Accelerating effects of pentobarbitone on the inactivation process of the calcium current in Helix neurones

Sense and Insensibility - An Appraisal of the Effects of Clinical Anesthetics on Gastropod and Cephalopod Molluscs as a Step to Improved Welfare of Cephalopods

Recent progress in animal welfare legislation stresses the need to treat cephalopod molluscs, such as Octopus vulgaris, humanely, to have regard for their wellbeing and to reduce their pain and suffering resulting from experimental procedures. Thus, appropriate measures for their sedation and analgesia are being introduced. Clinical anesthetics are renowned for their ability to produce unconsciousness in vertebrate species, but their exact mechanisms of action still elude investigators. In vertebrates it can prove difficult to specify the differences of response of particular neuron types given the multiplicity of neurons in the CNS. However, gastropod molluscs such as Aplysia, Lymnaea, or Helix, with their large uniquely identifiable nerve cells, make studies on the cellular, subcellular, network and behavioral actions of anesthetics much more feasible, particularly as identified cells may also be studied in culture, isolated from the rest of the nervous system. To date, the sorts of study outlined above have never been performed on cephalopods in the same way as on gastropods. However, criteria previously applied to gastropods and vertebrates have proved successful in developing a method for humanely anesthetizing Octopus with clinical doses of isoflurane, i.e., changes in respiratory rate, color pattern and withdrawal responses. However, in the long term, further refinements will be needed, including recordings from the CNS of intact animals in the presence of a variety of different anesthetic agents and their adjuvants. Clues as to their likely responsiveness to other appropriate anesthetic agents and muscle relaxants can be gained from background studies on gastropods such as Lymnaea, given their evolutionary history.

Pentobarbital inhibition of human recombinant alpha1A P/Q-type voltage-gated calcium channels involves slow, open channel block

BACKGROUND AND PURPOSE

Pre-synaptic neurotransmitter release is largely dependent on Ca(2+) entry through P/Q-type (Ca(V)2.1) voltage-gated Ca(2+) channels (PQCCs) at most mammalian, central, fast synapses. Barbiturates are clinical depressants and inhibit pre-synaptic Ca(2+) entry. PQCC barbiturate pharmacology is generally unclear, specifically in man. The pharmacology of the barbiturate pentobarbital (PB) in human recombinant alpha(1A) PQCCs has been characterized.

EXPERIMENTAL APPROACH

PB effects on macroscopic Ca(2+)(I(Ca)) and Ba(2+)(I(Ba)) currents were studied using whole-cell patch clamp recording in HEK-293 cells heterologously expressing (alpha(1A))(human)(beta(2a)alpha(2)delta-1)(rabbit) PQCCs.

KEY RESULTS

PB reversibly depressed peak current (I(peak)) and enhanced apparent inactivation (fractional current at 800 ms, r(800)) in a concentration-dependent fashion irrespective of charge carrier (50% inhibitory concentration: I(peak), 656 microM; r(800), 104 microM). Rate of mono-exponential I(Ba) decay was linearly dependent on PB concentration. PB reduced channel availability by deepening non-steady-state inactivation curves without altering voltage dependence, slowed recovery from activity-induced unavailable states and produced use-dependent block. PB (100 microM) induced use-dependent block during physiological, high frequency pulse trains and overall depressed PQCC activity by two-fold.

CONCLUSION AND IMPLICATIONS

The results support a PB pharmacological mechanism involving a modulated receptor with preferential slow, bimolecular, open channel block (K(d)= 15 microM). Clinical PB concentrations (<200 microM) inhibit PQCC during high frequency activation that reduces computed neurotransmitter release by 16-fold and is comparable to the magnitude of Ca(2+)-dependent facilitation, G-protein modulation and intrinsic inactivation that play critical roles in PQCC modulation underlying synaptic plasticity. The results are consistent with the hypothesis that PB inhibition of PQCCs contributes to central nervous system depression underlying anticonvulsant therapy and general anaesthesia.

Potentiating effects of L-type Ca2+ channel blockers on pentobarbital-induced hypnosis are influenced by serotonergic system

In order to elucidate the mechanism(s) behind the interactions between barbiturates and Ca(2+) antagonists, the effects of three structurally diverse types of Ca(2+) antagonists combined or not with 5-HT on pentobarbital-induced hypnosis in mice were investigated. The results showed that dihydropyridine derivative nifedipine (10.0 and 20.0 mg/kg, p.o.) and other types of Ca(2+) antagonist, verapamil (5.0 and 10.0 mg/kg, p.o.) and diltiazem (2.5, 5.0 and 10.0 mg/kg, p.o.) increased both the sleeping time in hypnotic dosage of pentobarbital (45 mg/kg, i.p.) treated mice and the rate of sleep onset in the sub-hypnotic dosage of pentobarbital (28 mg/kg, i.p.) treated mice in a dose-dependent manner, respectively, and these effects were significantly augmented by 5-hydroxytryptophan (5-HTP), the immediate precursor of 5-hydroxytryptamine (5-HT). Pretreatment with p-chlorophenylalanine (PCPA, 300 mg/kg, s.c.), an inhibitor of tryptophan hydroxylase, significantly decreased pentobarbital-induced sleeping time and nifedipine (10.0 mg/kg, p.o.), verapamil (5.0 mg/kg, p.o.) and diltiazem (2.5 mg/kg, p.o.) abolished this effect. From these results, it should be presumed that the augmentative effect of L-type Ca(2+) channel blockers on pentobarbital-induced sleep may be influenced by serotonergic system.

Voltage-activated calcium channels: targets of antiepileptic drug therapy?

Voltage-gated calcium currents play important roles in controlling neuronal excitability. They also contribute to the epileptogenic discharge, including seizure maintenance and propagation. In the past decade, selective calcium channel blockers have been synthesized, aiding in the analysis of calcium channel subtypes by patch-clamp recordings. It is still a matter of debate whether whether any of the currently available antiepileptic drugs (AEDs) inhibit these conductances as part of their mechanism of action. We tested oxcarbazepine, lamotrigine, and felbamate and found that they consistently inhibited voltage-activated calcium currents in cortical and striatal neurons at clinically relevant concentrations. Low micromolar concentrations of GP 47779 (the active metabolite of oxcarbazepine) and lamotrigine reduced calcium conductances involved in the regulation of transmitter release. In contrast, felbamate blocked nifedipine-sensitive conductances at concentrations significantly lower than those required to modify N-methyl-D-aspartate (NMDA) responses or sodium currents. Aside from contributing to AED efficacy, this mechanism of action may have profound implications for preventing fast-developing cellular damage related to ischemic and traumatic brain injuries. Moreover, the effects of AEDs on voltage-gated calcium signals may lead to new therapeutic strategies for the treatment of neurodegenerative disorders.

The anesthetic agents pentobarbital and chloralose block phase shifts of a neuronal in vitro circadian pacemaker

The anesthetic pentobarbital (6 mM) is capable of blocking light or high K(+)-induced phase shifts of the circadian pacemaker in the isolated eye of Bulla. Pentobarbital alone was effective in generating phase shifts consistent with phase response curves obtained to either extracellular low Ca2+ or hyperpolarizing pulses. Patch clamp recordings from the circadian pacemaker cells indicate that pentobarbital reduces the Ca(2+)-dependent K+ current. Together, these data suggest that pentobarbital acts on the pacemaker by reducing an inward Ca2+ current. Chloralose (3 mM) was effective in blocking light, but not high K(+)-induced phase shifts, and did not generate phase shifts when applied alone, suggesting that chloralose may act as a weak Ca2+ channel inhibitor.

Calcium channel currents in bovine adrenal chromaffin cells and their modulation by anaesthetic agents

1. The calcium channel currents of bovine adrenal chromaffin cells were characterized using a variety of voltage pulse protocols and selective channel blockers before examination of their modulation by anaesthetic agents. 2. All the anaesthetics studied (halothane, methoxyflurane, etomidate and methohexitone) inhibited the calcium channel currents in a concentration-dependent manner and increased the rate of current decay. 3. The anaesthetics did not shift the current-voltage relation nor did they change the voltage for half-maximal channel activation derived from analysis of the voltage dependence of the tail currents. None of the anaesthetics appeared to alter the time constant of tail current decay. 4. To complement earlier studies of the inhibitory actions of anaesthetics on K(+)-evoked catecholamine secretion and the associated Ca2+ uptake, the IC50 values for etomidate and methohexitone were determined using a biochemical assay. The IC50 values for anaesthetic inhibition of calcium channel currents corresponded closely with those for inhibition of K(+)-evoked calcium uptake and catecholamine secretion. 5. The inhibitory effect of the volatile anaesthetics and etomidate is best explained by dual action: a reduction in the probability of channel opening coupled with an increase in the rate of channel inactivation. Methohexitone appeared to inhibit the currents by a use-dependent slow block.

Molecular and cellular mechanisms of general anaesthesia

General anaesthetics are much more selective than is usually appreciated and may act by binding to only a small number of targets in the central nervous system. At surgical concentrations their principal effects are on ligand-gated (rather than voltage-gated) ion channels, with potentiation of postsynaptic inhibitory channel activity best fitting the pharmacological profile observed in general anaesthesia. Although the role of second messengers remains uncertain, it is now clear that anaesthetics act directly on proteins rather than on lipids.

Inhibition of omega-conotoxin-sensitive Ca2+ channel currents by internal Mg2+ in cultured rat cerebellar granule neurones

The effects of changing the intracellular concentrations of either free Mg2+ ions ([Mg2+]i) or Mg(2+)-bound adenosine triphosphate ([Mg.ATP]i) on Ca2+ channel currents were assessed in cultured rat cerebellar granule neurones using the whole-cell patch-clamp technique. Raising [Mg2+]i from 0.06 mM to 1.0 mM inhibited Ca2+ channel currents by approximately 50%. The action of omega-conotoxin GVIA (omega-CgTX), a selective inhibitor of "N"-type Ca2+ channels was also investigated. With increasing [Mg2+]i, the proportion of current irreversibly blocked by omega-CgTX was reduced, and was negligible (approximately 5 pA of current) in the presence of [Mg2+]i values of 0.5 mM or greater. Block of the omega-CgTX-sensitive current accounted for the reduction in total current by concentrations of [Mg2+]i to 0.5 mM. Raising [Mg2+]i had no effect on the rate of decay of Ca2+ currents, but did produce a negative shift in current activation, possibly due to a non-specific interaction with negative surface charge. Altering [Mg.ATP]i from 0.3 to 5.0 mM caused a twofold increase in the size of currents without affecting the proportion of current sensitive to omega-CgTX. [Mg2+]i was also effective in inhibiting the Ca2+ channel current following potentiation by increasing [Mg.ATP]i. These data suggest that omega-CgTX-sensitive current in these cells is selectively inhibited by internal Mg2+ whereas both omega-CgTX-sensitive and -resistant components of current are potentiated by internal Mg.ATP. The mechanism by which Mg2+ inhibits "N"-type channels is unclear, but may involve an open channel block.

Differential effects of general anaesthetics on identified molluscan neurones in situ and in culture

1. The only unifying principle of general anaesthesia is that general anaesthetics interact with membrane components and no single cellular mechanism appears to explain their widespread effects in the central nervous system. 2. The gastropod mollusc, Lymnaea stagnalis, provides an excellent model system for studies on general anaesthetics because it has large, uniquely identifiable nerve cells. Several of these cells are interneurones with identified neurotransmitters and monosynaptic connections to other cells. 3. Recent work on Lymnaea neurones suggests that calcium currents are depressed by volatile general anaesthetics applied in the clinical range, whilst evidence from other preparations indicates that there is a rise in intracellular calcium concentration following application of these substances. 4. Identified Lymnaea neurones have different responses to applied anaesthetics, irrespective of the anaesthetic used. Following application of halothane, barbiturates and several other anaesthetic agents, some cells gradually become quiescent after a short period, whilst in others a series of paroxysmal depolarizing shifts occur prior to quiescence. 5. Cultured neurones of Lymnaea, Helisoma and related species retain their characteristic action potential types and neurotransmitter identity. Their responses to anaesthetics are similar to those in the intact brain. They may also form synapses in culture. Thus, they are a useful tool for studying the cellular and subcellular actions of general anaesthetics.

Barbiturates inhibit ATP-K+ channels and voltage-activated currents in CRI-G1 insulin-secreting cells

1. Patch-clamp recording techniques were used to examine the effects of barbiturates upon the ATP-K+ channel, and voltage-activated channels present in the plasma membrane of CRI-G1 insulin-secreting cells. 2. Thiopentone inhibited ATP-K+ channel activity when applied to cell-attached patches or the intracellular or extracellular surface of cell-free patches. Secobarbitone and pentobarbitone were also effective inhibitors of ATP-K+ channels in cell-free patches, whereas phenobarbitone was ineffective. 3. The diabetogenic agent, alloxan, which is structurally related to the barbiturates also produced an inhibition of ATP-K+ channel activity in outside-out patches. 4. Whole-cell ATP-K+ currents were used to quantify the effects of the barbiturates: concentration-inhibition curves for thiopentone, secobarbitone and pentobarbitone resulted in IC50 values of 62, 250 and 360 microM respectively. Phenobarbitone at a concentration of 1 mM was virtually ineffective. 5. Calculation of the apparent membrane concentrations for these drugs indicate that for a given degree of ATP-K+ channel inhibition a similar concentration of each barbiturate is present in the membrane. This suggests that hydrophobicity plays a primary role in their mechanism of action. The pH-dependence and additive nature of barbiturate block also indicates a membrane site of action. 6. Thiopentone, (100 microM) was also found to inhibit differentially voltage-activated whole-cell currents. The relative potency of thiopentone at this concentration was 0.64, 0.38 and 0.12 for inhibiting Ca2+, K+ and Na+ currents respectively when compared with its ability to inhibit the ATP-K+ channel.

Inhibition of the voltage-dependent calcium currents in isolated frog sensory neurons by GABA-related agonistic compounds

Effects of GABAA-, barbiturate- and benzodiazepine receptor agonists and GABAB agonist, baclofen, on voltage-dependent Ca2+ current (ICa) were studied in isolated frog sensory neurons after suppression of Na+ and K+ currents using single-electrode voltage-clamp. GABA, muscimol, taurine and pentobarbital (PB) dose-dependently induced a transient Cl- current (ICl), while baclofen and diazepam (DZP) did not elicit any currents. With GABAA agonists such as GABA, muscimol and taurine, ICa was suppressed transiently, and the maximum inhibition of ICa occurred within 1 min. The suppression of ICa by all GABAA agonists was neither voltage dependent nor attenuated in the presence of either bicuculline or picrotoxin. In addition, there was no correlation between GABA- and baclofen-induced suppressions of ICa. The results suggest that the inhibition of ICa by GABAA receptor agonists is not due to either GABAA or GABAB receptor activation at least. The inhibition of ICa by baclofen, PB and DZP was persistent. PB suppressed the amplitude of ICa and also facilitated the inactivation process, suggesting that PB behaves as a Ca channel blocker. However, the mechanisms of ICa suppression by baclofen and DZP are the subject for a future study. The potency order of the drugs in reducing ICa was muscimol greater than GABA = DZP greater than baclofen greater than PB greater than taurine.

Differential actions of pentobarbitone on calcium current components of mouse sensory neurones in culture

1. Using the single-electrode voltage clamp technique, three calcium current components were recorded at 35 degrees C from mouse dorsal root ganglion (DRG) neurones in culture. A transient low-threshold calcium current (T current) was recorded at clamp potentials (Vc) positive to -60 mV. Holding potentials (Vh) at or negative to -90 mV were required to fully remove inactivation. A large transient high-threshold calcium current component (N current) was recorded at Vc positive to -40 mV. Vh at or negative to -80 mV removed all steady-state inactivation. A slowly inactivating high-threshold calcium current component (L current) was recorded at Vc positive to -30 mV. Inactivation was removed by Vh at or negative to -60 mV. When currents were evoked at Vc positive to -20 mV from Vh negative to -60 mV, all three calcium current components were present. 2. Pentobarbitone (500 microM) had no effect on the isolated T current, but reduced the isolated L current 50-100% when evoked at Vc of -20 to 0 mV from Vh of -50 mV. Pentobarbitone had voltage-dependent effects on calcium currents containing all three calcium current components. Pentobarbitone produced small and equal reductions of the peak and late (greater than or equal to 300 ms) calcium currents evoked at -20 to 0 mV from Vh at or negative to -80 mV, but at more positive Vh there was a greater reduction in the peak current. The rate of current inactivation was increased in the presence of pentobarbitone. 3. Current-voltage plots were constructed from currents recorded in the absence and presence of 500 microM-pentobarbitone. Pentobarbitone reduced the magnitude of the calcium current without affecting the voltage dependence of the current-voltage relation. 4. Calcium current traces were fitted with a multiexponential function to determine the amplitudes and inactivation time constants (tau i) of the three calcium current components. Inactivation time constants decreased with more positive Vc for all three calcium current components. Pentobarbitone reduced only those tau i corresponding to the N current. 5. Recovery from inactivation of the N current was determined using a two-pulse protocol. In control neurones, recovery from inactivation occurring at 0 mV was slower at Vh = -65 mV than at Vh = -80 mV. In the presence of pentobarbitone, recovery from inactivation was faster, and occurred at a similar rate at both potentials. 6. Steady-state inactivation curves for the N current were derived from neurones in the absence and presence of pentobarbitone.(ABSTRACT TRUNCATED AT 400 WORDS)

Some properties of inhibitory action of lidocaine on the Ca2+ current of single isolated frog sensory neurons

The action of lidocaine on the Ca2+ current (ICa) was studied on single isolated neurons of frog dorsal root ganglia using a 'concentration-clamp' technique which combines intracellular perfusion with a rapid external solution change. Lidocaine decreased the peak amplitude of ICa at a threshold concentration of 10 microM. Higher concentrations gave a dose-dependent increase in inhibition of ICa. Lidocaine also depressed the Na+ current (INa) at a threshold concentration 10 times lower than that for decreasing the amplitude of ICa of neurons isolated from same ganglia. The rate of inhibitory action on ICa was slowed at more negative holding potentials (VH). Lidocaine appears to block both closed and open Ca2+ channels, but acts more profoundly on open channels.

Nimodipine block of calcium channels in rat anterior pituitary cells

1. Ca channels were studied in the GH4C1 clonal cell line derived from rat anterior pituitary cells. The whole-cell variation of the patch-electrode voltage-clamp technique was used. 2. Two types of Ca channels were found. One type ('slowly inactivating' channels) is insensitive to changes in holding potential, does not inactivate during test pulses lasting several seconds, and deactivates very quickly upon repolarization. For holding potentials less than -40 mV, a second type of Ca channel is available for opening. This population ('transient' channels) differs from the first type in that it activates at more negative potentials, inactivates rapidly with either Ca or Ba as the charge carrier, deactivates about 10 times more slowly upon repolarization, and is less selective for Ba over Cs. 3. Nimodipine preferentially blocks the slowly inactivating channels. Block of these channels is time- and voltage-dependent, such that block is maximized by long depolarizations. 4. A comparison of the voltage dependence of steady-state nimodipine block with the voltage dependence of channel activation indicates that channel block is directly proportional to the number of open channels. The results are accounted for by a model that postulates 1:1 high-affinity drug binding to open Ca channels. The apparent dissociation constant for binding to open channels is 517 pM. Similar binding constants were previously reported for the inhibition of high-K-induced hormone secretion and high-affinity ligand binding of [3H]nimodipine to isolated plasma membranes. 5. The rate of onset of nimodipine block increases with the test potential, in quantitative agreement with the model of open-channel block. The apparent association rate is about 9.6 X 10(7) M-1 s-1; the dissociation rate is about 0.050 s-1. At therapeutic concentrations (less than 10 nM) nimodipine block takes many seconds to reach equilibrium. 6. Nimodipine should have little effect on stimulus-secretion coupling in healthy pituitary cells in vivo because: (a) the drug binds very weakly to the transient channels that are open at normal resting potentials, and (b) negligible high-affinity binding occurs during spontaneous activity because the onset of block is very slow.

Reduction of the voltage-dependent calcium current in Aplysia neurons by pentobarbital

Effects of pentobarbital on the calcium current of Aplysia neurons were investigated under current- and voltage-clamp conditions using the conventional two-microelectrode technique. Pentobarbital attenuated the progressive broadening of repeated action potentials of somata, suggesting a reduction in the calcium current. When calcium ion was replaced with barium ion in the perfusing solution, in which neither sodium nor potassium ions carried transmembrane currents, the barium current (IBa) which flowed through the calcium channel of the cell membrane was generated by depolarizing pulses of several hundred milliseconds applied every 1 min from a holding potential of -50 mV. The IBa was not affected by tetrodotoxin (30 microM). The current was decreased by pentobarbital (0.1-5 mM) in a dose-dependent manner. The inhibition was much greater at a lower pH of the perfusate, indicating that the uncharged form of the agent was responsible. The voltage-dependent inactivation of the IBa proceeded with two time constants [190 +/- 21 and 2020 +/- 146 msec (N = 4) at -10 mV], both of which were shortened by adding 1 mM pentobarbital [to 120 +/- 18 and 540 +/- 51 msec (N = 4), respectively]. The IBa recovered from the inactivation with two time constants [60 +/- 7 and 871 +/- 76 msec (N = 3) at -50 mV]. The anesthetic (1 mM) prolonged both of them, to 124 +/- 20 and 1480 +/- 172 msec (N = 3), respectively, resulting in a use-dependent depression of the current at 2-Hz stimulation. Pentobarbital reduced the IBa to a greater extent when the holding potential was more positive (-30 instead of -50 mV), indicating a higher affinity of the drug to the inactivated state of the channel. These findings suggest that the attenuation of the progressive broadening of successive spikes by pentobarbital is due to a decrease in the voltage- and time-dependent calcium current, ending in depression of transmitter release from the nerve terminal.

Effects of n-alkanols on the calcium current of intracellularly perfused neurons of Helix aspersa

The effects of n-alkanols on the calcium current (ICa) were studied in molluscan neurons perfused intracellularly and voltage clamped using a suction pipette technique. All n-alkanols employed in this experiment (methanol, ethanol and butanol) decreased the peak amplitude of ICa and caused acceleration of the decay of ICa in a dose-dependent manner at all membrane potentials. The concentrations of n-alkanols required for these actions decreased as the hydrocarbon chain increased in length. The results suggest that these effects on the ICa of molluscan neurons may be related to the lipophilic properties of n-alkanols.

gamma-Aminobutyric-acid- and pentobarbitone-gated chloride currents in internally perfused frog sensory neurones

gamma-Aminobutyric-acid- (GABA) and pentobarbitone-induced Cl- currents (ICl) were studied in isolated frog sensory neurones after suppression of Na+, K+ and Ca2+ currents using a suction-pipette technique combining internal perfusion with voltage clamp. All GABA-sensitive neurones responded to pentobarbitone. Both GABA- and pentobarbitone-induced ICl reversed at the Cl- equilibrium potential (ECl). The dose-response curve for maxima of GABA-induced ICl was sigmoidal with a mean concentration producing a half-maximum response, Ka of 2 X 10(-5) M at a Hill coefficient of 1.8. In the presence of pentobarbitone, the GABA dose-response curve shifted to the left without affecting the saturating maximum current. At high concentrations, both GABA and pentobarbitone could also potentiate the pentobarbitone- and GABA-induced ICl respectively, while pre-treatment with one of the two markedly attenuated currents induced by the other, indicating a 'cross-desensitization'. In the presence of pentobarbitone, the augmented response was voltage dependent and this augmentation was much greater in the inward-current direction than outward. In producing ICl, pentobarbitone and its stereoisomers were potent in the order of (-) isomer greater than (+/-) racemic mixture greater than (+) isomer. A stereospecific facilitatory action of pentobarbitone on GABA responses was also found in the same order. Responses to GABA, homotaurine, taurine, beta-alanine, 5-aminovaleric acid, (+)- and (-)-gamma-amino-beta-hydroxybutyric acid and muscimol were equally enhanced by pentobarbitone, though its action on glycine-induced ICl was less effective. Picrotoxin inhibited the GABA- and pentobarbitone-induced ICl from either side of membrane, while internal application of GABA and pentobarbitone did not exert any effect. It was concluded that pentobarbitone binds to the 'barbiturate receptors' located close to the GABA receptor-Cl- channel complex, and directly affects the GABA-GABA receptor interactions rather than the ionic channels.

Contribution of restricted extracellular space to the inactivation of calcium current in the snail neuron

The mechanisms underlying the inactivation of calcium current (ICa) were investigated in isolated nerve cell bodies of Helix aspersa using a suction pipette technique that allowed voltage clamp and internal perfusion at the same time. ICa was recorded after eliminating the Na and K currents by removing Na+ and K+ both in external and internal solutions, and ICa inactivation due to intracellular Ca2+ accumulation was blocked by 5-25 mM EGTA. The inactivation rates of ICa, IBa and ISr corresponded to two exponential processes. The inactivation rates of the inward currents (IMn, ICd and IZn) less than 1/5 of ICa fitted a single exponential. However, when neurons were superfused with hypertonic external solution by adding 100 mM sucrose together with internal EGTA, the steady-state inactivation of ICa, IBa and ISr was reduced, and the inactivation processes changed to a single exponential similar to that of IMn, ICd and IZn. In contrast, internal perfusion with the hypertonic solution had no effect on the inactivation of ICa, IBa and ISr. Therefore, it was concluded that the inactivation process of ICa is dependent not only on the membrane voltage and the intracellular Ca2+ accumulation as described previously, but is also affected by the rapid fall in the concentration of Ca2+ in the restricted extracellular spaces (RES) which gets enlarged by the hypertonic external solution. The same is also true for IBa and ISr.

Barbiturates increase the rate of voltage-dependent inactivation of the calcium current in snail neurones

Effects of barbiturates (thiopentone, pentobarbitone, phenobarbitone and barbitone) on the calcium current (ICa) in identified Helix neurones were studied, using a conventional suction pipette technique. Barbiturates depressed the maximal peak amplitudes (MPA) of ICa in a dose-dependent manner without shifting the current-voltage relationships along the voltage axis. Barbiturates accelerated the decay phase of ICa at high concentrations (1 X 10(-4) to 3 X 10(-3) M), at which concentrations double-pulse experiments showed the increased rate of a voltage-dependent inactivation of ICa. It is concluded that the acceleration of the decay phase of ICa by barbiturates may be due to the increased rate of the voltage-dependent inactivation of ICa.