Ca2+-activated K+ current at 0 mV. 4. The time course of the slowly declining components of the Ca2+-activated K+ tail currents measured at the -50 mV holding potential was usually biexponential with time constants of 0.21 +/- 0.02 and 1.75 +/- 0.23 s (mean +/- S.E. of mean, n = 14). Both of the two slowly decaying components contribute to the TEA- and apamin-sensitive currents. 5. It is concluded that GH3 cells have at least two pharmacologically distinct Ca2+-activated K+ currents and a 4-AP-sensitive voltage-dependent K+ current.

Slow membrane currents in bursting pace-maker neurones of Tritonia

1. Membrane ionic currents in bursting pace-maker neurones of the marine mollusc Tritonia were studied in voltage-clamp experiments with emphasis on slow tail current relaxations after depolarizing pulses. 2. The slow tail current undergoes a complex transition from an initially inward current to an initially outward current as the duration of the depolarizing pulse is lengthened. It was found that the slow tail current is the sum of two separate and independent ionic currents. Methods were devised to study each current in isolation. 3. A slow inward tail current, termed IB, is activated by depolarization and decays exponentially on return to -55 mV with a time constant of 2-4 s. The voltage dependence and kinetics of IB activation were measured. Current amplitude is sensitive to removal of both Na+ and Ca2+ from the bathing medium but the current is not blocked by either tetrodotoxin (TTX) or replacement of Ca2+ by Co+. The amplitude of the current is independent of the external K+ concentration. 4. A slow outward tail current, termed IC, is also activated by depolarization. It is shown to be a K+ current whose activation results from an increase in the cytoplasmic Ca2+ concentration during depolarization. The decay of IC on repolarization requires more than 30 s to reach completion. 5. The slow rates of relaxation of IB and IC tail currents suggest that they are important determinants of the slow membrane potential variations characteristic of burst firing. IB activates more rapidly than IC during depolarization and is thought to be important for maintaining the depolarized phase of the burst cycle and for producing the depolarizing after-potential after each spike. IC activates more slowly but reaches greater amplitudes. It is thought to be important for adaptation in spike frequency during the burst, for burst termination, and for determining the duration of the interval between bursts.

Egg-laying hormone of Aplysia induces a voltage-dependent slow inward current carried by Na+ in an identified motoneuron

This report presents studies on ionic currents in Aplysia motoneuron B16 that are modulated by the neuropeptide egg-laying hormone (ELH) of Aplysia. ELH induces an inward current that persists in the presence of the peptide and that decays slowly after ELH is removed from the bath. The effect is not due to a decrease in the delayed potassium current, the calcium-activated potassium current, or the transient potassium current. Current-voltage measurements indicate that ELH produces increased inward currents from -80 mV to approximately equal to 0 mV. The effect is particularly enhanced in the region from -40 mV to -25 mV where a negative slope conductance due to voltage-dependent slow inward current is observed. The slow inward current and the response to ELH persist in saline solutions in which Ca2+ is replaced with Co2+ but are eliminated when Na+ is replaced with equimolar concentrations of either Tris or N-methyl-D-glucamine. The response to ELH is unaffected by replacing chloride with equimolar acetate; by increasing the potassium concentration; or by adding tetraethylammonium chloride, CsCl, 4-amino-pyridine, or tetrodotoxin to the saline bath. In addition, the reversal potentials for the ELH response (range, -28 to +46 mV), obtained from difference current-voltage relationships, are consistent with an increase in the Na+-dependent slow inward current. We conclude that at least one of the effects of ELH on B16 is to increase a slow inward current carried by Na+.

The development of calcium and potassium currents during oogenesis in the starfish, Leptasterias hexactis

The development of membrane electrical properties of oocytes of the starfish Leptasterias hexactis during oogenesis was studied using voltage- and current-clamp techniques. Two voltage-dependent K currents--the fast transient and inwardly rectifying--are present early in oogenesis, before the rapid growth phase, and are maintained throughout oogenesis at the same current density and kinetics. The inward current, which is composed of a Ca current and a slower Ca-dependent inward sodium current, is also present early in oogenesis, but at very low current density. Late in oogenesis, after the oocyte has grown to full size, the inward current increases in amplitude by about fivefold, and undergoes major changes in kinetics. These changes are closely associated with the migration of the germinal vesicle to the cell periphery. The relationship of these events to electrophysiological changes during subsequent maturation and fertilization of the oocytes is discussed.

Synaptic block of a calcium-activated potassium conductance in Aplysia neurones

In the preceding paper (Kehoe, 1985) it was shown that the firing of any one of three neurones (I, II, III) presynaptic to the medial cells of the pleural ganglion of Aplysia californica causes a diminution of the cholinergically controlled K conductance in those cells. Firing of the same three presynaptic neurones was shown here to cause a similar diminution in a depolarization-induced K-dependent conductance in the same post-synaptic cells. The depolarization-induced K conductance was found to disappear when Ca ions were removed from the sea water bathing the ganglion or when the cell was injected with the Ca chelator ethyleneglycol-bis-(beta-aminoethylether)N,N'-tetra-acetic acid (EGTA). The diminution in this Ca-activated, K-dependent current occurred even when the presynaptic neurone was fired a few seconds after the end of the depolarizing voltage step to the post-synaptic neurone, showing that the diminution in K conductance was not an indirect effect of a transmitter-induced diminution in Ca influx during the depolarizing pulse. The two K conductances affected by the 'blocking neurones' could be selectively eliminated. The cholinergic conductance could be blocked by receptor-specific cholinergic antagonists (e.g. 1 mM concentrations of phenyltrimethylammonium (PTMA), choline and tetraethylammonium (TEA]. Even at 10 mM concentrations, none of these compounds (including TEA, which is known to block certain Ca-activated K conductances) had an effect on the depolarization-induced, Ca-activated K conductance studied here. This latter conductance, on the other hand, was selectively blocked by an intracellular injection of EGTA. The three blocking neurones continued to diminish the K conductance (cholinergic or depolarization induced) that remained intact under these different experimental conditions. The depolarization-induced influx of Ca was shown to block the cholinergically controlled K conductance, but Ca was excluded as the possible mediator of the diminution in K conductance caused by the three blocking neurones. An intracellular injection of Ca ions into the medial cells was shown to activate a variety of changes in membrane conductance; in particular, two K-conductance increases: an early, TEA-sensitive one, and a slowly developing, TEA-insensitive one. Both the permeant cyclic AMP analogue p-chlorophenylthioadenosine 3',5'-monophosphate (CPT-cyclic AMP) and the phosphodiesterase inhibitors amino-phylline and isobutyl-1-methylxanthine (IBMX) were shown to block the depolarization-induced K conductance, and to reduce, though not eliminate, the slowly developing K conductance activated by an intracellular injection of Ca.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic calcium diffusion from various arrays of single channels. Implications for transmitter release and synaptic facilitation

A one-dimensional model of presynaptic calcium diffusion away from the membrane, with cytoplasmic binding, extrusion by a surface pump, and influx during action potentials, can account for the rapid decay of phasic transmitter release and the slower decay of synaptic facilitation following one spike, as well as the very slow decline in total free calcium observed experimentally. However, simulations using this model, and alternative versions in which calcium uptake into organelles and saturable binding are included, fail to preserve phasic transmitter release to spikes in a long tetanus. A three-dimensional diffusion model was developed, in which calcium enters through discrete membrane channels and acts to release transmitter within 50 nm of entry points. Analytic solutions of the equations of this model, in which calcium channels were distributed in active zone patches based on ultrastructural observations, were successful in predicting synaptic facilitation, phasic release to tetanic spikes, and the accumulation of total free calcium. The effects of varying calcium buffering, pump rate, and channel number and distribution were explored. Versions appropriate to squid giant synapses and frog neuromuscular junctions were simulated. Limitations of key assumptions, particularly rapid nonsaturable binding, are discussed.

Calcium dependence of A-currents in perfused Aplysia neurons

Transient outward currents were studied in neurons in the visceral ganglion of Aplysia californica, using intracellular perfusion and voltage-clamp techniques. The early outward currents in response to depolarizations from holding potentials near -90 mV were activated in the range -60 to -20 mV, below the threshold for the delayed outward current. Resting inactivation of the early outward currents was removed by prehyperpolarizations in the range -130 to -70 mV. A-currents produced in this manner were blocked by external application of CoCl2 and augmented by increasing external Ca-concentration. They were also blocked by treatment with 4-aminopyridine. The currents were reduced by treatment with verapamil hydrochloride, further suggesting a role for calcium in the current-generating mechanism. A model with a fourth-power activation process and first-power inactivation process could fit the early outward currents reasonably well. The effect of application of Ca-free, cobalt-containing solution was modeled as a decrease in peak conductance and an increase in the time constants of activation and inactivation.

Calcium-induced inactivation of calcium current causes the inter-burst hyperpolarization of Aplysia bursting neurones

A triphasic series of tail currents which follow depolarizing voltage-clamp pulses in Aplysia neurones L2-L6 was described in the preceding paper (Kramer & Zucker, 1985). In this paper, we examine the nature of the late outward component of the tail current (phase III) which generates the inter-burst hyperpolarization in unclamped cells. The phase III tail current does not reverse between -30 and -90 mV, and is relatively insensitive to the external K+ concentration. In contrast, Ca2+-dependent K+ current (IK(Ca)), elicited by intracellular Ca2+ injection, reverses near -65 mV, and the reversal potential is sensitive to the external K+ concentration. Addition of 50 mM-tetraethylammonium (TEA) to the bathing medium causes a small increase in the phase III tail current. In contrast, IK(Ca) is completely blocked by addition of 50 mM-TEA. The phase III tail current is suppressed by depolarizing pulses which approach ECa, is blocked by Ca2+ current antagonists (Co2+ and Mn2+), and is blocked by intracellular injection of EGTA. The phase III tail current is reduced by less than 10% after complete removal of extracellular Na+. These bursting neurones have a voltage-dependent Ca2+ conductance which exhibits steady-state activation at a membrane potential similar to the average resting potential of the unclamped cell (i.e. -40 mV). The steady-state Ca2+ conductance can be inactivated by Ca2+ injection, or by depolarizing pre-pulses which generate a large influx of Ca2+. The steady-state Ca2+ conductance has a voltage dependence similar to that of the phase III tail current. The Ca2+-dependent inactivation of the steady-state Ca2+ conductance occurs in parallel with the phase III tail current; both have a similar sensitivity to Ca2+ influx, and both processes decay with similar rates after a depolarizing pulse. Hence, we propose that the phase III tail current is due to the Ca2+- dependent inactivation of a steady-state Ca2+ conductance. The decay of IK(Ca) following simulated spikes or bursts of spikes is rapid (less than 1 s) compared to the time course of the phase III tail current and the inter-burst hyperpolarization (tens of seconds). Thus, we conclude that IK(Ca) does not have a major role in terminating bursts or generating the inter-burst hyperpolarization in these cells. We present a qualitative model of the ionic basis of the bursting pace-maker cycle. The central features of the model are the voltage-dependent activation and the Ca2+-dependent inactivation of a Ca2+ current.

Effect of ethanol on subthreshold currents of Aplysia pacemaker neurons

The subthreshold currents in bursting pacemaker neurons of the Aplysia abdominal ganglion were individually studied with the voltage clamp technique for sensitivity to 4% ethanol. The most prevalent effect of ethanol on unclamped bursting neurons was a hyperpolarization. This was shown to be due to a decrease of a voltage independent inward leakage current. Direct measurement of the Na-dependent slow inward current showed that this current was eliminated by 4% ethanol. Direct measurement of the Ca-dependent slow inward current showed that this current was substantially reduced by 4% ethanol. Injection of EGTA into cell bodies did not eliminate the ethanol-induced block of the slow inward calcium current. Thus, ethanol cannot be reducing the Ca-dependent slow inward current solely by an increase of internal calcium concentration. The effect of ethanol on voltage dependent outward current was measured by blockage of all inward current. The peak outward current was increased by ethanol. The rate of inactivation of this outward current was also increased. Calcium activated potassium current (IK(Ca)) is particularly complicated in its response to ethanol because it is dependent on both Ca and voltage for its activation. The level of IK(Ca) elicited in response to constant Ca injection was increased by ethanol treatment. The level of this current as activated by voltage clamp pulses was either increased or decreased depending on the neuron type. Ca2+ activated potassium conductance increased e-fold for a 26 mV depolarization in membrane holding potential. Ethanol decreased this voltage dependence to e-fold for a 55 mV change in potential. This result was interpreted to mean that ethanol shifted an effective Ca2+ binding site of these channels from about halfway through the membrane field to one quarter of the way across. The same theoretical approach allowed the further conclusion that ethanol caused an increased internal free calcium concentration probably by decreasing calcium binding by intracellular buffers.

Two components of Ca-dependent potassium current in identified neurons of Aplysia californica

Outward tail currents measured in Aplysia neurones after termination of depolarizing voltage-clamp pulses consist of rapidly decaying voltage-dependent K currents and slow tail currents of much slower time course. The rapidly decaying voltage-dependent tail currents were blocked with aminopyridines, and measurements of the slow tail currents were made following decay of any residual rapid tail currents. The slow tail current exhibited two components of differing sensitivity to externally applied tetraethylammonium (TEA) ions. In some neurones of the abdominal ganglion (L-2, L-4), virtually all of the slow tail current was resistant to blockage by TEA, while in others (L-3, L-6) 80% or more of the slow tail current was blocked by low TEA concentrations (KD less than 1 mM), the remaining slow tail current being resistant to TEA. This TEA-resistant slow tail current was identified as a K current because it reversed near the K equilibrium potential (EK), the reversal potential was shifted by changes in the external K concentration, and it could be blocked by injection of Cs+. It was abolished by replacement of external Ca2+ by Co2+ or Ba2+, by addition of Cd2+, or by injection of EGTA, and thus determined to be a Ca-dependent current. Intracellular injection of TEA or external application of aminopyridine or apamine had little or no effect on the TEA-resistant slow tail current. Quinidine reduced the TEA-sensitive, but not the TEA-resistant current. Both the TEA-sensitive and the TEA-resistant components of the slow tail current exhibited similar time courses of decay.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple actions of a molluscan cardioexcitatory neuropeptide and related peptides on identified Helix neurones

The effects of the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (FMRF amide) and related peptides (Price & Greenberg, 1977) were tested on Helix aspersa neurones. Ionophoretic application of FMRFamide depolarized and excited some neurones, but hyperpolarized and inhibited others. In some neurones the sign of the response was dependent on the membrane potential. Two responses resulted from an increase in membrane conductance, a depolarizing response mediated mainly by an increase in Na+ ion permeability, and a hyperpolarizing response mediated by an increase in K+ ion permeability. In the C1 neurone a voltage-dependent response was observed, which only occurred when the neurone was depolarized from its resting level. This response was recorded as an inward current during voltage clamp and resulted from a decrease in K current(s), possibly Ca-activated K current. More than one response may occur in a single neurone. In the C1 neurone, the K-mediated hyperpolarization occurred as well as the voltage-dependent response, while the depolarization seen in the F2 neurone was a combination of an increase in Na conductance and an increase in K conductance.

A mutation that alters properties of the calcium channel in Paramecium tetraurelia

The membrane properties of a new mutant of Paramecium tetraurelia, dancer, were compared under voltage clamp with those of the wild type. The Ca2+ current was isolated and examined using CsCl-filled electrodes and tetraethylammonium in the bath solution to block K+ channels. The amplitude of the Ca2+ transient was not altered by the mutation. However, the Ca2+ current in the mutant inactivated more slowly and less extensively: hence a larger sustained Ca2+ current remained in the mutant. A change in the time course of the deactivation of the Ba2+ current was observed in the mutant. This mutational change is not likely to be the consequence of the Ca2+-channel inactivation because it is seen in the Ba2+ solution where there is little inactivation of the current. Other measured properties of the Ca2+ channel, the voltage-dependent K+ current, and the resting properties of the membrane were normal in the mutant. The Ca2+-activated K+ current and the Ca2+-activated Na+ current were larger in the mutant than in the wild type, consistent with a greater elevation of free intracellular Ca2+ during depolarization in the mutant. It is likely that the mutation causes an alteration in the Ca2+-channel structure or in its immediate environment and thereby affects the inactivation and deactivation processes of the Ca2+ channel. As would be expected from the greater Ca2+ current, the mutant tends to generate all-or-none Ca action potentials as opposed to the graded action potentials in the wild type.

Sodium-dependent calcium efflux from single Aplysia neurons

45Ca2+ efflux from single Aplysia somata was measured. Replacement of external Na with Tris caused a reduction in the efflux following a transient increase. CCMP, a metabolic poison, caused a reversible increase in the efflux. The results suggest that Na+/Ca2+ exchange and mitochondrial uptake can act to regulate Ca2+ in Aplysia neurons.

Calcium domains associated with individual channels can account for anomalous voltage relations of CA-dependent responses

Computer-assisted modeling of calcium influx through voltage-activated membrane channels predicted that buffer-limited elevation of cytoplasmic free calcium ion concentration occurs within microscopic hemispherical "domains" centered upon the active Ca channels. With increasing depolarization, the number of activated channels, and hence the number of Ca domains, should increase; the single-channel current should, however, decrease, thereby decreasing Ca2+ accumulation in each domain relative to the macroscopic current. Such voltage dependence of the microscopic distribution of Ca2+ may influence relations between total Ca2+ entry and Ca-dependent processes. Ca-mediated inactivation of Ca channels in Aplysia neurons exhibits behavior consistent with the calcium domain hypothesis.

Localization of neuronal Ca2+ buffering near plasma membrane studied with different divalent cations

Absorbance changes associated with divalent cation binding to arsenazo III were used to measure changes in Ca2+, Sr2+, and Ba2+ concentrations under a variety of experimental conditions. The rate of the falling phase of an absorbance change signal, measured in nerve cell bodies injected with arsenazo III and under membrane potential control, was taken as an index of divalent cation buffering. With influx of ions through the membrane or with ionophoretic injection, we found the buffering, i.e., the dye-absorbance signal's falling rate, to be greatest for Ca2+ ions: the sequence was Ca2+ greater than Sr2+ much greater than Ba2+. Injecting Ca2+ or Sr2+ into the center of a nerve cell produced a significantly greater amplitude of arsenazo III signal than the same injection near the cell membrane. We did not find this to be the case for Ba2+ or Mg2+ injections. We conclude that the Ca2+ regulatory system binds Ca2+ most strongly compared to the other ions tested, and there is a variable distribution of buffering machinery within the nerve soma, with increased buffer capacity near the plasma membrane of the cell. A preliminary report of some of the results presented in this paper has appeared previously ( Tillotson and Gorman, 1980).