ATP and G proteins affect the runup of the Ca2+ current in bovine chromaffin cells

Protein phosphorylation maintains the normal function of cloned human Cav2.3 channels

Ca_v2.3 Ca²⁺ channels are subject to cytosolic regulation, which has been difficult to characterize in native cells. Neumaier et al. demonstrate the role of phosphorylation in the function of these channels and suggest a close relationship between voltage dependence and the phosphorylation state.

R-type currents mediated by native and recombinant Ca_v2.3 voltage-gated Ca²⁺ channels (VGCCs) exhibit facilitation (run-up) and subsequent decline (run-down) in whole-cell patch-clamp recordings. A better understanding of the two processes could provide insight into constitutive modulation of the channels in intact cells, but low expression levels and the need for pharmacological isolation have prevented investigations in native systems. Here, to circumvent these limitations, we use conventional and perforated-patch-clamp recordings in a recombinant expression system, which allows us to study the effects of cell dialysis in a reproducible manner. We show that the decline of currents carried by human Ca_v2.3+β₃ channel subunits during run-down is related to adenosine triphosphate (ATP) depletion, which reduces the number of functional channels and leads to a progressive shift of voltage-dependent gating to more negative potentials. Both effects can be counteracted by hydrolysable ATP, whose protective action is almost completely prevented by inhibition of serine/threonine but not tyrosine or lipid kinases. Protein kinase inhibition also mimics the effects of run-down in intact cells, reduces the peak current density, and hyperpolarizes the voltage dependence of gating. Together, our findings indicate that ATP promotes phosphorylation of either the channel or an associated protein, whereas dephosphorylation during cell dialysis results in run-down. These data also distinguish the effects of ATP on Ca_v2.3 channels from those on other VGCCs because neither direct nucleotide binding nor PIP₂ synthesis is required for protection from run-down. We conclude that protein phosphorylation is required for Ca_v2.3 channel function and could directly influence the normal features of current carried by these channels. Curiously, some of our findings also point to a role for leupeptin-sensitive proteases in run-up and possibly ATP protection from run-down. As such, the present study provides a reliable baseline for further studies on Ca_v2.3 channel regulation by protein kinases, phosphatases, and possibly proteases.

GTP-dependent run-up of Piezo2-type mechanically activated currents in rat dorsal root ganglion neurons

Rapidly adapting mechanically activated channels (RA) are expressed in primary afferent neurons and identified as Piezo2 ion channels. We made whole-cell voltage-clamp recordings from cultured dorsal root ganglion (DRG) neurons to study RA channel regulation. RA currents showed gradual increases in current amplitude (current "run-up") after establishing whole-cell mode when 0.33 mM GTP or 0.33 mM GTPγS was included in the patch pipette internal solution. RA current run-up was also observed in HEK293 cells that heterologously expressed Piezo2 ion channels. No significant RA current run-up was observed in DRG neurons when GTP was omitted from the patch pipette internal solution, when GTP was replaced with 0.33 mM GDP, or when recordings were made under the perforated patch-clamp recording configuration. Our findings revealed a GTP-dependent up-regulation of the function of piezo2 ion channels in DRG neurons.

Modulation by endogenously released ATP and opioids of chromaffin cell calcium channels in mouse adrenal slices

Modulation of high-threshold voltage-dependent calcium channels by neurotransmitters has been the subject of numerous studies in cultures of neurons and chromaffin cells. However, no studies on such modulation exist in chromaffin cells in their natural environment, the intact adrenal medullary tissue. Here we performed such a study in voltage-clamped chromaffin cells of freshly prepared mouse adrenal slices under the whole cell configuration of the patch-clamp technique. The subcomponents of the whole cell inward Ca(2+) current (I(Ca)) accounted for 49% for L-, 28% for N-, and 36% for P/Q-type channels. T-type Ca(2+) channels or residual R-type Ca(2+) currents were not seen. However, under the perforated-patch configuration, 20% of I(Ca) accounted for a toxin-resistant R-type Ca(2+) current. Exogenously applied ATP and methionine-enkephalin (Met-enk) inhibited I(Ca) by 33%. Stop-flow and Ca(2+) replacement by Ba(2+), which favored the release of endogenous ATP and opioids, also inhibited I(Ca), with no changes in activation or inactivation kinetics. This inhibition was partially voltage independent and insensitive to prepulse facilitation. Furthermore, in about half of the cells, suramin and naloxone augmented I(Ca) in the absence of exogenous application of ATP/Met-enk. No additional modulation of I(Ca) was obtained after bath application of exogenous ATP and opioids to these already inhibited cells. Augmentation of I(Ca) was also seen upon intracellular dialysis of guanosine 5'-[β-thio]diphosphate (GDPβS), indicating the existence in the intact slice of a tonic inhibition of I(Ca) in resting conditions. These results suggest that in the intact adrenal tissue a tonic inhibition of I(Ca) exists, mediated by purinergic and opiate receptors.

Presynaptic muscarinic receptor subtypes involved in the enhancement of spontaneous GABAergic postsynaptic currents in hippocampal neurons

We investigated the effects of muscarinic acetylcholine receptor (mAChR) activation on GABAergic synaptic transmission in rat hippocampal neurons. Current-clamp recordings revealed that methacholine produced membrane depolarization and action potential firing. Methacholine augmented the bicuculline-sensitive and GABA(A) -mediated frequency of spontaneous inhibitory postsynaptic currents (sIPSCs); the action of methacholine had a slow onset and longer duration. The increase in methacholine-evoked sIPSCs was completely inhibited by atropine and was insensitive to glutamatergic receptor blockers. Interestingly, methacholine action was not inhibited by intracellular perfusion with GDP-β-S, suggesting that muscarinic effects on membrane excitability and sIPSC frequency are mainly presynaptic. McN-A-343 and pirenzepine, selective agonist and antagonist of the m1 mAChR subtype, respectively, neither enhanced sIPSCs nor inhibited the methacholine effect. However, the m3-m5 mAChR antagonist 4-DAMP, and the m2-m4 mAChR antagonist himbacine inhibited the methacholine effect. U73122, an IP(3) production inhibitor, and 2APB, an IP(3) receptor blocker, drastically decreased the methacholine effect. Recording of miniature events revealed that besides the effect exerted by methacholine on membrane firing properties and sIPSC frequency, muscarinic receptors also enhanced the frequency of mIPSCs with no effect on their amplitude, possibly modulating the molecular machinery subserving vesicle docking and fusion and suggesting a tight colocalization at the active zone of the presynaptic terminals. These data strongly suggest that by activating presynaptic m2, m3, m4 and m5 mAChRs, methacholine can increase membrane excitability and enhance efficiency in the GABA release machinery, perhaps through a mechanism involving the release of calcium from the endoplasmic reticulum.

Depletion of ATP and release of presynaptic inhibition in the CA1 region of hippocampal slices during hypoglycemic hypoxia

Transient recovery (TR) of evoked synaptic potentials and ATP depletion during the late stage of hypoxic hypoglycemic insults were investigated in rat hippocampal slices. TR was observed not only in the late stage of insult, but also during recovery. The concentration of ATP corresponded to the appearance (27% of control) and disappearance (15% of control) of TR. Paired pulse studies showed the presynaptic nature of the release of inhibition of synaptic transmission during TR. Both N- and P/Q-type voltage-dependent calcium channels were involved in the appearance of TR. This evidence suggests that underlying mechanisms of TR appearance during hypoxic hypoglycemic insult might be related to ATP depletion and release of A1 adenosine receptor mediated inhibition of presynaptic voltage-dependent calcium channels.

Direct autocrine inhibition and cAMP-dependent potentiation of single L-type Ca2+ channels in bovine chromaffin cells

Using the cell-attached recording configuration, we found that in adult bovine chromaffin cells there exists a direct membrane-delimited inhibition of single Bay K-modified L-channels mediated by opioids and ATP locally released in the recording pipette. This autocrine modulation is mediated by pertussis toxin (PTX)-sensitive G-proteins and causes a 50 % decrease of the open channel probability (Po) and an equivalent percentage increase of null sweeps at +10 mV with no changes to the activation kinetics, single channel conductance and mean open time. The decrease in Po is mainly due to an increase in the occurrence and duration of slow closed times (> 40 ms). Addition of purinergic and opioidergic antagonists (suramin and naloxone) or cell pre-treatment with PTX removes the inhibition while addition of ATP and opioids inside the pipette, but not outside, mimics the effect. Strong pre-pulses (+150 mV, 280 ms) followed by short repolarizations are unable to remove the inhibition at test potential (+10 mV). Increasing the level of cAMP by either direct application of 8-(4-chlorophenylthio)-cAMP (8-CPT-cAMP) or mixtures of forskolin and 1-methyl-3-isobutylxanthine (IBMX) potentiates the activity of L-channels by increasing the mean open time and decreasing the mean closed time and percentage of null sweeps. The cAMP-induced potentiation occurs regardless of whether the G-protein-mediated inhibition is activated by ATP and opioids or inactivated by PTX. Protein kinase inhibitors (H7 and H89) prevent the effects of cAMP without altering the basal autocrine modulation associated with PTX-sensitive G-proteins. Our results provide new evidence for the coexistence of two distinct modulations that may converge on the same neuroendocrine L-channel: a direct G-protein-dependent inhibition and a cAMP-mediated potentiation, which may work in combination to regulate Ca2+ entry during neurosecretion.

Voltage-independent autocrine modulation of L-type channels mediated by ATP, opioids and catecholamines in rat chromaffin cells

The inhibition of L-type channels induced by either bath application of ATP, opioids and catecholamines or by endogenously released neurotransmitters was investigated in rat chromaffin cells with whole-cell recordings (5 mM Ba2+). In both cases, the L-type current, isolated pharmacologically using omega-toxin peptides and potentiated by Bay K 8644, was inhibited by approximately 50% with nearly no changes to the activation-inactivation kinetics. Inhibition was voltage independent at a wide range of potentials (-20 to +50 mV) and insensitive to depolarizing prepulses (+100 mV, 50 ms). Onset and offset of the inhibition were fast (time constants: tau(on) approximately 0.9 s, tau(off) approximately 3.6 s), indicating a rapid mechanism of channel modulation. Whether induced exogenously or from the released granules content in conditions of stopped cell superfusion, the neurotransmitter action was reversible and largely prevented by either intracellular GDP-beta-S, cell treatment with pertussis toxin or simultaneous application of P2y,2x delta/mu-opioidergic and alpha/beta-adrenergic antagonists. This suggests the existence of converging modulatory pathways by which autoreceptors-activated G-proteins reduce the activity of L-type channels through fast interactions. The autocrine inhibition of L-type currents, which was absent in superfused isolated cells, was effective on cell clusters, suggesting that L-type channels may be potently inhibited by cell exocytosis under physiological conditions resembling the intact adrenal glands.

Different types of calcium channels and secretion from bovine chromaffin cells

Bovine chromaffin cells possess several types of Ca2+ channels, and influx of Ca2+ is known to trigger secretion. However, discrepant information about the relative importance of the individual subtypes in secretion has been reported. We used whole-cell patch-clamp measurements in isolated cells in culture combined with fura-2 microfluorimetry and pharmacological manipulation to determine the dependence of secretion on different types of Ca2+ channels. We stimulated cells with relatively long depolarizing voltage-clamp pulses in a medium containing 60 mM CaCl2. We found that, within a certain range of pulse parameters, secretion as measured by membrane capacitance changes was mainly determined by the total cumulative charge of Ca2+ inflow and the basal [Ca2+] level preceding a stimulus. Blocking or reducing the contribution of specific types of Ca2+ channels using either 20 microM nifedipine plus 10 microM nimodipine or 1 microM omegaCTxGVIA (omega-conotoxin GVIA) or 2 microM omegaCTxMVIIC (omega-conotoxin MVIIC) reduced secretion in proportion to Ca2+ charge, irrespective of the toxin used. We conclude that for long-duration stimuli, which release a large fraction of the readily releasable pool of vesicles, it is not so important through which type of channels Ca2+ enters the cell. Release is determined by the total amount of Ca2+ entering and by the filling state of the readily releasable pool, which depends on basal [Ca2+] before the stimulus. This result does not preclude that other stimulation patterns may lead to responses in which subtype specificity of Ca2+ channels matters.

Identifying neuronal non-L Ca2+ channels--more than stamp collecting?

The pharmacology of the majority of Ca2+ channels in the nervous system is very limited. Although attempts have been made to constrain native Ca2+ channels into the framework provided by the six pore-forming molecules cloned to date, refined biophysical analysis of Ca2+ currents, expression techniques and the use of selective toxins have helped to identify unambiguously only a limited number of Ca2+ channels. In fact, many native Ca2+ channel activities remain as 'orphans', waiting for their molecular counterparts to be defined. In this article, Janet Nooney, Régis Lambert and Anne Feltz systematically delineate the well characterized non-L Ca2+ channel activities and the missing elements in our knowledge of the Ca2+ channel family.

G-protein-mediated desensitization of metabotropic glutamatergic and muscarinic responses in CA3 cells in rat hippocampus

1. Desensitization of a metabotropic response was investigated in CA3 pyramidal neurons in hippocampal slice cultures using the patch-clamp technique. 2. 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD), an agonist at metabotropic glutamate receptors (mGluRs), and metacholine (MCh), an agonist at muscarinic receptors, induced a cationic current that appears to be activated through a G-protein-independent transduction process, as previously shown. Prolonged or repetitive bath application of agonists led to rapid desensitization of the cationic current with a time constant of approximately 20 s. 3. Complete recovery from desensitization was observed within 6 min. 4. These responses mediated by mGluRs and muscarinic receptors cross-desensitized. 5. Preventing the activation of G-proteins by loading cells with GDP beta S strongly reduced or suppressed desensitization, and resulted in a sustained inward cationic current. When cells were filled with GTP gamma S to irreversibly activate G-proteins, the desensitization process was enhanced such that a first application of agonist caused a markedly reduced response. 6. These results show that a cationic current induced by metabotropic agonists in hippocampal pyramidal cells undergoes apparent desensitization and suggests that this process occurs through a G-protein-mediated inhibition of the underlying membrane conductance.

ADP exerts a protective effect against rundown of the Ca2+ current in bovine chromaffin cells

In isolated chromaffin cells, the high-voltage-activated Ca2+ current, recorded using 5 mM Ca2+ as the divalent charge carrier, exhibits rundown within 10 min, which is delayed for 1 h at least by the addition of 1 mM adenosine 5'-triphosphate (ATP) to the pipette medium. The mechanism of this stabilizing action of ATP has been examined. ATP action is dose dependent; the rundown process, which was delayed at concentrations below 0.4 mM, was totally abolished at higher concentrations. The requirement for ATP was shown to be quite strict: 2 mM inosine 5'-triphosphate (ITP) could not replace ATP, whereas guanosine 5'-triphosphate (GTP) could, but at higher concentrations. This effect of ATP was shown to require the presence of MgCl2 and the liberation of a phosphate group since the ATP analogue 5'-adenylyl-imidodiphosphate (AMP-PNP) could not act as a substitute for ATP, suggesting an action through either adenosine 5'-diphosphate (ADP) or a phosphorylation step. ADP, in the presence of Mg2+ only, could replace ATP in the same concentration range. This effect was shown to be specific to ADP; it was maintained after blocking the pathways which convert ADP into ATP, and could not be mimicked by guanosine 5'-diphosphate (GDP). Similarly, ATP and ADP effects were abolished at an increased internal Ca2+ concentration (pCa 6 instead of pCa 7.7, where pCa = -log10[Ca2+]). Nevertheless, the presence of 1 mM Mg-ADP in the bathing solution did not prevent the rundown of the Ca2+ channels when going to the inside-out patch recording configuration.(ABSTRACT TRUNCATED AT 250 WORDS)