Are dihydropyridine binding sites voltage sensitive calcium channels?

Diltiazem or verapamil prevents haloperidol-induced apomorphine supersensitivity in mice

Chronic thioridazine treatment in animals has been reported to produce less dopaminergic supersensitivity than other neuroleptics. This difference may be due to the potent calcium channel inhibitory effect of thioridazine. To test this hypothesis Swiss-Webster mice were treated chronically (28 d) with calcium channel inhibitors (CCI's) - diltiazem, nifedipine or verapamil - with or without haloperidol. Following three days of drug withdrawal, mice were tested for amphetamine-induced locomotion and apomorphine-induced cage climbing. Co-administration of diltiazem or verapamil (but not nifedipine) prevented the development of haloperidol-induced behavioral supersensitivity to apomorphine. Co-administration of CCI's with haloperidol did not affect the development of amphetamine supersensitivity. These data support the hypothesis that co-administration of haloperidol and a CCI (verapamil or diltiazem, but not nifedipine) would mimic the effects of thioridazine treatment alone.

Comparison of calcium antagonist properties of antispasmotic agents

Several agents commonly employed for the treatment of detrusor hyperreflexia or instability are characterized as antispasmotics. Their mechanism of action is not completely understood but it has been proposed that their actions are dependent on anticholinergic activity, CNS mediated relaxation, or local anesthetic properties. The purpose of this study was to determine if imipramine, flavoxate HCl, or oxybutynin HCl possess any calcium antagonist properties. This was accomplished by determining the ability of these agents to inhibit a standard cholinergic stimulus (200 uM bethanechol) over a range of extracellular calcium concentrations (0.5 to 10.0 mM). In-vitro isolated smooth muscle strips of rabbit bladder dome were utilized. Control tissues displayed a reproducible response to bethanechol stimulation at different calcium concentrations with an ED50 of 0.4 mM calcium and a peak response of 5.0+/-0.4 grams tension. Flavoxate (2.5 mM), oxybutynin (2.5 uM), and imipramine 25 uM) all significantly reduced peak tension generation. The ED 50's for extracellular calcium in the presence of flavoxate and oxybutynin were not significantly different from controls. Imipramine at both 3 and 25 uM significantly increased the ED50 for calcium. The above data demonstrate that imipramine possesses competitive calcium antagonism. The relative contribution of calcium antagonism toward the inhibitory effects of imipramine is unknown but may play a significant role in its clinical activity.

BAY K 8644 increases release of acetylcholine at the murine neuromuscular junction

In this study we describe the stimulatory effects of submicromolar concentrations of BAY K 8644 on neurally evoked and spontaneous release of transmitter at the vertebrate neuromuscular junction, a model cholinergic synapse. BAY K 8644 increases mean quantal content. This effect is blocked by pretreatment with the dihydropyridine (DHP) antagonist, nimodipine, but nimodipine itself had no effect on quantal content. Furthermore, BAY K 8644 induces a marked increase in spontaneous quantal release of transmitter as measured by miniature endplate potential frequency. These results are important in that they indicate that DHP-sensitive Ca2+ channels exist at motor nerve terminals and when activated can participate in transmitter release; second, they imply that these elements do not normally participate in transmitter release; third, they indicate a functional effect of DHPs on neurons under somewhat more relevant physiological conditions than those imposed by prolonged K+-induced depolarization, a technique used to evoke transmitter release in subcellular systems and fourth, they indicate an effect of BAY K 8644 on spontaneous release of transmitter, an effect not reported in previous neurochemical experiments.

Effect of calcium agonists and antagonists on cerebellar granule cells

In cerebellar cultures, comprising predominantly granule neurones, dihydropyridine (DHP) Ca2+ agonists were potent stimulators of voltage-sensitive 45Ca2+ uptake. Their effect was maximal in partially depolarized cells; at 15 mM K+e half maximal stimulation occurred at about 5 X 10(-8) M BAY K 8644 and 10(-7) M (+)-(S)-202791. Organic Ca2+ antagonists were effective inhibitors of voltage-sensitive calcium entry into granule cells: the order of potency in blocking uptake induced by sub-maximal concentration of K+ and BAY K 8644 was nifedipine greater than (-)-202791 greater than D600. BAY K 8644 also stimulated the release of glutamate, the transmitter of the granule cells, from depolarized cells. Granule cells are therefore a class of neurones whose responsiveness to organic Ca2+ effectors is similar to that of cardiac and smooth muscle. The discrepant findings on the effect of calcium effectors in various preparations of nervous tissues may thus reflect a differential distribution of voltage-sensitive Ca2+ channels in different neuronal cell types.

Multiple voltage-sensitive calcium channels are probably involved in endogenous GABA release from striatal neurones differentiated in primary culture

Calcium-dependent release of neurotransmitters is thought to be due to Ca2+ entry into nerve terminals, but the identities of the various voltage-sensitive Ca2+ channels (VSCC) involved in this process remain obscure. To elucidate the types of VSCCs involved in the release process, we studied the effects of various organic Ca2+ channel antagonists and agonists on the release of endogenous gamma-aminobutyric acid (GABA) from mouse striatal neurones differentiated in primary culture. Diltiazem, verapamil and methoxyverapamil (D 600) inhibited K+-evoked (30 mM) GABA release at very high concentrations (greater than 1 microM). The dihydropyridine (DHP) nifedipine, at low concentrations (0.01-1.00 microM), was able to inhibit part of the K+-evoked GABA release (25.6 +/- 7.3% inhibition at 1 microM). This is in agreement with the high affinity of nifedipine for DHP binding sites. The DHPs, BAY K 8644 (EC50 = 41 +/- 15 nM) and CGP 28.392, which possess agonist properties at VSCCs, increased the 15 mM K+-evoked GABA release. The release evoked by the combination of K+ (15 mM) and BAY K 8644 (up to 10 microM) remained smaller than the release elicited by 30 mM K+. The effect of BAY K 8644 (1 microM) was inhibited by nifedipine (IC50 0.55 +/- 0.05 microM). When Na+ ions were replaced by choline, basal and K+-evoked GABA release was significantly increased. Even in the absence of external Na+, nifedipine (1 microM) was not able to totally block the K+ effect. Moreover amiloride, a drug known to inhibit Na+/Ca2+ exchange, and tetrodotoxin (TTX), did not modify the 30 mM K+ response.(ABSTRACT TRUNCATED AT 250 WORDS)

Dihydropyridine inhibition of neuronal calcium current and substance P release

Dihydropyridine (DHP) calcium channel antagonists, which inhibit the slowly inactivating or L-type cardiac calcium (Ca) current, have been shown to be ineffective in blocking 45Ca influx and Ca-dependent secretion in a number of neuronal preparations. In the studies reported here, however, the antagonist DHP nifedipine inhibited both the L-type Ca current and potassium-evoked substance P (SP) release from embryonic chick dorsal root ganglion (DRG) neurons. These results suggest that, in DRG neurons, Ca entry through L-type channels is critical to the control of secretion. The inhibition of Ca current by nifedipine was both voltage and time-dependent, significant effects being observed only on currents evoked from relatively positive holding potentials maintained for several seconds. As expected from these results, nifedipine failed to inhibit L-type Ca current underlying the brief plateau phase of the action potential generated from the cell's normal resting potential; likewise, no significant effect of the drug was observed on action potential-stimulated SP release evoked by electrical field stimulation. The results of this work are discussed in terms of an assessment of the role of L-type Ca channels in neurosecretion.

Differential effects of calcium entry blockers on pre- and postsynaptic influx of calcium in the rat hippocampus in vitro

A decrease in extracellular free Ca ([Ca2+]o) in response to stimulation of Schaffer collaterals could be recorded in or near the stratum pyramidale even when synaptic transmission was completely blocked. Under the same conditions, alvear stimulation also evoked a decrease in [Ca2+]o at the same site. We attributed the former to influx of Ca2+ into presynaptic terminals and the latter to influx into postsynaptic (pyramidal) cells. Both pre- and postsynaptic Ca2+ influx were completely blocked by Ni2+ (2.5 mM). Nifedipine (5-10 microM). verapamil (50-100 microM) and fendiline (100-200 microM) reduced the postsynaptic influx of Ca2+ but did not alter Ca2+ loss from the extracellular space into presynaptic terminals. The calcium channel activators, BAY-K 8644 and CGP 28,392, had no consistent effect on either pre- or postsynaptic influx. Occasional enhancement of both pre- and postsynaptic responses was seen. In most studies the agents were without effect and on occasions a reduction in both responses was seen. The results could indicate that Ca-channels at pre- and postsynaptic sites in CA1 may be of different types.

Pituitary Ca2+ channels: blockade by conventional and novel Ca2+ antagonists

We have identified several new agents that block Ca2+ channels in the rat pituitary GH4C1 cell line. These drugs, which include the diphenylbutylpiperidine antipsychotic pimozide, the calmodulin antagonist calmidazolium, and the steroidal Na+ channel toxin veratridine, were compared with several conventional Ca2+ antagonist in 45Ca2+ uptake, prolactin secretion, and whole cell patch voltage-clamp experiments. Pimozide, the most potent of these novel Ca2+ antagonists, inhibited depolarization-dependent 45Ca2+ uptake and prolactin secretion half maximally at a concentration of 100 nM, whereas calmidazolium and veratridine produced 50% inhibition at concentrations of 500 nM and 1 microM. In comparison, the three organic Ca2+ antagonists nitrendipine, verapamil, and diltiazem blocked 45Ca2+ uptake half maximally at concentrations of 2.5 nM, 1 microM, and 2.5 microM, respectively. All of the antagonists inhibited Ca2+ uptake and prolactin secretion stimulated by the dihydropyridine Ca2+ agonist BAY-K 8644 less potently than KCl-stimulated responses. In patch-clamp experiments, pimozide, veratridine, and nitrendipine blocked Ca2+ current through the slowly inactivating Ca2+ channels of GH4C1 cells. These results demonstrate that Ca2+ channels in an endocrine cell line can be blocked by a variety of molecules including sodium channel toxins and calmodulin antagonists. The data extend the pharmacological similarity between Ca2+ channels in pituitary and other excitable cells and suggest a structural similarity among several cellular proteins.

Alpha-2 adrenergic modulation of norepinephrine secretion in the perfused rabbit iris-ciliary body

Clonidine and other selective alpha-2 adrenergic agonists have been found to lower intraocular pressure in the eyes of rabbits and primates, including humans. It has been suggested that the ocular hypotensive response to alpha-2 agonists may be mediated, in part, by prejunctional inhibition of norepinephrine secretion at intraocular synapses. In this study, we have investigated the effects of adrenergic agonists and antagonists on field-stimulated, Ca++-dependent release of 3H-norepinephrine (3H-NE) from isolated, perfused rabbit iris-ciliary bodies and have utilized radioligand binding methods to identify prejunctional adrenoceptors in this tissue. Clonidine (10(-9)-10(-5) M) produced a dosage-dependent inhibition of stimulation-evoked 3H-NE secretion (EC50 approximately equal to 3 X 10(-8) M), but did not alter basal secretion. Other adrenergic agonists capable of activating alpha-2 adrenoceptors (e.g., epinephrine, norepinephrine and xylazine) also significantly depressed 3H-NE secretion, whereas selective alpha-1 adrenergic or beta adrenergic agonists were without effect. Clonidine-mediated inhibition of 3H-NE release was reversed by the selective alpha-2 antagonist yohimbine (10(-7) M), but was unaffected by prazosin or timolol. Yohimbine alone markedly enhanced 3H-NE secretion, indicating tonic activation of prejunctional alpha-2 adrenoceptors by endogenous released norepinephrine. Forskolin or 8-bromo-cAMP, which alone enhanced norepinephrine secretion, failed to attenuate the inhibitory responses to alpha-2 agonists. 3H-rauwolscine binding measurements showed a small decrease in alpha-2 receptor sites in iris-ciliary body membranes following surgical sympathetic denervation. It is concluded that the rabbit iris-ciliary body contains functional, prejunctional alpha-2 adrenoceptors which may play an autoregulatory role in vivo and contribute to the ocular effects of adrenergic drugs.

Nimodipine block of calcium channels in rat anterior pituitary cells

1. Ca channels were studied in the GH4C1 clonal cell line derived from rat anterior pituitary cells. The whole-cell variation of the patch-electrode voltage-clamp technique was used. 2. Two types of Ca channels were found. One type ('slowly inactivating' channels) is insensitive to changes in holding potential, does not inactivate during test pulses lasting several seconds, and deactivates very quickly upon repolarization. For holding potentials less than -40 mV, a second type of Ca channel is available for opening. This population ('transient' channels) differs from the first type in that it activates at more negative potentials, inactivates rapidly with either Ca or Ba as the charge carrier, deactivates about 10 times more slowly upon repolarization, and is less selective for Ba over Cs. 3. Nimodipine preferentially blocks the slowly inactivating channels. Block of these channels is time- and voltage-dependent, such that block is maximized by long depolarizations. 4. A comparison of the voltage dependence of steady-state nimodipine block with the voltage dependence of channel activation indicates that channel block is directly proportional to the number of open channels. The results are accounted for by a model that postulates 1:1 high-affinity drug binding to open Ca channels. The apparent dissociation constant for binding to open channels is 517 pM. Similar binding constants were previously reported for the inhibition of high-K-induced hormone secretion and high-affinity ligand binding of [3H]nimodipine to isolated plasma membranes. 5. The rate of onset of nimodipine block increases with the test potential, in quantitative agreement with the model of open-channel block. The apparent association rate is about 9.6 X 10(7) M-1 s-1; the dissociation rate is about 0.050 s-1. At therapeutic concentrations (less than 10 nM) nimodipine block takes many seconds to reach equilibrium. 6. Nimodipine should have little effect on stimulus-secretion coupling in healthy pituitary cells in vivo because: (a) the drug binds very weakly to the transient channels that are open at normal resting potentials, and (b) negligible high-affinity binding occurs during spontaneous activity because the onset of block is very slow.

The calcium channel antagonists receptor from rabbit skeletal muscle. Reconstitution after purification and subunit characterization

The Ca2+ channel antagonists receptor from rabbit skeletal muscle was purified to homogeneity. Following reconstitution into phosphatidylcholine vesicles, binding experiments with (+)[3H]PN 200-110, (-)[3H]D888 and d-cis-[3H]diltiazem demonstrated that receptor sites for the three most common Ca2+ channel markers copurified with binding stoichiometries close to 1:1:1. Sodium dodecyl sulfate gel analysis of the purified receptor showed that it is composed of only one protein of Mr 170,000 under non-reducing conditions and of two polypeptides of Mr 140,000 and 32,000 under disulfide-reducing conditions. Iodination of the protein of Mr 170,000 and immunoblots experiments with antisera directed against the different components demonstrated that the Ca2+ channel antagonists receptor is a complex of Mr 170,000 composed of a polypeptide chain of Mr 140,000 associated to one polypeptide chain of Mr 32,000 by disulfide bridges. One of the problems concerning this subunit structure of the putative Ca2+ channel was the presence of smaller polypeptide chains of Mr 29,000 and 25,000. Peptide mapping of these polypeptide chains and analysis of their cross-reactivity with sera directed against the proteins of Mr 170,000 and 32,000 demonstrated that they were degradative products of the Mr 32,000 component. Both the large (140 kDa) and the small (32 kDa) component of the putative Ca2+ channel are heavily glycosylated. At least 20-22% of their mass were removed by enzymatic deglycosylation. Finally the possibility that both the 140-kDa and 32-kDa components originate from a single polypeptide chain of Mr 170,000 which is cleaved by proteolysis upon purification is discussed.

Neurochemical characteristics of a novel dorsal root ganglion X neuroblastoma hybrid cell line, F-11

We investigated the properties of the novel dorsal root ganglion (DRG) hybrid cell line F-11 to see how closely these cells resembled normal DRG cells. Under normal growth conditions, F-11 cells appeared to contain several short neurite-like processes. However, these cells could also be grown under conditions in which they showed a much more extensive neuronal morphology, exhibiting many long neurites. Several differentiated features of DRG cells were present on F-11 cells. These included the presence of delta-opioid receptors, receptors for prostaglandins and bradykinin, and dihydropyridine-sensitive calcium channels. F-11 cells also synthesized and released a substance P-like compound, as determined by immunoreactivity. Both the number of bradykinin receptors and the voltage-sensitive calcium influx increased on cell differentiation. Opioid agonists (delta-specificity) were found to decrease cyclic AMP levels in F-11 cells in a naloxone- and pertussis toxin-reversible fashion. Bradykinin stimulated the synthesis of inositol-1,4-bisphosphate and inositol-1,4,5-trisphosphate. Ca2+ channel agonists stimulated voltage-sensitive Ca2+ influx in a dose-dependent, stereospecific manner, whereas Ca2+ channel antagonists inhibited Ca2+ influx. F-11 cells should, therefore, prove useful as models for authentic DRG neurons.

Enhancement of vasoconstrictor and attenuation of vasodilator effects of 5-hydroxytryptamine by the calcium channel blockers nimodipine and nifedipine in the pig

As calcium (Ca2+) channel blockers are effective against the vasoconstrictor responses to 5-hydroxytryptamine (5-HT) in vitro, and a favourable response is claimed for these drugs in migraine prophylaxis, we studied the interaction between nimodipine or nifedipine, and 5-HT for effects on carotid haemodynamics in the anaesthetized pig. Intracarotid infusions of nimodipine (0.25 microgram X kg-1 X min-1), nifedipine (0.75 microgram X kg-1 X min-1) or 5-HT (2.0 micrograms X kg-1 X min-1) caused a redistribution of carotid blood flow in favour of the nutrient (capillary) fraction at the expense of the non-nutrient (arteriovenous anastomoses; AVA) fraction. Compared to those of 5-HT, the effects of the Ca2+ channel blockers on cranial AVAs were much weaker and the increase in the capillary fraction was observed mainly in the skeletal muscles, rather than in the skin and ears as with 5-HT. When 5-HT was infused in the presence of nimodipine or nifedipine, the amine-induced vasoconstrictor responses in the total carotid vascular bed and its AVA fraction were either not attenuated or were increased while the vasodilator responses were reduced. We conclude that: in contrast to what was found in vitro, the 5-HT-induced vasoconstriction in vivo, involving either '5-HT1-like' (AVAs) or 5-HT2 (arterioles) receptors, was not antagonized by nimodipine or nifedipine; the attenuation of the 5-HT-induced dermal vasodilatation by the two Ca2+ channel blockers is most likely to be the result of a 'steal' due to the profound vasodilatation in the skeletal muscle region; and the comparatively mild reduction in AVA conductance caused by the Ca2+ channel blockers may be one of the reasons for their inability to abort acute attacks of migraine. The increase in nutrient blood flow is of potential benefit, but whether this property of the Ca2+ channel blockers is linked to their usefulness in migraine prophylaxis remains to be ascertained.

Different target sizes of the voltage-dependent Ca2+ channel and the [3H]nitrendipine binding site in rat brain

High energy irradiation of frozen rat frontal cortex was used to determine the molecular target sizes of the [3H]nitrendipine binding site (93,500 +/- 9,700 daltons and of the voltage-dependent Ca2+ channel (340,500 +/- 34,000 daltons) determined as the K+-induced 45Ca2+ uptake into synaptosomes. The conclusion that could be drawn from the target size analysis was that 45Ca2+ flux occurs via a structure of molecular weight very different from that of the dihydropyridine binding site. Furthermore, it was found that [3H]nitrendipine binding to synaptosomes was not influenced by depolarization with K+.

Calcium channels and calcium channel antagonists

Changes in free intracellular Ca2+ levels provide signals that allow nerve and muscle cells to respond to a host of external stimuli. A major mechanism for elevating the level of intracellular Ca2+ is the influx of extracellular Ca2+ through voltage-dependent channels in the cell membrane. Recent research has yielded new insights into the physiological properties, molecular structure, biochemical regulation, and functional heterogeneity of voltage-dependent Ca2+ channels. In addition, Ca2+ channel antagonist drugs have been developed that are valuable both as probes of channel structure and function and as therapeutic agents. Preliminary evidence suggests that these drugs may be useful in the treatment of diverse neurological disorders, including headache, subarachnoid hemorrhage, stroke, and epilepsy.

Presynaptic Ca-antagonist omega-conotoxin irreversibly blocks N-type Ca-channels in chick sensory neurons

The present studies on electrophysiological and pharmacological differences of the three types of Ca-currents (N-, L- and T-types) in whole-cell clamped, cultured embryonic chick sensory neurons revealed that the majority (94%) of the Ca-currents in the nerve cells were the N-type, omega-Conotoxin (omega CTX, 5 microM), a blocker of transmitter release at the presynaptic terminals, induced a complete and irreversible blockage of Ca-currents elicited from the resting membrane potential (-60 mV) in 29 cells among 58. The Ca-currents thus irreversibly blocked by the omega CTX were determined as the N-type (neuronal), as they were insensitive to nifedipine (5 microM) or were reduced in amplitude by Bay K 8644 (5 microM). A small fraction (12%) of the total Ca-currents, which were still present after the omega CTX treatment (in the rest of 29 cells), were pure L-type (long-lasting) Ca-currents, as they were enhanced by the Bay K and were blocked by the nifedipine. omega CTX was a partial and reversible blocker of the L-type Ca-currents. Furthermore, T-type (transient) Ca-currents elicited in the hyperpolarized membrane (at -100 mV) were blocked by omega CTX in an incomplete and reversible manner. The N-type Ca-currents thus separated in the nerve cells exhibited various differences in features of the voltage-dependence and ionic selectivity from the L- and T-type Ca-currents.

Multiple calcium channels and neuronal function

Recent investigations have demonstrated that neurons have a number of different types of calcium channels, each with their own unique properties and pharmacology. These calcium channels may be important in the control of different aspects of nerve activity. Some of the possibilities can now be discussed.

Changes in duodenal contractility induced by "calcium antagonists" with different modes of action

The inhibitory action of nifedipine, verapamil, diltiazem and trifluoperazine has been examined on isolated duodenum from rats and rabbits. On rabbit duodenum Ca2+ antagonists caused a reduction of the spontaneous motility in very low concentrations (10(-12)-10(-6)M). On rat duodenum Ca2+ antagonists inhibited the contractile response to BaCl2, CaCl2 and to field stimulation, nifedipine being the most potent compound (threshold concentration down to 10(-12)M). The above results indicated that Ca2+ antagonists can markedly alter the duodenal motility, both basal and drug-stimulated. The high potency of nifedipine and the selective antagonism by Bay K 8644 against nifedipine suggest the presence of a specific receptor for the dihydropyridines (DHP receptor) in the duodenum.

Verapamil in refractory schizophrenia: a case report

Verapamil, a papaverine calcium channel blocker, has been used effectively and safely in the treatment of angina pectoris and auricular arrhythmias, and more recently in the treatment of mania. Many antipsychotic drugs show calcium channel blocking effects similar to verapamil's. A 41 year old male schizophrenic, only partially responsive to haloperidol decanoate and oral haloperidol, was given increasing doses of verapamil concomitantly, and monitored clinically and by the BPRS, electrocardiogramme, and other laboratory measures. The patient's total BPRS score dropped from 79 to 41 and remained stable, after initial worsening at lower doses, at verapamil 80 mg po qid. Mild fatigue was the only side effect. Further investigation of verapamil in the treatment of schizophrenia is warranted.

Effects of Ca antagonists on the action potential and their relationship to the muscarinic ACh actions in isolated sympathetic neurons of rabbits

Muscarinically induced depressions of the shoulder in the falling phase as well as the after-spike-hyperpolarization and -depolarization of the action potential in the isolated sympathetic neurons of rabbits were mimicked by a novel peptide Ca channel blocker, omega-conotoxin (synthetic; 0.1-0.5 microM). Cobalt ions (0.1-2 mM) showed bidirectional effects on the shoulder, an early depression followed by a later prolongation, while they consistently induced depressions of other components. Organic Ca channel blockers, verapamil and D-600 (1-50 microM) and nifedipine (0.1-1 microM) appeared to have other effects as they rather caused a prolongation of the falling phase that was shortened by further application of acetylcholine.

gamma-Aminobutyric acid release from synaptosomes as influenced by Ca2+ and Ca2+ channel blockers

We studied the correlation between the high affinity binding of Ca2+ channel blockers to purified synaptic plasma membranes (SPM) and the effect of these drugs in blocking the 45Ca2+ uptake and the release of [3H]gamma-aminobutyric acid [( 3H]GABA) by preloaded synaptosomes. The Ca2+ channel blocker binding sites were characterized by studying the binding of the dihydropyridine, [3H]nimodipine, and of the phenylalkylamine, (-)-[3H]desmethoxyverapamil, to purified SPM isolated from sheep brain cortex synaptosomes. The purified SPM had high affinity binding sites for both Ca2+ channel blockers. The binding parameters were similar to those previously reported for whole brain homogenates: KD = 0.64 nM and Bmax = 160 fmol/mg of protein for [3H]nimodipine, and KD = 7.9 nM and Bmax = 1,500 fmol/mg of protein for (-)-[3H]desmethoxyverapamil. The Ca2+ channel blockers inhibited the release of [3H]GABA induced by K+ depolarization in the presence or in the absence of Ca2+. The Ca2+-dependent component of [3H]GABA release was inhibited by verapamil, (-)-D 600, d-cis-diltiazem, nifedipine and PY 108-86 with IC50 values of 2.2 X 10(-5) M, 6.3 X 10(-5) M, 3 X 10(-4) M, greater than 10(-4) M and 3 X 10(-5) M, respectively. Furthermore, the Ca2+ channel blockers also inhibited the Ca2+-independent [3H]GABA release which occurred in the presence, but not in the absence, of external Na+. The Ca2+ channel blockers at concentrations which inhibited [3H]GABA release inhibited the entry of Ca2+ through the Ca2+ channels and also the entry of Ca2+ by Na+/Ca2+ exchange. We conclude that the concentrations of Ca2+ blockers necessary to block Ca2+ uptake through the Ca2+ channels and by Na+/Ca2+ exchange coincide with the concentrations at which they inhibit [3H]GABA release, but that their effect on the relationship between Ca2+ uptake and [3H]GABA release is different for the various blockers. The effects of the drugs on Ca2+ movements and [3H]GABA release are not specifically mediated through the high affinity binding of the drugs since relatively high concentrations were necessary (greater than 10(-5) M) for the effects reported here.

Bay K 8644 induces a reversible spasticity-like syndrome in rats

The dihydropyridine Bay K 8644 exerts a positive modulation of Ca2+ channels. Administration of Bay K 8644 3-5 mg/kg i.p. to rats induces within 15 min a severe spasticity syndrome consisting of stiff tail, arched back, stretching and twisting of forelimbs and hindlegs and backwards motility and rolling over. The syndrome was effectively antagonized by nifedipine 3-30 mg/kg but not by the other Ca2+ channel blockers flunarizine, diltiazem and verapamil. Diltiazem even enhanced the spasticity. Diazepam 10-30 mg/kg i.p. completely blocked the spasticity whereas the other muscle relaxants (-)-baclofen and the beta-carboline ZK 93423 were completely inactive. These findings with Bay K 8644 suggest that spasticity may be caused by changed Ca2+ homeostasis.

Ionic channels: II. Voltage- and agonist-gated and agonist-modified channel properties and structure

This article reviews the different forms of ionic channels: voltage-gated, agonist-gated, and agonist- and second messenger-modified channels. The recent advances in our knowledge of the amino acid sequence of the sodium channel and the nicotinic acetylcholine receptor and the relationship of the primary structure to the channels' quarternary structure and function are discussed.

Characteristics of a Ca2+/calmodulin-dependent binding of the Ca2+ channel antagonist, nitrendipine, to a postsynaptic density fraction isolated from canine cerebral cortex

Synaptic membrane (SM) and postsynaptic density (PSD) fractions isolated from the cerebral cortex (CTX) and cerebellum (CL) of the canine brain were found to contain one class of specific nitrendipine binding sites. The specific binding constants were: CTX-SM, Kd = 110 pM (Bmax = 126 fmol/mg protein); CTX-PSD, Kd = 207 pM (Bmax = 196 fmol/mg); CL-SM, Kd = 100 pM (Bmax = 65 fmol/mg); CL-PSD, Kd = 189 pM (Bmax = 80 fmol/mg). Treatment of the CTX-SM and CTX-PSD fractions with 0.5% deoxycholate and 1.0% N-lauroyl sarcosinate removed 88-91% and 42-51% of the nitrendipine binding, respectively, indicating that the major nitrendipine binding present in the SM fractions are of non-synaptic origin. Moreover, the percentages of total protein and specific nitrendipine binding removed from PSDs by these detergents were similar, indicating no preferential dissociation of the latter, and suggesting that the receptor protein is firmly bound and is probably an intrinsic component of the PSD fraction. Both Ca2+ and calmodulin were found to be important for the binding of nitrendipine to the CTX-SM and CTX-PSD fractions since: R24571, a calmodulin antagonist, was found to inhibit nitrendipine binding to the CTX-SM and CTX-PSD fractions with IC50 values of 1.1 microM and 0.9 microM, respectively; removal of Ca2+ from the CTX-SM and CTX-PSD fractions with 0.2 mM EGTA resulted in losses of specific nitrendipine binding of 80 and 90%, respectively; Ca2+ alone restored nitrendipine binding to EGTA-pretreated CTX-SM fractions and not to CTX-PSD fractions, with the latter needing both Ca2+ and calmodulin to restore nitrendipine binding; EGTA treatment removed 14-16% and 89-91% of nitrendipine bound to the CTX-SM and CTX-PSD fractions, respectively, suggesting that calmodulin (but not Ca2+) is needed to maintain the nitrendipine-nitrendipine receptor-calmodulin complex; Ca2+-reconstituted EGTA-pretreated CTX-SM fractions and the Ca2+ plus calmodulin-reconstituted EGTA-pretreated CTX-SM and CTX-PSD fractions were found to have similar binding constants to those for the corresponding native, untreated fractions; and the Ca2+/calmodulin dependency on nitrendipine binding was similar to the well-known Ca2+/calmodulin dependency on phosphorylation in EGTA-pretreated PSD fractions. It needed much less Ca2+ to saturate Ca2+/calmodulin-dependent phosphorylation of the pretreated CTX-PSD fractions than the nitrendipine binding. Yet, less calmodulin was needed to saturate nitrendipine binding than the phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroleptics of the diphenylbutylpiperidine series are potent calcium channel inhibitors

[3H]Fluspirilene, a neuroleptic molecule of the diphenylbutylpiperidine series, binds to skeletal muscle transverse tubule membranes with a high affinity corresponding to a Kd of 0.11 +/- 0.04 nM, A 1:1 stoichiometry was found between [3H]fluspirilene binding and the binding of (-)-[3H]desmethoxyverapamil [(-)[3H]D888], one of the most potent Ca2+ channel inhibitors. Ca2+ channel inhibitors such as D888, verapamil, gallopamil, bepridil, or diltiazem antagonize [3H]fluspirilene binding besides antagonizing (-)[3H]-D888 binding. Neuroleptics, especially those of the diphenylbutylpiperidine family, also antagonize both (-)[3H]D888 binding and [3H]fluspirilene binding. There is an excellent correlation between affinities found from [3H]fluspirilene binding experiments and those found from (-)[3H]D888 binding experiments. Analysis of the properties of these cross-inhibitions indicates that [3H]fluspirilene binds to a site that is not identical to that for phenylalkylamine derivatives (gallopamil, verapamil, diltiazem, and bepridil). Voltage-clamp experiments have shown that fluspirilene is an efficient inhibitor of the voltage dependent Ca2+ channel, achieving a half-maximal effect near 0.1-0.2 nM and nearly complete blockade at 1 nM. Fluspirilene blockade has little voltage dependence.

Antihypertensive mechanism of action and binding sites of nitrendipine

Members of the 1,4 dihydropyridine category of the calcium channel antagonists, including nitrendipine, have defined structure-activity relations that involve both their specificity and their sites of action. Other categories of calcium channel antagonists, such as D600, verapamil and diltiazem, differ from the 1,4 dihydropyridines in a number of aspects, including lower potency. All of the calcium channel antagonists share the ability to inhibit calcium currents passing through a voltage-dependent calcium channel. The 1,4 dihydropyridines are relatively selective for vascular smooth muscle. Because of this selectivity, nitrendipine has potent action as an antihypertensive drug. Although biochemical radioligand binding studies have shown that the same type of high affinity binding sites occur in cardiac muscle as in vascular smooth muscle, it is not fully established why the 1,4 dihydropyridines are not as potent in cardiac as in vascular muscle. Other members of the very large 1,4 dihydropyridine category of calcium channel antagonists include nifedipine, nimodipine and nisoldipine. The 1,4 dihydropyridines also include recently discovered calcium channel activators, including the compound Bay K 8644. These calcium channel activators are diametrically opposed in properties to nifedipine and nitrendipine. Thus, this single structural category of compounds has a complete spectrum of pharmacologic activity at Ca++ channels, from turning on to turning off.

Interaction of 1,4-dihydropyridines with somatic Ca currents in hippocampal CA1 neurones of the guinea pig in vitro

Two distinct Ca currents may be recorded from guinea pig hippocampal CA1 neuronal somata: a non-inactivating (persistent) current and a rapidly inactivating (transient) current. Nimodipine, a 'calcium antagonist', reduced both Ca currents though the transient current was somewhat less sensitive than the persistent current. BAY K 8644, a 'calcium agonist' enhanced the persistent current and shifted the activation threshold for this current to more negative potentials. BAY K 8644 also enhanced the transient Ca current though the effect was very variable. The results demonstrate that Ca channels in hippocampal CA1 neurones possess a dihydropyridine receptor in common with Ca channels in other tissues.

Modulation of depolarization stimulated 45Ca2+ uptake in cultured neuronal cells by calcium channel activators and antagonists

The effect of dihydropyridine calcium agonists and antagonists on 45Ca2+ uptake into primary neuronal cell cultures was investigated. K+ stimulated neuronal 45Ca2+ accumulation in a concentration dependent manner. This effect was further enhanced by the calcium agonists Bay K 8644 and (+)-(S)-202-791 with EC50 values of 21 nM and 67 nM respectively. The calcium antagonists PN 200-110 and (-)-(R)-202-791 inhibited Bay K 8644 (1 microM) stimulated uptake with IC50 values of 20 nM and 130 nM respectively. 45Ca2+ efflux from neuronal cells was measured in the presence and absence of Na+. Efflux occurred at a much greater rate from cells incubated in the presence of Na+, indicating the existence of an active Na+/Ca2+ exchanger in these neurons. The data suggests that voltage sensitive calcium channels of these neurons are sensitive to dihydropyridines and thus that dihydropyridine binding sites have a functional role in these neuronal cultures.

Stereospecific regulation of [3H]inositol monophosphate accumulation by calcium channel drugs from all three main chemical classes

Depolarization of [3H]inositol-prelabelled rat cortical slices through the elevation of extracellular K+ levels leads to increased accumulation of [3H]inositol phosphates. In the presence of 18 mM K+, Ca2+ channel activators selectively stimulated the formation of [3H]inositol monophosphate ([3H]IP1) whereas Ca2+ channel blockers were inhibitory. Blockade of the Na+ channel by 1 microM tetrodotoxin had no effect but chelation of extracellular Ca2+ abolished the response. The enantiomers of the benzoxadiazol 1,4-dihydropyridine 202-791 showed opposite stereospecific regulation of [3H]IP1 formation: (+)-(S)-202-791 stimulated (252%; ED50: 88 nM), whereas (-)-(R)-202-791 inhibited (65% inhibition, ED50: 602 nM). The (-) enantiomer of Bay K 8644 was a potent [3H]IP1 stimulator (258%; ED50: 82 nM). While (+)-Bay K 8644 was inactive in the presence of 18 mM K+, it completely inhibited the (-)-Bay K 8644-induced stimulation with a Ki of 103 nM. Representatives of the other two main classes of Ca2+ channel blockers (phenylalkylamines and benzothiazepines) inhibited K+ depolarization-induced and (-)-Bay K 8644 enhanced [3H]IP1 formation in a dose-dependent, stereospecific manner. The results show that Ca2+ channel blockers are efficient modulators of depolarization-induced and Ca2+ channel activator-induced [3H]inositol monophosphate formation in brain, and demonstrate the functional coupling of three distinct drug receptor sites on neuronal Ca2+ channels.

Multiple calcium channels mediate neurotransmitter release from peripheral neurons

We examined the effects of dihydropyridine drugs on evoked neurotransmitter release from cultured neonatal rat sensory and sympathetic neurons. Depolarization with K+-rich solutions increased the release of substance P from cultured sensory neurons. This release was enhanced by BAY K8644 and (+)-202791 and was blocked by a variety of other dihydropyridines including (-)-202791, by Co2+, or in Ca2+-free solutions. K+-rich solutions also stimulated the release of [3H]norepinephrine from cultured sympathetic neurons. This release was also completely blocked by Co2+ or in Ca2+-free solution. In contrast to the situation in sensory neurons, however, the evoked release of [3H]norepinephrine was completely resistant to the blocking effects of dihydropyridine such as nimodipine. However, BAY K8644 was able to enhance the evoked release of [3H]norepinephrine, and this enhancement was blocked by nimodipine. These results are discussed in relation to the possible participation of multiple types of calcium channels in the release of neurotransmitters.

In vivo effects of the Ca2+-antagonist nimodipine on dopamine metabolism in mouse brain

The effects of the Ca2+-antagonist nimodipine on central dopamine (DA) neurons in mice were investigated in vivo. Nimodipine caused a dose-dependent decrease in the DA metabolite 3-methoxytyramine (3-MT) in striatum and the limbic region. If the brains were microwave radiated immediately after decapitation in order to minimize post-mortal accumulation of 3-MT, the effect of nimodipine was less pronounced and statistically not significant. Nimodipine markedly decreased the accumulation of 3-MT induced by pargyline, an inhibitor of monoamine oxidase, a phenomenon that was not attenuated by microwave radiation. Furthermore, whereas nimodipine had no effect on mouse motor activity when given alone it readily blocked the pargyline-induced increase in activity. The concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) in striatum and the limbic region were also reduced by nimodipine as was the accumulation of 3,4-dihydroxyphenylalanine (DOPA) measured after inhibition of the aromatic amino acid decarboxylase by 3-hydroxybenzylhydrazine (NSD 1015). In addition, nimodipine caused decreased concentrations of DA and homovanillic acid (HVA) in the limbic region but not in striatum. Nimodipine caused an increase in the striatal concentrations of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA); these changes were not seen in the limbic region. In conclusion, nimodipine appears to reduce the release as well as the synthesis of DA in mouse brain. These effects are believed to be related to the Ca2+-antagonism of nimodipine.

Pharmacological characteristics of receptor-operated and potential-operated Ca2+ channels in rat aorta

In the rat aorta activation of the potential-operated Ca2+ channels by 100 mM K+ resulted in a greater 45Ca2+ influx than stimulation of the receptor-operated Ca2+ channels by norepinephrine (NE, 3 X 10(-7) M) or angiotensin II (AII, 10(-7) M). 45Ca2+ influx induced by NE was inhibited by prazosin (10(-7) M) but not by yohimbine (10(-6) M) while that by AII was abolished by [Sar1, Ile8]AII (10(-8) M). These receptor antagonists had no effect on the 45Ca2+ influx produced by K+. Bay k 8644 enhanced the influxes to low concentrations of NE and K+ while it was additive with the maximal concentration of NE but not with K+. 3-Isobutyl-1-methyl-xanthine and forskolin inhibited both the influx and efflux of 45Ca2+ elicited by NE but were ineffective against those caused by K+. Nifedipine blocked the efflux of 45Ca2+ induced by K+ but not that evoked by NE. However, both types of Ca2+ channel exhibited the same sensitivity to inhibition by Ca2+ entry blockers (nifedipine/verapamil) on 45Ca2+ influxes. These data suggest that in the rat aorta, the receptor-operated calcium channels and potential-operated calcium channels share similar structural characteristics. However, they are gated separately and distinctly by their respective activators.

Calcium antagonists in mania: a double-blind study of verapamil

Five of seven acutely manic patients improved significantly when taking verapamil but not placebo in a double-blind crossover study. This finding adds support to two other formal studies of verapamil, several case reports of verapamil in manic patients, and one small study of nifedipine suggesting that some calcium antagonists may have antimanic properties. Indirect evidence links this antimanic action to correction of a disturbance of intracellular calcium dynamics in affective disorders. However, the effectiveness of calcium antagonist drugs could also be related to some property other than interference with the action of calcium within brain neurons.

PK 11195, an antagonist of peripheral type benzodiazepine receptors, modulates Bay K8644 sensitive but not beta- or H2-receptor sensitive voltage operated calcium channels in the guinea pig heart

In a partially depolarized guinea pig papillary muscle preparation, BAY K8644 stimulated voltage-operated calcium channels, promoting slow action potentials; this effect was dose-dependent over a concentration range of 3 X 10(-7) M to 3 X 10(-6) M. Isoproterenol and histamine also induced slow action potentials by stimulating beta or H2 receptors, respectively. PK 11195, the antagonist of peripheral type benzodiazepine receptors, inhibited the effect of BAY K8644, but not those of histamine or isoproterenol. Moreover, PK 11195 "dose-dependently" antagonized the ability of RO5-4864 to inhibit the slow action potentials elicited by barium chloride. Thus, in the heart, PK 11195, an antagonist of peripheral type benzodiazepine receptors, can modulate voltage-operated calcium channels when they are activated directly, but not when they are activated by stimulation of neurotransmitter receptors.

Calcium channel agonists and antagonists regulate protein phosphorylation in intact synaptosomes

Protein phosphorylation in intact synaptosomes is highly sensitive to alterations in calcium fluxes and was used to probe the possible mechanism of action of the calcium channel agonist BAY K 8644 and antagonists verapamil and nifedipine. These agents (at 1 microM) all increased the basal phosphorylation of a specific set of 4 synaptosomal phosphoproteins termed P139, P124, P96 and P60, but did not alter depolarization-dependent protein phosphorylation. The increases could not be explained by a direct stimulation of protein kinases and appears unrelated to the known effects of these drugs on K+-stimulated neurotransmitter release. This finding may reveal a possible new mechanism of action for drugs which interact with calcium channels.

[3H]Nitrendipine binding sites are decreased in the substantia nigra and striatum of the brain from patients with Parkinson's disease

We examined the degree of binding of the calcium antagonist, [3H]nitrendipine ([3H]NDP), in the prefrontal cortex, caudate nucleus, putamen, pallidum and substantia nigra obtained at autopsy from patients with Parkinson's disease. The specific bindings of [3H]NDP were significantly reduced in the caudate nucleus, putamen and substantia nigra, as compared to the relevant controls. Scatchard analyses revealed that these reductions resulted from decreases in the apparent maximum number of binding sites (Bmax). The affinity constants (Kd) remained unchanged. Thus, it is highly likely that calcium channel antagonist binding sites on nigral dopamine (DA) neurons may be lost in the degenerative process of Parkinson's disease.

Calcium entry into cultured mouse astrocytes

The barium-sensitive component of transmembrane influx of 45Ca into cultured mouse astrocytes was investigated in order to find clues for an involvement of a glial Ca2+ uptake system in Ca2+ movements of the CNS. The Ca2+ turnover was very high amounting to a rate constant of 1.4 min-1. The magnitude of the Ca2+ flux was, however, only about 2% of the K+ flux. It had the same magnitude as the Cl- flux. The Ca2+ influx was not sensitive to the organic Ca2+ channel blockers verapamil and nifedipine; however, cadmium, cobalt and barium blocked the flux in millimolar concentrations. The Ca2+ influx was a linear function of the external Ca2+ concentration up to 1.8 mM. A further increase in external Ca2+ did not increase the magnitude of the influx. Increases in external K+ to the physiological ceiling level of 12 mM (which corresponds to a 15 mV depolarization) and to 54 mM (48 mV depolarization) had no effect on the system. We conclude that the Ca2+ entry system of glial cells is not involved in the decrease of the external Ca2+ concentration during neuronal activity. The barium-evoked spontaneous depolarizations of astrocytes reported in the literature are probably due to the blockade of K+ channels by barium, since 50 microM barium did not affect the Ca2+ flux and increased the total Ca2+:K+ flux ratio from 1:50 to 1:2.5.

Nifedipine and flunarizine block amphetamine-induced behavioral stimulation in mice

Sixteen calcium channel inhibitors (CCI's) were tested in a model utilizing amphetamine-induced behavioral stimulation in mice. Nifedipine, flunarizine and possibly PY 108-068 were effective in blocking amphetamine-induced locomotor stimulation. Verapamil, diltiazem and many other CCI's were ineffective in this experimental paradigm.

Phosphorylation and dephosphorylation of dihydropyridine-sensitive voltage-dependent Ca2+ channel in skeletal muscle membranes by cAMP- and Ca2+-dependent processes

The phosphorylation and dephosphorylation of the dihydropyridine-sensitive Ca2+ channel was studied in transverse-tubule membranes isolated from rabbit skeletal muscle. Exposure of these membranes to either the cAMP-dependent protein kinase or a Ca2+/calmodulin-dependent protein kinase resulted in a rapid phosphorylation of a protein with properties similar to the major component of the skeletal muscle Ca2+ channel. The molecular mass of the phosphoprotein was 140 or 160 kDa, depending on the electrophoretic conditions. The stoichiometry of the phosphorylation was calculated to be 0.4-1.0 mol of phosphate per mol of protein. Neither the rate nor the extent of phosphorylation was affected by dihydropyridines. Limited proteolytic digestion of the protein that had been phosphorylated by either or both protein kinases yielded a single phosphopeptide of approximately equal to 5.4 kDa. The Ca2+-dependent phosphatase calcineurin dephosphorylated the membrane-bound Ca2+ channel that had been previously phosphorylated by either protein kinase. The results suggest that the major component of the dihydropyridine-sensitive Ca2+ channel from skeletal muscle can be effectively phosphorylated and dephosphorylated in its native state by cAMP- and Ca2+-dependent processes.

Nerve growth factor modulates the drug sensitivity of neurotransmitter release from PC-12 cells

The release of catecholamines from adrenal chromaffin cells is known to be blocked by dihydropyridines, such as nitrendipine, and enhanced by others, such as BAY K8644. On the other hand, release from sympathetic neurons is predominantly insensitive to these agents. Release of [3H]norepinephrine from undifferentiated PC-12 pheochromocytoma cells resembles that from chromaffin cells in that it is extremely sensitive to dihydropyridines. Following differentiation, however, release of catecholamine becomes predominantly insensitive to both nitrendipine and BAY K8644. Under both growth conditions, release remains completely blocked by 3 mM Co2+ or by removal of Ca2+ from the release media. Dose-response curves to K+ show that following differentiation, cells become more sensitive, releasing transmitter at lower K+ concentrations. In contrast, depolarization-induced uptake of 45Ca2+ remains sensitive to dihydropyridines and shows similar sensitivity to K+ stimulation in both growth conditions. These results can be explained by invoking a model involving dihydropyridine-sensitive and -insensitive types of voltage-sensitive calcium channels.

Maitotoxin-induced release of gamma-[3H]aminobutyric acid from cultures of striatal neurons

The potent marine toxin, maitotoxin, induced the release of gamma-[3H]aminobutyric acid (GABA) from reaggregate cultures of striatal neurons in a dose-dependent manner. Maitotoxin-induced release occurred following a lag period of several minutes and was persistent. Release induced by 70 mM K+ on the other hand was immediate and transient in nature. Co2+ (3 mM) and Cd2+ (1 mM) inhibited maitotoxin-induced release of GABA as did removal of extracellular Ca2+. However, the organic calcium antagonists nisoldipine, nitrendipine, and D-600 at concentrations of 10(-6) M did not block maitotoxin-induced or 70 mM K+-induced release. High concentrations of D-600 (10(-4) M) partially blocked both maitotoxin- and 70 mM K+-induced release. The dihydropyridine calcium agonist BAY K8644 (10(-6) M) did not enhance maitotoxin-induced or 70 mM K+-induced release. Replacement of Na+ in the incubation medium with choline led to an increased basal output of GABA and an apparent inhibition of the effect of maitotoxin. These data are discussed with reference to the hypothesis that maitotoxin can directly activate voltage-sensitive calcium channels.