Nimodipine block of calcium channels in rat anterior pituitary cells

Molecular Basis for Ligand Modulation of a Mammalian Voltage-Gated Ca2+ Channel

The L-type voltage-gated Ca²⁺ (Ca_v) channels are modulated by various compounds exemplified by 1,4-dihydropyridines (DHP), benzothiazepines (BTZ), and phenylalkylamines (PAA), many of which have been used for characterizing channel properties and for treatment of hypertension and other disorders. Here, we report the cryoelectron microscopy (cryo-EM) structures of Ca_v1.1 in complex with archetypal antagonistic drugs, nifedipine, diltiazem, and verapamil, at resolutions of 2.9 Å, 3.0 Å, and 2.7 Å, respectively, and with a DHP agonist Bay K 8644 at 2.8 Å. Diltiazem and verapamil traverse the central cavity of the pore domain, directly blocking ion permeation. Although nifedipine and Bay K 8644 occupy the same fenestration site at the interface of repeats III and IV, the coordination details support previous functional observations that Bay K 8644 is less favored in the inactivated state. These structures elucidate the modes of action of different Ca_v ligands and establish a framework for structure-guided drug discovery.

A bout analysis reveals age-related methylmercury neurotoxicity and nimodipine neuroprotection

Age-related deficits in motor and cognitive functioning may be driven by perturbations in calcium (Ca(2+)) homeostasis in nerve terminals, mechanisms that are also thought to mediate the neurotoxicity of methylmercury (MeHg). Calcium-channel blockers (CCBs) protect against MeHg toxicity in adult mice, but little is known about their efficacy in other age groups. Two age groups of BALB/c mice were exposed to 0 or 1.2mg/kg/day MeHg and 0 or 20mg/kg/day of the CCB nimodipine for approximately 8.5 months. Adults began exposure on postnatal day (PND) 72 and the retired breeders on PND 296. High-rate operant behavior was maintained under a percentile schedule, which helped to decouple response rate from reinforcer rate. Responding was analyzed using a log-survivor bout analysis approach that partitioned behavior into high-rate bouts separated by pauses. MeHg-induced mortality did not depend on age but nimodipine neuroprotection was age-dependent, with poorer protection occurring in older mice. Within-bout response rate (a marker of sensorimotor function) was more sensitive to MeHg toxicity than bout-initiation rate (a marker of motivation). Within-bout rate declined almost 2 months prior to overt signs of toxicity for the MeHg-only retired breeders but not adults, suggesting greater delay to toxicity in younger animals. Motor-based decrements also appeared in relatively healthy adult MeHg+NIM animals. Aging appeared to alter the processes underlying Ca(2+) homeostasis thereby diminishing protection by nimodipine, even in mice that have not reached senescence. The study of MeHg exposure presents an experimental model by which to study potential mechanisms of aging.

Aging, motor function, and sensitivity to calcium channel blockers: An investigation using chronic methylmercury exposure

Methylmercury (MeHg) neurotoxicity is thought to be mediated, in part, by dysregulation of calcium (Ca(2+)) homeostasis, a mechanism that may also slowly and progressively degrade neuronal function during normal aging. Longitudinal studies of MeHg exposure provide a powerful approach to studying neural and behavioral mechanisms by which both MeHg toxicity and aging affect motor function. Wheel-running and rotarod performance were assessed in two age groups of BALB/c mice chronically exposed to 0 or 1.2mg/kg/day MeHg and 0 or 20mg/kg/day nimodipine, a 1,4-dihyrdopyridine L-type calcium channel blocker (CCB), for approximately 8.5 months. Adults began exposure on postnatal day (PND) 72 and retired breeders on PND 296. A log-survivor bout analysis partitioned wheel-running into bouts that identified motor (within-bout rates) and motivational (bout-initiation rates) influences. Retired breeders ran farther, because of a higher bout-initiation rates, but performed more poorly on the rotarod than younger adults, a difference unaffected by nimodipine. MeHg produced relatively age-independent deficits in wheel-running and rotarod performance, whereas nimodipine afforded greater protection to adult mice than to retired breeders. Rotarod performance and within-bout response rate were more sensitive to and more reliable predictors of MeHg toxicity than bout-initiation rate, which was least affected by MeHg exposure. Thus the motivation to run was unimpaired as the ability to do so declined. While chronic MeHg exposure produced functionally similar behavior deficits between age groups, the age-dependent neuroprotection by nimodipine supports the notion that underlying neurobiological systems mediated by Ca(2+) signaling, are differentially affected in older adults.

Elevations of intracellular calcium reflect normal voltage-dependent behavior, and not constitutive activity, of voltage-dependent calcium channels in gastrointestinal and vascular smooth muscle

In smooth muscle, the gating of dihydropyridine-sensitive Ca²⁺ channels may either be stochastic and voltage dependent or coordinated among channels and constitutively active. Each form of gating has been proposed to be largely responsible for Ca²⁺ influx and determining the bulk average cytoplasmic Ca²⁺ concentration. Here, the contribution of voltage-dependent and constitutively active channel behavior to Ca²⁺ signaling has been studied in voltage-clamped single vascular and gastrointestinal smooth muscle cells using wide-field epifluorescence with near simultaneous total internal reflection fluorescence microscopy. Depolarization (−70 to +10 mV) activated a dihydropyridine-sensitive voltage-dependent Ca²⁺ current (I_Ca) and evoked a rise in [Ca²⁺] in each of the subplasma membrane space and bulk cytoplasm. In various regions of the bulk cytoplasm the [Ca²⁺] increase ([Ca²⁺]_c) was approximately uniform, whereas that of the subplasma membrane space ([Ca²⁺]_PM) had a wide range of amplitudes and time courses. The variations that occurred in the subplasma membrane space presumably reflected an uneven distribution of active Ca²⁺ channels (clusters) across the sarcolemma, and their activation appeared consistent with normal voltage-dependent behavior. Indeed, in the present study, dihydropyridine-sensitive Ca²⁺ channels were not normally constitutively active. The repetitive localized [Ca²⁺]_PM rises (“persistent Ca²⁺ sparklets”) that characterize constitutively active channels were observed rarely (2 of 306 cells). Neither did dihydropyridine-sensitive constitutively active Ca²⁺ channels regulate the bulk average [Ca²⁺]_c. A dihydropyridine blocker of Ca²⁺ channels, nimodipine, which blocked I_Ca and accompanying [Ca²⁺]_c rise, reduced neither the resting bulk average [Ca²⁺]_c (at −70 mV) nor the rise in [Ca²⁺]_c, which accompanied an increased electrochemical driving force on the ion by hyperpolarization (−130 mV). Activation of protein kinase C with indolactam-V did not induce constitutive channel activity. Thus, although voltage-dependent Ca²⁺ channels appear clustered in certain regions of the plasma membrane, constitutive activity is unlikely to play a major role in [Ca²⁺]_c regulation. The stochastic, voltage-dependent activity of the channel provides the major mechanism to generate rises in [Ca²⁺].

Hormone effects on specific and global brain functions

The first demonstration of how biochemical changes in neurons in specific parts of the brain direct a complete mammalian behavior derived from the effects of estrogens in hypothalamic neurons that facilitate lordosis behavior, the primary reproductive behavior of female quadrupeds (Pfaff. Estrogens and Brain Function. 1980; Pfaff. Drive: Neurobiological and Molecular Mechanisms of Sexual Motivation. 1999). Sex behaviors depend on sexual arousal that in turn depends on a primitive function: generalized CNS arousal (Pfaff. Brain Arousal and Information Theory. 2006). Here we summarize one of the ways in which a generalized arousal transmitter, norepinephrine, can influence the electrical excitability of ventromedial hypothalamic cells in a way that will foster female sex behavior.

Voltage-dependent calcium channels in ventromedial hypothalamic neurones of postnatal rats: modulation by oestradiol and phenylephrine

Oestradiol actions in the hypothalamus play an important role in reproductive behaviour. Oestradiol treatment in vivo induces alpha(1b)-adrenoceptor mRNA and increases the density of alpha(1B)-adrenoceptor binding in the hypothalamus. Oestradiol is also known to modulate neuronal excitability, in some cases by modulating calcium channels. We assessed the effects of phenylephrine, an alpha(1)-adrenergic agonist, on low-voltage-activated (LVA) and high-voltage-activated (HVA) calcium channels in ventromedial hypothalamic (VMN) neurones from vehicle- and oestradiol-treated female rats. Whole-cell and gramicidin perforated-patch recordings were obtained, with barium as the charge carrier. In the absence of phenylephrine, oestradiol treatment increased the magnitude of LVA currents compared to controls, but had no effect on HVA currents. Phenylephrine enhanced HVA currents in a significantly greater proportion of neurones from oestradiol-treated rats (76%) than from vehicle-treated (41%) rats. The L-channel blocker nifedipine abolished this oestradiol effect on phenylephrine-enhanced HVA currents. Preincubating slices with the N-type channel blocker omega-conotoxin GVIA completely blocked the phenylephrine response, suggesting that the N-type channel is essential. Phenylephrine also stimulated LVA currents in approximately two-thirds of neurones in slices from both vehicle- and oestradiol-treated rats. Our data show that oestradiol increases LVA currents in the VMN. Oestradiol also amplifies alpha(1)-adrenergic signalling by increasing the proportion of neurones showing phenylephrine-stimulated HVA currents mediated by N- and L-type calcium channels. In this way, oestradiol may increase excitatory responses to arousing adrenergic inputs to VMN neurones governing oestradiol-dependent reproductive behaviour.

Voltage-dependent calcium channels of dog basilar artery

Electrophysiological and molecular characteristics of voltage-dependent calcium (Ca(2+)) channels were studied using whole-cell patch clamp, polymerase chain reaction and Western blotting in smooth muscle cells freshly isolated from dog basilar artery. Inward currents evoked by depolarizing steps from a holding potential of -50 or -90 mV in 10 mm barium consisted of low- (LVA) and high-voltage activated (HVA) components. LVA current comprised more than half of total current in 24 (12%) of 203 cells and less than 10% of total current in 52 (26%) cells. The remaining cells (127 cells, 62%) had LVA currents between one tenth and one half of total current. LVA current was rapidly inactivating, slowly deactivating, inhibited by high doses of nimodipine and mibefradil (> 0.3 microM), not affected by omega-agatoxin GVIA (gamma100 nM), omega-conotoxin IVA (1 microM) or SNX-482 (200 nM) and probably carried by T-type Ca(2+) channels based on the presence of messenger ribonucleic acid (mRNA) and protein for Ca(v3.1) and Ca(v3.3) alpha(1) subunits of these channels. LVA currents exhibited window current with a maximum of 13% of the LVA current at -37.4 mV. HVA current was slowly inactivating and rapidly deactivating. It was inhibited by nimodipine (IC(50) = 0.018 microM), mibefradil (IC(50) = 0.39 microM) and omega-conotoxin IV (1 microM). Smooth muscle cells also contained mRNA and protein for L- (Ca(v1.2) and Ca(v1.3)), N- (Ca(v2.2)) and T-type (Ca(v3.1) and Ca(v3.3)) alpha(1) Ca(2+) channel subunits. Confocal microscopy showed Ca(v1.2) and Ca(v1.3) (L-type), Ca(v2.2) (N-type) and Ca(v3.1) and Ca(v3.3) (T-type) protein in smooth muscle cells. Relaxation of intact arteries under isometric tension in vitro to nimodipine (1 microM) and mibefradil (1 microM) but not to omega-agatoxin GVIA (100 nM), omega-conotoxin IVA (1 microM) or SNX-482 (1 microM) confirmed the functional significance of L- and T-type voltage-dependent Ca(2+) channel subtypes but not N-type. These results show that dog basilar artery smooth muscle cells express functional voltage-dependent Ca(2+) channels of multiple types.

Allosteric interactions required for high-affinity binding of dihydropyridine antagonists to Ca(V)1.1 Channels are modulated by calcium in the pore

Dihydropyridines (DHPs) are an important class of drugs, used extensively in the treatment of angina pectoris, hypertension, and arrhythmia. The molecular mechanism by which DHPs modulate Ca(2+) channel function is not known in detail. We have found that DHP binding is allosterically coupled to Ca(2+) binding to the selectivity filter of the skeletal muscle Ca(2+) channel Ca(V)1.1, which initiates excitation-contraction coupling and conducts L-type Ca(2+) currents. Increasing Ca(2+) concentrations from approximately 10 nM to 1 mM causes the DHP receptor site to shift from a low-affinity state to a high-affinity state with an EC(50) for Ca(2+) of 300 nM. Substituting each of the four negatively charged glutamate residues that form the ion selectivity filter with neutral glutamine or positively charged lysine residues results in mutant channels whose DHP binding affinities are decreased up to 10-fold and are up to 150-fold less sensitive to Ca(2+) than wild-type channels. Analysis of mutations of amino acid residues adjacent to the selectivity filter led to identification of Phe-1013 and Tyr-1021, whose mutation causes substantial changes in DHP binding. Thermo-dynamic mutant cycle analysis of these mutants demonstrates that Phe-1013 and Tyr-1021 are energetically coupled when a single Ca(2+) ion is bound to the channel pore. We propose that DHP binding stabilizes a nonconducting state containing a single Ca(2+) ion in the pore through which Phe-1013 and Tyr-1021 are energetically coupled. The selectivity filter in this energetically coupled high-affinity state is blocked by bound Ca(2+), which is responsible for the high-affinity inhibition of Ca(2+) channels by DHP antagonists.

The effect of mebudipine and dibudipine, two new Ca2+ channel blockers, in comparison with nifedipine on Ca2+ spikes of F1 neuronal soma membrane inHelix aspersa

Mebudipine and dibudipine are two new dihydropyridine (DHP) Ca2+ channel blockers that have been synthesized by Mahmoudian et al. (1997). In previous studies, they showed considerable relaxant effect on vascular and ileal smooth muscles. These two compounds also reduced the contraction force of rat left atrium (20, 22). In the present study, we attempted to compare the inhibitory actions of these new DHPs and nifedipine on the high threshold Ca2+ spikes of F1 neuronal soma membrane in the subesophageal ganglia of Helix aspersa, using current-clamp method. At a concentration of 1 microM, two new DHP compounds (mebudipine and dibudipine) were tested for their L-type Ca2+ channel blocker activity. Both compounds reversibly reduced the peak amplitude of action potential and after hyperpolarization potential and markedly decreased the duration of Ca2+ spikes. The most potent of these DHPs was mebudipine. Neither the two new DHPs nor nifedipine changed the resting membrane potential in a statistically significant way.

Differential discrimination of fast and slow synaptic waveforms by two low-voltage-activated calcium channels

Electrophysiological analysis of human embryonic kidney 293 cells stably expressing recombinant channels was used to compare how the biophysical properties of the low-voltage-activated Ca(2+) channels encoded by the alpha(1G) (Ca(V)3.1) or alpha(1I) (Ca(V)3.3) subunits shape their responses to excitatory synaptic potentials. In medium containing 2 mM extracellular Ca(2+) standard current-voltage relationships demonstrated both channel types to be clearly low-voltage activated with significant slowly activating current responses being observed at -66 mV. At all test potentials examined, activation of Ca(V)3.3 was substantially slower than that of Ca(V)3.1. To probe how these different T-type channels might respond to excitatory postsynaptic potentials (EPSPs), mock EPSPs with different kinetic profiles were created from the sum of exponentials. These waveforms were then used as command templates in voltage-clamp experiments. Ca(V)3.1-mediated channels responded effectively to both rapidly decaying mock EPSPs and slowly decaying EPSPs. In contrast, Ca(V)3.3-mediated channels were poorly gated by rapidly decaying EPSPs but were effectively activated by the more prolonged synaptic response. When activated with mock EPSPs Ca(V)3.3-mediated currents were more resistant to steady-state depolarisation of the pre-stimulus holding potential. Ca(V)3.3 currents were also more resistant to repetitive application of prolonged EPSPs, which caused substantial inactivation of Ca(V)3.1-mediated currents. The addition of a single mock action potential to the peak of a rapidly decaying EPSP voltage-clamp template greatly enhanced the currents produced by either Ca(V)3.1 or Ca(V)3.3-expressing cells. This facilitatory effect was considerably greater for Ca(V)3.3-mediated channels. From these data we suggest that the slow activation kinetics of Ca(V)3.3-mediated T-type channels enable them to respond selectively to either slow or suprathreshold synaptic potentials.

T-type Ca(2+) channels mediate neurotransmitter release in retinal bipolar cells

Transmitter release in neurons is thought to be mediated exclusively by high-voltage-activated (HVA) Ca(2+) channels. However, we now report that, in retinal bipolar cells, low-voltage-activated (LVA) Ca(2+) channels also mediate neurotransmitter release. Bipolar cells are specialized neurons that release neurotransmitter in response to graded depolarizations. Here we show that these cells express T-type Ca(2+) channel subunits and functional LVA Ca(2+) currents sensitive to mibefradil. Activation of these currents results in Ca(2+) influx into presynaptic terminals and exocytosis, which we detected as a capacitance increase in isolated terminals and the appearance of reciprocal currents in retinal slices. The involvement of T-type Ca(2+) channels in bipolar cell transmitter release may contribute to retinal information processing.

Regulation of L-type calcium channels in pituitary GH(4)C(1) cells by depolarization

The neurosecretory anterior pituitary GH(4)C(1) cells exhibit the high voltage-activated dihydropyridine-sensitive L-type and the low voltage-activated T-type calcium currents. The activity of L-type calcium channels is tightly coupled to secretion of prolactin and other hormones in these cells. Depolarization induced by elevated extracellular K(+) reduces the dihydropyridine (+)-[(3)H]PN200-110 binding site density and (45)Ca(2+) uptake in these cells (). This study presents a functional analysis by electrophysiological techniques of short term regulation of L-type Ca(2+) channels in GH(4)C(1) cells by membrane depolarization. Depolarization of GH(4)C(1) cells by 50 mm K(+) rapidly reduced the barium currents through L-type calcium channels by approximately 70% and shifted the voltage dependence of activation by 10 mV to more depolarized potentials. Down-regulation depended on the strength of the depolarizing stimuli and was reversible. The currents recovered to near control levels on repolarization. Down-regulation of the calcium channel currents was calcium-dependent but may not have been due to excessive accumulation of intracellular calcium. Membrane depolarization by voltage clamping and by veratridine also produced a down-regulation of calcium channel currents. The down-regulation of the currents had an autocrine component. This study reveals a calcium-dependent down-regulation of the L-type calcium channel currents by depolarization.

Permanently charged chiral 1,4-dihydropyridines: molecular probes of L-type calcium channels. Synthesis and pharmacological characterization of methyl(omega-trimethylalkylammonium) 1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedicarboxylate iodide, calcium channel antagonists

We report the synthesis of the single enantiomers of permanently charged dihydropyridine derivatives (DHPs with alkyl linker lengths of two and eight carbon atoms) and their activities on cardiac and neuronal L-type calcium channels. Permanently charged chiral 1,4-dihydropyridines and methyl (omega)-trimethylalkylammonium) 1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedicarboxylate iodides were synthesized in high optical purities from (R)-(-) and (S)-(+)-1,4-dihydro-2,6-dimethyl-5-methoxycarbonyl-4-(3-nitrophenyl)-3-+ ++pyridinecarboxylic acid, obtained by resolution of racemic 1,4-dihydro-2,6-dimethyl-5-methoxycarbonyl-4-(3-nitrophenyl)-3-pyridi necarboxylic acid. Competition binding experiments with radioligand [3H]-(+)-PN200-110 and the block of whole cell barium currents through L-type calcium channels in GH4C1 cells show that the compounds with the eight-carbon alkyl linker optimally block the L-type Ca2+ channels, and that the S-enantiomer is more potent than the R-enantiomer.

Role of neuropeptide-sensitive L-type Ca(2+) channels in histamine release in gastric enterochromaffin-like cells

Peptides release histamine from enterochromaffin-like (ECL) cells because of elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) by either receptor-operated or voltage-dependent Ca(2+) channels (VDCC). To determine whether VDCCs contribute to histamine release stimulated by gastrin or pituitary adenylate cyclase-activating polypeptide (PACAP), the presence of VDCCs and their possible modulation by peptides was investigated in a 48-h cultured rat gastric cell population containing 85% ECL cells. Video imaging of fura 2-loaded cells was used to measure [Ca(2+)](i), and histamine was assayed by RIA. Cells were depolarized by increasing extracellular K(+) concentrations or by 20 mM tetraethylammonium (TEA(+)). Cell depolarization increased transient and steady-state [Ca(2+)](i) and resulted in histamine release, dependent on extracellular Ca(2+). These K(+)- or TEA(+)-dependent effects on histamine release from ECL cells were coupled to activation of parietal cells in intact rabbit gastric glands, and L-type channel blockade by 2 microM nifedipine inhibited 50% of [Ca(2+)](i) elevation and histamine release. N-type channel blockade by 1 microM omega-conotoxin GVIA inhibited 25% of [Ca(2+)](i) elevation and 14% of histamine release. Inhibition was additive. The effects of 20 mM TEA(+) were fully inhibited by 2 microM nifedipine. Both classes of Ca(2+) channels were found in ECL cells, but not in parietal cells, by RT-PCR. Nifedipine reduced PACAP-induced (but not gastrin-stimulated) Ca(2+) entry and histamine release by 40%. Somatostatin, peptide YY (PYY), and galanin dose dependently inhibited L-type Ca(2+) channels via a pertussis toxin-sensitive pathway. L-type VDCCs play a role in PACAP but not gastrin stimulation of histamine release from ECL cells, and the channel opening is inhibited by somatostatin, PYY, and galanin by interaction with a G(i) or G(o) protein.

Dihydropyridine enantiomers block recombinant L-type Ca2+ channels by two different mechanisms

1. The molecular basis of the state-dependent block of L-type Ca2+ channels by dihydropyridines is still poorly understood. Therefore, we studied the enantioselectivity of Ca2+ channel block by isradipine enantiomers at three holding potentials (-80, -60 and -40 mV) in Chinese hamster ovary (CHO) cells stably expressing the rabbit lung alpha1C-b-subunit. 2. The extent of enantioselectivity did not markedly change with the holding potential (IC50 ratios of 104-138), whereas the potency of both isradipine enantiomers increased with depolarisation of the holding potential. 3. In addition to its block of the peak Ca2+ channel current, Ipeak, (-)-isradipine inhibited the relative current at the end of the test pulse, the so-called Ilate, normalised to Ipeak (Ilate/Ipeak). This effect was unaffected by the holding potential and revealed distinct kinetics compared to the development of conventional block of Ipeak. 4. When these effects were studied using an alpha1C-b-mutant lacking the high-affinity dihydropyridine binding site, expressed in human embryonic kidney (HEK 293) cells, both enantiomers blocked Ilate/Ipeak to a similar degree. 5. Our data are discussed within the framework of the 'guarded receptor' and the 'modulated receptor' hypotheses. The very different properties of the block of Ilate/Ipeak compared to those of the conventional high-affinity block of Ipeak suggest the existence of an additional mechanism possibly mediated via a second, distinct binding site.

Control of the low voltage-activated calcium channel of mouse sperm by egg ZP3 and by membrane hyperpolarization during capacitation

Sperm adhesion to egg zonae pellucidae initiates sperm acrosome reactions, an exocytotic event that is an early step during fertilization. Previously, it was suggested that zona pellucida-evoked Ca2+ entry into sperm through low voltage-activated Ca2+ channels is an essential step in acrosome reactions, based on the inhibitory effects of Ca2+ channel antagonists. However, analysis of this channel is limited by the inability to apply electrophysiological methods directly to sperm. In this report, optical methods of determining membrane potential and internal Ca2+ levels were used to demonstrate that (i) contact with zonae pellucidae activates a transient Ca2+ response in sperm that has a time course and antagonist sensitivity anticipated of low voltage-activated Ca2+ channels; (ii) these channels are unavailable for opening in uncapacitated sperm because of voltage-dependent, steady state inactivation; (iii) membrane hyperpolarization during sperm capacitation is sufficient to recruit channels into a closed state, from which they are available for opening during fertilization; and (iv) channel conductance state may be a factor in determines the efficacy with which channel antagonists inhibit fertilization. This study provides evidence for the activation of sperm Ca2+ channels during gamete adhesion and offers a mechanism that may account for aspects of the regulation of sperm fertility during capacitation through the control of channel availability. Finally, these results suggest that channel conductance state may be a central feature in the design of channel antagonists that inhibit sperm function.

Akt-dependent potentiation of L channels by insulin-like growth factor-1 is required for neuronal survival

The insulin-like growth factor-1 (IGF-1)/receptor tyrosine kinase recently has been shown to mediate neuronal survival and potentiate the activity of specific calcium channel subtypes; survival requires Akt, a serine/threonine kinase. We demonstrate here that Akt mediates the IGF-1-induced potentiation of L channel currents, but not that of N channels. Transient expression of wild-type, dominant-negative, and constitutively active forms of Akt in cerebellar granule neurons causes, respectively, no change in IGF-1/L channel potentiation, complete inhibition of potentiation, and a dramatic increase in basal L currents accompanied by the loss of ability to induce further increases. In no case is the IGF-1 potentiation of N currents affected. We additionally find that IGF-1 partially mediates granule neuron survival via L channel activity and that Akt-dependent L channel modulation is a necessary component. Interestingly, very brief exposure (1 min) to IGF-1 triggers nearly complete survival and requires L channel activity. These results strongly suggest that neuronal receptor tyrosine kinases can control long-term calcium-dependent processes via the rapid control of voltage-sensitive channels.

Akt-dependent potentiation of L channels by insulin-like growth factor-1 is required for neuronal survival

The insulin-like growth factor-1 (IGF-1)/receptor tyrosine kinase recently has been shown to mediate neuronal survival and potentiate the activity of specific calcium channel subtypes; survival requires Akt, a serine/threonine kinase. We demonstrate here that Akt mediates the IGF-1-induced potentiation of L channel currents, but not that of N channels. Transient expression of wild-type, dominant-negative, and constitutively active forms of Akt in cerebellar granule neurons causes, respectively, no change in IGF-1/L channel potentiation, complete inhibition of potentiation, and a dramatic increase in basal L currents accompanied by the loss of ability to induce further increases. In no case is the IGF-1 potentiation of N currents affected. We additionally find that IGF-1 partially mediates granule neuron survival via L channel activity and that Akt-dependent L channel modulation is a necessary component. Interestingly, very brief exposure (1 min) to IGF-1 triggers nearly complete survival and requires L channel activity. These results strongly suggest that neuronal receptor tyrosine kinases can control long-term calcium-dependent processes via the rapid control of voltage-sensitive channels.

Inhibition by local anesthetics of Ca2+ channels in rat anterior pituitary cells

The characteristics of local anesthetic inhibition of voltage-dependent Ca2+ channels in a rat pituitary clonal cell line were investigated by whole-cell voltage clamp and compared with inhibition by the dihydropyridine Ca2+ channel antagonist, nicardipine. With extracellular Ba2+ (10 mM) as the current carrier, depolarization above -40 mV evoked a slowly inactivating I(Ba). Extracellularly applied lidocaine inhibited I(Ba) without changing the activation threshold, the voltage of peak current, or the reversal potential. Inhibition was greater at a holding potential of -60 mV (IC50 = 1.2 mM) than at -80 mV (IC50 = 2.6 mM). This depolarization-induced potentiation in I(Ba) inhibition developed over 0.1-10 s after membrane depolarization began. Nicardipine also dose-dependently inhibited I(Ba) with an IC50 = 90 nM (at a holding potential = -80 mV). Both lidocaine and nicardipine shifted the I(Ba) steady-state inactivation (availability) curves to the left. Double-pulse protocols revealed that lidocaine (1 mM) accelerated the depolarization-induced inhibition (inactivation) of I(Ba) over the rate in drug-free solutions, but had no effect on the hyperpolarization-induced removal of channel inactivation. Nicardipine also accelerated the depolarization-induced inactivation of I(Ba) but, in addition, it slowed the hyperpolarization-induced inactivation removal. The relative inhibitory action of lidocaine in suppressing I(Ba) was unchanged in the presence of nicardipine. These results suggest that lidocaine has a direct action on membrane Ca2+ channels, similar to the voltage-dependent action of dihydropyridine, but acting at a separate and independent site.

Endogenous pacemaker activity of rat tumour somatotrophs

1. Cells derived from a rat pituitary tumour (GC cell line) that continuously release growth hormone behave as endogenous pacemakers. In simultaneous patch clamp recordings and cytosolic Ca2+ concentration ([Ca2+]i) imaging, they displayed rhythmic action potentials (44.7 +/- 2.7 mV, 178 +/- 40 ms, 0.30 +/- 0.04 Hz) and concomitant [Ca2+]i transients (374 +/- 57 nM, 1.0 +/- 0.2 s, 0.27 +/- 0.03 Hz). 2. Action potentials and [Ca2+]i transients were reversibly blocked by removal of external Ca2+, addition of nifedipine (1 microM) or Ni2+ (40 microM), but were insensitive to TTX (1 microM). An L-type Ca2+ current activated at -33.6 +/- 0.4 mV (holding potential (Vh), -40 mV), peaked at -1.8 +/- 1.3 mV, was reduced by nifedipine and enhanced by S-(+)-SDZ 202 791. A T/R-type Ca2+ current activated at -41.7 +/- 2.7 mV (Vh, -80 or -60 mV), peaked at -9.2 +/- 3.0 mV, was reduced by low concentrations of Ni2+ (40 microM) or Cd2+ (10 microM) and was toxin resistant. Parallel experiments revealed the expression of the class E calcium channel alpha1-subunit mRNA. 3. The K+ channel blockers TEA (25 mM) and charybdotoxin (10-100 nM) enhanced spike amplitude and/or duration. Apamin (100 nM) also strongly reduced the after-spike hyperpolarization. The outward K+ tail current evoked by a depolarizing step that mimicked an action potential reversed at -69. 8 +/- 0.3 mV, presented two components, lasted 2-3 s and was totally blocked by Cd2+ (400 microM). 4. The slow pacemaker depolarization (3.5 +/- 0.4 s) that separated consecutive spikes corresponded to a 2- to 3-fold increase in membrane resistance, was strongly Na+ sensitive but TTX insensitive. 5. Computer simulations showed that pacemaker activity can be reproduced by a minimum of six currents: an L-type Ca2+ current underlies the rising phase of action potentials that are repolarized by a delayed rectifier and Ca2+-activated K+ currents. In between spikes, the decay of Ca2+-activated K+ currents and a persistent inward cationic current depolarize the membrane, activate the T/R-type Ca2+ current and initiate a new cycle.

Stimulation of Ca2+ entry in lactotrophs and somatotrophs from immature rat pituitary by N-terminal fragments of proopiomelanocortin

We have previously shown that 10-12 kDa N-terminal fragments of rat proopiomelanocortin (POMC) and human POMC1-76 stimulate mitosis and/or differentiation in lactotrophs of early postnatal rat pituitary. A truncated form, POMC1-26, mimics the differentiation-inducing but not the mitogenic action of the former peptides. To further characterize these two biological responses, the present study compared changes in the intracellular free calcium concentration ([Ca2+]i) in response to POMC1-76 and POMC1-26 in isolated pituitary cells from 14-day-old female rats. Calcium (Ca2+) responses were also used as a guide to determine whether the responsive cells belong to the lactosomatotroph lineage. Application of POMC1-76 or POMC1-26 induced a maintained oscillating [Ca2+]i increase in a small population of cells. Increasing doses of the peptides did not affect the magnitude and the frequency of [Ca2+]i oscillations but clearly augmented the number of responding cells. Approximately 2% of the cells responded at 0.1 nM POMC1-76 or 5 nM POMC1-26, and 11-13% of the cells responded at 10 nM and 500 nM of the respective peptides. About one-third of the cells responsive to these peptides also showed a [Ca2+]i increase in response to growth hormone-releasing peptide-6 (GHRP-6) while, in a small number of responsive cells, [Ca2+]i was depressed by dopamine, suggesting that the former cells are somatotrophs and the latter lactotrophs. This interpretation was confirmed by immunocytochemical identification of prolactin and growth hormone (GH) in the cells. Of the cells showing Ca2+ response to POMC1-76, approximately one-third contained GH and another third prolactin. The remainder contained neither GH nor prolactin. Comparable results were obtained with POMC1-26. The rise of [Ca2+]i induced by the N-terminal POMC peptides persisted after depletion of intracellular Ca2+ stores by thapsigargin. Removal of Ca2+ from the extracellular medium or addition of cadmium completely abolished both the POMC1-76- and POMC1-26-induced [Ca2+]i increase. Nifedipine inhibited the Ca2+ response to both peptides, although only in 55% of the responsive cells. Depletion of some isoforms of protein kinase C by preincubation with the phorbol ester PMA for 24 h did not modify the Ca2+ responses. In contrast, blockade of the protein kinase A pathway with Rp-cAMPs partially inhibited the POMC1-76- or POMC1-26-induced [Ca2+]i increase. The present data show that, in immature pituitary cells, POMC1-76 induces an increase in [Ca2+]i through extracellular Ca2+ influx, possibly mediated in part by protein kinase A activation. The active domain of POMC1-76 seems to comprise its N-terminal moiety. The data support the hypothesis that POMC1-76 exerts a specific function in the development of different members of the lactosomatotroph lineage and that the peptide mobilizes different subsets of cells within this lineage, by a mechanism determined by its concentration.

Electrical properties of a cockroach motor neuron soma depend on different characteristics of individual Ca components

Electrical properties of a cockroach motor neuron soma depend on different characteristics of individual Ca components. J. Neurophysiol. 78: 2455-2466, 1997. The "fast" coxal depressor motor neuron (Df) of the cockroach is among the most extensively studied of insect neurons. It has been shown that the cell body of this neuron can exhibit active electrical properties, which may change over time or with chemical modulation. To further understand these electrical events and their modulation, inward currents in Df have been characterized under conditions in which outward currents have been suppressed. The inward current activated at potentials positive to -60 mV and peaked between -10 and 0 mV when measured in barium saline and between 0 and +10 mV when measured in calcium saline. The inward current was insensitive to Ni2+ (100 mu M) but reduced by verapamil (50 mu M) and abolished by Cd2+ (1 mM). Two components of ICa were identified by their sensitivity to either 50 mu M nifedipine or micromolar Cd2+. The nifedipine-sensitive component activated positive to -60 mV and peaked between -10 and 0 mV, whereas the Cd2+-sensitive component activated positive to -40 mV and peaked between +10 and +20 mV. Immediately after dissection, depolarization of Df evoked plateau potentials, whereas 1-4 h after dissection, depolarization evoked action potentials. The plateau potentials were insensitive to 100 mu M Cd2+ but blocked by 50 mu M nifedipine, whereas the spikes required a combination of nifedipine (50 mu M) and Cd2+ (100 mu M) for complete suppression, indicating that only one component of ICa contributes to the plateau potential, whereas both components contribute to action potentials. Currents measured in calcium saline decayed faster than currents measured in barium saline. The inactivation characteristics were investigated with the use of double-pulse voltage-clamp experiments. ICa showed a greater degree of inactivation and slower recovery from inactivation than did IBa. Current decay and the extent of inactivation were reduced after injection of the calcium-chelator 1,2-bis(2-aminophenoxy)ethane-N,N, N',N'-tetraacetic acid (BAPTA). This suggests that the calcium current of this neuron displays calcium-dependent inactivation. An additional mechanism, most probably voltage-dependent inactivation, also occurs because IBa, even in neurons injected with BAPTA, displayed some inactivation. The inactivation characteristics may be important in determining activity displayed by Df. Indirect evidence suggests that intracellular calcium is high immediately after dissection. At this time, the calcium current may therefore be reduced due to calcium-dependent inactivation. This may, at least partly, explain why the cell does not spike shortly after dissection.

Generation of action potentials in a mathematical model of corticotrophs

Corticotropin-releasing hormone (CRH) is an important regulator of adrenocorticotropin (ACTH) secretion from pituitary corticotroph cells. The intracellular signaling system that underlies this process involves modulation of voltage-sensitive Ca2+ channel activity, which leads to the generation of Ca2+ action potentials and influx of Ca2+. However, the mechanisms by which Ca2+ channel activity is modulated in corticotrophs are not currently known. We investigated this process in a Hodgkin-Huxley-type mathematical model of corticotroph plasma membrane electrical responses. We found that an increase in the L-type Ca2+ current was sufficient to generate action potentials from a previously resting state of the model. The increase in the L-type current could be elicited by either a shift in the voltage dependence of the current toward more negative potentials, or by an increase in the conductance of the current. Although either of these mechanisms is potentially responsible for the generation of action potentials, previous experimental evidence favors the former mechanism, with the magnitude of the shift required being consistent with the experimental findings. The model also shows that the T-type Ca2+ current plays a role in setting the excitability of the plasma membrane, but does not appear to contribute in a dynamic manner to action potential generation. Inhibition of a K+ conductance that is active at rest also affects the excitability of the plasma membrane.

Contrasting biophysical and pharmacological properties of T-type and R-type calcium channels

In contrast to other kinds of voltage-gated Ca2+ channels, the underlying molecular basis of T-type and R-type channels is not well-understood. To facilitate comparisons with cloned Ca2+ channel subunits, we have carried out a systematic analysis of the properties of T-type currents in undifferentiated NG108-15 cells and R-type currents in cerebellar granule neurons. Marked differences were found in their biophysical and pharmacological features under identical recording conditions. T-type channels became activated at potentials approximately 25 mV more negative than R-type channels; however, T-type channels required potentials approximately 15 mV less negative than R-type channels to be available. Accordingly, T-type channels display a much larger overlap between the curves describing inactivation and activation, making them more suitable for generating sustained Ca2+ entry in support of secretion or pacemaker activity. In contrast, R-type channels are not equipped to provide a steady current, but are very capable of supplying transient surges of Ca2+ influx. In response to a series of increasingly strong depolarizations T-type and R-type Ca2+ channels gave rise to very different kinetic patterns. T-type current records crossed each other in a characteristic pattern not found for R-type currents. These biophysical distinctions were independent of absolute membrane potential and were, therefore, complementary to the conventional categorization of T- and R-type Ca2+ channels as low- and high-voltage activated. R-type channels deactivated approximately eight-fold more quickly than T-type channels, with clear consequences for the generation of divalent cation influx during simulated action potentials. Pharmacological comparisons revealed additional contrasts. R-type current was responsive to block by omega-Aga IIIA but not nimodipine, while the opposite was true for T-type current. Both channel types were potently inhibited by the non-dihydropyridine compound mibefradil. In all respects examined, R-type currents were similar to currents derived from expression of the alpha1E subunit whereas T-type currents were not.

Molecular determinants of drug binding and action on L-type calcium channels

The crucial role of L-type Ca2+ channels in the initiation of cardiac and smooth muscle contraction has made them major therapeutic targets for the treatment of cardiovascular disease. L-type channels share a common pharmacological profile, including high-affinity voltage- and frequency-dependent block by the phenylalkylamines, the benz(othi)azepines, and the dihydropyridines. These drugs are thought to bind to three separate receptor sites on L-type Ca2+ channels that are allosterically linked. Results from different experimental approaches implicate the IIIS5, IIIS6, and IVS6 transmembrane segments of the alpha 1 subunits of L-type Ca2+ channels in binding of all three classes of drugs. Site-directed mutagenesis has identified single amino acid residues within the IIIS5, IIIS6, and IVS6 transmembrane segments that are required for high-affinity binding of phenylalkylamines and/or dihydropyridines, providing further support for identification of these transmembrane segments as critical elements of the receptor sites for these two classes of drugs. The close proximity of the receptor sites for phenylalkylamines, benz(othi)azepines, and dihydropyridines raises the possibility that individual amino acid residues may be required for high-affinity binding of more than one of these ligands. Therefore, we suggest that phenylalkylamines and dihydropyridines bind to different faces of the IIIS6 and IVS6 transmembrane segments and, in some cases, bind to opposite sides of the side chains of the same amino acid residues. The results support the domain interface model for binding and channel modulation by these three classes of drugs.

High affinity block by nimodipine of the internal calcium elevation in chronically depolarized rat cerebellar granule neurons

Chronic depolarization enhances survival of cultured rat cerebellar granule neurons by elevating the internal calcium concentration ([Ca2+]i). In Fura2-loaded cells maintained in 25 mM KCl, [Ca2+]i was close to 150 nM and decreased to approximately 50 nM when KCl was lowered to 5.4 mM. The effect of nimodipine (IC50 = 0.45 nM) was similar to depolarization removal, while agatoxin IVA (up to 500 nM) was ineffective. In whole-cell-clamp experiments, the IC50 for current inhibition was 57 nM, while with transient, KCl-induced depolarizations, the dose dependence of nimodipine inhibition had a 'double-affinity' shape, with IC50(1) = 0.30 nM and IC50(2) = 71 nM. We concluded that L-type calcium channels are the main responsible of the elevated internal calcium level necessary for survival in these neurons. These channels do not inactivate and bind nimodipine with different affinity, depending on their state.

Maitotoxin, a calcium channel activator, inhibits cell cycle progression through the G1/S and G2/M transitions and prevents CDC2 kinase activation in GH4C1 cells

Calcium regulates progression through several checkpoints in the cell cycle, including the G1/S-phase transition, G2/M-phase transition, and exit from mitosis. In the GH4C1 rat pituitary cell line, calcium mobilizing polypeptides and calcium channel activation inhibit cell proliferation. This report examines the effects of maitotoxin (MTX), an activator of type L voltage-dependent calcium channels (L-VDCC), on calcium influx and cell cycle progression in GH4C1 cells. MTX causes both a block from G1 to S-phase and a concentration-dependent accumulation of cells in G2+M. MTX does not increase the mitotic index; thus, sustained calcium channel activation by MTX results in an accumulation of cells in G2. In order to temporally localize the MTX-induced G2 block relative to cell cycle regulatory events at the G2/M transition, we assessed the relative activity of two cell cycle regulatory protein kinases, CDC2 and CDK2, in MTX-treated cells. CDC2-specific histone kinase activity in MTX-treated cells is lower than either in cells blocked in mitosis with the microtubule destabilizing agent demecolcine or in randomly cycling cells. In contrast, the activity of CDK2 is highest in MTX-treated cells, consistent with a G2 block prior to CDC2 activation. Together, these results implicate with a G2 block prior to CDC2 activation. Together, these results implicate calcium as an intracellular signal required for progression through G2 phase of the cell cycle prior to CDC2 kinase activation.

L- and N-type Ca2+ channels in adult rat carotid body chemoreceptor type I cells

1. Whole-cell voltage-dependent Ca2+ currents recorded from chemoreceptor type I cells of the adult rat carotid body had maximum amplitudes of -94 pA in 10 mM Ca2+ and were half-inactivated at a holding potential of -38 mV. Somatostatin and dopamine inhibited whole-cell Ca2+ current in type I cells. 2. The dihydropyridine agonist (+)202-791 increased the Ca2+ current amplitude by 106% at a step potential of -18 mV. The dihydropyridine antagonist nimodipine decreased the Ca2+ current amplitude by 40% from a holding potential of -80 mV, and by 74% from a holding potential of -60 mV. The nimodipine-sensitive current had a maximum amplitude at a membrane potential of -12 mV. omega-Conotoxin GVIA (omega-CgTX GVIA) blocked the whole-cell Ca2+ current by 40%. The omega-CgTX GVIA-sensitive current had a maximum amplitude at a membrane potential of +2 mV. 3. In summary, type I cells of the adult rat carotid body have dihydropyridine-sensitive L-type and omega-conotoxin GVIA-sensitive N-type voltage-dependent Ca2+ channels. These channels may play a role in the voltage-gated entry of Ca2+ necessary for stimulus-secretion coupling in response to changes in arterial PO2, PCO2 and pH. Inhibition of the Ca2+ currents by somatostatin and dopamine may alter the chemotransduction signal in type I cells.

Short-term regulation of neuronal calcium channels by depolarization

The 1,4-dihydropyridine-sensitive voltage-gated Ca2+ channel is widely distributed in excitable cells. The channel and its several associated drug binding sites are known to be up- and downregulated by a variety of homologous and heterologous influences including membrane depolarization. The neurosecretory GH4C1 cell line possesses L-type channels. Depolarization of these cells by elevated K+ increases the binding affinity of 1,4-dihydropyridines and decreases the number of 1,4-dihydropyridine binding sites and functional channels. There is a coordinate upregulation of the number of muscarinic receptors. This membrane potential- and Ca(2+)-calmodulin-dependent process of channel downregulation may involve internalization of the channel heteromeric complex or, more plausibly, a dissociation of the complex and a concomitant loss of both binding and permeation functions.

Mechanisms of the enhanced contractile response to a low concentration of phorbol 12,13-dibutyrate in thoracic aorta isolated from rats with dietary magnesium deficiency

The mechanisms underlying the enhanced contractile response to phorbol 12,13-dibutyrate (PDBu) were examined in de-endothelialized thoracic aortas isolated from rats with dietary magnesium (Mg) deficiency. PDBu (1.0 nM)-induced contractions were significantly larger in Mg-deficient rats than in the controls. The contraction was completely inhibited by nifedipine, removal of external Ca2+ or by l-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7). PDBu (1.0 nM) and phorbol 12-myristate 13-acetate (1.0 microM) significantly decreased the KD value and increased the Bmax for the binding of [3H]PN200-110 to the aortas. The degree of the decrease in the KD value was significantly greater in Mg-deficient rats than in the controls. The PDBu-induced decrease in the KD value was abolished by H7. These results suggest that activation of protein kinase C by phorbol esters may participate in the activation of L-type Ca2+ channels, which increases both the affinity of [3H]PN200-110 binding and the magnitude of the external Ca(2+)-dependent contraction. Dietary Mg-deficiency may enhance these processes.

Evidence that TRH controls prolactin release from rat lactotrophs by stimulating a calcium influx

Prolactin (PRL) release and intracellular free calcium concentration [Ca2+]i were measured in two populations of normal rat lactotrophs (light and heavy fractions) in culture. Spontaneous PRL release of heavy fraction cells was more sensitive to dihydropyridines (DHPs; Bay K 8644 and nifedipine) when compared to the light fraction lactotrophs. The stimulatory effect of thyrotropin-releasing hormone (TRH) on PRL release from heavy fraction cells was inhibited by Cd2+ and mimicked by Bay K 8644. Indo-1 experiments revealed that TRH-increased [Ca2+]i was reversibly inhibited by Cd2+. In a Ca(2+)-free EGTA-containing medium, TRH did not modify [Ca2+]i.

Actions of growth-hormone-releasing hormone on rat pituitary cells: intracellular calcium and ionic currents

Actions of growth-hormone-releasing hormone (GHRH) on single rat anterior pituitary cells were studied using indo-1 fluorescence to monitor changes in intracellular calcium, [Ca2+]i, and perforated-patch recording to measure changes in membrane potential and ionic currents. GHRH elevated [Ca2+]i in non-voltage-clamped cells by a mechanism that was dependent upon extracellular Na+ and Ca2+ and was blocked by the dihydropyridine Ca(2+)-channel blocker, nitrendipine. Resting cells had a fluctuating membrane potential whose a mean value depolarized by 9 mV in response to GHRH. The membrane-permeant cAMP analogue, 8-(4-chlorophenylthio)cAMP, mimicked the action of GHRH on membrane potential. Under voltage clamping, GHRH activated a small inward current (1-5 pA). Two types of response could be distinguished. The type I response had an inward current that was largest at more negative potentials (-90 mV), and the type II response had inward current that was larger at more positive potentials (-40 to -70 mV). Both types of response were reversible and blocked by removal of extracellular Na+. These results suggest that the rise in [Ca2+]i produced by GHRH in non-voltage-clamped cells results from the activation via cAMP of a Na(+)-dependent conductance, which depolarizes the cell and increases the Ca2+ influx through voltage-gated Ca2+ channels.

Pharmacology of high-threshold calcium currents in GH4C1 pituitary cells and their regulation by activation of human D2 and D4 dopamine receptors

1. The objective of this study was to characterize the pharmacology of calcium currents in GH4C1 pituitary cells and determine whether activation of heterologously expressed human dopamine receptors can regulate their function. Human D2(short), D3 and D4.2 receptor cDNA's were separately transfected into GH4C1 cells and whole cell calcium currents were recorded by use of nystatin-perforated patch clamp techniques. 2. High-threshold calcium currents were antagonized in a biphasic manner by the dihydropyridine, nisoldipine. The half-maximally effective concentration for each site was 0.2 nM (pIC50 = 9.78 +/- 0.21, n = 4) and 339 nM (pIC50 = 6.47 +/- 0.12, n = 4). The component of current inhibited by 10 nM nisoldipine was also blocked by omega-conotoxin GVIA (30 +/- 9% at 30 nM, n = 6) or by omega-agatoxin IVA (34 +/- 7% at 100 nM, n = 4). 3. Activation of either D2 or D4 receptors by dopamine (10 microM) or quinpirole (0.1 to 10 microM) reduced the peak calcium current by ca. 20% in the majority of cells studied. No inhibition was observed in control or D3 transfected GH4C1 cell lines. 4. The mobilisation of intracellular calcium by thyrotropin releasing hormone in hD4-GH4C1 cells was also studied using Fura-2 AM microspectrofluorimetry. Thyrotropin releasing hormone caused a concentration-dependent increase in calcium mobilisation with an EC50 of 7 nM. D4 receptor activation had no effect upon either basal or hormone-induced [Ca2+]i transients. 5. These results demonstrate that GH4C1 pituitary cells have at least two types of dihydropyridine sensitive high-threshold calcium currents and that like D2 receptors, human D4 receptors can also regulate calcium channel function.

Calcium channel subtypes in cat chromaffin cells

1. Using the patch-clamp technique we have investigated the kinetic and pharmacological properties of high-voltage-activated (HVA) Ca2+ channels in short-term-cultured cat chromaffin cells. 2. In 10 mM Ba2+, HVA currents activated around -40 mV, reached maximal amplitude at 0 mV and reversed at about +60 mV. At 0 mV, HVA current activation was fast (mean tau act, 2.45 ms), and followed by either an incomplete inactivation or by a second slow phase of activation (mean tau slow, 36.8 ms) that was lost when Ba2+ was replaced by Ca2+. HVA Ba2+ currents deactivate quickly on repolarization to -50 mV (mean tau deact, 0.36 ms). 3. In most cells, HVA currents were sensitive to common dihydropyridine (DHP) derivatives. Nisoldipine blocked the currents maximally at low membrane potentials (mean block 76% at -30 mV, 3 microM) and gradually less at higher voltages. Nisoldipine block was clearly time dependent (33 and 56% after 30 and 600 ms, respectively, to 0 mV). 4. Bay K 8644 (3 microM) action was variable and caused (1) a 2- to 4-fold increase of Ba2+ currents at -40 to -20 mV, (2) a -15 mV shift of the current-voltage relationship and (3) a 10- to 20-fold prolongation of HVA channel deactivation at -50 mV. 5. Nisoldipine block and Bay K 8644 potentiation of HVA currents increased markedly in omega-conotoxin GVIA (omega-CgTX)-pretreated cells, suggesting an increased fraction of DHP-sensitive currents in these cells. Nisoldipine block of residual omega-CgTX-resistant currents was almost complete (mean block, 82%) during pulses of 1 s to 0 mV. 6. The degree of inhibition produced by omega-CgTX (2 microM for 1 min) varied from cell to cell (mean block, 46%) and was partly reversible. Residual omega-CgTX-resistant currents exhibited faster activation-deactivation kinetics than control currents. 7. The slow phase of HVA current activation was abolished if a conditioning prepulse of 40 ms to +70 mV preceded a test pulse to 0 mV. Double-pulse protocols caused an average current increase (facilitation) of 37% that was voltage dependent and which correlated with the slow phase of Ca2+ channel activation. Facilitation was lost in most omega-CgTX-treated cells and was little affected by nisoldipine (3 microM) and Bay K 8644 (1 microM). Facilitation was potentiated in cells dialysed with 100 microM guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and fully prevented by 1 mM guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S).(ABSTRACT TRUNCATED AT 400 WORDS)

Localized L-type calcium channels control exocytosis in cat chromaffin cells

Depolarizing 1-s pulses to 0 mV from a holding potential of -70 mV, induced whole-cell currents through Ca2+ channels (ICa) in patch-clamped cat adrenal medulla chromaffin cells. The dihydropyridine (DHP) furnidipine (3 microM) reduced the peak current by 47% and the late current by 80%. omega-Conotoxin GVIA (CgTx, 1 microM) reduced the peak ICa by 42% and the late ICa by 55%. Pulses (10 s duration) with 70 mM K+/2.5 mM Ca2+ solution (70 K+/2.5 Ca2+), applied to single fura-2-loaded cat chromaffin cells increased the cytosolic Ca2+ concentration ([Ca2+]i) from 0.1 to 2.21 microM; this increase was reduced by 43.7% by furnidipine and by 42.5% by CgTx. In the perfused cat adrenal gland, secretion evoked by 10-s pulses of 70 K+/2.5 Ca2+ was reduced by 25% by CgTx and by 96% by furnidipine. Similar results were obtained when secretion from superfused isolated cat adrenal chromaffin cells was studied and when using a tenfold lower [Ca2+]o. The results are compatible with the existence of DHP-sensitive (L-type) as well as CgTx-sensitive (N-type) voltage-dependent Ca2+ channels in cat chromaffin cells. It seems, however, that though extracellular Ca2+ entry through both channel types leads to similar increments of averaged [Ca2+]i, the control of catecholamine release is dominated only by Ca2+ entering through L-type Ca2+ channels. This supports the idea of a preferential segregation of L-type Ca2+ channels to localized "hot spots" in the plasmalemma of chromaffin cells where exocytosis occurs.

Block by capsaicin of voltage-gated K+ currents in melanotrophs of the rat pituitary

1. Whole-cell recordings of macroscopic K+ currents were made from acutely dissociated and cultured melanotrophs isolated from the pars intermedia of the adult rat pituitary. 2. In acutely dissociated cells, external capsaicin reversibly decreased the amplitude both of the fast-activating, fast-inactivating potassium current IK(f) and the slowly-activating, slowly-inactivating potassium current IK(s). To simplify the investigation of the mechanism of action of capsaicin experiments were conducted on cultured melanotrophs that express only IK(s). 3. In control cells the activation rate and the amplitude of IK(s) increased with depolarization and the current showed very little inactivation at any voltage during pulses lasting for 100-300 ms. In capsaicin, the decrease of the current amplitude was associated with an increased rate of current decay ('inactivation'). At a given voltage, the extent and the rate of the capsaicin-induced inactivation was proportional to the capsaicin concentration; and, at a given concentration, the extent and rate of the inactivation increased with membrane depolarization. 4. The fit of the Hill equation to data derived from the steady-state block of IK(s) evoked at 10 mV indicated an equilibrium dissociation constant (KD) of 17.4 microM (95% confidence limits 15.8-19.0) and a Hill coefficient of 1.8 (95% Cl 1.5-2.2) suggesting that at least two molecules of capsaicin must bind to the channel to block it. 5. Analysis of the voltage-dependence of the steady-state block in 100 microM capsaicin showed that half-maximal block occurred at -29 +/- 2 mV (n = 10). Two-pulse experiments designed to study the time-dependence of channel block in 100 MicroM capsaicin indicated that the blocking kinetics were well fitted by a single exponential and that the rate of block increased with depolarization. The value for Tblock at 0mV was 24 +/-7ms (n=4).6. Recovery from block in 100 MicroM capsaicin was also well fitted by a single exponential. The recovery time constant ( was 708 +/- 140 ms at - 50 mV, 70 +/- 6 ms at - 70 mV and 19 +/- 1.3 ms at-90 mV (n = 4).7. In 50-100 MicroM capsaicin, the decay of the tail current was biexponential, the values for fast and Tslow being, respectively, less than and greater than the single time constant fitted to the control tail current.Inward and outward K+ currents were equally affected by capsaicin.8. Most of these effects of capsaicin on the IK(S) of melanotrophs can be accounted for by a kinetic scheme in which capsaicin binds to and blocks open K+ channels.

Some properties of the UL-FS 49 block of the hyperpolarization-activated current (i(f)) in sino-atrial node myocytes

Block of the hyperpolarization-activated "pacemaker" current (i(f)) by the bradycardic agent UL-FS 49 was studied in isolated sino-atrial (SA) node myocytes. Using repetitive activation/deactivation protocols, micromolar concentrations of UL-FS 49 blocked i(f) in a dose-dependent fashion. Block development was slow, with time constants decreasing with drug concentration and ranging from 25.8 s at 10 microM to 75.5 s at 1 microM UL-FS 49. Block did not develop in cells held at -35 mV, at which voltage i(f) channels are closed, indicating that channels must open before blocking occurs. Apparently in contrast with the requirement of negative voltages for block development, block was relieved by hyperpolarization with a time course slower than current kinetics. Due to the hyperpolarization-induced block relief, current/voltage (I/V) relations in the presence of UL-FS 49 displayed inward-going rectification. Experimental data fitted the hypothesis that UL-FS 49 behaves as an "open channel" blocker of "single-ion" i(f) channels. Block occurs within the pore, at a distance of about 39% of the membrane thickness from its internal side.

Mechanisms of the enhanced contractile response to phenylephrine in thoracic aorta isolated from rats with dietary magnesium deficiency

The mechanisms underlying the enhanced contractile response to phenylephrine (PE) and increased susceptibility to nifedipine of de-endothelialized thoracic aorta isolated from rats with dietary magnesium deficiency (Mg-deficient rats) were examined by functional and radioligand binding studies. Enhanced PE-induced contractions and increased susceptibility to nifedipine in Mg-deficient rats were not observed in the presence of 10 microM H7. PE significantly decreased the KD value without changing Bmax in the binding of [3H]PN200-110 to de-endothelialized aortic strips. The KD value obtained in the Mg-deficient rats was significantly smaller than that in the controls. Nifedipine displaced the binding of [3H]PN200-110 concentration-dependently, and the pKi value in Mg-deficient rats was significantly larger than that in the controls. A combination of PE and H7 abolished this difference. These results indicate that the modulation of L-type Ca2+ channels via the stimulation of alpha 1-adrenoceptors may be involved in the enhancement of vasoconstriction and increased susceptibility to nifedipine in aortas isolated from Mg-deficient rats. The H7-sensitive mechanisms may play an important role in these phenomena.

Glucocorticoid modulation of calcium currents in growth hormone 3 cells

Clonal malignant pituitary growth hormone 3 cells were used for the analysis of the influence of hydrocortisone and dexamethasone on voltage-gated calcium currents and hormone secretion. The whole-cell patch-clamp technique was used. The presence of low-threshold inactivating and high-threshold persisting components in the total calcium current was shown; they could be separated at less negative holding potential level. Some increase in current densities of both components was observed as early as 30 min after treatment with 10(-6) mol/l glucocorticoids. The increase was maximal for both types of currents after 2 h of incubation; however, the high-threshold component was affected much more strongly (current density increased by more than four-fold) than the low-threshold one (current density increased by about a three-fold). Potentiation of currents was blocked by actinomycine D (10(-4) M), suggesting that protein synthesis was required. A substantial increase in growth hormone secretion (measured by radioimmunoassay method) was observed in the same cells after 2 h of incubation with hydrocortisone, while the secretion of prolactin remained even slightly depressed.

Sensitivity to dihydropyridines, omega-conotoxin and noradrenaline reveals multiple high-voltage-activated Ca2+ channels in rat insulinoma and human pancreatic beta-cells

High-voltage-activated (HVA) Ba2+ currents of rat insulinoma (RINm5F) and human pancreatic beta-cells were tested for their sensitivity to dihydropyridines (DHPs), omega-conotoxin (omega-CgTx) and noradrenaline. In RINm5F cells, block of HVA currents by nimodipine, nitrendipine and nifedipine was voltage- and dose-dependent (apparent KD < 37 nM) and largely incomplete even at saturating doses of DHPs (mean 53%, at 10 microM and 0 mV). Analysis of slow tail currents in Bay K 8644-treated cells indicated the existence of Bay K 8644-insensitive channels that turned on at slightly more positive voltages and deactivated more quickly than Bay K 8644-modified channels. DHP Ca2+ agonists and antagonists in human beta-cells had similar features to RINm5F cells except that DHP block was more pronounced (76%, at 10 microM and 0 mV) and Bay K 8644 action was more effective, suggesting a higher density of L-type Ca2+ channels in these cells. In RINm5F cells, but not in human beta-cells, DHP-resistant currents were sensitive to omega-CgTx. The toxin depressed 10-20% of the DHP-resistant currents sparing a "residual" current (25-35%) with similar voltage-dependent characteristics and Ca2+/Ba2+ permeability. Noradrenaline (10 microM) exhibited different actions on the various HVA current components: (1) it prolonged the activation kinetics of omega-CgTx-sensitive currents, (2) it depressed by about 20% the size of DHP-sensitive currents, and (3) it had little or no effects on the residual DHP- and omega-CgTx-resistant current although intracellularly applied guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) prolonged its activation time course.(ABSTRACT TRUNCATED AT 250 WORDS)

Stable transfection of calbindin-D28k into the GH3 cell line alters calcium currents and intracellular calcium homeostasis

Previous work demonstrating the presence and differential distribution of Ca(2+)-binding proteins in the CNS has led to the proposal that cytosolic proteins, such as calbindin-D28k (CB), may play a pivotal role in neurons. We have used a retrovirus containing the full-length cDNA for CB to transfect the pituitary tumor cell line GH3, to generate CB-expressing GH3 cells and to investigate whether ionic channel activities as well as the concentration of intracellular free Ca2+ ([Ca2+]i) homeostasis could be altered by the presence of this Ca(2+)-binding protein. We show that CB-transfected GH3 cells exhibited lower Ca2+ entry through voltage-dependent Ca2+ channels and were better able to reduce [Ca2+]i transients evoked by voltage depolarizations than the wild-type parent cell line. These observations provide a mechanism by which CB may protect tissues against Ca(2+)-mediated excitotoxicity.