Inhibition of Ca-channels by diazepam compared with that by nicardipine in pheochromocytoma PC12 cells

The triakontatetraneuropeptide TTN increases [CA2+]i in rat astrocytes through activation of peripheral-type benzodiazepine receptors

Astrocytes synthesize a series of regulatory peptides called endozepines, which act as endogenous ligands of benzodiazepine receptors. We have recently shown that one of these endozepines, the triakontatetraneuropeptide TTN, stimulates DNA synthesis in astroglial cells. The purpose of the present study was to determine the mechanism of action of TTN on cultured rat astrocytes. Binding of the peripheral-type benzodiazepine receptor ligand [3H]Ro5-4864 to intact astrocytes was displaced by TTN, whereas its C-terminal fragment (TTN[17-34], the octadecaneuropeptide ODN) did not compete for [3H]Ro5-4864 binding. Microfluorimetric measurement of cytosolic calcium concentrations ([Ca2+]i) with the fluorescent probe indo-1 showed that TTN (10(-10) to 10(-6) M) provokes a concentration-dependent increase in [Ca2+]i in cultured astrocytes. Simultaneous administration of TTN (10(-8) M) and Ro5-4864 (10(-5) M) induced an increase in [Ca2+]i similar to that obtained with Ro5-4864 alone. In contrast, the effects of TTN (10(-8) M) and ODN (10(-8) M) on [Ca2+]i were strictly additive. Chelation of extracellular Ca2+ by EGTA (6 mM) or blockage of Ca2+ channels with Ni2+ (2 mM) abrogated the stimulatory effect of TTN. The calcium influx evoked by TTN (10(-7) M) or by Ro5-4864 (10(-5) M) was not affected by the N- and T-type calcium channel blockers omega-conotoxin (10(-6) M) and mibefradil (10(-6) M), but was significantly reduced by the L-type calcium channel blocker nifedipine (10(-7) M). Patch-clamp studies showed that, at negative potentials, TTN (10(-7) M) induced a sustained depolarization. Reduction of the chloride concentration in the extracellular solution shifted the reversal potential from 0 mV to a positive potential. These data show that TTN, acting through peripheral-type benzodiazepine receptors, provokes chloride efflux, which in turn induces calcium influx via L-type calcium channels in rat astrocytes.

Diazepam, gamma-aminobutyric acid, and progesterone open K(+) channels in myocytes from coronary arteries

Benzodiazepines enhance coronary blood flow and lower blood pressure, but the cellular basis of this action remains unclear. The present study now demonstrates a direct effect of diazepam, gamma-aminobutyric acid (GABA), and progesterone on the large conductance, Ca(2+)- and voltage-activated K(+) channel (BK(Ca)) in single myocytes isolated from porcine coronary arteries. These GABA receptor agonists significantly increased whole-cell (perforated patch) K(+) currents and stimulated the activity of single BK(Ca) channels in cell-attached patches dramatically. This effect is not mediated via cyclic AMP or cyclic GMP, but involves stimulation of Ca(2+) influx in response to activation of a bicuculline-sensitive GABA(A)-like receptor. We propose that localized, subsarcolemmal increases in Ca(2+) levels open BK(Ca) channels, thereby promoting K(+) efflux, membrane repolarization, and coronary relaxation. This transduction pathway can now account, at least in part, for the direct vasodilatory effects of diazepam, progesterone, and GABA.

The effect of calcium channel blockers on cerebral oxygenation during tracheal extubation

Calcium channel blockers are effective in stabilizing systemic hemodynamics during tracheal extubation. However, they may increase cerebral blood flow (CBF) during tracheal extubation because of cerebral vasodilation, even if systemic arterial blood pressure decreases. In this study, we observed changes in cerebral oxygenation during tracheal extubation by using near-infrared spectroscopy and evaluated the effect of nicardipine and diltiazem on the resultant changes. We studied 45 women undergoing elective gynecologic surgery. After surgery, the patients were randomly allocated to three groups (n = 15 each): saline (control), 0.02 mg/kg nicardipine, and 0.2 mg/kg diltiazem. After 2 min, we started to aspirate secretions for 2 min and then, extubated the trachea. Changes in cerebral oxygenated hemoglobin (HbO(2)) and deoxygenated hemoglobin were measured during the extubation procedure for 9 min after drug treatment. Systemic hemodynamics, including mean arterial blood pressure, heart rate, end-tidal CO(2), end-tidal sevoflurane concentration, and peripheral arterial oxygen saturation were also monitored. During extubation, HbO(2) increased significantly, presumably caused by the increase in CBF. Changes in deoxygenated hemoglobin were minimal. Compared with the control, nicardipine and diltiazem significantly inhibited the increase in mean arterial blood pressure. On the contrary, they significantly enhanced the increase in HbO(2). In conclusion, calcium channel blockers may increase CBF during extubation, even if these drugs stabilize systemic hemodynamics.

IMPLICATIONS

This study is a preliminary report evaluating the changes in cerebral oxygenation during the tracheal extubation. Cerebral oxygenated hemoglobin increased significantly, presumably caused by the increase in cerebral blood flow during extubation. In addition, these changes were enhanced by calcium channel blockers.

Inhibition by antipsychotic drugs of L-type Ca2+ channel current in PC12 cells

Inhibition by antipsychotic drugs of voltage-gated L-type Ca2+ channels was characterized in rat neuronal cell line pheochromocytoma PC12 cells. Under whole-cell voltage-clamp, haloperidol and chlorpromazine (1-100 microM) inhibited Ba2+ current permeating through Ca2+ channels. Fluspirilene and pimozide the Ba2+ current at lower concentrations (fluspirilene, 0.1 pM to 1 nM; pimozide 10 pM to 1 microM). Effects of dopamine receptor antagonists and calmodulin antagonists were tested because antipsychotic drugs are known to exhibit these pharmacological activities. Sulpiride (1 and 10 microM), an antagonist to dopamine D2 receptors, and SCH-23390 (R(+)-7-chloro-8-hydroxy-3-methyl-l-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine; 1 and 10 microM), an antagonist to dopamine D1 receptors, also inhibited the Ba2+ current. As for calmodulin antagonists, W-7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide; 10 and 100 microM) as well as calmidazolium (10 nM to 1 microM) reduced the Ba2+ current. The inhibition by haloperidol or fluspirilene of the Ba2+ current was not affected when GTP in intracellular solution was replaced with GDP beta S. These properties of the Ca2+ channel inhibition are discussed by comparing with those of the K+ channel inhibition and in relation to therapeutic relevance.

The endogenous benzodiazepine receptor ligand ODN increases cytosolic calcium in cultured rat astrocytes

We have investigated the production of diazepam-binding inhibitor (DBI)-related peptides by astrocytes in primary culture and we have determined the effect of the octadecaneuropeptide DBI[33-50] (ODN) on the intracellular calcium concentration ([Ca2+]i) in astrocytes. Immunocytochemical labeling with antibodies against ODN showed that cultured astrocytes retain their ability to synthesize DBI in vitro. Cultured astrocytes were also found to release substantial amounts of ODN-immunoreactive material, and a brief exposure of astrocytes to a depolarizing potassium concentration resulted in a 5-fold increase in the rate of release of the ODN-like peptide. Microfluorimetric measurement of [Ca2+]i with the fluorescent probe indo-1 showed that nanomolar concentrations of ODN induced a marked increase in [Ca2+]i. The stimulatory effect of ODN on [Ca2+]i was not affected by calcium channel blockers or by incubation in Ca(2+)-free medium. In contrast, thapsigargin, an inhibitor of microsomal Ca(2+)-ATPase activity, totally abolished the ODN-induced increase in [Ca2+]i. Repeated pulses of ODN caused attenuation of the response, indicating the existence of a desensitization phenomenon. Preincubation of astrocytes with pertussis toxin totally blocked the effect of ODN on [Ca2+]i. The present study indicates that ODN-related peptides are synthesized and released by glial cells. Our results also show that synthetic ODN induces calcium mobilization from an intracellular store through stimulation of pertussis toxin-sensitive G protein. Taken together, these data suggest that endozepines act as paracrine and/or autocrine factors controlling the activity of astroglial cells.

Diazepam pretreatment suppresses morphine withdrawal signs in the mouse

The effect of diazepam on the development of physical dependence on morphine and on the naloxone-precipitated increase in cortical NA turnover were investigated in mice. Co-administration of diazepam (1-4 mg/kg, i.p.) during chronic morphine treatment suppressed the expression of naloxone (3 mg/kg, s.c.)-precipitated withdrawal signs (jumping, exploratory rearing and weight loss). However, a single injection of diazepam (4 mg/kg, i.p.) in morphine-dependent mice did not affect the expression of naloxone-precipitated withdrawal signs. The 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) level and noradrenaline (NA) turnover (MHPG/NA) in the cerebral cortex were increased by naloxone (3 mg/kg) challenge. These increases in the cortical MHPG level and NA turnover were significantly prevented by co-administration of diazepam (4 mg/kg, i.p.) during chronic morphine treatment. These findings suggest that the co-administration of diazepam during chronic morphine treatment may prevent some neurochemical changes in the central noradrenergic system during chronic morphine treatment, and may suppress the development of physical dependence on morphine. Therefore, the inhibitory action of GABA via benzodiazepine binding sites may play an important role in the development of physical dependence on morphine.

Characterization of inhibition by haloperidol and chlorpromazine of a voltage-activated K+ current in rat phaeochromocytoma cells

1. Inhibition by haloperidol and chlorpromazine of a voltage-activated K+ current was characterized in rat phaeochromocytoma PC12 cells by use of whole-cell voltage-clamp techniques. 2. Haloperidol or chlorpromazine (1 and 10 microM) inhibited a K+ current activated by a test potential of +20 mV applied from a holding potential of -60 mV. The K+ current inhibition did not exhibit voltage-dependence when test potentials were changed between -10 and +40 mV or when holding potentials were changed between -120 and -60 mV. 3. Effects of compounds that are related to haloperidol and chlorpromazine in their pharmacological actions were examined. Fluspirilene (1 and 10 microM), an antipsychotic drug, inhibited the K+ current, but pimozide (1 and 10 microM), another antipsychotic drug did not significantly inhibit the K+ current. Sulpiride (1 or 10 microM), an antagonist of dopamine D2 receptors, did not affect the K+ current whereas (+)-SCH-23390 (10 microM), an antagonist of dopamine D1 receptors, reduced the K+ current. As for calmodulin antagonists, W-7 (100 microM), but not calmidazolium (1 microM), reduced the K+ current. 4. The inhibition by haloperidol or chlorpromazine of the K+ current was abolished when GTP in intracellular solution was replaced with GDP beta S. Similarly, the inhibition by pimozide, fluspirilene, (+)-SCH-23390 or W-7 was abolished or attenuated in the presence of intracellular GDP beta S. The inhibition by haloperidol or chlorpromazine was not prevented when cells were pretreated with pertussis toxin or when K-252a, an inhibitor of a variety of protein kinases, was included in the intracellular solution. 5. Haloperidol and chlorpromazine reduced a Ba2+ current permeating through Ca2+ channels. Inhibition by haloperidol or chlorpromazine of the Ba2+ current was not affected by GDP beta S included in the intracellular solution. 6. It is concluded that haloperidol and chlorpromazine inhibit voltage-gated K+ channels in PC12 cells by a mechanism involving GTP-binding proteins. The inhibition may not be related to their activity as antagonists of dopamine D2 receptors or calmodulin antagonists.

Modulation by alkyl p-hydroxybenzoates of voltage- and ligand-gated channels in peripheral neuronal cells

Effects of alkyl p-hydroxybenzoates (APHBs), which are used as preservatives, on ion channels were investigated in rat pheochromocytoma PC12 cells. Methyl p-hydroxybenzoate (MPHB; 300 microM) and butyl p-hydroxybenzoate (BPHB; 300 microM) inhibited Ba2+ current passing through Ca2+ channels, and facilitated the inactivation of the Ba2+ current. K+ current obtained with a depolarizing voltage-step was also suppressed by 300 microM MPHB or 300 microM BPHB. The extent of the suppression of the K+ current was not affected by extracellular Cd2+, suggesting that the suppression of the K+ current is not a secondary effect arising from the Ca2+ channel inhibition. An inward current activated by acetylcholine (ACh; 100 microM) was abolished by 300 microM BPHB, and it was partially blocked by 300 microM MPHB. In contrast to the ACh-activated current, an inward current activated by ATP (30 microM) was markedly potentiated by 300 microM BPHB. The results suggest that APHBs exert significant effects on the voltage- and ligand-gated channels. The significance of these channel modifications were discussed in relation to reported effects of APHBs, including induction of minor irritation.

Pharmacological relevance of peripheral type benzodiazepine receptors on motor nerve and skeletal muscle

1. Effects of agonists and antagonists of peripheral and central benzodiazepine receptors (pBZR and cBZR) on neuromuscular transmission were studied in mouse isolated phrenic nerve-diaphragm preparations. 2. Ro5-4864, a pBZR agonist, had no effect on the neuromuscular transmission but increased muscle contractility and antagonized the tetanic fade induced by neostigmine. 3. Ro5-4864 inhibited the regenerative tonic endplate depolarization caused by repetitive stimulation in the presence of neostigmine without affecting the amplitude and decay time of miniature and evoked single endplate potentials. 4. All the effects of Ro5-4864 were shared by PK11195, a pBZR antagonist, but not by clonazepam and flumazenil, a cBZR agonist and antagonist, respectively. 5. It is suggested that peripheral type benzodiazepine receptors modulate presynaptic function and muscle contraction.

Organic hydroperoxide-induced activation of liver microsomal glutathione S-transferase of rats in vitro

The effect of t-butyl hydroperoxide (t-BuOOH), cumene hydroperoxide (CuOOH) or linoleic acid hydroperoxide (linoleic-OOH) on liver microsomal glutathione S-transferase of rats was studied in vitro. When microsomes were incubated with either 100 microM t-BuOOH or 25 microM CuOOH, glutathione S-transferase activity was increased 1.5-fold; activity was further increased to 2.2-fold in the presence of small amounts of glutathione. The same amounts of dithiothreitol or cysteine did not enhance the t-BuOOH or CuOOH-induced increase in transferase activity. The transferase activity was also increased 1.4-fold by 10 microM linoleic-OOH plus 1 microM glutathione. The increase in microsomal glutathione S-transferase activity after treatment of microsomes with t-BuOOH in the presence of glutathione was completely reversed by addition of dithiothreitol, whereas the activation of the transferase caused by t-BuOOH in the absence of glutathione was not reversed. Although microsomal glutathione S-transferase also possesses glutathione peroxidase activity, only transferase activity was increased by t-BuOOH in either the presence or absence of glutathione. These data indicate that microsomal glutathione S-transferase is activated by organic hydroperoxides in either the absence or presence of small amounts of glutathione, suggesting an activation of the transferase by thiol oxidation of the cysteine residue.

Differential inhibition of transient and long-lasting calcium channel currents by benzodiazepines in neuroblastoma cells

The effects of diazepam, nitrazepam, clonazepam, and Ro5-4864 on transient (type I) and long-lasting (type II) calcium channels associated with low-affinity benzodiazepine receptors were investigated using the whole-cell patch-clamp technique. Clonazepam (100 microM), a specific agonist for the central-type benzodiazepine receptor, reduced transient currents through the type I calcium channel by 40% without affecting long-lasting currents through the type II calcium channel. Diazepam and nitrazepam (100 microM), non-specific agonists for both the central- and peripheral-type benzodiazepine receptors, reduced both transient and long-lasting currents equally by 25-30%. A similar non-selective inhibition was observed by Ro5-4864 (1-10 microM), a specific agonist for the peripheral-type benzodiazepine receptor. It is concluded that the two calcium channel types are regulated differentially by two different kinds of benzodiazepines; central-type for type I channel and peripheral-type for both type I and type II channels.