Release of calcium ions linked to the activation of potassium conductance in a caffeine-treated sympathetic neurone

GABA-induced depolarizing responses of the frog spinal cord can be either enhanced or antagonized by the benzodiazepine midazolam and the methylxanthine caffeine

The isolated frog spinal cord was used as a test system to investigate the interactions of midazolam and caffeine with GABA-evoked responses recorded from dorsal afferent fibres. Midazolam was a potent stimulator (ED50 = 1 nM) of GABA effectiveness on this preparation since the ED50 value for GABA was nearly halved. The enhancing action of midazolam was apparently produced via activation of benzodiazepine receptor mechanisms with no detectable receptor cooperativity and with a rather high sensitivity to the selective benzodiazepine antagonist Ro 14-7437 (antagonist ED50 = 2.5 nM). Responses to glutamate, glycine or high K+ were unchanged in midazolam solutions. When midazolam was applied in concentrations equal or larger than 100 nM, antagonism of GABA responses was evident. Caffeine (50 microM) also potentiated GABA responses through a mechanism distinct from benzodiazepine receptor activation as this action of caffeine was insensitive to Ro 14-7437. Mixtures of low doses of caffeine and midazolam frequently antagonized GABA responses. It is suggested that the blockade of GABA responses by caffeine-midazolam mixtures might account for the reversal of some behavioural actions of these substances even if they probably operate through separate pharmacological mechanisms.

(+)-Tubocurarine blocks the Ca2+-dependent K+-channel of the bullfrog sympathetic ganglion cell

(+)-Tubocurarine [+)-Tc: 10-100 microM) reduced the duration of the afterhyperpolarization, which was induced by the activation of Ca2+-dependent K+-conductance (GK,Ca) following an action potential in the bullfrog sympathetic ganglion cell, but did not affect the maximum rates of rise and fall of Na+- and Ca2+-dependent action potentials. The amplitudes of slow rhythmic membrane hyperpolarizations produced by rhythmic rises in the GK,Ca were also decreased by (+)-Tc without a change in their intervals. Thus, (+)-Tc appears to block the Ca2+-dependent K+-channel of the bullfrog sympathetic ganglion cell.

Morphine augments calcium-dependent potassium conductance in guinea-pig myenteric neurones

Intracellular recordings were made from myenteric neurones removed from guinea-pig ileum and maintained in vitro. Action potentials were elicited by passing brief depolarizing currents through the recording electrode. In AH cells they were followed by afterhyperpolarizations resulting from an increase in potassium conductance (GK,Ca). Morphine (1 nM - 1 microM), applied by superfusion, increased the duration of the afterhyperpolarization (and the underlying GK,Ca) which followed from 1 to 30 action potentials. Morphine did not change the peak amplitude of the afterhyperpolarization. This action of morphine occurred both in cells which showed no change in resting membrane potential or resistance and in cells which were hyperpolarized. It was prevented by naloxone (10 nM - 1 microM). The possibility is proposed that morphine inhibits one of the mechanisms by which myenteric neurones control their free intracellular calcium concentration close to the plasma membrane.

Effects of dantrolene and methylxanthines on the sensory nerve terminal of the frog muscle spindle

The application of 1.5-4 microM dantrolene decreased the threshold and the current sensitivity of the rhythmic hyperpolarizations that occur during depolarization of the sensory nerve terminal in the frog muscle spindle. The higher concentration provoked spontaneous rhythmic changes even without depolarization. Methylxanthines (5 mM caffeine, theophylline or pentylene-tetrazole) increased the threshold and the sensitivity. Electron microscopic observations of the dantrolene-treated spindles revealed numerous electron-dense deposits associated with the cytoplasmic membrane of the sensory terminals and with mitochondrial membranes. The deposits were found to contain K+ and Ca2+ by energy dispersive X-ray microanalysis. Electron-dense deposits containing Ca2+ were usually observed in the inner capsular space and in the mitochondria of the sensory terminals perfused by normal or high Ca2+ Ringer solutions. They were reduced in number following incubation with methylxanthines. The amplitudes of afferent spikes and the spindle potential were increased by methylxanthines in much the same way as by K+ channel blockers, suggesting that GK of the terminal membrane may be reduced by methylxanthines. We suggest that methylxanthines may modulate the terminal responses both as a K+ channel blocker and by enhancing the release of Ca2+ from a storage site, perhaps in the inner capsular space, whereas dantrolene has the opposite effect.

Spontaneous voltage and current fluctuations in tissue cultured mouse dorsal root ganglion cells

Fetal mouse dorsal root ganglion (DRG) neurons were maintained in primary dissociated cell culture for periods of 7 days to 3 months. Intracellular recordings from these cells revealed the presence of spontaneous subthreshold potentials in 101/177 neurons studied. When measured at the resting membrane potential, these spontaneous voltage events took two forms: (a) high frequency potential fluctuations several millivolts in peak-to-peak amplitude and (b) small, discrete hyperpolarizations. Neurons exhibiting either type of event were designated as 'active' DRG cells. No spontaneous potentials were seen in DRG cells hyperpolarized to membrane voltages more negative than -64 +/- 11.5 mV (n = 5 cells). Under voltage-clamp conditions, the subthreshold potentials of active DRG cells were replaced by fluctuations in outward current. The power spectral density, S(f) of these current fluctuations was approximated by an equation of the form S(f) = (S(o)/[1 + (f/fc) alpha] where 2 less than or equal to a less than or equal to 3 and the half-power frequency fc = 11.3 +/- 3.1 Hz at 23 degrees C (n = 17 cells). The spontaneous voltage fluctuations of active DRG cells were abolished in Ca2+-free saline, and of the divalent metal cations Sr2+, Mg2+, Ba2+, Co2+ and Mn2+, only Sr2+ could substitute for Ca2+ in the maintenance of this activity. Tetraethylammonium ions (1-10 mM) reversibly blocked the spontaneous potentials, while caffeine (10 mM) increased the frequency of these events. The spontaneous voltage fluctuations were not dependent on the presence of spinal cord neurons in the culture plate, and they were also observed in cultured DRG cells derived from adult mice.

Ca-activated potassium current in vertebrate sympathetic neurons

Ca-activated K-currents (IC) in sympathetic neurones have been triggered by intracellular Ca-injection or by activating ICa. IC is strongly voltage-dependent, with a peak slope of 11 mV/e-fold depolarization above -50 mV. Relaxation, fluctuation and single channel analysis suggests this to result from voltage-dependent opening and closing rates. Time-constants for channel opening and closing are about 15 msec near zero mV. Single channel conductance is about 100 pS. Currents can be blocked by TEA. IC is activated very rapidly (less than or equal to 5 msec) and sometimes transiently by a depolarizing voltage-step. It is suggested that IC contributes to both spike repolarization and spike after-hyperpolarization. Spontaneous miniature ICs have also been recorded, probably activated by the release of packets of intracellular Ca.

Origin of calcium ions involved in the generation of a slow afterhyperpolarization in bullfrog sympathetic neurones

The origin of Ca2+ that activates the Ca2+-dependent K+ conductance which is responsible for the slow afterhyperpolarization (a.h.p.) following an action potential was studied in bullfrog sympathetic ganglia. The decay phase of the a.h.p. was a graded function of the extracellular Ca2+, and showed a voltage sensitivity opposite to that of the Ca2+-dependent K+-current reported previously (Pallotta et al. 1981), indicating that it reflected the time course of an increase in intracellular free Ca2+. An a.h.p. of longer duration was generated in cells which showed more pronounced rhythmic hyperpolarizations induced by intracellular Ca2+ release. The duration of the a.h.p. recorded with electrodes filled with K3-citrate [a.h.p. (citrate)], which favors Ca2+ release, was longer than the a.h.p. recorded with KCl-filled electrodes [a.h.p. (C1)]. D-600 (50-100 microM) drastically reduced the a.h.p. (C1), but had less effect on the a.h.p. (citrate). Caffeine which facilitates Ca2+ release prolonged the a.h.p. (C1), but had less effect on the a.h.p. (citrate). The a.h.p. (citrate) showed a greater sensitivity to a low temperature than the a.h.p. (C1). Mn2+ (1-3 mM) depressed both types of a.h.ps to the same extent. These results suggest that the origin of intracellular Ca2+ for a.h.p. (C1) is mainly Ca2+ influx during an action potential, while that for the a.h.p. (citrate) is both Ca2+ entry and intracellular Ca2+ release, although the effect of Mn2+ is difficult to explain fully.

Voltage oscillations in mammalian metaphase II oocytes

The membrane potential has been measured in ovulated mouse oocytes using conventional electrophysiological techniques. Temporal oscillations in membrane voltage have been observed in the oocytes, with periods of about 6 h. This oscillatory pattern, peculiar to oocytes in metaphase II, might explain the differences in membrane potential values reported in several studies on mammalian oocytes.

Detection of intracellular Ca2+ transients in sympathetic neurones using arsenazo III

Changes in cytosolic calcium ion concentration ([Ca2+]i) have been implicated in a wide variety of cellular stimulus--transduction roles. In nerve cells, it is believed that electrical activity raises [Ca2+]i by allowing influx of Ca2+ through voltage-dependent channels in the surface membrane. Elevation of neuronal [Ca2+]i may also occur due to release of Ca2+ from intracellular storage sites. Transient increases in [Ca2+]i are thought to trigger neurotransmitter release, and to modulate axonal transport, energy metabolism and growth cone movement. Intracellular Ca2+ also appears to regulate membrane potassium channels and thereby to regulate electrical excitability. Although [Ca2+]i transients have been measured in a few giant invertebrate neurones, detection of such transients in a vertebrate neurone has not been previously reported. We have measured [Ca2+]i in bullfrog sympathetic neurones by photometry of a microinjected calcium indicator dye, arsenazo III (refs 14-16), and report here that action potentials and voltage-clamped depolarizations cause long-lasting increases in [Ca2+]i. Aslo, exposure to the drug theophylline can cause spontaneous periodic increases in [Ca2+]i. Comparisons of [Ca2+]i signals with simultaneous intracellular recordings of membrane potential suggest that the kinetics of the post-tetanic hyperpolarization (PTH) following a series of action potentials or the spontaneous hyperpolarizations induced by theophylline directly reflect the kinetics of the [Ca2+]i transient.

Rhythmic membrane potential changes in hamster parasympathetic neurons

Two types of rhythmic membrane potentials in hamster submandibular neurons: (i) slow oscillations of membrane potential (SOMP); and (ii) spontaneous or caffeine-induced rhythmic hyperpolarizing potentials (C-HPs), have been analyzed. SOMPs occurred spontaneously, roughly in sinusoidal forms, between the subthreshold range and the potassium equilibrium potential (EK, approximately -85 mV). The average amplitude of SOMPs from crest to trough was 12 mV with an average crest to crest interval of 6 min. The largest amplitude of SOMPs was seen when their median membrane potentials were between -65 and -70 mV; values outside this range attenuated the amplitude of SOMPs. SOMPs were hardly discernible at or near EK. The membrane resistance was, in general, higher at the crest than at the trough. In eserine-treated preparations, SOMPs of varying durations following postsynaptic potentials were triggered by preganglionic repetitive stimulation. Reduction of extracellular K+ concentration increased the amplitude of SOMP without changing its frequency. This effect was noted at times before K+-free induced membrane depolarization occurred. The amplitude of the SOMP decreased in Ca2+-free saline with concomitant depolarization; conversely, in saline in which the Ca2+ concentration was doubled the membrane potential (Em) was found to be again stable near the EK level. A transient hyperpolarization occurred following intracellular Ca2+ injection when the Em of the preinjected state was between -45 and -60 mV. Among K+-conductance (GK) blockers (TEA, 3- and 4-aminopyridine, Cs+ and Ba2+) examined, only Ba2+ at 5 mM reduced both amplitude and frequency of C-HPs significantly. All Ca2+-conductance (GCa) blockers (Co2+ and Mn2+ at 5 mM, Cd2+ and La3+ at 1 mM, and D-600 at 0.4 mg/ml) prevented synaptic transmission and abolished spike-induced late hyperpolarizing afterpotential. C-HPs were nearly abolished by these agents in 4 mM Ca2+-containing saline. Mitochondrial inhibitors (DNP, CCCP, KCN, NaN3) in a concentration range between 10(-4) M and 10(-5) M, hyperpolarized the membrane before depolarizing and abolishing C-HPs. However, the plasma membrane Na+-pump inhibitor ouabain, at concentrations up to 5 X 10(-4) M, did not affect C-HPs during 1 h perfusion in the majority of neurons; no membrane hyperpolarization was induced, although a gradual depolarization did occur. Both ruthenium red (5 mM) and quinine (5 X 10(-4) M) abolished C-HPs. It is assumed that the two above types of membrane potential changes are generated by the Ca2+-activated GK increase, which, in turn, is under the control of mitochondrial Ca2+ regulatory activity.

Effects of altered extracellular and intracellular calcium concentration on hyperpolarizing responses of the hamster egg

Upon fertilization the hamster egg shows transient, periodic hyperpolarizing responses (h.r.s) due to a Ca-activated K conductance; these are superimposed on a gradual, hyperpolarizing shift of the resting potential (h.s.) (Miyazaki & Igusa, 1981a, 1982a). The h.r.s and h.s. were further analysed by changing external divalent cations or by injection of Ca2+ into the egg, to study the mechanisms of the increase in the intracellular Ca2+ concentration ([Ca2+]i). The series of h.r.s was abolished by the removal of external Ca2+. The frequency of the h.r. was decreased by lowering the [Ca2+]o or by adding Mn2+ or Co2+, and it was increased by raising the [Ca2+]o in a time- and concentration-dependent manner. The h.r. frequency was decreased on sustained depolarization with steady current, and increased on hyperpolarization. In contrast to the h.r. frequency, the amplitude, conductance increase and reversal potential of each h.r. were little affected by [Ca2+]o, Mn2+ or Co2+. The h.s. was decreased by lowering the [Ca2+]o, by adding Mn2+ or Co2+, or by injection of EGTA. The h.s. may reflect continuous Ca influx stimulating a Ca-activated K conductance (GK). In unfertilized eggs a regenerative h.r. was induced by Ca injection with an apparent threshold. The relationship between GK and the injected Ca2+ showed a steep jump at the critical current, associated with a four-fold increase in GK. The regenerative h.r. was followed by a refractory period of 1-2 min. In inseminated eggs the periodic sperm-mediated h.r.s. (s.-h.r.s) were interrupted by interposed h.r.(s) induced by Ca injection(s): the periodicity of s.-h.r.s was reset by Ca-induced h.r. In inseminated eggs the regenerative h.r. was induced by Ca injection with a much smaller pulse than necessary in unfertilized eggs. The refractory period was shortened to 40-50 sec, comparable to the period of s.-h.r.s. In inseminated eggs periodic h.r.s similar to s.-h.r.s were produced by continuous, repetitive injections of Ca2+ with constant pulses. The frequency of these h.r.s was dependent on the injection current. It is concluded that each h.r. indicates an enhancement of the increase in [Ca2+]i, probably the result of Ca-induced Ca release from intracellular stores. A possible mechanism for periodic increase in [Ca2+]i reflected in s.-h.r.s is proposed, based on a linkage of the continuous Ca influx to Ca release.

Does intracellular release of Ca2+ participate in the afterhyperpolarization of a sympathetic neurone?

The afterhyperpolarization (AHP) of an action potential in the bullfrog sympathetic ganglion cell was highly sensitive to anions (a factor affecting Ca2+ release) filled in a recording electrode; it was slower for citrate ion than for Cl-. The AHP recorded with a 'KCl-electrode' was suppressed drastically by D-600 (Ca2+-antagonist) and prolonged significantly by caffeine (promoting Ca2+ release), while the AHP recorded with a 'K3-citrate-electrode' was affected only slightly by these agents. Thus, these results suggest that Ca2+ entry during an action potential is the main origin of Ca2+ for the AHP recorded with a 'KCl-electrode', and favour the idea that the intracellular release of Ca2+ by an action potential as well as the Ca2+ influx participates in the mechanism of the AHP recorded with a 'K3-citrate-electrode'.

The ionic basis of adenosine receptor actions on post-ganglionic neurones in the rat

Adenosine inhibited three Ca2+-dependent potentials recorded intracellularly from post-ganglionic neurones of the rat superior cervical ganglion. A shoulder on the falling phase of the action potential elicited in normal Locke solution, a hyperpolarizing after-potential (h.a.p.) that follows the spike, and a regenerative Ca2+ spike elicited in Locke solution containing TTX and TEA were all reversibly inhibited by adenosine analogues in a dose-dependent fashion. The maximum rate of rise of the Ca2+ spike (dV/dt) was markedly reduced suggesting that the underlying mechanism of adenosine action is inhibition of the Ca2+ conductance mechanism and thus, the voltage-sensitive Ca2+ current. I/V curves in low Ca2+, high Mg2+, TTX, TEA, and Co2+ to block the Ca2+ current show no change in resistance in the presence of 2-chloroadenosine. The actions of adenosine were nearly eliminated in the presence of 1 mM-theophylline, an adenosine receptor antagonist. The order of agonist potency on the inhibition of the h.a.p. was: N-6-[L-phenylisopropyl] adenosine (L-PIA) greater than 2-chloroadenosine greater than adenosine greater than cyclic AMP = 5' AMP. The concentration of L-PIA which produced a half-maximal effect (EC50) was 0.5 microM and that for cyclic AMP was 100 microM. Dipyridamole, an adenosine uptake blocker, potentiated the effects of low concentrations of adenosine and shifted the dose-response curve for adenosine towards that of 2-chloroadenosine (EC50 = 1 microM). These results are consistent with the concept of an external adenosine receptor, but we are unable to assign a receptor subtype. Cyclic AMP mimicked the effects of adenosine, but these effects were eliminated by adenosine deaminase. Our results suggest that the electrogenic effects of bath-applied cyclic AMP may result from the metabolism of cyclic AMP to adenosine by ganglionic tissue. We conclude that adenosine activates a receptor on the neuronal cell surface to inhibit the voltage-dependent Ca2+ current.

Caffeine-induced potentiation of GABA effects on frog spinal cord: an electrophysiological study

A parasagittal slice of the frog spinal cord was kept in vitro for electrophysiological recordings from dorsal and ventral roots. Low concentrations of caffeine (50 microM) which had relatively small effects on baseline electrical activity, increased the depolarizing action of GABA on dorsal root fibres by 50%. A similar result was also obtained when GABA motoneuronal responses were tested. On dorsal roots the potentiation of GABA responses by caffeine was reflected by a significant decrease in GABA ED50 value without change in the maximal response amplitude; this enhancing action of caffeine was not blocked by bicuculline (5 microM) but was abolished by flurazepam (5 microM) or by Ca2+ antagonists (Mn2+ and Cd2+). Blockade of interneuronal activity by procaine left the potentiating action of caffeine unchanged. High doses of caffeine (up to 1 mM) produced a seemingly non-competitive antagonism of GABA responses. We suggest that caffeine can modulate GABA responses through two different mechanisms: a potentiation of GABA effects (seen with low doses of caffeine) probably due to Ca2+ mobilization and an antagonism of GABA responses (typically seen with large doses of caffeine) perhaps caused by block of GABA receptor-activated channels. This novel caffeine-GABA interaction may be useful to interpret some of the effects of caffeine on mammalian behaviour.

GK(Ca)-dependent cyclic potential changes in the sensory nerve terminal of frog muscle spindle

Spontaneous cyclic hyperpolarizations along the sensory nerve terminal of frog muscle spindles were observed during the application of 1-9 nA depolarizing currents across an air-gap on which the axon was bridged. An increase in the current intensity increased the amplitude and duration of the cyclic changes. Upon subthreshold depolarization, single or repetitive hyperpolarizations could be elicited after a brief electric pulse or during stretch of the receptors, respectively. The threshold was decreased in higher Ca2+, Sr2+ or Ba2+ solutions. The cyclic changes were reversibly blocked by K+- or Ca2+-blockers and quinine. These results suggest that the changes are due to GK(Ca). The site of origin of the changes was at the branching node in the capsule, as confirmed by the following results: (1) the cyclic changes were abolished upon inactivating the node by UV-irradiation; (2) in normal Ringer's solution, the rate of afferent impulses, which reflects the membrane potential at the encoding site along the non-myelinated filaments, was unmodified by the cyclic changes and was independent of the intensity of the polarizing currents within a certain range; however, it was sensitively dependent on this intensity after treatment with K+-blockers; (3) the amplitude of the impulses reaching the branching node was attenuated during the cyclic changes, but not after GK-blockade.

A study of calcium compartments in rat brain cortex thin slices: effects of veratridine, lithium and of a mitochondrial uncoupler

The efflux kinetics of 45Ca from rat brain cortex thin slices previously equilibrated with it, was studied in a superfusion system. Two first order kinetic components of efflux from the tissue were found: k2 = 0.0667 min-1, that was unchanged by lowering the temperature from 37 degrees C to 15 degrees, and k3 = 0.0167 min-1 at 37 degrees C, that was reduced to 0.0897 min-1 at 15 degrees C. This suggests that k2 represents efflux from the extracellular space, and k3 that from the cellular compartment. Addition of the mitochondrial uncoupler carbonyl cyanide, m-chlorophenylhydrazone (CCCP) (10(-5)M) increased the efflux fractional rate constant of 45Ca by 35%, while no change in efflux was induced by 10 mM caffeine. Veratridine (10(-5)M) drastically reduced 45Ca efflux if superfusion was with physiological salt solution (150 mM sodium present), but not if 50 mM lithium replaced an equivalent amount of sodium in the superfusion fluid. This lithium-containing solution did not affect 45Ca efflux in the absence of veratridine. These results indicate that mitochondria accumulate only a minor fraction of intracellular 45Ca; that 45Ca possibly turns over very rapidly in the endoplasmic reticulum, and that most of 45Ca is present in a different, non-mitochondrial, non endoplasmic reticular compartment, the nature of which can be only conjectured.

Identification of gK systems activated by [Ca2+]

Rhythmic caffeine hyperpolarizations generated in bullfrog sympathetic ganglion cells are assumed to be caused by periodic increase in gK due to rise in [Ca2+]i7--9,13. Caffeine-induced outward currents seem to be composed of two different components, which show different pharmacological natures and also different dependencies on membrane potential changes. These two components may be generated by activation of two voltage-dependent K+ currents, namely IK1 (the delayed rectifier K+ current) and IK2 (IM) of ganglion cells. These results suggested that at least two different gK systems were activated by [Ca2+]i in sympathetic ganglion cells.

Modulation of voltage-dependent currents by muscarinic receptor in sympathetic neurones of bullfrog

The muscarinic actions of acetylcholine (ACh) on the action potentials of bullfrog sympathetic ganglion cells were studied with voltage-clamp experiments. The slow inward current (Isi) carried by Ca2+ was markedly depressed by ACh. ACh also markedly depressed the time-dependent outward current following Isi. The outward current was composed of two components, a TEA-sensitive rectifier K+ current (IK1) and a TEA-insensitive slow rectifier K+ current (IK2). Both of these components were depressed by ACh.

The calcium-activated potassium conductance in guinea-pig myenteric neurones

1. Intracellular recordings were made from guinea-pig myenteric neurones in vitro.2. From one to sixty action potentials were followed by an afterhyperpolarization, the amplitude and duration of which increased with the number of preceding action potentials.3. The afterhyperpolarization reversed its polarity at a membrane potential of -91 mV. This value changed by 58 mV when the potassium concentration of the perfusing solution was changed ten-fold.4. The afterhyperpolarization was abolished in calcium-free solutions. It was shortened in low calcium (1.2 mM) solutions and prolonged in solutions which contained high (5.0 mM) calcium concentrations, TEA (1 mM) or caffeine (1 muM).5. The conductance increase during the afterhyperpolarization (g(K, Ca)) was calculated from the amplitude of electrotonic potentials, taking advantage of the lack of membrane rectification in the range -60 to -90 mV. Peak g(K, Ca) increased as the number of action potentials was increased, but was relatively independent of membrane potential in this range.6. g(K, Ca) declined with a time course which was single exponential (time constant 1.5-5 s) following one to six action potentials, and double exponential (time constants about 3 and 12 s) following fifteen to sixty action potentials.7. It is concluded that the calcium which enters the neurone during the action potential elevates the membrane potassium conductance. The time course of this conductance increase probably reflects the free intracellular calcium concentration, and therefore describes the calcium sequestration or extrusion process.

Calcium channel and calcium pump involved in oscillatory hyperpolarizing responses of L-strain mouse fibroblasts

1. In fibroblastic L cells, spontaneously repeated hyperpolarizing responses (oscillation of membrane potential) and hyperpolarizing responses evoked by electrical stimuli were suppressed by the external application of a K(+) channel blocker, nonyltriethylammonium (C(9)). This hydrophobic TEA-analogue also inhibited the hyperpolarization induced by intracellular Ca(2+) injection.2. Quinine or quinidine, known inhibitors of the Ca(2+)-activated K(+) channel of red cells, instantaneously inhibited these hyperpolarizations. Thus, these hyperpolarizations are likely to be caused by the operation of Ca(2+)-sensitive K(+) channels.3. Azide, which is known to inhibit the mitochondrial Ca(2+) uptake in fibroblasts, and caffeine, dantrolene Na and oxalate, which affect the microsomal Ca(2+) transport, did not exert any effects upon the electrical potential profiles.4. On the other hand, Ca(2+) channel blockers (nifedipine, D 600 and Co(2+)) suppressed the hyperpolarizing responses, but not the hyperpolarizations produced by intracellular Ca(2+) injection, suggesting that the calcium ions responsible for the hyperpolarizing responses are mainly derived from outside the cell through Ca(2+) channels.5. Flavones of plant origin, which are known to inhibit Ca(2+)-ATPase, prolonged the duration of the hyperpolarizing phase of the oscillation or produced a sustained hyperpolarization.6. It is concluded that the Ca(2+) channel and the Ca(2+) pump play essential roles in the generation of the hyperpolarizing response and of the membrane potential oscillation in L cells, and that these hyperpolarizations are brought about by a transient elevation of cytosolic Ca(2+) level which, in turn, activates Ca(2+)-dependent K(+) channels.

Cyclic AMP, 5-HT, and the modulation of transmitter release at the crayfish neuromuscular junction

In this study it was found that several agents which elevate cAMP levels in cells also increase dramatically the quantity of transmitter released from crayfish excitatory nerve terminals in response to a stimulus. With respect to time course and magnitude, the increase produced by one of these agents, the cyclic nucleotide phosphodiesterase inhibitor Squibb 20,009 (SQ 20,009), is unlike any reported for such a drug at a synapse. Additionally, SQ 20,009 potentiated the facilitation of transmitter release produced by serotonin (5-HT) at this synapse. These results establish a possible role for cAMP in the control and modulation of transmitter release at the crayfish neuromuscular junction (NMJ). They further suggest that 5-HT functions here by activation of a presynaptically located adenylate cyclase.

Caffeine blocks the delayed K+ outward current of molluscan neurons

Extracellular application of caffeine inhibits the delayed K+ outward current of Aplysia neurons in a dose dependent manner without changing the kinetics. Half-maximum blockade is produced with a concentration of 16.0 +/- 0.7 mM (S.E.M.) caffeine after 1-2 min. Intracellular injection of caffeine has an almost immediate blocking effect. The evidence suggests that the blocking site is at or close to the inner surface of the cellular membrane.

Altered burst responses in hippocampal CA3 neurons injected with EGTA

Intracellular injection of EGTA abolished the hyperpolarization seen after spontaneous and evoked bursts of action potentials in hippocampal CA3 neurons. Simultaneously recorded, orthodromically elicited IPSPs were relatively unaffected by this treatment. It is concluded that CA3 neurons generate at least two types of hyperpolarization: one resulting from a calcium-mediated potassium conductance and the other from increased conductance to chloride.

Inhibitory action of Ca2+ on spontaneous transmitter release at motor nerve terminals in a high K+ solution

The inhibitory effect of a high external Ca2+ ([Ca2+]o) on spontaneous transmitter release in a high K+ solution (Gage and Quastel 1966; Birks et al. 1968) was studied at the frog neuromuscular junction, based on the hypothesis that an increased intracellular free Ca2+ ([Ca2+]i) in the nerve terminal plays a key role in the depression. Three procedures were employed to increase [Ca2+]i; increasing [Ca2+]o, application of caffeine and tetanic nerve stimulation. All of these procedures increased m.e.p.p. frequency in normal Ringer. However, as the basic m.e.p.p. frequency was increased by raising the external K+ concentration (7--15 mM), their facilitatory effects on m.e.p.p. frequency decreased, dissapeared and eventually reversed to depressant actions. Since a rise in the external K+ concentration would increase the steady state level of [Ca2+]i, it is suggested that when the [Ca2+]i is preset at a high level, manipulations so as to further increase [Ca2+]i depress spontaneous release of transmitter. Possible mechanisms for this inhibition was discussed in relation to a question whether or not the rate of spontaneous transmitter release is a monotonic function of [Ca2+]i.