Intracellular protein kinase and calcium inward currents in perfused neurones of the snail Helix pomatia

Signaling mechanisms of down-regulation of voltage-activated Ca2+ channels by transient receptor potential vanilloid type 1 stimulation with olvanil in primary sensory neurons

Olvanil ((N-vanillyl)-9-oleamide), a non-pungent transient receptor potential vanilloid type 1 agonist, desensitizes nociceptors and alleviates pain. But its molecular targets and signaling mechanisms are little known. Calcium influx through voltage-activated Ca(2+) channels plays an important role in neurotransmitter release and synaptic transmission. Here we determined the effect of olvanil on voltage-activated Ca(2+) channel currents and the signaling pathways in primary sensory neurons. Whole-cell voltage-clamp recordings were performed in acutely isolated rat dorsal root ganglion neurons. Olvanil (1 microM) elicited a delayed but sustained inward current, and caused a profound inhibition (approximately 60%) of N-, P/Q-, L-, and R-type voltage-activated Ca(2+) channel current. Pretreatment with a specific transient receptor potential vanilloid type 1 antagonist or intracellular application of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid abolished the inhibitory effect of olvanil on voltage-activated Ca(2+) channel current. Calmodulin antagonists (ophiobolin-A and calmodulin inhibitory peptide) largely blocked the effect of olvanil and capsaicin on voltage-activated Ca(2+) channel current. Furthermore, calcineurin (protein phosphatase 2B) inhibitors (deltamethrin and FK-506) eliminated the effect of olvanil on voltage-activated Ca(2+) channel current. Notably, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, calmodulin antagonists, and calcineurin inhibitors each alone significantly increased the amplitude of voltage-activated Ca(2+) channel current. In addition, double immunofluorescence labeling revealed that olvanil induced a rapid internalization of Ca(V)2.2 immunoreactivity from the membrane surface of dorsal root ganglion neurons. Collectively, this study suggests that stimulation of non-pungent transient receptor potential vanilloid type 1 inhibits voltage-activated Ca(2+) channels through a biochemical pathway involving intracellular Ca(2+)-calmodulin and calcineurin in nociceptive neurons. This new information is important for our understanding of the signaling mechanisms of desensitization of nociceptors by transient receptor potential vanilloid type 1 analogues and the feedback regulation of intracellular Ca(2+) and voltage-activated Ca(2+) channels in nociceptive sensory neurons.

A cGMP-dependent cascade enhances an L-type-like Ca2+ current in identified snail neurons

We studied the impact of an NO-cGMP dependent signalling pathway on the high-voltage-activated (HVA) Ca(2+) current in identified neurons of the pulmonate snail, Helix pomatia, using Ba(2+) as charge carrier. The 3',5'-cyclic guanosine monophosphate (cGMP) analogues, dibutyryl-cGMP and 8-bromo-cGMP, consistently induced a biphasic response, consisting of an increase superseded by a decline of the Ba(2+) current. The NO donor, sodium nitroprusside (SNP), modulated only in a minority of neurons the Ba(2+) current. Blockade of protein kinase activity with 1-[5-isoquinolinesulfonyl]-2 methyl piperazine (H 7), a nonselective protein kinase inhibitor, or Rp-8-pCPT-cGMP, a selective protein kinase G (PKG) inhibitor, decreased, whereas Rp-cAMP, a selective protein kinase A (PKA) inhibitor, increased the Ba(2+) current upon application of cGMP analogues or SNP. Okadaic acid or calyculin, inhibitors of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A), augmented the Ba(2+) current. Under these conditions, cGMP analogues or SNP had an additive-enhancing effect on the Ba(2+) current. When neurons were exposed to the nonselective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), cGMP analogues induced a persistent increase of the Ba(2+) current, whereas SNP induced a biphasic response. These data suggest coexistence of cGMP-PKG and cGMP-PDE pathways as well as crosstalk between cGMP and 3',5'-cyclic adenosine monophosphate (cAMP) pathways, which converge on HVA Ca channels in Helix neurons. In this model, augmentation of the Ba(2+) current through HVA Ca channels is accomplished by PKA and PKG, whereas attenuation is mediated by PDEs, which prevent activation of protein kinases via hydrolysis of cyclic nucleotides.

Control of Ca2+ channel current and exocytosis in rat lactotrophs by basally active protein kinase C and calcineurin

Modulation of voltage-activated Ca2+ channel activity by phosphorylation was studied in metabolically intact voltage-clamped rat lactotrophs. Experiments using Ba2+ as a charge carrier indicated that a phorbol ester protein kinase C activator stimulates high-voltage-activated Ca2+ channel currents, but has no effect on low-voltage-activated currents. Extracellular application of structurally and mechanistically distinct protein kinase C inhibitors (staurosporin, H7, calphostin C, chelerythrine and Ro 31-8220) preferentially inhibited the high-voltage-activated Ba2+ current. This suggests that protein kinase C is required for maintainance of Ca2+ channel activity even in the absence of modulators. Cyclosporin A, an inhibitor of the Ca2+/calmodulin-dependent protein phosphatase calcineurin, increased the high-voltage-activated Ca2+ channel current, and staurosporin reversed this effect. Thus, dephosphosphorylation by calcineurin may limit basal Ca2+ channel activity. Time-domain monitoring of cellular capacitance changes demonstrated that cyclosporin A and 12-O-tetradecanoyl-phorbol-13-acetate do not affect exocytosis at a hyperpolarized potential, but each enhances depolarization-induced exocytosis. Facilitation of exocytosis by cyclosporin A differed from 12-O-tetradecanoyl-phorbol-13-acetate in that it was biphasic. The delayed facilitation induced by cyclosporin A could be accounted for by stimulation of the voltage-gated Ca2+ current. These results suggest that the high-voltage activated Ca2+ channel current in rat lactotrophs is determined by the opposing basal activities of protein kinase C and calcineurin. Furthermore, it is concluded that the regulation of Ca2+ channels by protein kinase C and calcineurin affects depolarization-induced exocytosis.

Voltage gated calcium channels in molluscs: classification, Ca2+ dependent inactivation, modulation and functional roles

Molluscan neurons and muscle cells express transient (T-type like) and sustained LVA calcium channels, as well as transient and sustained HVA channels. In addition weakly voltage sensitive calcium channels are observed. In a number of cases toxin or dihydropyridine sensitivity justifies classification of the HVA currents in L, N or P-type categories. In many cases, however, pharmacological characterization is still preliminary. Characterization of novel toxins from molluscivorous Conus snails may facilitate classification of molluscan calcium channels. Molluscan preparations have been very useful to study calcium dependent inactivation of calcium channels. Proposed mechanisms explain calcium dependent inactivation through direct interaction of Ca2+ with the channel, through dephosphorylation by calcium dependent phosphatases or through calcium dependent disruption of connections with the cytoskeleton. Transmitter modulation operating through various second messenger mediated pathways is well documented. In general, phosphorylation through PKA, cGMP dependent PK or PKC facilitates the calcium channels, while putative direct G-protein action inhibits the channels. Ca2+ and cGMP may inhibit the channels through activation of phosphodiesterases or phosphatases. Detailed evidence has been provided on the role of sustained LVA channels in pacemaking and the generation of firing patterns, and on the role of HVA channels in the dynamic changes in action potentials during spiking, the regulation of the release of transmitters and hormones, and the regulation of growth cone behavior and neurite outgrowth. The accessibility of molluscan preparations (e.g. the squid giant synapse for excitation release studies, Helisoma B5 neuron for neurite and synapse formation) and the large body of knowledge on electrophysiological properties and functional connections of identified molluscan neurons (e.g. sensory neurons, R15, egg laying hormone producing cells, etc.) creates valuable opportunities to increase the insight into the functional roles of calcium channels.

Enhancement by muscarinic agonists of a high voltage-activated Ca2+ current via phosphorylation in a snail neuron

1. In previous work we have shown that in the snail Helix aspersa neuron F1 carbamylcholine (CCh) and other muscarinic agonists enhance the inward current carried through high voltage-activated Ca2+ channels by Ba2+ (HVA-ICa). It was also found that cyclic nucleotides, inositol trisphosphate or arachidonic acid are not involved in this modulation. Moreover, despite the effect of CCh being blocked by intracellular injection of EGTA, neither protein kinase C nor Ca(2+)-calmodulin-dependent protein kinase II appeared to play a role. 2. In the present paper, the intracellular mechanism of this muscarinic modulation was investigated further by studying the effects of inhibitors of Ser-Thr protein phosphatases (PP) on both the HVA-ICa of neuron F1 and its enhancement by CCh. 3. Intracellular injections in the F1 neuron of either microcystin LR or okadaic acid, both inhibitors of PP1 and PP2A, mimic the action of CCh on the HVA-ICa and occlude the effects of CCh on this current. In contrast, cyclosporin A, an inhibitor of PP2B (calcineurin), affects neither the HVA Ca2+ current itself nor its modulation by CCh. 4. The efficacy of PP inhibitors was tested in F1 neurons in which serotonin (5-HT) induces an inward current involving intracellular increases in cAMP and a protein kinase A-dependent closing of K+ channels. We found that intracellular injection of either microcystin LR or okadaic acid mimicked the 5-HT-induced inward current and occluded the effect of further application of 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple second messenger routes enhance two high-voltage-activated calcium currents in molluscan neuroendocrine cells

Two types of high-voltage-activated calcium currents were identified in whole-cell voltage-clamp recordings of the neuroendocrine caudodorsal cells, which control egg-laying in the freshwater snail Lymnaea stagnalis. The currents were: (i) a rapidly inactivating high-voltage-activated current, with an activation threshold of -40 mV and maximal amplitude at +10 mV; and (ii) a slowly inactivating high-voltage-activated current, with a threshold of -10 mV and a peak at +30 mV. Both currents were reduced by nifedipine and verapamil, but not by omega-conotoxin GVIA, suggesting that they belong to the L-type family of calcium currents. The voltage-dependence of inactivation of the rapidly inactivating high-voltage-activated current was bell-shaped. Time-constants of inactivation ranged from 10 to 25 ms. Steady-state inactivation was characterized by a potential of half maximal inactivation of -21.7 +/- 3.4 mV and a slope factor of 8.1 +/- 1.7 mV. The voltage-dependence of inactivation of the slowly inactivating high-voltage-activated current was S-shaped. Time-constants of inactivation increased with depolarization up to a maximum of 300 ms. The steady-state inactivation parameters were a potential of half maximal inactivation of +6.8 +/- 2.2 mV and a slope factor of 6.0 +/- 1.1 mV. The membrane-permeable analog of cAMP, 8-chlorophenylthio-cyclic AMP, predominantly increased the slowly inactivating high-voltage-activated current, and shifted its voltage-dependence of activation and inactivation 10 mV to the left. The rapidly inactivating high-voltage-activated current was slightly increased by 8-chlorophenylthio-cyclic AMP. 8-Bromo-cyclic GMP and the phorbol ester, 12-O-tetradecanoyl-13-phorbol acetate, had qualitatively similar effects. Both agents enhanced the rapidly inactivating current and, to a lesser degree, the slowly inactivating current, without affecting their voltage-dependence. The cyclic AMP-dependent protein kinase inhibitor, Walsh inhibitor peptide, antagonized the stimulating effect of 8-chlorophenylthio-cyclic AMP. The broad-spectrum protein kinase inhibitor 1-(5-isoquino-linylsulfonyl)-2-methyl-piperazine (H-7) strongly attenuated the effects of 8-chlorophenylthio-cyclic AMP, 8-bromo-cyclic GMP and 12-O-tetradecanoyl-13-phorbol acetate, suggesting that all treatments increase both types of high-voltage-activated calcium currents through phosphorylation of the channel-complex.

Inactivation of the Ba2+ current in dissociated Helix neurons: voltage dependence and the role of phosphorylation

The rate of inactivation of the voltage-dependent Ba2+ current in dissociated neurons from the snail Helix aspersa was found to be modulated by phosphorylation. Conditions were chosen such that the most likely mechanism of inactivation of the Ba2+ current was a voltage-dependent/calcium-independent inactivation process. If adenosine-triphosphate (ATP) was not included in the patch electrode filling solution, or if alkaline phosphatase was added, the Ba2+ current rapidly ran down and the rate of inactivation greatly increased with time. Dialysis with either ATP gamma S or the phosphatase inhibitor okadaic acid (OA) either enhanced the amplitude or greatly reduced the rate of run-down of the Ba2+ current (depending upon the presence of ATP), as well as reducing the rate of inactivation. However, dialysis with either the catalytic subunit of the cyclic-adenosine-mono-phosphate-dependent protein kinase (cAMP-PK), a synthetic peptide inhibitor of this enzyme, or staurosporine (a potent inhibitor of protein kinase C), did not have any significant effect on the amplitude or kinetics of the Ba2+ current. Surprisingly, dialysis with a peptide inhibitor (CKIP) of the Ca2+/calmodulin-dependent protein kinase II (Ca(2+)-CaM-PK) significantly reduced the rate of inactivation of this current. These results suggest that phosphorylation may exert its effect by modulating the gating properties of the Ca2+ channels.

Novel suppression of an L-type calcium channel in neurones of murine dorsal root ganglia by 2,3-butanedione monoxime

1. Voltage-activated currents through calcium channels in primary cultures of murine dorsal root ganglion cells (DRG) were studied with the whole-cell and cell-attached patch recording techniques. 2. The chemical phosphatase 2,3-butanedione monoxime (BDM) reversibly reduced the amplitude of L-type calcium current (ICa) in a dose-dependent manner; at a concentration of 20 mM, BDM caused a 47% suppression of ICa. 3. Application of 10 mM-8-bromo-cyclic AMP or 50 microM-isoprenaline onto DRG treated with BDM completely restored ICa to the pre-BDM level. 4. In striking contrast, bath application of Bay K 8644 (0.5-5 microM) had no effect on the BDM-suppressed ICa. As expected, Bay K 8644 alone caused a two- to threefold increase of the maximal ICa and shifted its I-V relationship to the left. Interestingly, if a cell was first exposed to Bay K 8644 further treatment with 20 mM-BDM resulted in 100% suppression of ICa. This suggests that Bay K 8644 changes the conformation of the calcium channel to one which is more sensitive or more accessible to the action of the phosphatase. 5. Pre-treatment of DRG with an activator of protein kinase C, 12-O-tetradecanoyl-phorbol-13-acetate, did not antagonize BDM's effect on ICa. 6. The depressant action of BDM on ICa was distinct from that of nifedipine in that it did not exhibit use dependence. 7. When single calcium channel currents were recorded in cell-attached patches (barium as the charge carrier), bath application of BDM reduced the percentage of time that the channel spent in the open state. 8. Superfusion with 8-bromo-cyclic AMP restored the ensemble macroscopic 'ICa' to the pre-BDM amplitude. This was due to a dramatic enhancement of the frequency of channel openings. 9. We suggest that BDM acts through the cytoplasm to alter cyclic AMP-dependent protein kinase modulation of neuronal L-type calcium channels. The brief, high-frequency openings which 8-bromo-cyclic AMP activates in the presence of BDM may reflect a rapid phosphorylation-dephosphorylation sequence which controls channel gating.

Parathyroid hormone enhances calcium current in snail neurones--simulation of the effect by phorbol esters

Effects of parathyroid hormone substance (PTH) on the voltage-activated calcium current (ICa) were studied on intracellularly perfused neurones of the snail, Helix pomatia, under voltage-clamp conditions. Application of 0.1 nM PTH produced a marked potentiation of the current. The effect developed slowly (60-70 min) and remained after removal of PTH. Potentiation could be observed in most neurones, but varied considerably from cell to cell; in some neurones ICa was increased 2- to 3-fold. Addition of ethylenebis(oxonitrilo)tetraacetate (EGTA, 10 mM) to, or removal of adenosine 5'-triphosphate (ATP, 2 mM) from the intracellular perfusing solution resulted in a suppression or attenuation of the potentiating effect. The effect could be reproduced by the synthetic 1-34 amino acid fragment of PTH. Extracellularly applied protein kinase-C (PK-C) activator phorbol ester phorbol 12-myristate 13-acetate (PMA, 0.1-10 microM) produced a similar slow increase in ICa (up to 1.5- to 2-fold), while its inactive analogue (4 alpha-phorbol ester) had no effect on ICa. The effects of PTH and PMA were not additive. PK-C inhibitors [1-(5-isoquinoline-sulphonyl)-2-methylpiperazine hydrochloride] (H-7, 100 microM) and staurosporine (100 microM) as well as calcium channel antagonists Cd2+, verapamil, nifedipine and nimodipine depressed the effect of PTH. The chloride channel blocker 4,4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS, 1 mM) did not affect the potentiating action of PTH. Activation of the adenylate cyclase system also potentiated ICa in some neurones, but this effect had a different time course and was additive to the effect of PTH.2=

Neuronal selectivity of ATP-sensitive potassium channels in guinea-pig substantia nigra revealed by responses to anoxia

1. Two sub-populations of pars compacta substantia nigra neurones were identified with very different electrophysiological properties and rostral-caudal distribution. Both cell types were identified by biocytin intracellular dye injection and found to be located within pars compacta containing tyrosine hydroxylase-positive cells. These sub-populations displayed distinctly different responses to transient anoxia. 2. The first group ('Phasic' neurones) exhibited a low threshold calcium conductance LTS gCa associated with bursts of action potentials, were located at the level of the mammillary bodies and were highly sensitive to anoxia. The second group ('rhythmic' neurones) fired in a rhythmic pattern, were located at the level of the accessory optic tract and were relatively insensitive to anoxia. 3. The anoxic response of phasic cells was characterized by membrane hyperpolarization (mean 12 mV), a decrease in input resistance (mean 36%) and cessation of action potential firing. The axonic response of these neurones was not blocked by TEA (5-10 mM), haloperidol (100 microM), the removal of extracellular calcium or depletion of endogenous dopamine. However, this effect was blocked by both the sulphonylurea tolbutamide (50-500 microM), and also by quinine (100 microM) and could be mimicked by application of diazoxide (1 mM). 4. Rhythmic cells displayed a variable response to anoxia consisting of either modest depolarization, hyperpolarization or no change in membrane potential, in all cases accompanied by little or no change in input resistance. The polarity of the membrane potential shift during anoxia was reversed by TEA (5-10 mM) or the removal of calcium. These cells were also relatively insensitive to diazoxide (1 mM). 5. It is concluded from the neuronal responses to anoxia and the pharmacological modification of these responses, that the ATP-sensitive potassium channel (KATP channel) is functionally operative in the substantia nigra and is primarily distributed on the phasically discharging cells of the rostral pars compacta. The relevance of this recently discovered ionic channel is discussed with regard to the normal and abnormal functioning of the substantia nigra.

Concentration-related effects of extracellular application of ATP on the action potential and membrane currents of the guinea-pig vas deferens

The ionic mechanisms underlying concentration-related alterations in the action potential configuration caused by ATP were studied using preparations of the guinea-pig vas deferens voltage-clamped by a double sucrose gap method. Under current-clamp conditions, ATP at concentration of 1.6 microM enhanced the rates of rise and of repolarisation of the action potential whereas at concentration of 1.6 mM it reduced both rates. Under voltage-clamp conditions, lower concentrations increased the maximum inward Ca current without altering kinetics or reversal potential. Higher concentrations reduced the maximum inward Ca current with slowing of rates of activations and inactivation, but also caused a negative shift in reversal potential without affecting conductance. These results suggest that a low ATP concentration activates the voltage-dependent Ca current channels and that the action of a high ATP concentration is related to the internal Ca ion concentration.

Muscarinic enhancement of the voltage-dependent calcium current in an identified snail neuron

1. In the F1 neuron of the snail Helix aspersa bathed in a Ba2+ and 4-aminopyridine-containing saline, carbamylcholine (CCh) enhanced the inward current carried by Ba2+ through the voltage-dependent Ca2+ channels. 2. This effect of CCh on the F1 neuron was not affected by the nicotinic antagonists (+)-tubocurarine and hexamethonium, but it was mimicked by oxotremorine and blocked by both atropine and pirenzepine. 3. The intracellular injection of GTP gamma S (guanosine 5'-O-(3- thiotriphosphate] into the F1 neuron caused both a decrease in Ca2+ current and a blockade of the CCh-induced enhancement of the Ca2+ current. 4. Neither cyclic AMP, cyclic GMP nor arachidonic acid mimicked the effect of CCh on the Ca2+ current in the F1 neuron. In contrast, the intracellular injection of EGTA blocked the CCh-induced enhancement of the Ca2+ current thus suggesting that cytosolic Ca2+ is involved in the CCh-induced response. 5. We then investigated the possible role of inositol 1,4,5-trisphosphate (InsP3) and Ca(2+)-dependent protein kinases in the CCh-induced enhancement of the Ca2+ current. The intracellular injection of InsP3 in the F1 neuron elicited no consistent change in the Ca2+ current. Diacylglycerol analogues (OAG and DOG) decreased the Ca2+ current amplitude, i.e. an effect opposite to that produced by CCh. This effect of the diacylglycerol analogues resulted from the activation of protein kinase C (PKC) since it was blocked by staurosporine. In addition, staurosporine did not affect the CCh-induced increase in Ca2+ current. 6. The intracellular injection of either Ca(2+)-calmodulin-dependent protein kinase II (Ca(2+)-CaM-PK) or a peptide inhibitor of this enzyme into the F1 neuron affected neither the Ca2+ current nor its enhancement by CCh. 7. We conclude that the CCh-induced enhancement of the Ca2+ current in the snail F1 neuron involves the activation via muscarinic receptors of an intracellular transduction mechanism in which cytosolic Ca2+ plays a key role. However, InsP3, protein kinase C and Ca(2+)-CaM-PK do not appear to be directly involved in this CCh-induced response.

Activation process of calcium-dependent potassium channel in Euhadra neurons: involvement of calcium/calmodulin and subsequent protein phosphorylation

1. The activation process of Ca(2+)-dependent potassium channel was studied electrophysiologically and pharmacologically using identified neurons of the land snail, Euhadra peliomphala. 2. Ca(2+)-mediated delayed outward K current (IKD) was dose-dependently reduced by the calmodulin inhibitors, N-(6-aminohexyl)-1-naphthalenesulfonamide (W-5, week) and N-(6-aminohexyl)-5-chloro-naphthalenesulfonamide (W-7, potent). These antagonists also caused a slight membrane depolarization and increase in impulse discharge frequency with decrease in the amplitude of both action potential and after hyperpolarization. 3. The cAMP-dependent protein kinase inhibitor N-[2-(methylamino) ethyl]-5-isoquinoline-sulfonamide (H-8) did not produce any significant effect on IKD and membrane potential. 4. Calmodulin, when injected into the neuron which had been treated with either W-5 or W-7, transiently restored the suppressed IKD nearly to the pretreatment level, and caused hyperpolarization of the cell. In contrast, calcium chloride, intracellularly injected in the same way, had little effect on both the IKD and the membrane potential shifted by these antagonists. 5. Intracellular injection of kinase II, a Ca2+/calmodulin-dependent protein kinase, caused an increase in the IKD and membrane hyperpolarization. Similar but weak effects were produced when a catalytic subunit (CS) of cAMP-dependent protein kinase was intracellularly injected. However, the neurons pretreated with W-7 no longer had any detectable increase in the IKD and hyperpolarization of the membrane. 6. These results suggest the possibility that Ca2+/camodulin-dependent protein phosphorylation may finally mediate the activation of a certain number of potassium channels.

Role of cyclic adenosine monophosphate in simple forms of plasticity in the edible snail

The participation of the adenylate cyclase system in the short-lived changes in the efficiency of synaptic transmission of a functionally identified synapse of the edible snail has been investigated. It was established that imidazole (a phosphodiesterase activator) in a concentration of 5 mM and tolbutamide (an inhibitor of cAMP-dependent phosphorylation) in a concentration of 2 mM do not alter the rate of the depression of EPSPs which is elicited by rhythmic stimulation at a frequency of 0.1 Hz, and do not block heterosynaptic facilitation. At the same time, both of these substances decrease the amplitude of EPSPs. The possibility of the modulation of the efficiency of synaptic transmission through the adenylate cyclase system is discussed.

Parallel processing and selection of the responses to serotonin during reinnervation of an identified leech neuron

In an attempt to define the mechanism of synaptic specificity, we have been studying pairs of identified leech neurons isolated in tissue culture. The cultured neurons reform specific synapses when paired with appropriate partners in the absence of other cell types. In recent studies, we have examined in detail the reformation of a serotoninergic synapse between the Retzius cell and one of its targets, the pressure sensitive (P) cell. The P cell in vivo and its soma in vitro have two types of responses to serotonin (5-HT). From voltage clamp analysis of cultured P cells, we demonstrated the parallel activation of chloride (gCls) and monovalent cation (gCations) channels coupled to distinct receptor subtypes and gated by separate second messengers. Only gCls was activated by 5-HT released from the presynaptic Retzius cell both in vivo and in vitro. This demonstrates the remarkable specificity of the reformation of this synapse in culture since only the correct 5-HT receptor subtype is activated. An 80% reduction of gCations was observed in P cells that had failed to be innervated by Retzius cells in culture, suggesting that gCations may be lost prior to synapse formation. Retzius cells depleted of 5-HT also reduced gCations in the paired P cells and incubating single P cells in 5-HT did not reduce gCations. In addition, aldehyde-fixed Retzius cells were able to selectively reduce gCations when paired with P cells. We conclude that the loss of gCations was due to contact between the neurons. The early clearing of counter-effective receptor subtypes may be a prelude to synapse formation.

Calcium, neuronal hyperexcitability and ischemic injury

Due to tight regulatory controls, a 10,000-fold concentration gradient exists between intracellular and extracellular free Ca2+ concentrations. With appropriate stimulus Ca2+ will rapidly flow into neurons through various types of membrane channels including voltage-dependent and receptor-operated channels. Intracellular Ca2+ concentrations are then quickly restored primarily through Ca2+-ATPase, Na+/Ca2+ exchange, and endoplasmic reticulum sequestration. It is well-known that Ca2+ is essential for neurotransmitter release. More recent investigations indicate that Ca2+ influx is essential for neuronal excitability independent from synaptic function. In fact, abnormal Ca2+ metabolism may play a dominant role in both the initiation and propagation of seizure discharge. Accordingly, Ca2+ channel blockers may represent a new therapeutic modality to treat epilepsy. Analyzed in this article are the major mechanisms by which neurons control Ca2+ fluxes and the evidence supporting the role of Ca2+ in seizure phenomena. Thereafter, an integrative theory for the role of calcium in neuronal hyperexcitability and ischemic cell death is constructed.

Modulation of K channels in dialyzed squid axons. ATP-mediated phosphorylation

In squid axons, internally applied ATP potentiates the magnitude of the potassium conductance and slows down its activation kinetics. This effect was characterized using internally dialyzed axons under voltage-clamp conditions. Both amplitude potentiation and kinetic slow-down effects are very selective towards ATP, other nucleotides like GTP and ITP are ineffective in millimolar concentrations. The current potentiation Km for ATP is near 10 microM with no further effects for concentrations greater than 100 microM. ATP effect is most likely produced via a phosphorylative reaction because Mg ion is an obligatory requirement and nonhydrolyzable ATP analogues are without effect. In the presence of ATP, the K current presents more delay, resembling a Cole-Moore effect due to local hyperpolarization of the channel. ATP effect induces a 10-20 mV shift in both activation and inactivation parameters towards more depolarized potentials. As a consequence of this shift, conductance-voltage curves with and without ATP cross at approximately -40 mV. This result is consistent with the hyperpolarization observed with ATP depletion, which is reversed by ATP addition. At potentials around the resting value, addition of ATP removes almost completely K current slow inactivation. It is suggested that a change in the amount of the slow inactivation is responsible for the differences in current amplitude with and without ATP, possibly as a consequence of the additional negative charge carried by the phosphate group. However, a modification of the local potential is not enough to explain completely the differences under the two conditions.

Calcium channel regulation by calcineurin, a Ca2+-activated phosphatase in mammalian brain

The enzymatic addition or removal of phosphate esters on serine and threonine hydroxyls alters the activity of many proteins that contribute to the characteristic structure and function of nerve cells. Recently, calcineurin, a major calmodulin-binding protein in mammalian brain, has been purified and identified as a Ca2+-activated protein phosphatase. Preliminary experiments suggest that calcineurin may limit Ca2+ influx through dihydropyridine-sensitive Ca2+ channels in the plasma membrane by dephosphorylating the channel, or a closely associated protein, and inactivating it.

Diversity of calcium ion channels in cellular membranes

Successful introduction of techniques for separation of different ionic currents and recording of single channel activity has demonstrated the diversity of membrane structures responsible for generation of calcium signal during various forms of cellular activity. In excitable cells the electrically-operated calcium channels have been separated into two types functioning in different membrane potential ranges (low- and high-threshold ones). The low-threshold channels are ontogenetically primary and may play a role in regulation of cell development and differentiation. A similar function may also be characteristic of chemically-operated channels in some highly specialized cells (lymphocytes). The high-threshold channels in excitable cells generate an intracellular signal coupling membrane excitation and intracellular metabolic processes responsible for specific cellular reactions (among them retention of traces of previous activity in neurons--"learning"--being especially important). Chemically-operated N-methyl-D-aspartate-channels also participate in this function. The calcium signal can be potentiated by activation of calcium-operated channels in the membranes of intracellular structures, resulting in the liberation of calcium ions from the intracellular stores. Although different types of calcium channels have some common features in their structure which may indicate their genetic similarity, their specific properties make them well suited for participation in a wide range of cellular mechanisms.

Inactivation of calcium channels

Rapid progress in our understanding of the properties and functions of voltage-gated calcium channels had produced the need for an update to our previous review of calcium inactivation. The major elements of change included in this review are: 1. The existence of multiple forms of voltage-sensitive Ca+ channels, with distinctive single channel properties, thus necessitating a reappraisal of properties deduced from macroscopic current recordings, particularly of the processes of activation and inactivation. 2. The differences in biochemical properties between channel types are reflected in their differences in divalent selectivity, their requirement for metabolic maintenance and their mechanism of inactivation. These properties appear to divide the channels into two categories which may relate to their molecular structures. Further subgroupings, based upon the voltage thresholds, have also been observed. 3. Molecular properties of one class of channels have been elucidated, which correlate with the observed biochemistry of channel modulation and inactivation. 4. An enzymatic process underlying the mechanism of Ca2+-dependent inactivation has been elucidated and may serve as a model for other modulatory systems. The interweaving of the properties of these Ca2+ channels, with their spatial distributions and their influence upon other channel types, acts to transduce and integrate information within cells.

"Run-down" of the Ca current during long whole-cell recordings in guinea pig heart cells: role of phosphorylation and intracellular calcium

We examined by a statistical approach the decrease of the Ca current ("run-down") during long-lasting recordings with the whole-cell patch-clamp technique in guinea pig ventricular myocytes. The results are as follows. (1) Run-down of the Ca current (ICa) occurs in three phases (T1-T3). T1 (38 +/- 19 min, n = 135) and T3 (35 +/- 17 min, n = 23) are characterized by a slow rate of decay of ICa [90 +/- 20 and 60 +/- 20 nA.cm-2.min-1, respectively]. T1 and T3 are separated by T2 (6 +/- 4 min, n = 135) during which the current decays quickly [1200 +/- 230 nA.cm-2.min-1]. Between the onsets of T1 and T3, ICa decreases from 11 +/- 3 to 3.5 +/- 1 microA/cm2. (2) Normalized current-voltage relationship, reversal potential and voltage-dependencies of steady-state activation and inactivation of ICa are globally shifted toward more negative potentials during the run-down process by 10-15 mV. (3) ICa3 measured during T3 retains the pharmacological properties (blockade by D600, NiCl2 and CoCl3, increase by isoprenaline and insensitivity to tetrodotoxin) of the original ICa. (4) Intracellular perfusion of the nonhydrolysable ATP analogue AMP-PNP does not prevent the occurrence of T2, suggesting that a phosphorylation-dephosphorylation process is not involved in the fast run-down of ICa. (5) With 0.1 mM EGTA in the pipette, addition of 3 mM ATP significantly prolongs ICa survival. No improvements are obtained by increasing the ATP concentration to 10 mM or replacing ATP with creatine phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Reciprocal modulation of calcium current by serotonin and dopamine in the identified Aplysia neuron R15

Voltage-clamp methods were employed to study the effects of serotonin (5-HT) and dopamine on the pharmacologically isolated calcium current in the identified Aplysia neuron R15 grown in cell culture. Neurons were obtained from juvenile animals and had not yet developed the bursting pacemaker pattern of activity characteristic of R15 in mature animals. In R15 5-HT elicits a biphasic response consisting of excitatory depolarization followed by an inhibitory hyperpolarization and dopamine elicits an inhibitory hyperpolarization. 5-HT increased the Ca2+ current without affecting its voltage dependence. The 5-HT effect persisted when Ba2+ was employed to carry current through Ca2+ channels. 5-HT did not affect the rate of Ca2+-dependent Ca2+ current inactivation other than through its effect on the magnitude of the Ca2+ current. The adenylate cyclase activator forskolin, in the presence of a phosphodiesterase inhibitor, also increased the magnitude of the Ca2+ or Ba2+ current. This result suggested that the 5-HT-induced enhancement of Ca2+ current was mediated by cAMP. Dopamine inhibited Ca2+ current when either Ca2+ or Ba2+ was employed as the current carrier. Dopamine did not affect the rate of Ca2+-dependent inactivation of Ca2+ current other than through its effect on the magnitude of the Ca2+ current. Intracellular injection of the Ca2+ chelator EGTA inhibited serotonergic modulation of the Ca2+ current but not dopaminergic modulation. These results indicated that the putative neurotransmitters 5-HT and dopamine may regulate bursting activity in mature R15 neurons through modulation of Ca2+ current.

Dual actions of phorbol esters to decrease calcium and potassium conductances of mouse neurons

Phorbol esters, which substitute for diacylglycerol to activate protein kinase C, were applied to mouse dorsal root ganglion and cerebral hemisphere neurons in cell culture. The phorbol esters, phorbol 12,13-dibutyrate and 12-O-tetradecanoyl-phorbol-13-acetate, prolonged calcium-dependent action potential duration at resting membrane potential and at more negative membrane potentials but decreased action potential duration following membrane depolarization to less than -45 mV. When calcium and potassium currents were recorded using the single electrode voltage-clamp technique, the phorbol esters were shown to reduce both voltage-dependent calcium and potassium currents. These studies have demonstrated directly that phorbol esters, presumably by activating protein kinase C, can modify more than one membrane conductance in individual neurons.

Modulation of ionic currents in smooth muscle balls of the rabbit intestine by intracellularly perfused ATP and cyclic AMP

The effects of intracellularly perfused ATP and cyclic-AMP (c-AMP) on ionic currents recorded from fragmented smooth muscle cells (smooth muscle ball; SMB) were investigated, using the single electrode whole cell voltage clamp method. The Ca2+ current was distinguished from K+ currents, using pipette solution containing Cs+, TEA+ and 4 mM EGTA. ATP enhanced the Ca2+ current dose-dependently between 0.3 and 10 mM, and slightly slowed the slow component of the decay of the Ca2+ current, while the steady-state inactivation curve remained unaffected. Intracellular application of 5'-adenylyl-imidodiphosphate (AMP-PNP; 1 mM) inhibited the Ca2+ current by competition with ATP, but c-AMP (up to 300 microM) had no effect. With a high-K+ solution containing 0.3 mM EGTA and ATP in the pipette and physiological salt solution in the bath, a net inward current with transient (Ca2+ dependent) and delayed (Ca2+ independent) K+ outwart currents were evoked. Increased concentrations of ATP (above 1 mM) but not c-AMP (up to 100 microM) in the pipette enhanced the transient K+ outward current. Neither agent had any effect on the delayed outward current. When repetitive stimulations of intervals shorter than 5 s were applied, the amplitude of the transient outward current was markedly reduced, and 100 microM c-AMP partially prevented this attenuation. ATP may act on the Ca2+ channel either by phosphorylating the channel protein or by other ATP requiring mechanisms, independently from those induced by the action of c-AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of an alpha subunit of dihydropyridine-sensitive brain calcium channels

Voltage-sensitive calcium channels in different tissues have diverse functional properties. Polyclonal antibodies (PAC-2) against the alpha subunits of purified rabbit skeletal muscle calcium channels immunoprecipitated calcium channels labeled with the dihydropyridine PN200-110 from both skeletal muscle and brain. The immunoreactivity of PAC-2 with the skeletal muscle channel was greater than that with the brain calcium channel and was absorbed only partially by prior treatment with the brain channel. PAC-2 specifically recognized a large peptide in synaptic plasma membranes of rabbit brain with an apparent molecular size of 169,000 daltons. This protein resembles an alpha subunit of the skeletal muscle calcium channel in apparent molecular weight, antigenic properties, and electrophoretic behavior after reduction of disulfide bonds. Thus, the dihydropyridine-sensitive calcium channel of rabbit brain has an alpha subunit that is homologous, but not identical, to those of the skeletal muscle calcium channel. The different functional properties of these two calcium channels may result from minor variations in structurally similar components.

Neuronal phosphoproteins. Mediators of signal transduction

This article summarizes some of our knowledge concerning intracellular protein phosphorylation pathways in nerve cells. It also summarizes, very briefly, recent direct experimental evidence involving intracellular injection of protein kinases, protein kinase inhibitors, and substrates, indicating that protein phosphorylation mediates the actions of a variety of neurotransmitters on their target cells. Finally, it summarizes in somewhat greater detail the results of studies of three different types of substrate proteins that appear to regulate different types of biological responses in nerve cells: synapsin I, a substrate protein present in virtually all nerve terminals, which appears to regulate neurotransmitter release from those nerve terminals; the acetylcholine receptor, the phosphorylation of which regulates its rate of desensitization in the presence of acetylcholine; and DARPP-32, the phosphorylation of which converts it into a very potent phosphoprotein phosphatase inhibitor that may be involved in the regulation by the neuromodulator dopamine of the effects of the neurotransmitter glutamate. The identification and characterization of additional neuronal phosphoproteins can be expected to lead to the clarification of numerous additional molecular mechanisms by which signal transduction is carried out in nerve cells.

Dual effects of ATP on K+ currents of mouse pancreatic beta-cells

K+ currents through ATP-dependent channels were recorded from inside-out patches of beta-cell membrane as previously described (Rorsman and Trube 1985). Channels were opened by removing ATP from the intracellular side of the membrane. The open probability and/or the number of active channels declined spontaneously ("run-down") when ATP was absent for periods longer than about 30 s. Channels subject to the run-down could be activated again after applying a blocking concentration (greater than 0.1 mM) of ATP in presence of 1 mM MgCl2 for at least 2 min. ATP in absence of Mg and the ATP-analogues AMP-PNP, AMP-PCP and ATP gamma S were ineffective in reactivating the channels. This suggests that phosphorylation of the channels or associated proteins or hydrolysis of ATP may be necessary for keeping the channels available. In contrast to the differential effects on the run-down, ATP in presence and absence of Mg and the ATP analogues were similarly effective in blocking the channels at concentrations above 0.1 mM. Using an experimental protocol avoiding the run-down the dose-inhibition curve for ATP was found to reach 50% at 18 microM.

Dephosphorylation of synaptosomal proteins P96 and P139 is regulated by both depolarization and calcium, but not by a rise in cytosolic calcium alone

Depolarization of intact synaptosomes activates calcium channels, leads to an influx of calcium, and increases the phosphorylation of several neuronal proteins. In contrast, there are two synaptosomal phosphoproteins labeled in intact synaptosomes with 32Pi, termed P96 and P139, which appear to be dephosphorylated following depolarization. Within intact synaptosomes P96 was found in the cytosol whereas P139 was present largely in membrane fractions. Depolarization-stimulated dephosphorylation was fully reversible and continued for up to five cycles of depolarization/repolarization, suggesting a physiological role for the phenomenon. The basal phosphorylation of these proteins was at least partly regulated by cyclic AMP, since dibutyryl cyclic AMP produced small but significant increases in P96 and P139 labeling, even in the presence of fluphenazine at concentrations that inhibited calcium-stimulated protein kinases. Depolarization-dependent dephosphorylation was independent of a rise in intracellular calcium, since agents such as guanidine and low concentrations of A23187, which increase intracellular calcium without activating the calcium channel, did not initiate P96 or P139 dephosphorylation. These agents did sustain increases in the phosphorylation of a number of other proteins including synapsin I and protein III. The results suggest that the phosphorylation of these two synaptosomal proteins is intimately linked to the membrane potential and that their dephosphorylation is dependent on both the mechanism of calcium entry and calcium itself, rather than simply on a rise in intracellular free calcium.

An enzymatic mechanism for calcium current inactivation in dialysed Helix neurones

'Wash-out' and inactivation of the Ca current were examined in dialysed, voltage-clamped neurones of Helix aspersa under conditions that isolate the Ca current virtually free of other currents. EGTA or other internal Ca2+ chelators were routinely omitted from the dialysate. The time-dependent loss, or wash-out, of Ca current was slowed by addition to the dialysing solution of agents, such as dibutyryl adenosine 3'-5'-cyclic monophosphate (dibutyryl cyclic AMP), Mg adenosine 5'-triphosphate (ATP) and the catalytic subunit of cyclic-AMP-dependent protein kinase, that promote protein phosphorylation and by EGTA. However, neither the phosphorylation-promoting agents nor internal EGTA prevented wash-out entirely, nor did they significantly restore previously 'washed-out' current. With phosphorylating agents in the dialysing solution, the irreversible development of wash-out was greatly reduced by introduction of leupeptin, an inhibitor of protease activity. Thus, the irreversible component of wash-out appears to result from a Ca-dependent proteolytic process. In the presence of leupeptin alone, Ca current amplitude continued to decline: however, the current could be largely or fully restored with addition of catalytic subunit, dibutyryl cyclic AMP, and Mg ATP to the dialysing solution. Thus, inhibition of proteolysis revealed a reversible component of wash-out that appears to result from dephosphorylation. During perfusion with leupeptin, Mg ATP, dibutyryl cyclic AMP and catalytic subunit the Ca current remained stable for up to several hours without addition of internal Ca2+ buffer. The rate of inactivation of the current that occurs during a depolarizing step showed only a very gradual decline during this time. Under these conditions, perfusion with calcineurin, a Ca-calmodulin-dependent phosphatase, caused a significant increase in the rate of Ca current inactivation. This inactivation was virtually eliminated by introduction of EGTA or by replacement of external Ca2+ with Ba2+, which is consistent with the ion dependency for calmodulin-dependent activation of calcineurin. When ATP in the dialysate was replaced with ATP-gamma-S (adenosine 5'-O-(thiotriphosphate], an analogue that donates a thiophosphate group resistant to hydrolysis, the rate of inactivation slowed. Since Ca-dependent inactivation during step depolarizations is enhanced by conditions that promote dephosphorylation, and Ca current wash-out is slowed by conditions that promote phosphorylation, inactivation and reversible wash-out appear to be related.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium channel currents in pars intermedia cells of the rat pituitary gland. Kinetic properties and washout during intracellular dialysis

Ca channel currents in primary cultured pars intermedia cells were studied using whole-cell recording with patch pipettes. Experiments were carried out at 18-21 degrees C in cells internally dialyzed with K-free, EGTA-containing solutions and in the presence of 10 mM Ca or 10 mM Ba in the external solution. Ca and Ba currents depended on the activity of two main populations of channels, SD and FD. With Ca as the charge carrier, these two populations differed in their closing time constants at -80 mV (SD, 1.8 ms; FD, 110 microseconds), apparent activation levels (SD, -40 mV; FD, -5 mV), half-maximal activation levels (SD, +5 to +10 mV; FD, +20 to +25 mV), half-times of activation at +20 mV (SD, 2.5-3.5 ms; FD, 1.0-1.3 ms), and time courses of inactivation (SD, fast; FD, slow). Functional FD channels were almost completely lost within 20-25 min of breaking into a cell, whereas SD channels retained most of their functional activity. In addition, the conductance-voltage curve for FD channels shifted approximately 15 mV toward more negative membrane potentials within 11-14 min under whole-cell recording. At that time, 60-70% of the FD channel maximum conductance was lost. However, the conductance-voltage curve for SD channels shifted less than 5 mV within 25 min. The addition of 3 mM MgATP and 40 microM GTP to the internal solution slowed down the loss of FD channels and prevented the shift in their activation curve. It was also found that the amplitude of the current carried by FD channels tends to increase as a function of the age of the culture, with no obvious changes in the kinetic properties of the channels or in SD channel activity.

Action potentials, macroscopic and single channel currents recorded from growth cones of Aplysia neurones in culture

Action potentials, macroscopic ionic currents and single channel currents were recorded from growth cones of Aplysia right upper quadrant (r.u.q.) cells in culture, using the patch-clamp technique. Recordings were obtained from both intact growth cones and from growth cones that had been mechanically isolated from the rest of the neurone. In current-clamp mode, greater than half of the isolated growth cones display an all-or-none action potential when depolarized above 0 mV with outward current pulses. The remaining growth cones display only a graded depolarization that is unaffected by tetrodotoxin (TTX). In whole-cell voltage clamp almost all isolated growth cones display a rapidly activating and inactivating inward current followed by a delayed outward current in response to depolarizations positive to -20 mV. The rapid inward current reverses direction at around +70 to +80 mV and is completely suppressed by 100 microM-TTX, which suggests that this current is carried by the fast Hodgkin-Huxley sodium current channels. The delayed outward current appears to result from the activation of both the delayed rectifier potassium current, IK, and the calcium-activated potassium current, IC. The growth cones do not display any prominent early transient outward current, IA. The sodium current, INA, was studied in isolation by substituting caesium for potassium ions in the pipette solution. INa is half-inactivated at a holding potential of -36 mV, reaches half-maximal activation with a depolarization to 0 mV, and has a mean peak current density of 13 microA/cm2. The time course of inactivation is well described by a single exponential (tau = 3 ms at 0 mV). In cell-attached patches, a rapidly activating and inactivating inward current channel was recorded with an average unit conductance of 6.9 pS. The activation and inactivation parameters of the ensemble averaged current closely match the measured values from the macroscopic sodium current. At very positive potentials we recorded a voltage-dependent outward current channel with a conductance of around 35 pS. No significant inward calcium current was observed in whole-cell measurements and few single calcium channel currents were measured in cell-attached patches, suggesting a sparse distribution of calcium channels in the r.u.q. growth cones.

Calcium-dependent currents in cultured rat dorsal root ganglion neurones are inhibited by an adenosine analogue

Dorsal root ganglion neurones from 2-day-old rats were grown in dissociated culture. The effect of the adenosine analogue 2-chloroadenosine (2-CA) was investigated on action potential duration and on Ca2+ current (ICa) activation. 2-CA (0.5 microM) shortened both control action potentials and those prolonged by tetraethylammonium (TEA), Ba2+, or intracellular Cs+. This effect was prevented by two adenosine antagonists isobutylmethylxanthine (IBMX, 1-2 mM) and 8-phenyltheophylline (8-PT, 2.5 microM). The inward current, ICa, recorded using the 'whole-cell' patch-clamp technique in medium containing 2.5 microM-tetrodotoxin, 25 mM-TEA and 2.5 mM-Ba2+ was reduced by 2-CA (0.05 microM). The activation of ICa was decreased, but its reversal potential was unchanged. The effect of 2-CA was antagonized by IBMX (1 mM) and 8-PT (1 microM). 2-CA also reduced the large inward tail currents which occurred at the termination of the depolarizing voltage step command in a proportion of neurones. Brief application of 2-CA (0.05 microM) did not affect the inward holding current required to maintain the cells at -80 mV. In the presence of TTX (2.5 microM) and Ca2+ (5 mM), 2-CA decreased the activation of outward K+ currents caused by 5 s depolarizing voltage commands from -80 mV or -40 mV. The GABAB agonist (-)-baclofen (50-100 microM) also shortened the action potential duration and reduced ICa. 8-PT (1 microM) did not prevent the effect of baclofen on ICa. It is concluded that in cultured rat dorsal root ganglion neurones 2-CA reduces ICa activation by a direct effect on an A1 adenosine receptor.

Apparent loss of calcium-activated potassium current in internally perfused snail neurons is due to accumulation of free intracellular calcium

Internal perfusion of Helix neurons with a solution containing potassium aspartate, MgCl2, ATP, and HEPES causes the calcium-activated potassium current (IK(Ca)) evoked by depolarizing voltage steps to decrease with time. When internal free Ca++ is strongly buffered to 10(-7) M by including 0.5 mM EGTA and 0.225 mM CaCl2 in the internal solution, IK(Ca) remains constant for up to 3 hours of perfusion. In cells where IK(Ca) is small at the start of perfusion, perfusion with the strongly buffered 10(-7) M free Ca++ solution produces increases in IK(Ca) which ultimately saturate. In cells perfused with solutions buffered to 10(-6) M free Ca++, IK(Ca) is low and does not change with perfusion. These results lead us to conclude that IK(Ca) is stable in perfused Helix neurons and that the apparent loss of IK(Ca) seen initially with perfusion is due to accumulation of cytoplasmic calcium. Since the calcium current (ICa) provides the Ca++ which activates IK(Ca) during a depolarizing pulse, ICa is also stable in perfused cells when free intracellular Ca++ is buffered. Perfusion with 1 microM calmodulin (CaM) produces no effect on IK(Ca) with either 10(-7) or 10(-6) M free internal calcium. Inhibiting endogenous CaM by including 50 microM trifluoperazine (TFP) in both the bath and the internal perfusion solution also produces no effect on IK(Ca) with 10(-7) M free internal calcium. It is concluded that CaM plays no role in IK(Ca) activation.

Intracellular factors for the maintenance of calcium currents in perfused neurones from the snail, Lymnaea stagnalis

Isolated nerve cell bodies from Lymnaea stagnalis were internally perfused and voltage-clamped. The magnitude of the Ca2+ current was monitored while perfusing with various intracellular solutions. When the intracellular perfusate was unenriched (containing only inorganic ions, 100 mM-HEPES and 5 mM-EGTA), the Ca2+ current was found to 'wash out', falling to half of its maximum value approximately 30-40 min from the beginning of perfusion. Stopping the flow of the perfusing solution increased this half-time to more than 50 min. The current-voltage relationship changed only slightly during wash-out. The addition of 2 mM-ATP and 1 mM-Mg2+ to the internal perfusate prevented, and even reversed, wash-out of the Ca2+ current. Both ATP and Mg2+ were necessary for maximal effect. Such current loss as occurred in the presence of ATP and Mg2+ was associated with a decrease in the capacitance of the cell and probably resulted from membrane being pulled into the pipette. The rate of inactivation of the Ca2+ current increased during perfusion with an unenriched internal solution, but decreased to initial values when ATP and Mg2+ were added to the internal perfusate. Although intracellular Mg2+ was necessary for the prevention of wash-out, levels higher than 1 mM had a blocking effect on the Ca2+ current. Certain factors that promote cyclic AMP-dependent protein phosphorylation (internal: cyclic AMP, theophylline and catalytic subunit of cyclic AMP-dependent protein kinase; external: dibutyryl cyclic AMP, 8-bromo cyclic AMP and forskolin) had no effect on the magnitude of the Ca2+ current in cells perfused with ATP and Mg2+. Externally applied theophylline blocked the Ca2+ current. The mechanism through which ATP and Mg2+ act to prevent wash-out of the Ca2+ current may be to enhance the ability of the cell to lower the Ca2+ concentration near the inner surface of the plasma membrane. This would prevent both the reversible block of Ca2+ current by intracellular Ca2+ and an irreversible loss of current due to high levels of intracellular Ca2+.

Phosphorylation of ion channels

The introduction of highly specific reagents such as enzymes and inhibitors directly into living cells has proven to be a powerful tool in studying the modulation of cellular activity by protein phosphorylation. The use of exogenous kinases can be thought of as a pharmacological approach: this demonstrates that phosphorylation can produce modulation, but does not address the question of whether the cell actually uses this mechanism under normal physiological conditions. The complementary approach, the introduction of highly specific inhibitors such as R subunit or PKI, does ask whether endogenous kinase activity is necessary for a given physiological response. Together these two approaches have provided rather compelling evidence that cAMP-dependent and calcium/phospholipid-dependent protein phosphorylations can regulate membrane excitability. In several cases single-channel analysis has allowed the demonstration that an ion channel itself or something very close to the channel is the phosphorylation target, and it seems reasonable to assume that this will also be the case for many if not all of the other systems described above. Have any general principles emerged from the results to date? Certainly it seems clear that protein phosphorylation regulates not one but many classes of ion channels. As summarized in the Table, different channels can be modulated in different cells, some channels are activated while others are inhibited, and in some cells more than one channel is subject to modulation by phosphorylation. The list in the Table is probably not yet complete, and indeed it is not inconceivable that all ion channels can under appropriate conditions be regulated by phosphorylation. What aspect of channel function is altered by phosphorylation? The total membrane current, I, carried by a particular species of ion channel is given by Npi, where N is the number of active channels in the membrane, p is the probability that an individual channel will be open, and i is the single-channel current. In principle a change in I, the quantity measured in whole cell experiments, could be caused by a change in any one (or more) of the parameters, N, p or i (see Fig. 1). In the two cases in which single-channel measurements have allowed this question to be investigated, changes in N (Shuster et al., 1985) and p (Ewald et al., 1985) have been observed. Here again it seems unlikely that any one mechanism operates in all cases, and it would not be surprising to find that phosphorylation of some other channel results in a change in i.(ABSTRACT TRUNCATED AT 400 WORDS)