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

EFFECT OF TRPA1 RECEPTOR ACTIVATION ON TRPV1 CHANNEL DESENSITIZATION IN RAT DORSAL GANGLION NEURONS

The activity of TRPA1 and TRPV1 channels, their sensitivity to selective activators - allyl isothiocyanate (AITC) and capsaicin (Caps), especially their interaction were studied. The method of microfluorescent microscopy and Ca2+ sensitive dye fura- 2AM. Registration of changes in the concentration of intracellular Ca2+ was performed by using the ratio of fluorescence signals measured at two wavelengths (R = F1/ F2). Researches were conducted on cultured neurons of rat dorsal ganglia (DRG neurons). Application of AITC and Caps on soma of DRG neurons resulted in an increase in intracellular Ca2+. Consistent repeated Caps applications resulted in a significant reduction in the amplitude of Ca2+ transients ( desensitization of TRPV1 channels), which accounted 20,7% of initial value. Further application of selective TRPA1 channel agonist (AITC) resulted in restoration of sensitivity to capsaicin TRPV1 channels ( resensitization TRPV1 channels). Thus, we have established the presence of regulation of TRPV1 channel activity by TRPA1 channels.

Action of hypoxia on different types of calcium channels in hippocampal neurons

Whole-cell patch clamp and polarographic oxygen partial pressure (pO2) measurements were used to establish the sensitivity of high-voltage-activated (HVA) Ca2+ channel subtypes of CA1 hippocampal neurons of rats to hypoxic conditions. Decrease of pO2 to 15-30 mm Hg induced a potentiation of HVA Ca2+ currents by 94%. Using selective blockers of N- and L-types of calcium channels, we found that inhibition of L-type channels decreased the effect by 54%, whereas N-type blocker attenuated the effect by 30%. Taking into account the ratio of currents mediated by these channel subtypes in CA1 hippocampal neurons, we concluded that both types of HVA Ca2+ channels are sensitive to hypoxia, however, L-type was about 3.5 times more sensitive to oxygen reduction.

Effect of hypoxia on calcium channels depends on extracellular calcium in CA1 hippocampal neurons

Our previous studies have shown that short lasting hypoxia induces an increase of Ca(2+) influx into the cell through high voltage-activated Ca(2+) channels in hippocampal neurons. This effect was abolished by removing of free Ca(2+) from intracellular solution. The aim of this study was to compare hypoxic responses at different extracellular Ca(2+) concentrations ([Ca(2+)](e)) in hippocampal neurons to ascertain whether the hypoxic sensitivity is restricted to Ca(2+) ions. Whole-cell patch-clamp recordings were made from acutely dissociated CA1 hippocampal neurons of rats. Polarographic method for measurements of O(2) partial pressure was used. Here we found that at 2 mM [Ca(2+)](e) the hypoxic effect was significant (up to approximately 94%), whereas [Ca(2+)](e) elevations to 5 and 15 mM resulted in gradual decreasing of the effect. We found, that total Ca(2+) charge carried into the cell under the hypoxia was similar at all [Ca(2+)](e), whereas Ca(2+) charge carried at normoxia was different for different [Ca(2+)](e), being larger at higher [Ca(2+)](e). These data indicated a saturation of the hypoxic effect due to limitation in the channel conductance. Therefore, we suggested that the hypoxic effect can be connected with increase of channel conductance, and the level of channel conductance at normoxia can determine the amplitude of hypoxic effect.

Intracellular mechanisms of hypoxia-induced calcium increase in rat sensory neurons

Elevation of cytosolic level of Ca(2+) was measured by spatial screening of freshly isolated dorsal root ganglion neurons loaded with Fura-2AM after subjecting them to a moderate hypoxic solution (pO(2)=10-40 mmHg). Short exposure of neurons to hypoxia resulted in a reversible elevation of intracellular Ca(2+) to about 120% in the cell center and to 80% in the cell periphery. Such elevation could be almost completely eliminated by removal of Ca(2+) or Na(+) from external medium or application of nifedipine, an L-type calcium channel blocker. Remarkable antihypoxic efficiency (58%) was achieved by preapplication of mitochondrial protonophore CCCP. A conclusion is made that in sensory neurons the hypoxia-induced elevation of cytosolic Ca(2+) is induced by combined changes of function in three cell substructures: voltage-operated L-type Ca(2+) and Na(+) channels and Ca(2+) accumulation by mitochondria. Mitochondria are important for spatial difference in the hypoxia-induced Ca(2+) elevation due to their specific location in these neurons.

Mechanisms of up-regulation of single calcium channels by serotonin in Helix pomatia neurons

Action of serotonin (5-HT) on single Ca(2+) channel activity was studied in identified neurons of snail Helix pomatia. Only one type of Ca(2+) channels of 5 pS unitary conductance was determined under patch-clamp cell-attached mode. Kinetic analysis have shown a monotonically declining distribution of channel open times (OT) with mean time constant of 0.2 ms. The distribution of channel closed times (CT) could be fitted by double-exponential curve with time constants 1 and 12 ms. We established that 5-HT acts on Ca(2+) channel activity indirectly via cytoplasm. 5-HT prolonged the OT (up to 0.3 ms) and shortened the CT proportionally for both constants to 0.4 and 6 ms correspondingly. A conclusion is made that enhancement of Ca(2+) macro-current by 5-HT is determined by kinetic changes, increase of the number of active channels, and increase of the probability of OT. At the same time the transmitter did not affect the unitary channel conductance.

Dual action of cytosolic calcium on calcium channel activity during hypoxia in hippocampal neurones

The role of cytoplasmic calcium level (Ca(i)) in regulation of Ca channel activity during hypoxia was studied in hippocampal neurones from rats. Whole-cell patch clamp recordings in combination with measurements of O(2) partial pressure (pO(2)) were used. Lowering of pO(2) induced a potentiation of HVA Ca channel activity by 25.7% at Ca(i) = 75 nM in comparison with Ca(2+)-free solution. Increase of Ca(i) up to 410 nM slightly increased the effect and significantly slowed the Ca(2+) current run-down. On the other hand, hypoxia increased a steady-state channel inactivation and speeded up the kinetics of Ca(2+) current decay by about 30%. We conclude that moderate hypoxia induces dual action on Ca channels: intracellularly mediated augmentation of Ca influx via Ca channels and their Ca(2+)-dependent inactivation.

High-threshold calcium channel activity in rat hippocampal neurones during hypoxia

Whole-cell patch clamp recordings in combination with direct control and measurements of O2 tension (pO2) in bath solution were used to determine the sensitivity of Ca2+ channels of cultured hippocampal neurones to hypoxia in glucose free solution. In all tested neurones, a lowering of pO2 to 4/50 mmHg did not induce changes either in magnitude, kinetics or voltage-current relations of total Ca2+ currents, which composed mainly from two types, L-type (64%) and N-type (31%) components. Hypoxia only induced a delay of Ca2+ current run-down about 27.5% and 39% at 50 and 4 mmHg pO2 respectively that presumably depended on changes in cytoplasmic channel-modulatory metabolites. The obtained results demonstrate that Ca2+ channel molecules in cultured hippocampal neurones are themselves insensitive to short-lasting (10-20 min) oxygen and glucose deprivation, and that they are not a principal target for hypoxic influences on hippocampal function.

Oxygen deprivation inhibits Na+ current in rat hippocampal neurones via protein kinase C

1. Hippocampal neurones respond to acute oxygen deprivation (hypoxia) with an inhibition of whole-cell Na+ current (INa), although the mechanism of the inhibition is unknown. Kinases can modulate INa and kinases are activated during hypoxia. We hypothesized that kinase activation may play a role in the hypoxia-induced inhibition of INa. 2. Single electrode patch clamp techniques were used in dissociated hippocampal CA1 neurones from the rat. INa was recorded at baseline, during exposure to kinase activators (with and without kinase inhibitors), and during acute hypoxia (with and without kinase inhibitors). 3. Hypoxia (3 min) reduced INa to 38.1 +/- 4.5% of initial values, and shifted steady-state inactivation in the negative direction. Hypoxia produced no effect on activation or fast inactivation. 4. Protein kinase A (PKA) activation with 2.5 mM adenosine 3',5'-cyclic adenosine monophosphate, N6,O2-dibutyryl, sodium salt (db-cAMP) resulted in reduction of INa to 62.8 +/- 5.5% without an effect on activation or steady-state inactivation. INa was also reduced by activation of protein kinase C (PKC) with 5 nM phorbol 12-myristate 13-acetate (PMA; to 40.0 +/- 3.7%) or 50 microM 1-oleoyl-2-acetyl-sn-glycerol (OAG; to 46.1 +/- 2.8%). In addition, steady-state inactivation was shifted in the negative direction by PKC activation. Neither the activation curve nor the kinetics of fast inactivation was altered by PKC activation. 5. The response to PKA activation was blocked by the PKA inhibitor N-[2-p-bromocinnamyl-amino) ethyl]-5-isoquinolinesulphonamide (H-89; 30 microM) and by 30 microM of PKA inhibitory peptide PKA5-24 (PKAi). PKC activation was blocked by the kinase inhibitor 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7; 100 microM), by the PKC inhibitor calphostin C (10 microM) and by 20 microM of the inhibitory peptide PKC19-31 (PKCi). 6. The hypoxia-induced inhibition of INa and shift in steady-state inactivation were greatly attenuated with H-7, calphostin C, or PKCi, but not with H-89 or PKAi. 7. We conclude that hypoxia activates PKC in rat CA1 neurones, and that PKC activation leads to the hypoxia-induced inhibition of INa.

Activity-dependent expression of parathyroid hormone-related protein (PTHrP) in rat cerebellar granule neurons. Requirement of PTHrP for the activity-dependent survival of granule neurons

To identify genes whose expression is neuronal activity-dependent, we used an mRNA differential display technique and discovered that parathyroid hormone-related protein (PTHrP) is expressed in an activity-dependent manner in primary cultures of rat cerebellar granule neurons. PTHrP mRNA was expressed as early as 1 h by the addition of KCl to a final concentration of 25 mM to the culture medium. This expression was induced by Ca2+ influx through voltage-dependent L-type Ca2+ channels and regulated at the transcriptional step. PTHrP mRNA was persistently expressed before and after the time of commitment of granule neurons to apoptosis when they are cultured in the presence of 25 mM KCl or both 150 microM N-methyl-D-aspartic acid and 15 mM KCl, both of which promote the survival of these neurons. PTHrP was rapidly secreted into the culture medium in a depolarization-dependent manner. Parathyroid hormone/PTHrP receptor mRNA was also expressed in the primary cultures, and its expression was up-regulated by KCl and/or N-methyl-D-aspartic acid. The addition of anti-PTHrP antiserum to the culture medium resulted in a reduction of the activity-dependent survival of the granule neurons. These results suggest that PTHrP is involved in an autocrine loop and required for the survival of granule neurons.

Regulated calcium channel in apical membranes renal proximal tubule cells

Using the single-channel patch-clamp technique, we identified a Ca2+ channel in the apical membranes rabbit cultured proximal tubule cells. The channel is permeable to both Ca2+ and Ba2+ but not to monovalent cations. In on-cell patches, the channel opened infrequently and had a conductance of 4.6 +/- 0.9 pS (n = 5) with 105 mM CaCl2 in the pipette. Although addition of forskolin (12.5 microM) Cl2 is without effect, addition of phorbol 12-myristate 13-acetate (PMA, 1 microM) activated the channel. At 0 mV pipette voltage (resting state) in the on-cell patches, PMA increased open probability (P0) from 0 to 6.9 +/- 2.3% (n = 5) within 1-3 min of PMA application. Likewise, stretching the membrane patch (-10 to -30 mmHg) activated this channel (P0 increased to 5.3 +/- 2.1%, n = 3, at 0 mV applied pipette potential), with results consistent with a mechanosensitive channel. The channel displayed only modest voltage sensitivity, with mild activation on membrane hyperpolarization but with inactivation on strong depolarization. The addition of the L-type Ca2+ channel blocker (antagonist), nifedipine (10 microM), completely blocked this channel in both on-cell and inside-out patches, whereas the agonist, BAY K 8644 (5 microM) was without effect. It is concluded that this channel is a nifedipine-sensitive, protein kinase C-regulated Ca2+ channel and that it may play a role in Ca2+ signaling in the proximal tubule cells, particularly during periods of mechanical stress.

The effect of the PKC inhibitor GF109203X on the release of Ca2+ from internal stores and Ca2+ entry in DDT1 MF-2 cells

1. The effects of the specific protein kinase C (PKC) inhibitor, GF109203X, were measured on the cytoplasmic Ca2+ concentration ([Ca2+]i), and on histamine H1 receptor- and thapsigargin-mediated increases in [Ca2+]i in DDT1 MF-2 smooth muscle cells. 2. After pretreatment of cells with GF109203X (5 microM, 45 min), the histamine (100 microM)-induced initial rise in [Ca2+]i, representing Ca2+ mobilization from internal stores, was inhibited (by 59 +/- 7%). The slowly declining phase of the histamine induced Ca2+ response, reflecting Ca2+ entry, was enhanced (83 +/- 26%) in the presence of the PKC inhibitor. 3. The histamine induced release of Ca2+ from internal stores, measured after blocking Ca2+ entry with LaCl3 was inhibited by GF109203X in a concentration-dependent manner (IC50: 3.1 +/- 1.1 microM). 4. Histamine-induced formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) was not changed in the presence of GF109203X. 5. The PKC activating phorbol ester, phorbol 12-myristate 13-acetate (PMA, 1 microM), strongly reduced histamine-induced Ins(1,4,5)P3 formation (58 +/- 16%). This effect was reversed by GF109203X (5 microM). Furthermore, PMA diminished histamine evoked Ca2+ release (50 +/- 6%) and blocked Ca2+ entry completely. 6. The rise in [Ca2+]i caused by blocking endoplasmic reticulum Ca2(+)-ATPase with thapsigargin (1 microM), was strongly reduced (57 +/- 3%) after pretreatment of cells with GF109203X. Downregulation of PKC by long-term pretreatment of cells with PMA (1 microM, 48 h) did not abolish this effect of GF109203X (48 +/- 3% inhibition). 7. In permeabilized DDT, MF-2 cells preloaded with 45Ca2+ in the presence of GF109203X, the amount of 45Ca2+ released by Ins(1,4,5)P3 (10 microM) was markedly reduced (42 +/- 9%). GF109203X did not release Ca2+ itself and did not impair Ins(1,4,5)P3 receptor function. 8. Uptake of 45Ca2+ by intact cells, representing Ca2+ entry, was enhanced by GF109203X (65 +/- 11%), by histamine (24 +/- 6%) and also by thapsigargin (121 +/- 10%). The GF109203X- and the thapsigargin-induced uptake of 45Ca2+ were not additive. 9. These data suggest that GF109203X reduces the filling-state of intracellular Ins(1,4,5)P3 sensitive Ca2+ stores by inhibiting the Ca2+ uptake into these stores, thereby promoting store-dependent (capacitive) Ca2+ entry.

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.

Protein kinase-independent inhibition of muscarinic K+ channels by staurosporine

Acetylcholine (ACh) binding to atrial muscarinic receptors activates an inwardly rectifying K+ current (IK[ACh]) via a pertussis toxin-sensitive GTP-binding protein (GK). The muscarinic K+ channel (termed GIRK1) has been cloned, and the nucleotide sequence contains nine consensus sites for protein kinase C (PKC) phosphorylation (16). Dephosphorylation of the muscarinic K+ channel has been implicated in rapid IK[ACh] desensitization in the presence of agonist (13). Staurosporine is a widely used membrane-permeant inhibitor of PKC and other protein kinases (7), including G protein-coupled receptor kinases. We investigated the role of phosphorylation in the regulation of IK[ACh] by examining the effect of a variety of protein kinase inhibitors. Staurosporine produced a rapid and reversible dose-dependent decrease in IK[ACh], activated by either GTP or guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S). Other PKC inhibitors, including calphostin C and K-252b, were without effect on GTP gamma S-activated IK[ACh]. In excised patches of atrial membrane under nonphosphorylating conditions (0 ATP, 1 mM 5'-adenylylimidodiphosphate), staurosporine reversibly reduced muscarinic K+ channel activity without altering single-channel current amplitude. These results suggest that staurosporine inhibits IK[ACh] by a mechanism independent of intracellular protein kinases.

Parathyroid hormone binding sites in the brain

Parathyroid hormone (PTH) has been shown to have actions within the brain, suggesting the presence of central PTH receptors. This possibility was examined by determining the binding of 125I-labeled [Nle8,18,Tyr34]bovine PTH to the plasma membranes of rat and rabbit brains. Specific binding of the tracer to membranes of the whole brain was time and tissue dependent, and was greater with membranes from the hypothalamus than with membranes from the cerebellum, cerebrum, or brain stem. The binding of the tracer to rat hypothalamic membranes was saturable and competitively displaced by unlabeled PTH(1-34), PTH(3-34), [Nle8,18,Tyr34]PTH(1-34), and by PTH-related protein, indicating the presence of a single class of high-affinity (dissociation constant = 2-5 nM), low-capacity (maximum binding capacity, Bmax = 110-250 fmol/mg protein) binding site. The binding of radiolabeled PTH to these sites was not displaced by unrelated peptides of comparable molecular size (calcitonin, calcitonin-gene related peptide, adrenocorticotropin). The binding of PTH to these sites did not, however, appear to stimulate adenylate cyclase activity, as in peripheral PTH target sites. Thus, although these results indicate the presence of PTH receptors in the brain, these binding sites have a lower affinity than those in peripheral tissues and may utilize a different signal transduction system.

Mechanisms of antagonistic action of internal Ca2+ on serotonin-induced potentiation of Ca2+ currents in Helix neurones

The influence of internal Ca2+ ions has been investigated during intracellular perfusion of isolated neurones from pedal ganglia of Helix pomatia in which serotonin (5-HT) induces a cyclic-adenosine-monophosphate-(cAMP)-dependent enhancement of high-threshold Ca2+ current (ICa). Internal free Ca2+ ([Ca2+]i) was varied between 0.01 and 10 microM by addition of Ca(2+)-EGTA [ethylenebis(oxonitrilo)tetraacetate] buffer. Elevation of [Ca2+]i depressed the 5-HT effect. The dose/effect curve for the Ca2+ blockade had a biphasic character and could be described by the sum of two Langmuir's isotherms for tetramolecular binding with dissociation constants Kd1 = 0.063 microM and Kd2 = 1 microM. Addition of calmodulin (CM) antagonists (50 microM trifluoperazine or 50 microM chlorpromazine), phosphodiesterase (PDE) antagonists [100 microM isobutylmethylxanthine (IBMX) or 5 mM theophylline] and protein phosphatase antagonists [2 microM okadaic acid (OA)] in the perfusion solution caused "anticalcium" action and modified the Ca2+ binding isotherm. Using the effect of OA and IBMX, two components of the total Ca2+ inhibition were separated and evaluated. In the presence of one of these blockers tetramolecular curves with Kd1 = 0.04 microM and Kd2 = 0.69 microM were obtained describing the activation of the retained unblocked enzyme--PDE or calcineurin (CN) correspondingly. The sum of these isotherms gave a biphasic curve similar to that in control. Leupeptin (100 microM), a blocker of Ca(2+)-dependent proteases did not influence the amplitude of 5-HT effect, indicating that channel proteolysis is not involved in the depression. Our findings show that the molecular mechanism of Ca(2+)-induced suppression of the cAMP-dependent upregulation of Ca2+ channels is due to involvement of two Ca(2+)-CM-dependent enzymes: PDE reducing the cAMP level, and CN causing channel dephosphorylation. No other processes are involved in the investigated phenomenon at a Ca2+ concentration of less than or equal to 10 microM.