Inhibition of N- and L-type Ca2+ currents by dopamine in lamprey spinal motoneurons

Dopamine is co-localized with 5-hydroxytryptamine in a ventromedially located plexus in the lamprey spinal cord and reduces Ca2+ currents in motoneurons that express high-voltage activated Ca2+ currents of the N-, L- and P/Q-types. Blockade of L- and P/Q- type channels leaving N-type channels intact reduced but did not prevent the inhibition of the Ca2+ current by dopamine. Dopamine also reduced the L-type current potentiated by BAY K 8644. During simultaneous blockade of N-type and L-type currents, dopamine was unable to affect the remaining Ca2+ current. In addition, blockade of G-proteins abolished the dopaminergic modulation. The inhibition was unaffected by depolarizing prepulses. Thus, dopamine mediates inhibition of N- and L-type currents through a G-protein-dependent, voltage-independent pathway in lamprey spinal motoneurons.     

5-HT1B receptor-mediated presynaptic inhibition at the calyx of Held of immature rats

5-hydroxytryptamine (5-HT) inhibits transmitter release via activating GTP-binding proteins, but the target of 5-HT receptors in the nerve terminal is not determined. We addressed this question at the calyx of Held synapse in the brainstem slice of immature rats. Bath-application of 5-HT attenuated the amplitude of nerve-evoked excitatory postsynaptic currents (EPSCs) associated with an increase in the paired-pulse ratio, whereas it had no effect on the amplitude of spontaneous miniature EPSCs. The 5-HT1B receptor agonist CP93129 mimicked the inhibitory effect of 5-HT, but the 5-HT1A agonist (R)-(+)-8-hydroxy-DPAT (8-OHDPAT) had no effect. The 5-HT1B receptor antagonist NAS-181 blocked the inhibitory effect of 5-HT. These results suggest that 5-HT activated 5-HT1B receptors in calyceal nerve terminals, thereby inhibiting transmitter release. In direct whole-cell recordings from calyceal nerve terminals, 5-HT attenuated voltage-dependent Ca2+ currents, but had no effect on voltage-dependent K+ currents. When EPSCs were evoked by presynaptic Ca2+ currents during simultaneous pre- and postsynaptic recordings, the magnitude of the 5-HT-induced inhibition of Ca2+ currents fully explained that of EPSCs. Upon repetitive applications, 5-HT showed tachyphylaxis, with its effect on both EPSCs and presynaptic Ca2+ currents becoming weaker in the second application. 1,2-bis(o-aminophenoxy)ethane-N-N'-N'-N'-tetraacetic acid (BAPTA; 10 mm) loaded into the nerve terminal abolished this tachyphylaxis. The presynaptic inhibitory effect of 5-HT was prominent at postnatal day 5, but became weaker as animals matured. We conclude that activation of 5-HT1B receptors inhibits voltage-gated Ca2+ channels, thereby inhibiting transmitter release at immature calyceal nerve terminals, and that 5-HT1B receptors undergo Ca2+-dependent tachyphylaxis on repetitive activations.     

Adenosine inhibits L-type Ca2+ current and catecholamine release in the rabbit carotid body chemoreceptor cells

In an in vitro preparation of the intact carotid body (CB) of the rabbit, adenosine (100 microM) inhibited hypoxia-induced catecholamine release by 25%. The specific A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 1 microM) prevented the inhibition and increased the response to hypoxia further. In isolated chemoreceptor cells from the same species, adenosine inhibited voltage-dependent Ca2+ currents by 29% at 1 microM (concentration producing half-maximal inhibition, IC50 = 50 nM). This inhibition was mimicked by R(-)N6-(2-phenylisopropyl)-adenosine and 2-chloroadenosine (1 microM), two purinergic agonists poorly active at the intracellular ('P') site, and persisted in the presence of dipyridamole (a blocker of adenosine uptake; 1 microM) and was fully inhibited by 8-phenyltheophylline (10 microM). The A1 antagonists DPCPX (10 microM) and 8-cyclopentyl-1,3-dimethylxantine (0.1 microM) inhibited the effect of adenosine by 93% (IC50 = 0.14 microM) and 59%, respectively. The inhibition of the Ca2+ current (I(Ca)) was reduced by nisoldipine (an L-type Ca2+ channel antagonist) by nearly 50%, and was unaltered by omega-conotoxin GVIA, a blocker of N-type Ca2+ channels. Adenosine did not affect the voltage-dependent Na+ current (I(Na)) or K+ current (I(K)). We conclude that adenosine A1 receptors are located in chemoreceptor cells and mediate the inhibition of L-type Ca2+ channels and thereby the release of catecholamines produced by hypoxia. The data also indicate that endogenous adenosine acts as a physiological negative modulator of the chemoreceptor cell function. The previously reported excitatory action of adenosine on the activity of the sensory nerve of the CB is discussed in terms of a balance between the inhibition mediated by A1 receptors and the excitation mediated by A2 receptors.     

Halothane inhibits two components of calcium current in clonal (GH3) pituitary cells.

The effect of halothane on isolated calcium (Ca2+) current of clonal (GH3) pituitary cells was investigated using standard whole-cell clamp techniques at room temperature. Halothane (0.1-5.0 mM) reversibly reduced both the low-threshold, transient [low-voltage-activated (LVA)] component and the high-threshold [high-voltage-activated (HVA)] component of Ca2+ current. Halothane had little effect on the voltage dependence of activation or inactivation of either component of Ca2+ current. Inhibition of the peak high-threshold Ca2+ current was half-maximal at about 0.8 mM halothane, with maximal inhibition (100%) occurring with 5 mM halothane. When measured at the end of a 190-msec command step, half-maximal reduction of high-threshold current occurred at less than 0.5 mM halothane. The low-threshold transient current was less sensitive to halothane, with half-maximal inhibition of peak transient current activated at -30 mV occurring at approximately 1.3 mM. The effect of halothane on the HVA current was apparently not mediated by changes in intracellular Ca2+ concentration. The ability of halothane to inhibit Ca2+ current was unaffected by either the inclusion of the rapid Ca2+ buffer 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA) in the recording pipette or exposure of the cell to 10 mM caffeine. To assess the selectivity of the effect of halothane, the actions of halothane on two components of voltage-activated potassium (K+) current observed in the absence of extracellular Ca2+ and on voltage-dependent sodium (Na+) current were also examined. Halothane had no effect on the voltage-dependent, inactivating K+ current of GH3 cells at concentrations up to 1.2 mM. In contrast, the non-inactivating K+ current, though less sensitive to halothane than either Ca2+ current, was reduced by about 40% by 1.2 mM halothane at +20 mV. Peak Na+ current was also blocked by halothane, but 50% block required around 2.6 mM halothane with little effect at 1.6 mM. Reduction of Na+ current was associated with a substantial negative shift in the steady-state inactivation curve. Although the results indicate that a number of voltage-dependent ionic currents are sensitive to halothane, both components of Ca2+ current exhibit a greater sensitivity to halothane than any of three other voltage-dependent currents in GH3 cells. These results show that GH3 cell Ca2+ currents are selectively inhibited by clinically appropriate concentrations of halothane and that the reduction of Ca2+ current can account for the inhibition by halothane of TRH- or KCl-induced prolactin secretion in GH3 cells.     

N-type Ca2+ channels in cultured rat sphenopalatine ganglion neurons: an immunohistochemical and electrophysiological study.

Results from pharmacological studies have suggested that presynaptic N-type Ca2+ channels play an important role in regulating neuronal Ca2+ influx and transmitter nitric oxide (NO) release in isolated cerebral arteries. However, the presence of N-type Ca2+ channels in cerebral perivascular nerves has not been directly demonstrated. As a major source of cerebral perivascular NOergic innervation is the sphenopalatine ganglion (SPG), adult rat SPGs were cultured and examined by whole-cell patch-clamp technique. One week after growing in the culture medium, significant neurite outgrowth from the SPG neuronal cells was observed. Both soma and neurites of these cells were immunoreactive for N-type Ca2+ channels, transmitter-synthesizing enzymes (choline acetyltransferase and NO synthase), and several neuropeptides (vasoactive intestinal peptide, neuropeptide Y, calcitonin gene-related peptide, substance P, and pituitary adenylate cyclase-activating peptide-38) that had been found in cerebral perivascular nerves in whole-mount vascular preparations. In current-clamp recordings, injection of a small depolarizing current caused action potential firing. In voltage-clamp recordings, the fast inward currents were blocked by tetrodotoxin and outward currents by tetraethylammonium, which is typical for neurons. Most Ca2+ currents isolated by blockade of sodium and potassium currents were blocked by omega-conotoxin, indicating that N-type Ca2+ channels are the dominant voltage-dependent Ca2+ channels regulating Ca2+ influx during membrane depolarization of SPG neurons. The ability to culture postganglionic SPG neurons provides an opportunity to directly study the electrophysiological and pharmacological properties of these neurons.     

Zonisamide blocks T-type calcium channel in cultured neurons of rat cerebral cortex.

We investigated the effect of zonisamide, a new antiepileptic drug, on voltage-dependent Ca2+ currents in cultured neurons of rat cerebral cortex. Whole-cell voltage-clamp recordings demonstrated at least two distinct voltage-dependent Ca2+ currents: (1) a low-threshold, rapidly inactivating component, T-type Ca2+ current, which is sensitive to 100 microM Ni2+, and (2) a high-threshold, slowly inactivating (long-lasting) component, L-type Ca2+ current. Zonisamide, a new anticonvulsant effective against maximal electroshock (MES) seizures in mice reduced T-type Ca2+ current in a dose-dependent manner. The mean percentage of reduction was 59.5 +/- 7.2% at 500 microM, but zonisamide had no effect on L-type Ca2+ current. A methylated analog of zonisamide, which is ineffective against MES seizures in mice, was tested at a concentration of 500 microM, and reduced neither T-type nor L-type Ca2+ current. These findings suggest that the effects of zonisamide against MES seizures might occur through the reduction of T-type Ca2+ current. Because drugs that are effective against MES seizures are thought to prevent seizure discharge spread, T-type Ca2+ channels could underlie a cellular mechanism of spreading activity in epileptic seizures.     

Dopamine-D2 Actions on Voltage-Dependent Calcium Current and Gonadotropin-II Secretion in Cultured Goldfish Gonadotrophs

Dopamine D₂-receptor activation directly inhibits GnRH-induced gonadotropin-II (maturational gonadotropin, GTH-II) secretion from goldfish pituitary cells. In this study, we show that dopamine and its D₂ agonist, quinpirole, reduced GTH-II secretion induced by either high extracellular K⁺ concentration or the voltage-gated Ca²⁺ channel agonist, Bay K 8644. These actions of dopamine were blocked by addition of the dopamine D₂-receptor antagonist, spiperone. The actions of dopamine on Ca²⁺ current in single identified goldfish gonadotrophs were assessed in voltage-clamp experiments using Ba²⁺ as the charge carrier through voltage-gated Ca²⁺ channels. Dopamine caused a concentration-dependent reduction in Ba²⁺ current amplitude with an EC₅₀ of 1.0±0.3 nM, but did not shift the current-voltage relationship. The D₂ agonist quinpirole also caused a dose-dependent reduction in the Ba²⁺ current amplitude with an EC₅₀ of 2.7±1.4 nM. Quinpirole slowed the activation and inactivation kinetics, as well as removing the steady-state inactivation properties of the Ba²⁺ current. In contrast to the actions of quinpirole, the dopamine D₁-receptor agonist, SKF 38393, did not affect the Ba²⁺ current. The inhibitory action of dopamine on voltage-dependent Ca²⁺ currents was reversed by spiperone, but not by the D₁ antagonist SKF 83566. Voltage-dependent Na⁺ and K⁺ currents were not affected by dopamine or dopamine agonists. These data indicate that dopamine D₂-receptor activation reduces Ca²⁺ influx through voltage-dependent Ca²⁺ channels to inhibit GTH-II secretion.     

Expression of membrane currents in rat diencephalic neurons in serum-free culture

The whole-cell gigaseal voltage clamp technique has been used to investigate the timing of expression and type of voltage-dependent ionic currents in dissociated primary cultures of fetal rat (E17) diencephalic neurons grown in a serum-free defined medium. The expression of membrane currents varied among cells at any particular time in culture. Despite this variability, certain characteristics of the appearance of ionic currents emerge from this study. These are: (i) the earliest appearing membrane current is a voltage-dependent outward current carried by K+. In some cells, it is the classical delayed rectifier current, whereas in others it is the transient outward current (IA). (ii) The earliest appearing inward current is carried by Na+. In some cells the channels are first expressed in the neurites and then in or near the cell body. The early neuritic Na+ channels are blocked by cobalt or cadmium as well as by tetrodotoxin (TTX). In others, the early Na+ channels appear in or near the cell body and are only blocked by TTX. (iii) With additional time in culture, a majority of cells exhibit a Ca2+ current at the time of Na+ channel appearance in or near the cell body as well as a transient Ca2+-dependent outward current. The Ca2+ current is only a small fraction of the total inward current. These inward currents show the classical pharmacologic profile. The complex pattern of expression of ionic current may reflect multiple populations of neurons with different developmental sequences resulting from differences in cell age and lineage.     

Identification and characterization of an L-type Cav1.2 channel in spiral ligament fibrocytes of gerbil inner ear

Intracellular free Ca2+ levels are critical to the activity of BK channels in inner ear type I spiral ligament fibrocytes. However, the mechanisms for regulating intracellular Ca2+ levels in these cells are currently poorly understood. Using patch-clamp technique, we have identified a voltage-dependent L-type Ca2+ channel in type I spiral ligament fibrocytes cultured from gerbil inner ear. With 10 mM Ba2+ as the conductive cation, an inwardly rectifying current was elicited with little inactivation by membrane depolarization. The voltage activation threshold and the half-maximal voltage activation were -40 and -6 mV, respectively. This inward whole-cell current reached its peak at around 10 mV of membrane potential. The amplitude of the peak current varied among cells ranging from 50 to 274 pA with an average of 132.4 +/- 76.2 pA (n = 19); 10(-6) M nifedipine significantly inhibited the inward currents by 90.3 +/- 1.2% (n = 11). RT-PCR analysis revealed that cultured type I spiral ligament fibrocytes express the alpha1C isoform of the L-type Ca2+ channels encoded by the Cav1.2 gene. The expression of this channel in gerbil inner ear was confirmed by RT-PCR analysis using freshly isolated spiral ligament tissues. The Cav1.2 channel may function in conjunction with a previously identified intracellular Ca-ATPase (SERCA) to regulate intracellular free Ca2+ levels in type I spiral ligament fibrocytes, and thus modulate BK channel activity in these cells.     

Voltage- and Ca(2+)-activated ionic currents in acutely dissociated cells of the chick pineal gland.

Previous research on cultured chick pineal cells suggests that melatonin production is modulated by Ca2+ influx through voltage-dependent Ca2+ channels. The possible existence of other ionic currents was investigated by means of whole-cell recordings from acutely isolated cells. Several different inward and outward currents were identified. Inward currents included L-type Ca2+ currents and voltage-activated tetrodotoxin (TTX)-sensitive Na+ currents. Sodium currents have not been reported previously in pineal cells of any species. These two inward currents were present in the majority of cells. Chick pineal cells also expressed several types of voltage-dependent and Ca(2+)-dependent K+ currents that differed in voltage dependence, kinetics, and pharmacology. These included two Ca(2+)-dependent outward currents which differed in sensitivity to tetraethylammonium chloride (TEA), and at least two distinct voltage-activated K+ currents. Considerable cell-to-cell variation in the amplitude and nature of the evoked outward currents was observed. These ionic currents may be important for the regulation of melatonin synthesis and the modulation of circadian rhythmicity.     

Dopamine modulates a voltage-gated calcium channel in rat olfactory receptor neurons

Dopamine D2 receptors exist in the soma of rat olfactory receptor neurons. Actions of dopamine on the voltage-gated Ca(2+) channels in the neurons were investigated using the perforated whole-cell voltage-clamp. In 10 mM Ba(2+) solution, rat olfactory receptor neurons displayed the inward currents elicited by the voltage ramp (167 mV/s) and depolarizing step pulses from a holding potential of -91 mV. The inward Ba(2+) currents were greatly reduced by 10 microM nifedipine (L-type Ca(2+) channel blocker). The Ba(2+) currents were inhibited by the external application of dopamine. The IC(50) for the inhibition was about 1 microM. Quinpirole (10 microM, a D2 dopamine agonist) also inhibited the Ba(2+) currents. Quinpirole did not affect the activation and inactivation kinetics of the Ba(2+) currents. The results suggest that dopamine modulates the L-type Ca(2+) channels in rat olfactory receptor neurons via the mechanism independent of voltage.     

Synaptic modulation by dopamine of calcium currents in rat pars intermedia

Melanotrophs of the rat pars intermedia are innervated by dopaminergic fibers traveling through the pituitary stalk which inhibit secretion via an action on D-2 receptors. As secretion from the melanotroph has been shown to be calcium (Ca2+) dependent, it is possible that dopamine may have an action to inhibit Ca2+ currents in these cells. This possibility was tested by examining the effects of exogenously applied dopaminergic agonists or synaptically released dopamine upon Ca2+ currents recorded under single electrode voltage clamp in intact rat pars intermedia in vitro. Following blockade of sodium and potassium currents in melanotrophs, Ca2+ spikes were elicited with intracellular injection of depolarizing currents; electrical stimulation of the pituitary stalk caused an inhibition of the Ca2(+)-based action potentials which lasted for several seconds. Using single-electrode voltage-clamp techniques, we recorded inward Ca2+ currents corresponding to the T, N, and L types (see Williams et al., 1990). Stimulation of the pituitary stalk inhibited both the low- and high-threshold peak inward Ca2+ currents elicited from a holding potential of -90 mV. In contrast, when noninactivating Ca2+ currents were elicited from a holding potential of -30 mV, the currents were not altered by stalk stimulation. This pattern of inhibition of the Ca2+ currents was consistent with the preferential inhibition, by stalk stimulation, of the N and T Ca2+ currents, while sparing the L current. We observed that inhibition of Ca2+ currents due to stalk stimulation was completely reversed by bath perfusion of domperidone (1 microM), an antagonist of dopamine at the D-2 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dopamine enhances a voltage-dependent transient K+ current in the MMQ cell, a clonal pituitary line expressing functional D2 dopamine receptors

The influence of dopamine on voltage-dependent K+ current (IK) was studied in cultured MMQ cells using the whole-cell patch-clamp technique. IK in nearly all MMQ cells revealed a transient outward current component and inactivated during maintained depolarization lasting 60 ms. The transient component was inhibited by prepulse potentials more positive than -40 mV or by addition of 4 mM 4-aminopyridine to the bathing solution and was insensitive to the external Ca2+ concentration. Thus, this transient K+ current resembled the A-current (IA) found in other cells. Dopamine at 1 microM increased by 50% (P less than 0.001) the peak of IK evoked by a test potential to +80 mV and the response was prevented by pretreatment with 100 nM haloperidol, a D2 receptor antagonist. These data suggest that MMQ clonal pituitary cells possess a voltage-gated K+ A-current and that this current can be modulated by dopamine via D2 receptors.     

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

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

Cobalt-dependent potentiation of net inward current density in Helix aspersa neurons.

A low concentration of transition metal ions Co2+ and Ni2+ increases the inward current density in neurons from the land snail Helix aspersa. The currents were measured using a single electrode voltage-clamp/internal perfusion method under conditions in which the external Na+ was replaced by Tris+, the predominant external current carrying cation was Ca2+, and the internal perfusate contained 120 mM Cs+/0 K+; 30 mM tetraethylammonium (TEA) was added externally to block K+ current. In the presence of Co2+ (3 mM) or Ni2+ (0.5 mM) inward Ca2+ currents were stimulated normally by voltage-dependent activation of Ca2+ channels. There was a 5-10% decrease in the rate of rise of the inward current. The principal effect of Co2+ and Ni2+ in increasing the current density seems to be a decrease in the rate at which the inward currents decline during a depolarizing voltage pulse. The results may be due to a decrease in a voltage-dependent or Ca(2+)-dependent outward current and/or an inhibition of Ca2+ channel inactivation. Outward current under these conditions (zero internal K+) was significant and most likely due to Cs+ efflux through the voltage-activated or Ca(2+)-activated nonspecific cation channels. Co2+ is an extremely effective blocker of this outward current. These results are not an artifact of internal perfusion or the special ionic conditions. Intracellular recording of unperfused neurons in normal Helix Ringer's solution showed that the Ca(2+)-dependent action potential duration was increased significantly by low concentrations of Co2+. This result is consistant with the Co(2+)-dependent increase in inward (depolarizing) current seen in voltage-clamp experiments.     

A guanine nucleotide-binding protein mediates the inhibition of voltage-dependent calcium currents by dopamine in rat lactotrophs

The present study examines the effect of dopamine (DA), known to inhibit prolactin (PRL) release, on voltage-activated calcium currents in identified rat lactotrophs. Two types of voltage-dependent Ca2+ currents were recorded using the whole-cell mode of the patch-clamp technique. Both were reversibly inhibited by DA application. The inhibitory action of DA was reduced by (i) sulpiride (D2 antagonist), (ii) preincubation of the cells with pertussis toxin (PTX), and (iii) inclusion of guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) in the pipette solution, whereas it was potentiated by guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S). This DA-induced response could not be overcome by changing the adenosine 3',5'-cyclic monophosphate level. These findings suggest that DA can inhibit Ca2+ entry through voltage-activated Ca2+ channels via a PTX-sensitive G protein(s) pathway thereby affecting PRL release from rat lactotrophs.     

Opioid Inhibition of Ca2+ Channel Subtypes in Bovine Chromaffin Cells: Selectivity of Action and Voltage-dependence

Bovine chromaffin cells possess a mixture of high-voltage-activated Ca²⁺ channel subtypes: L-type, dihydropyridine-sensitive channels, and N-, P- and Q-types, ω-conotoxin MVIIC-sensitive channels. In these cells, we studied the reversible, naloxone-antagonized inhibition of Ba²⁺ currents by the opioid agonist met-enkephalin (IC₅₀= 272 nM). This inhibition could be resolved into a voltage-dependent and a voltage-independent component. The first was revealed by its slow Ba²⁺ current activation kinetics at 0 mV and by the current facilitation induced by short prepulses to +90 mV. The second was estimated as the residual inhibition persisting after the facilitation protocol. The two inhibitory components varied markedly from cell to cell and each contributed to about half of the total inhibition. Replacement of internal GTP by GDP-β-S or cell pretreatment with pertussis toxin completely abolished the voltage-dependent inhibition by opioids, partially preserving the voltage-independent component. The opioid-induced inhibition was not selective for any Ca²⁺ channel subtype, being not prevented after the addition of specific Ca²⁺ channel antagonists. However, when separately analyzing the contribution of each channel type to the voltage-dependent and voltage-independent modulation, a clear-cut distinction could be achieved. The voltage-independent inhibition was effective on all Ca²⁺ channel subtypes but predominantly on L-type Ca²⁺ channels. The voltage-dependent process was abolished by ω-conotoxin-MVIIC, but unaffected by nifedipine, and was thus sharply restricted to non-L-type channels (N-, P- and Q-types). Our data suggest a functionally distinct opioid receptor-mediated modulation of L- and non-L-type channels, i.e. of the two channel classes sharing major control of catecholamine secretion from bovine chromaffin cells.     

Calcium channels and nifedipine inhibition of serotonin-induced [3H]thymidine incorporation in cultured cerebral smooth muscle cells.

Cultures of smooth muscle cells were prepared from the basilar artery of adult guinea pigs. Passaged cultures (10-30 passages) that expressed serotonin receptors were studied using [3H]thymidine incorporation. When tested in quiescent medium, serotonin potently stimulated [3H]thymidine incorporation (EC50 of 31 nM) by as much as 400% at 24 h. The number of cells was not significantly increased at 24 or 48 h. At concentrations of 10(-8)-10(-5) M 5-HT, [3H]thymidine uptake was reduced 40-50% by the dihydropyridine Ca2+ channel blocker, nifedipine (1 microM). To demonstrate a possible mechanism for the sensitivity to nifedipine, Ca2+ currents were measured using the whole cell patch clamp technique. The cells expressed dihydropyridine-sensitive L-type Ca2+ channels, but not other subtypes of Ca2+ channels, as indicated by the kinetic and voltage-dependent characteristics of the current and by the stimulatory effect of Bay K 8644. The magnitude of the Ca2+ currents was related exponentially to the membrane surface area, measured as cell capacitance. These data support the association of dihydropyridine-sensitive Ca2+ channels with mitogenesis in vascular smooth muscle, and suggest an alternate mechanism of action for the beneficial effect of dihydropyridines in prophylaxis of cerebral vasospasm.