Molecular diversity of voltage-dependent Ca2+ channels

Novel variants of voltage-operated calcium channel alpha 1-subunit transcripts in a rat liver-derived cell line: deletion in the IVS4 voltage sensing region

Using reverse transcriptase-PCR and Northern analysis, we have shown that the H4IIE cell line, derived from the Reuber H35 rat hepatoma, contains significant amounts of transcripts for the CaCh3 (neuroendocrine) and CaCh1 (skeletal muscle) L-type voltage-operated calcium channel alpha 1-subunits. Two of the CaCh3 transcripts have a 45 bp deletion in the IVS4 membrane-spanning region which is the result of a mutation in genomic DNA. The deduced amino acid sequences of the PCR-derived clones of CaCh3 indicate that the mutation causes the loss of 15 amino acids from the IVS4 region, including three of the six positively charged residues, which are thought to be part of the voltage-sensing mechanism of voltage-operated Ca2+ channels. Quantitative-PCR and Northern analysis indicate that one of the novel CaCh3 transcripts is present in sufficient amounts to imply it could play a functional role in Ca2+ inflow. RT-PCR analysis of hepatocytes isolated from rat liver detected transcripts of CaCh3 (without the IVS4 mutation) and CaCh2, but at considerably lower levels than observed for the isoforms in the H4IIE cell line. Transcripts of CaCh1 and CaCh2 were also detected at low levels in Jurkat T lymphocytes. Fluorimetric studies with the Ca(2+)-sensitive probe, Fluo-3, have shown that H4IIE cells exhibit receptor-activated and store-activated (thapsigarin-induced), but not depolarisation (extracellular KCl)-induced Ca2+ inflow. The mutant transcripts are unlikely to produce Ca2+ channels that are opened by membrane depolarisation. The idea that they may be opened by other mechanisms is briefly discussed.

Effect of calcium antagonists on rat liver during extended cold preservation-reperfusion

BACKGROUND

Nisoldipine, a calcium antagonist, has been reported to improve the quality of grafted rat livers. We thus assessed the protective effect of two calcium antagonists, nisoldipine and nickel, during extended cold ischemia-reperfusion.

METHODS

Rat livers were isolated and perfused before or after 24 hr of cold ischemia in University of Wisconsin solution (4 degrees C) with or without nisoldipine or nickel. Sinusoidal endothelial cell and hepatocyte functions were measured by hyaluronic acid and taurocholate elimination, respectively.

RESULTS

Similar alterations in hepatocyte and sinusoidal cell functions were found in all groups after cold ischemia with or without calcium antagonists. In a second set of experiments, liver transplantation was performed in two groups of rats with livers stored under identical conditions with or without nisoldipine. Seven of 12 animals (62.5%) in both groups survived for over 10 days after 24-hr preservation in University of Wisconsin solution. Survival rates were similar in both groups.

CONCLUSIONS

Calcium antagonists do not appear to have a direct protective effect on sinusoidal endothelial cell and hepatocyte functions, nor on the overall liver preservation after extended cold preservation-reperfusion.

The monomeric G-proteins Rac1 and/or Cdc42 are required for the inhibition of voltage-dependent calcium current by bradykinin

Although regulation of voltage-dependent calcium current (ICa,V) by neurotransmitters is a ubiquitous mechanism among nerve cells, the signaling pathways involved are not well understood. We have determined previously that in a neuroblastoma-glioma hybrid cell line (NG108-15), the heterotrimeric G-protein G13 mediates the inhibition of ICa,V produced by bradykinin (BK) via an unknown mechanism. Various reports indicate that G13 can couple to RhoA, Rac1, and Cdc42, which are closely related members of the Rho family of monomeric G-proteins. We have investigated their role as signaling intermediates in the pathway used by BK to inhibit ICa,V. Using immunoblot analysis and the PCR, we found evidence that RhoA, Rac1, and Cdc42 all are expressed in NG108-15 cells. Intracellularly perfused recombinant Rho-GDI (an inhibitor of guanine nucleotide exchange specific for the Rho family) attenuated the inhibition of ICa,V by BK. These findings indicate that activation of RhoA, Rac1, or Cdc42 may be required for the response to BK. To determine whether any of these monomeric G-proteins mediate the response to BK, we have intracellularly applied blocking antibodies specific for each of the candidate proteins. Only the anti-Rac1 antibody blocked the response to BK. In parallel experiments, peptides corresponding to the C-terminal regions of Rac1 and Cdc42 blocked the same response. These data indicate a novel functional contribution of Rac1 and possibly also of Cdc42 to the inhibition of ICa,V by neurotransmitters.

Purification of a new dimeric protein from Cliona vastifica sponge, which specifically blocks a non-L-type calcium channel in mouse duodenal myocytes

Marine sponges are synthesizing a wide variety of peptidic and organic molecules with biological activities. Multiple-step purification of Cliona vastifica extract led to a new dimeric peptide (mapacalcine; M(r) = 19,064) that is composed of two homologous chains, each containing nine cysteins. This protein has been found to selectively block a new calcium conductance characterized in mouse duodenal myocytes with an IC50 value of approximately 0.2 microM. The mapacalcine-sensitive current was a non-L-type calcium current activated from a holding potential of -80 mV that persisted during stimulation of the cell at high frequencies (0.1-0.2 Hz) within 5-10 min. Time constants of inactivation were similar for both L-type and non-L-type calcium currents. The non-L-type calcium current of duodenal myocytes was not blocked by the pharmacological agents specific for N-, L-, P-, or Q-type calcium channels. Mapacalcine was unable to block T-type calcium current in portal vein myocytes as well as voltage-dependent potassium currents and calcium-activated chloride currents in duodenal and portal vein cells. Mapacalcine did not affect caffeine-induced calcium responses, indicating that it did not interfere with intracellular calcium stores. Competition experiments on mouse intestinal membranes showed that mapacalcine did not interact with dihydropyridines receptors. These data suggest that mapacalcine may be a specific inhibitor of a new type of calcium current, first identified in duodenal myocytes.

Binding and distribution of three prototype calcium channel blockers in perfused rat liver

This work represents a study of the binding and distribution of three different calcium channel blockers in the Sprague-Dawley rat liver, using an in situ perfusion technique. For this purpose, [3H] desmethoxyverapamil, [3H] PN200-110 (isradipine) and [3H] azidopine were used as binding probes interacting with calcium channels. The perfusion steps of the liver involved both portal vein and thoracic inferior vena cava cannulations as inlet and outlet respectively. The subhepatic inferior vena cava was ligated to prevent leakage of the perfusate. Buffer, containing the tracer drug, was administered via the portal vein at a rate of 1 mL/min and perfusate collected at the same rate within specified time intervals during 50 min. The concentration of the tracer solutes in the perfusate's outlet increased with time, and steady state was observed for all tracers at > or = 40 min. The effect of adding cold isradipine to tracer desmethoxyverapamil, or cold verapamil to tracer PN200-110 were also assessed. First order rate constants for hepatocellular influx, efflux and calcium channel binding of the tracer substances were obtained using a simplified model from Goresky et al. These constants were mathematically manipulated and changed into permeability constants, second order binding constants, and residency times. Tracer solute influx across hepatocellular membranes is solubility-diffusion controlled, is inversely related to the molecular weights and is different in value from the efflux constants. Cold isradipine reduced the binding constant of desmethoxyverapamil by 36%, while cold verapamil reduced the binding constant of PN200-110 by 23%. Azidopine cellular distribution was low, however, binding to its receptor was analogous to desmethoxyverapamil and PN200-110. Moreover, PN200-110 had the highest residency time with no effect of cold verapamil on its receptor binding, while desmethoxyverapamil had the lowest residency time which significantly increased in the presence of cold isradipine.

L-type calcium channels: binding domains for dihydropyridines and benzothiazepines are located in close proximity to each other

We investigated the binding of a fluorescent diltiazem analogue (3R,4S)-cis-1-[2-[[3-[[3-[4,4-difluoro-3a,4-dihydro-5,7-dimethyl-4-bo ra-3a,4a-diaza-s-indacen-3-yl]propionyl]amino]propyl]amin o]ethy]-1,3,4,5-tetrahydro-3-hydroxy-4-(4-methoxyphenyl)-6-(triflu oromethyl)-2H-1-benzazepin-2-one (DMBODIPY-BAZ) to L-type Ca2+ channels in the presence of different 1,4-dihydropyridines (DHPs) by using fluorescence resonance energy transfer (FRET) [Brauns, T., Cai, Z.-W., Kimball, S. D., Kang, H.-C., Haugland, R. P., Berger, W., Berjukov, S., Hering, S., Glossmann, H., & Striessnig, J. (1995) Biochemistry 34, 3461]. When channels are occupied with DMBODIPY-BAZ, a rapid fluorescence change occurred upon addition of different DHPs. The direction of this intensity modulation was found to be only dependent on the chemical composition of the dihydropyridine employed. DHPs containing a nitro group decreased, whereas others (e.g., isradipine) enhanced the fluorescence signal. In addition, all DHPs markedly decreased the association rate constant for DMBODIPY-BAZ without affecting equilibrium binding. Both observations together are best explained by a steric model where the DHP binding site is located in close proximity to the accession pathway of DMBODIPY-BAZ.

Slow wave sleep is accompanied by release of certain amino acids in the thalamus of cats

To determine whether EEG synchronization in sleep has a metabolic equivalent, we investigated state-dependent changes in extracellular concentrations of amino acids. In vivo microdialysis studies were performed in the ventroposterolateral (VPL) nuclei of the thalamus of cats during natural slow wave sleep (SWS), waking (W) and carbachol-induced paradoxical sleep (PS) like episodes. About two-fold increases in aspartate, glutamate, asparagine, glycine, alanine and gamma-aminobutyric acid (GABA) were observed in SWS compared with control samples collected in W, but serine increased to 487 +/- 211%. In the PS-like state, glutamine increased and GABA decreased. These results suggest changes in intracellular processes reflected by amino acid release in the thalamus, specific to slow wave generation in EEG during natural sleep.

Dynorphin A-mediated reduction in multiple calcium currents involves a G(o) alpha-subtype G protein in rat primary afferent neurons

We examined the effect of antisera directed at specific G-protein subtype(s) on dynorphin A (Dyn A)-mediated reduction of calcium currents in rat dorsal root ganglia (DRG) neurons. Whole cell patch-clamp recordings were performed on acutely dissociated neurons. Dyn A (1 microM)-mediated decrease in calcium currents was inhibited > 90% by the preferential kappa-receptor antagonist norbinaltorphimine. Dyn A (300-1,000 nM)-mediated reduction in calcium currents was examined during intracellular administration of antisera directed against specific regions of G(o) alpha, G(i) 1 alpha/G(1) 2 alpha, and G(i) 3 alpha subunits. Intracellular dialysis with an antiserum specific for G(o) alpha for 20 min decreased calcium current inhibition by Dyn A (1 microM) in 13 of 15 neurons by an average of 75%. Dialysis with nonimmune serum did not affect Dyn A's action to reduce calcium currents. Intracellular dialysis with either anti-G(i) 1 alpha/G(i) 2 alpha or anti-G(i) 3 alpha antisera did not affect Dyn A-induced changes in calcium currents. In the presence of the N-type calcium channel antagonist omega-conotoxin GVIA, the P-type calcium channel antagonist omega-Aga IVA, and omega-Aga MVIIC applied subsequent to the other toxins, the effect of Dyn A to reduce calcium currents was inhibited by 52, 28, and 16%, respectively. The L channel antagonist nifedipine did not affect the ability to Dyn A to inhibit calcium currents. These results suggest that in rat DRG neurons coupling of kappa-opioid receptors to multiple transient, high-threshold calcium currents involves the G(o) alpha subclass of G proteins.

Molecular structures involved in L-type calcium channel inactivation. Role of the carboxyl-terminal region encoded by exons 40-42 in alpha1C subunit in the kinetics and Ca2+ dependence of inactivation

The pore-forming alpha1C subunit is the principal component of the voltage-sensitive L-type Ca2+ channel. It has a long cytoplasmic carboxyl-terminal tail playing a critical role in channel gating. The expression of alpha1C subunits is characterized by alternative splicing, which generates its multiple isoforms. cDNA cloning points to a diversity of human hippocampus alpha1C transcripts in the region of exons 40-43 that encode a part of the 662-amino acid carboxyl terminus. We compared electrophysiological properties of the well defined 2138-amino acid alpha1C,77 channel isoform with two splice variants, alpha1C,72 and alpha1C,86. They contain alterations in the carboxyl terminus due to alternative splicing of exons 40-42. The 2157-amino acid alpha1C,72 isoform contains an insertion of 19 amino acids at position 1575. The 2139-amino acid alpha1C,86 has 80 amino acids replaced in positions 1572-1651 of alpha1C,77 by a non-identical sequence of 81 amino acids. When expressed in Xenopus oocytes, all three splice variants retained high sensitivity toward dihydropyridine blockers but showed large differences in gating properties. Unlike alpha1C,77 and alpha1C,72, Ba2+ currents (IBa) through alpha1C,86 inactivated 8-10 times faster at +20 mV, and its inactivation rate was strongly voltage-dependent. Compared to alpha1C,77, the inactivation curves of IBa through alpha1C,86 and alpha1C,72 channels were shifted toward more negative voltages by 11 and 6 mV, respectively. Unlike alpha1C,77 and alpha1C,72, the alpha1C,86 channel lacks a Ca2+-dependent component of inactivation. Thus the segment 1572-1651 of the cytoplasmic tail of alpha1C is critical for the kinetics as well as for the Ca2+ and voltage dependence of L-type Ca2+ channel gating.

Low-threshold Ca2+ currents in dendritic recordings from Purkinje cells in rat cerebellar slice cultures

Voltage-dependent Ca2+ conductances were investigated in Purkinje cells in rat cerebellar slice cultures using the whole-cell and cell-attached configurations of the patch-clamp technique. In the presence of 0.5 mM Ca2+ in the extracellular solution, the inward current activated with a threshold of -55 +/- 1.5 mV and reached a maximal amplitude of 2.3 +/- 0.4 nA at -31 +/- 2 mV. Decay kinetics revealed three distinct components: a fast (24.6 +/- 2 msec time constant), a slow (304 +/- 46 msec time constant), and a nondecaying component. Rundown of the slow and sustained components of the current, or application of antagonists for the P/Q-type Ca2+ channels, allowed isolation of the fast-inactivating Ca2+ current, which had a threshold for activation of -60 mV and reached a maximal amplitude of 0.7 nA at a membrane potential of -33 mV. Both activation and steady-state inactivation of this fast-inactivating Ca2+ current were described with Boltzmann equations, with half-activation and inactivation at -51 mV and -86 mV, respectively. This Ca2+ current was nifedipine-insensitive, but its amplitude was reduced reversibly by bath-application of NiCl2 and amiloride, thus allowing its identification as a T-type Ca2+ current. Channels with a conductance of 7 pS giving rise to a fast T-type ensemble current (insensitive to omega-Aga-IVA) were localized with a high density on the dendritic membrane. Channel activity responsible for the ensemble current sensitive to omega-Aga-IVA was detected with 10 mM Ba2+ as the charge carrier. These channels were distributed with a high density on dendritic membranes and in rare cases were also seen in somatic membrane patches.

Dihydropyridine- and neurotoxin-sensitive and -insensitive calcium currents in acutely dissociated neurons of the rat central amygdala

The central amygdala (CeA) is an area involved in emotional learning and stress, and identification of Ca2+ currents is essential to understanding interneuronal communication through this nucleus. The purpose of this study was to separate and characterize dihydropyridine (DHP)- and neurotoxin-sensitive and -resistant components of the whole cell Ca2+ current (ICa) in acutely dissociated rat CeA neurons with the use of whole cell patch-clamp recording. Saturating concentrations of nimodipine (NIM, 5 microM), a DHP antagonist, blocked 22% of ICa: this NIM-sensitive (L-type) current was recorded in 68% of CeA neurons. The DHP agonist Bay K 8644 (5 microM) produced a 36% increase in ICa in a similar proportion of CeA neurons (70%). omega-Conotoxin GVIA (CgTx GVIA, 1 microM) in saturating concentrations inhibited 30% of ICa, whereas omega-agatoxin IVA (Aga IVA, 100 nM), in concentrations known to block P-type currents, did not affect ICa. Higher concentrations of Aga IVA (1 microM) alone reduced ICa by 34%, but in the presence of NIM (5 microM) and CgTx GVIA (1 microM) blocked only 18% of ICa. omega-Conotoxin MVIIC (CgTx MVIIC, 250 nM) reduced ICa by 13% in the presence of CgTx GVIA (1 microM). Application of NIM (5 mM), CgTx GVIA (1 microM); and Aga IVA (1 microM) blocked approximately 67% of ICa. A similar portion (63%) of Ca2+ current was blocked with CgTx MVIIC (250 nM) in the presence of NIM (5 microM) and CgTx GVIA (1 microM). The current resistant to NIM and the neurotoxins represented 37% of ICa, whereas in neurons not having L-type currents the resistant current made up approximately 53% of ICa (49 +/- 2%, mean +/- SE). The resistant current activated at around -40 mV and peaked at approximately 0 mV with half-activation and -inactivation potentials of -17 and -58 mV and slopes for activation and inactivation of -5 and 13 mV, respectively. The resistant current was sensitive to Cd2+ (IC50 = 2.5 microM) and Ni2+ (IC50 = 86 microM), was larger in Ca2+ than in Ba2+ (ratio = 1.31:1), and showed a moderate rate of decay. In summary, our results show that the high-voltage-activated calcium current in rat CeA neurons is composed of at least four pharmacologically distinct components: L-type current (NIM sensitive, 22%), N-type current (CgTx GVIA sensitive, 30%), Q-type current [Aga IVA (1 microM) and CgTx MVIIC sensitive, approximately 13-18%], and a resistant current (Non-L, -N, and -Q current, 33 approximately 37%), amounting to 37-53% of the total current. The resistant current has some electrophysiological and pharmacological characteristics in common with doe-1, alpha 1E, and R-type calcium currents, but remains unclassified.

Molecular cloning and characterization of a putative neural calcium channel alpha1-subunit from squid optic lobe

The complete amino acid sequence of a putative calcium channel alpha1-subunit, SQCC1, from the optic lobe of the squid Loligo bleekeri has been deduced by cloning and sequence analysis of the complementary DNA. The open reading frame encodes 2206 amino acids, which corresponds to a molecular weight of 251,451. The deduced amino acid sequence shares general structural features with the other voltage-dependent calcium channels; it consists of four repeated units of homology. Each motif has five hydrophobic segments and one positively charged segment. The transcriptional products were detected in all nervous systems examined; optic lobe, cerebral ganglia and giant stellate ganglia. However, it was not detected in the mantle muscle, heart and stomach, indicating SQCC1 is a calcium channel alpha1-subunit specific for squid nervous system. SQCC1 is more closely related in its amino acid sequence patterns to dihydropyridine-insensitive calcium channels rather than dihydropyridine-sensitive ones.

Inhibition of calcium channels in rat hippocampal CA1 neurons by conantokin-T

Effects of conantokin-T, a 21 amino acid peptide toxin isolated from the fish-hunting cone snail Conus tulipa, on the high-voltage-activated Ca2+ channel currents were studied in acutely dissociated rat hippocampal CA1 neurons using whole-cell voltage clamp-recording technique with 5 mM Ba2+ as the charge carrier. Conantokin-T inhibited the whole-cell Ba2+ current (IBa) in a concentration-dependent manner. The nimodipine (20 microM) and omega-agatoxin-IVA (0.2 microM) block of IBa were abolished in the presence of conantokin-T (3 microM); however, conantokin-T (3 microM) did not affect the block of IBa induced by 3 microM omega-conotoxin-GVIA. These results indicate that conantokin-T is a potent but wide-spectrum Ca2+ channel antagonist.

Aging changes in voltage-gated calcium currents in hippocampal CA1 neurons

Previous current-clamp studies in rat hippocampal slice CA1 neurons have found aging-related increases in long-lasting calcium (Ca)-dependent and Ca-mediated potentials. These changes could reflect an increase in Ca influx through voltage-gated Ca channels but also could reflect a change in potassium currents. Moreover, if altered Ca influx is involved, it is nuclear whether it arises from generally increased Ca channel activity, lower threshold, or reduced inactivation. To analyze the basis for altered Ca potentials, whole-cell voltage-clamp studies of CA1 hippocampal neurons were performed in nondissociated hippocampal slices of adult (3- to 5-month-old) and aged (25- to 26-month-old) rats. An aging-related increase was found in high-threshold Ca and barium (Ba) currents, particularly in the less variable, slowly inactivating (late) current at the end of a depolarization step. Input resistance of neurons did not differ between age groups. In steady-state inactivation and repetitive-pulse protocols, inactivation of Ca and Ba currents was not reduced and, in some cases, was slightly greater in aged neurons, apparently because of larger inward current. The current blocked by nimodipine was greater in aged neurons, indicating that some of the aging increase was in L-type currents. These results indicate that whole-cell Ca currents are increased with aging in CA1 neurons, apparently attributable to greater channel activity rather than to reduced inactivation. The elevated Ca influx seems likely to play a role in impaired function and enhanced susceptibility to neurotoxic influences.

Expression of a rapidly inactivating Ca2+ channel in Pleurodeles oocytes during the resting season

Two kinds of Ca2+ channel activities have been recorded in unfertilized Pleurodeles oocytes during the resting season: the previously described L-type and a transient one. The transient Ba current (I(Ba-t)) exhibited a voltage threshold of -36+/-7 mV, peaked at -18+/-8 mV and reversed around +50 mV. It showed a fast monoexponential decay with an inactivation time course of 31.4+/-1.7 ms at -20 mV. I(Ba-t) was insensitive to nifedipine and omega-conotoxin-GVIA but blocked by Ni2+ (50 microM). Moreover, Cd2+ also reduced I(Ba-t) but was less efficient than Ni2+. When using Ca2+ as the charge carrier, the amplitude and the decay of I(ca-t) were largely similar to those of I(Ba-t). These data demonstrated that the population of Ca2+ channels could be seasonally modulated in Pleurodeles oocytes.

The molecular identity of Ca channel alpha 1-subunits expressed in rat sympathetic neurons

Much of our understanding of the mechanisms of the gating, modulation, and function of neuronal Ca channels has its origins in investigations of sympathetic neurons. In this article, we use molecular analyses to identify the three Ca channel alpha 1-subunits that are the likely counterparts to the pharmacologically defined: omega-Conotoxin GVIA-sensitive N-type; dihydropyridine-sensitive L-type, and omega-Conotoxin GVIA-insensitive, dihydropyridine-insensitive Ca channel currents observed in sympathetic neurons. With a combination of degenerate and exact primers, small regions of Ca channel alpha 1-subunit sequences were amplified by the polymerase chain reaction (PCR). Although all five Ca channel alpha 1-subunit genes were expressed in rat sympathetic ganglia, alpha 1B-, alpha 1D-, and alpha 1E-derived cDNAs were the dominant species. No novel Ca channel alpha 1-sequences were identified in the regions selected for amplification, and we conclude that alpha 1B, alpha 1D, and alpha 1E likely encode, respectively, N-type, L-type, and non-N/non-L-type channel currents of rat sympathetic neurons. In addition, we show that Ca channel beta 2-, beta 3-, and beta 4-subunit sequences are strongly represented in sympathetic ganglia. The results of this study also suggest that alpha 1D, and not alpha 1C, regulates Ca influx through dihydropyridine-sensitive Ca channel currents.

Why are endocrine pituitary cells excitable?

Since 1975, endocrine pituitary cells have been known to be excitable neuronlike cells. Using powerful single-cell approaches, in particular the patch clamp electrophysiological recording technique and the monitoring of Ca(2+) with fluorescent probes, solid evidence has been provided in the last 10 years that intracellular Ca(2+) signals are produced by stimulators and inhibitors of secretion via the modulation of action potentials in isolated pituitary cells. As cytosolic Ca(2+) changes are thought to control numerous cellular functions (for example, secretion, protein synthesis, gene expression, and proliferation) over a long time scale-milliseconds to hours-it is now time to address the long-standing question of what functions would be physiologically controlled by electrical excitability in intact pituitary tissue.

Absence epilepsy in tottering mutant mice is associated with calcium channel defects

Mutations at the mouse tottering (tg) locus cause a delayed-onset, recessive neurological disorder resulting in ataxia, motor seizures, and behavioral absence seizures resembling petit mal epilepsy in humans. A more severe allele, leaner (tg(la)), also shows a slow, selective degeneration of cerebellar neurons. By positional cloning, we have identified an alpha1A voltage-sensitive calcium channel gene that is mutated in tg and tg(la) mice. The alpha1A gene is widely expressed in the central nervous system with prominent, uniform expression in the cerebellum. alpha1A expression does not mirror the localized pattern of cerebellar degeneration observed in tg(la) mice, providing evidence for regional differences in biological function of alpha1A channels. These studies define the first mutations in a mammalian central nervous system-specific voltage-sensitive calcium channel and identify the first gene involved in absence epilepsy.

Smooth muscle cell cycle and proliferation. Relationship between calcium influx and sarco-endoplasmic reticulum Ca2+ATPase regulation

The role of Ca2+ influx in the regulation of the sarco-endoplasmic reticulum Ca2+ATPases (SERCA) associated with intracellular Ca2+ pools was investigated during smooth muscle cell (SMC) proliferation induced by platelet-derived growth factor (PDGF). We first defined that the previously described up-regulation of the SERCA2a isoform found in vascular SMC after a 24-h stimulation with PDGF (Magnier, C. , Papp, B., Corvazier, E., Bredoux, R., Wuytack, F., Eggermont, F., Maclouf, J., and Enouf, J. (1992) J. Biol. Chem. 267, 15808-15815) was precisely associated with SMC entry into S phase as it appeared linked with [3H]thymidine incorporation. This was further confirmed by testing the effect of transforming growth factor-beta1, which inhibited both aortic SMC proliferation associated with G1 cell cycle arrest and PDGF-induced SERCA2a up-stimulation. Then, we tested the role of Ca2+ influx by using SR 33805, a new Ca2+ channel blocker, which was characterized with regard to the voltage Ca2+ channel blocker nifedipine and the capacitative entry Ca2+ blocker SKF 96365. SR 33805 was found to be the most potent inhibitor of both PDGF-induced SMC proliferation and the associated rise in intracellular Ca2+ concentration with IC50 values of 0.2 +/- 0.1 and 0.31 +/- 0. 04 microM, respectively. Finally, by examining in parallel both SERCA2a and SERCA2b isoforms, in terms of activity and expression, we could determine that PDGF-induced stimulation of total SERCA activity (detected by formation of the phosphorylated intermediate, E approximately P) and of SERCA2a expression (Western blotting) were abolished when extracellular Ca2+ entry was prevented by SR 33805. This study demonstrates that SERCA2a up-regulation is: 1) related to the G1/S transition step of cell cycle and 2) dependent on Ca2+ entry during PDGF-induced SMC proliferation.

Molecular pharmacology of voltage-dependent calcium channels

Voltage-dependent Ca2+ channels serve as the only link to transduce membrane depolarization into cellular Ca(2+)-dependent reactions. A wide variety of chemical substances that have the ability to modulate Ca2+ channels have been demonstrated both for their clinic utility and for importance in elucidating the molecular basis of various biological responses. Recently, introduction of molecular biology to pharmacology has brought a great deal of information about the molecular basis of drug action in Ca2+ channels. In this review, we attempt to overview recent progress in understanding the interactions between Ca2+ channels and their blockers, namely Ca2+ antagonists, from a molecular and structural point of view.

Chronic lithium does not alter human myo-inositol or phosphomonoester concentrations as measured by 1H and 31P MRS

Lithium may act by decreasing intracellular concentrations of myo-inositol. The present study measured the effects of chronic lithium on myo-inositol concentrations in volunteers. Eleven subjects received either lithium (n = 7) or placebo (n = 4) for 7 days in a double-blind study. Myo-inositol concentrations at baseline and day 8 were measured in vivo using 1H magnetic resonance spectroscopy (MRS). The results showed that lithium did not alter brain myo-inositol concentrations compared to placebo. In 5 other subjects we used 1H MRS and 31P MRS to measure changes in both myo-inositol and phosphomonoester concentrations. This second study showed that lithium did not alter myo-inositol or phosphomonoester concentrations. Thus, the present studies do not support the hypothesis that lithium significantly affects the brain concentrations of myo-inositol or phosphomonoesters; however, it is possible these findings represent an inability to detect the changes in myo-inositol and phosphomonoester concentrations that may have occurred following lithium administration.

Effects of prenatal ethanol exposure on voltage-dependent calcium entry into neonatal whole brain-dissociated neurons

The effect of prenatal ethanol exposure on voltage-dependent calcium entry into neonatal-dissociated neurons was studied. Dissociated whole brain cells were isolated from neonates of prenatally ethanol-treated (ET), pair-fed (PF) control, and ad libitum (AL) control groups and loaded with fura-2. Prenatal ethanol exposure resulted in a significant reduction of calcium entry into K(+)-depolarized cells, compared with AL and PF control treatments. Initially, in dissociated cells from AL control animals, it was found that nifedipine (1 microM), omega-agatoxin (100 nM), and omega-conotoxin (500 nM), to a much lesser extent, significantly inhibited the 45 mM KCl-stimulated calcium entry. To determine the inhibitory action of prenatal ethanol exposure on N-, P-, and L-type voltage-dependent calcium channels, treatment of neonatal-dissociated neurons with different combinations of omega-conotoxin, omega-agatoxin, and nifedipine, respectively, was compared in the prenatal ethanol and control treatment groups. The inhibition of K(+)-stimulated increase in calcium entry by prenatal ethanol exposure was significantly less in the presence or absence of single antagonist conditions (ET < AL and PF). There was no apparent interaction of ethanol exposure and antagonist condition. However, the reduced calcium entry after prenatal ethanol exposure was superseded by the stronger inhibition in dual and triple antagonist conditions. The magnitude of the calcium response inhibition by the antagonist combinations was similar among the ET, PF, and AL groups. Thus, these results suggest that prenatal ethanol exposure decreases voltage-dependent calcium entry into neonatal-dissociated neurons in a manner that does not seem to involve the selective inhibition of any individual N-, P-, or L-type calcium channel.

Extracellular Mg2+ modulates intracellular Ca2+ in acutely isolated hippocampal CA1 pyramidal cells of the guinea-pig

Using digital imaging microscopy and fluorescent probes, isolated hippocampal CA1 pyramidal neurons of the guinea-pig were used to examine the roles of [Mg2+]o in regulation of [Ca2+]i and [Mg2+]i. Low extracellular Mg ([Mg2+]o) (0.3 mM) significantly increased [Ca2+]i compared to 1.2 and 4.8 mM [Mg2+]o. In contrast, [Mg2+]i levels remained relatively constant, irrespective of alterations of [Mg2+]o. The sustained rise in [Ca2+]i induced by low [Mg2+]o was reduced 70% by 1 microM verapamil and 42% by 1 mM Ni2+, and completely abolished by 5 mM Ni2+. The data suggest that [Mg2+]o regulates [Ca2+]i in hippocampal neurons, probably by modulating Ca2+ entry via voltage-sensitive Ca2+ channels, which may play important roles in epileptogenesis, memory, learning and brain trauma. Furthermore, the results demonstrate that intracellular Mg2+ concentration does not follow passively the concentration of Mg2+ in the extracellular solution.

Cardiovascular characterization of pyrrolo[2,1-d][1,5]benzothiazepine derivatives binding selectively to the peripheral-type benzodiazepine receptor (PBR): from dual PBR affinity and calcium antagonist activity to novel and selective calcium entry blockers

The synthesis and cardiovascular characterization of a series of novel pyrrolo[2,1-d][1,5]-benzothiazepine derivatives (54-68) are described. Selective peripheral-type benzodiazepine receptor (PBR) ligands, such as PK 11195 and Ro 5-4864, have recently been found to possess low but significant inhibitory activity of L-type calcium channels, and this property is implicated in the cardiovascular effects observed with these compounds. In functional studies both PK 11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxa mide) and Ro 5-4864 (4'-chlorodiazepam) did not display selectivity between cardiac and vascular tissue. Therefore, several 7-(acyloxy)-6-arylpyrrolo[2,1-d][1,5]benzothiazepines, potent and selective peripheral-type benzodiazepine receptor ligands recently developed by us (3, 7-20), were subjected to calcium channel receptor binding assay. Some of these compounds showed an unexpected potency in displacing the binding of [3H]nitrendipine from L-type calcium channels, much higher than that reported for PK 11195 and Ro 5-4864 and equal to or higher than that of reference calcium antagonists such as verapamil and (+)-cis-diltiazem. Specifically, in rat cortex homogenate, our prototypic PBR ligand 7-acetoxy-6-(p-methoxyphenyl)pyrrolo[2,1-d][1,5]benzothiazepine (3) showed an IC50 equal to 0.13 nM for inhibition of [3H]nitrendipine binding. Furthermore, in functional studies this compound displayed a clear-cut selectivity for cardiac over vascular tissue. Comparison of calcium antagonist activity on guinea pig aorta strips with the negative inotropic activity, determined by using isolated guinea pig left atria, revealed that 3 displayed higher selectivity than the reference (+)-cis-diltiazem. Thus, the pyrrolobenzothiazepine 3 might represent a new tool for characterizing the relationship between the PBR and cardiac function. Furthermore, we have also investigated the structural dependence of binding to PBR and L-type calcium channels, and this study allowed us to identify a new class of potent calcium channel blockers selective for cardiac over vascular tissue, with no affinity for PBR. A number of structure-activity relationship trends have been identified, and a possible explanation is advanced in order to account for the observed differences in selectivity. Three structural features, namely, (i) the saturation of the C(6)-C(7) double bond, with a consequent higher molecular flexibility, (ii) the presence of a substituent in the benzofused ring, and (iii) a basic side chain at C-10 of the pyrrolobenzothiazepine ring system, were found to be responsible for potent L-type calcium channel antagonism and clear-cut selectivity for cardiac over vascular tissue. Among the synthesized compounds the pyrrolobenzothiazepine 62 was found to be the most promising selective calcium channel blocker. Additionally, the molecular structure determination of the key intermediate 48 by X-ray diffraction, molecular modeling, and NMR analysis is reported.

The dihydropyridine-sensitive calcium channel subtype in cone photoreceptors

High-voltage activated Ca channels in tiger salamander cone photoreceptors were studied with nystatin-permeabilized patch recordings in 3 mM Ca2+ and 10 mM Ba2+. The majority of Ca channel current was dihydropyridine sensitive, suggesting a preponderance of L-type Ca channels. However, voltage-dependent, incomplete block (maximum 60%) by nifedipine (0.1-100 microM) was evident in recordings of cones in tissue slice. In isolated cones, where the block was more potent, nifedipine (0.1-10 microM) or nisoldipine (0.5-5 microM) still failed to eliminate completely the Ca channel current. Nisoldipine was equally effective in blocking Ca channel current elicited in the presence of 10 mM Ba2+ (76% block) or 3 mM Ca2+ (88% block). 15% of the Ba2+ current was reversibly blocked by omega-conotoxin GVIA (1 microM). After enhancement with 1 microM Bay K 8644, omega-conotoxin GVIA blocked a greater proportion (22%) of Ba2+ current than in control. After achieving partial block of the Ba2+ current with nifedipine, concomitant application of omega-conotoxin GVIA produced no further block. The P-type Ca channel blocker, omega-agatoxin IVA (200 nM), had variable and insignificant effects. The current persisting in the presence of these blockers could be eliminated with Cd2+ (100 microM). These results indicate that photoreceptors express an L-type Ca channel having a distinguishing pharmacological profile similar to the alpha 1D Ca channel subtype. The presence of additional Ca channel subtypes, resistant to the widely used L-, N-, and P-type Ca channel blockers, cannot, however, be ruled out.

Characterization of a voltage-dependent L-type calcium channel from rabbit and turtle brain

The binding of [3H]nitrendipine to membrane preparation from turtle and rabbit brain was studied. A single population of [3H]nitrendipine binding sites was detected in both species. [3H]nitrendipine bound with high affinity to brain membrane from both rabbit and turtle, revealing a significant population of binding sites (K(D) values of 0.55 +/- 0.05 nM and 0.56 +/- 0.04 nM and Bmax values of 122 +/- 11 and 275 +/- 18 fmol/mg of protein, respectively). Displacement studies showed a similar order of potency of various unlabeled ligands against [3H]nitrendipine both in rabbit or in turtle: nitrendipine > nifedipine >or= nicardipine >> verapamil >or= diltiazem. Our results show that a two fold increment of [3H]nitrendipine binding sites exists in the turtle brain respect to the rabbit.

Voltage-dependent modulation of single N-Type Ca2+ channel kinetics by receptor agonists in IMR32 cells

The voltage-dependent inhibition of single N-type Ca(2+) channels by noradrenaline (NA) and the delta-opioid agonist D-Pen(2)-D-Pen (5)-enkephalin (DPDPE) was investigated in cell-attached patches of human neuroblastoma IMR32 cells with 100 mM Ba(2+) and 5 microM nifedipine to block L-type channels. In 70% of patches, addition of 20 microM NA + 1 microM DPDPE delayed markedly the first channel openings, causing a four- to fivefold increase of the first latency at +20 mV. The two agonists or NA alone decreased also by 35% the open probability (P(o)), prolonged partially the mean closed time, and increased the number of null sweeps. In contrast, NA + DPDPE had little action on the single-channel conductance (19 versus 19.2 pS) and minor effects on the mean open time. Similarly to macroscopic Ba(2+) currents, the ensemble currents were fast activating at control but slowly activating and depressed with the two agonists. Inhibition of single N-type channels was effectively removed (facilitated) by short and large depolarizations. Facilitatory pre-pulses increased P(o) significantly and decreased fourfold the first latency. Ensemble currents were small and slowly activating before pre-pulses and became threefold larger and fast decaying after facilitation. Our data suggest that slowdown of Ca(2+) channel activation by transmitters is mostly due to delayed transitions from a modified to a normal (facilitated) gating mode. This single-channel gating modulation could be well simulated by a Monte Carlo method using previously proposed kinetic models predicting marked prolongation of first channel openings.

Ionic basis of the membrane potential responses of rat dorsal vagal motoneurones to HEPES buffer

The effects of 10 mM HEPES (N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid) buffered artificial cerebrospinal fluid (aCSF) on membrane potential and the action potential were studied in 93 dorsal vagal motoneurones (DVMs) using an in vitro slice preparation of the rat medulla. Changing from bicarbonate/CO2 aCSF to HEPES aCSF resulted in a depolarisation of 6.0 +/- 0.6 mV and an increase in input resistance (RIn; n = 61). In the presence of 5 mM 4-AP, HEPES either had little effect (n = 9) or hyperpolarised the membrane (n = 10). Mn2+ (3 mM) or Ni2+ (200 microM) abolished the hyperpolarisation and its associated increase in RIn. In voltage-clamp studies 5 mM 4-AP eliminated a transient outward current and Ni2+ blocked an inactivating inward current. It is concluded that HEPES buffer reduces the contribution of the A current to resting membrane potential and also reduces a Ni(2+)-sensitive transient ICa.

Antisense oligodeoxynucleotide inhibition of a swelling-activated cation channel in osteoblast-like osteosarcoma cells

By patch-clamp analysis, we have shown that chronic, intermittent mechanical strain (CMS) increases the activity of stretch-activated cation channels of osteoblast-like UMR-106.01 cells. CMS also produces a swelling-activated whole-cell conductance (Gm) regulated by varying strain levels. We questioned whether the swelling-activated conductance was produced by stretch-activated cation channel activity. We have identified a gene involved in the increase in conductance by using antisense oligodeoxynucleotides (ODN) derived from the alpha 1-subunit genes of calcium channels found in UMR-106.01 cells (alpha1S, alpha1C, and alpha1D). We demonstrate that alpha 1C antisense ODNs abolish the increase in Gm in response to hypotonic swelling following CMS. Antisense ODNs to alpha1S and alpha1D, sense ODNs to alpha1C, and sham permeabilization had no effect on the conductance increase. In addition, during cell-attached patch-clamp studies, antisense ODNs to alpha1c completely blocked the swelling-activated and stretch-activated nonselective cation channel response to strain. Antisense ODNs to alpha1S treatment produced no effect on either swelling-activated or stretch-activated cation channel activity. There were differences in the stretch-activated and swelling-activated cation channel activity, but whether they represent different channels could not be determined from our data. Our data indicate that the alpha1C gene product is involved in the Gm and the activation of the swelling-activated cation channels induced by CMS. The possibility that swelling-activated cation channel genes are members of the calcium channel superfamily exists, but if alpha1c is not the swelling-activated cation channel itself, then its expression is required for induction of swelling-activated cation channel activity by CMS.

Developmental changes in beta-adrenergic modulation of L-type Ca2+ channels in embryonic mouse heart

In the adult mammalian myocardium, cellular Ca2+ entry is regulated by the sympathetic nervous system. L-type Ca2+ channel currents are markedly increased by beta-adrenergic (beta-A) agonists, which contribute to changes in pacing and contractile activity of the heart. In the developing mammalian heart, the regulation of Ca2+ entry by this enzyme cascade has not been clearly established, because changes in receptor density and coupling to downstream elements of the signaling cascade are known to occur during embryogenesis. In this study, we systematically investigated the regulation of L-type Ca2+ channel currents during development of the murine embryonic heart. We used conventional whole-cell and perforated-patch-clamp procedures to study modulation of L- type Ca2+ channel currents and to assay functional activity of distinct steps in the beta-A signaling cascade in murine embryonic myocytes at different stages of gestation. Our data indicate that the L-type Ca2+ channels in early-stage (day-11 to -13) myocytes are unresponsive to either isoproterenol or cAMP. L-type Ca2+ channels in late-stage (day-17 to -19) murine myocytes, however, exhibit responses to isoproterenol and cAMP similar to responses in adult cells, providing evidence that the beta-A cascade becomes functionally active during this period of embryonic development. We found that L-type Ca2+ channel activity in early-stage cells is increased by cell dialysis with the catalytic subunit of cAMP-dependent protein kinase (cA-PK) and that dialysis of early-stage cells with the holoenzyme of cA-PK restores functional responses to forskolin and cAMP, but not to isoproterenol. Our results provide strong evidence that a key factor in the early-stage insensitivity of L-type Ca2+ channels to cAMP is the absence, or low expression level, of the holoenzyme of cA-PK but that in addition, another element in the signaling cascade upstream from adenylate cyclase is expressed at a nonfunctional level or is uncoupled from the cascade and thus contributes to L-type Ca2+ channel insensitivity to beta-A agonists in early stages of the developing murine heart.

Model predictions of myoelectrical activity of the small bowel

A mathematical model for the periodic electrical activity of a functional unit of the small intestine is developed. Based on real morphological and electrophysiological data, the model assumes that: the functional unit is an electromyogenic syncytium; the kinetics of L, T-type Ca2+, mixed Ca(2+)-dependent K+, potential sensitive K+ and Cl- channels determines electrical activity of the functional unit; the basic neural circuit, represented by a single cholinergic neurone, provides an excitatory input to the functional unit via receptor-linked L-type Ca2+ channels. Numerical simulation of the model has shown that it is capable of displaying the slow waves and that slight modifications of some of the parameters result in different electrical responses. The effects of the variations of the main parameters have been analyzed for their ability to reproduce various electrical patterns. The results are in good qualitative and quantitative agreement with results of experiments conducted on the small intestine.

The alpha 2/delta subunit of voltage sensitive Ca2+ channels is a single transmembrane extracellular protein which is involved in regulated secretion

The membrane topology of alpha 2/delta subunit was investigated utilizing electrophysiological functional assay and specific anti-alpha 2 antibodies. (a) cRNA encoding a deleted alpha 2/delta subunit was coinjected with alpha 1C subunit of the L-type calcium channel into Xenopus oocytes. The truncated form, lacking the third putative TM domain (alpha 2/delta delta TMIII), failed to amplify the expressed inward currents, normally induced by alpha 1C coinjected with intact alpha 2/delta subunit. Western blot analysis of alpha 2/delta delta TMIII shows the appearance of a degraded alpha 2 protein and no expression of the full-size two-TM truncated-protein. The improper processing of alpha 2/delta delta TMIII suggests that the alpha 2/delta is a single TM domain protein and the TM region is positioned at the delta subunit. (b) External application of anti-alpha 2 antibodies, prepared for an epitope within the alternatively spliced and 'intracellular' region, inhibits depolarization induced secretion in PC12, further supporting an external location of the alpha 2 subunit and establishing delta subunit as the only membrane anchor for the extracellular alpha 2 subunit.

Structural model of a synthetic Ca2+ channel with bound Ca2+ ions and dihydropyridine ligand

Grove et al. have demonstrated L-type Ca2+ channel activity of a synthetic channel peptide (SCP) composed of four helices (sequence: DPWNVFDFLI10VIGSIIDVIL20SE) tethered by their C-termini to a nanopeptide template. We sought to obtain the optimal conformations of SCP and locate the binding sites for Ca2+ and for the dihydropyridine ligand nifedipine. Eight Ca2+ ions were added to neutralize the 16 acidic residues in the helices. Eight patterns of the salt bridges between Ca2+ ions and pairs of the acidic residues were calculated by the Monte Carlo-with-energy-minimization (MCM) protocol. In the energetically optimal conformation, two Ca2+ ions were bound to Asp-1 residues at the intracellular side of SCP, and six Ca2+ ions were arrayed in two files at the diametrically opposite sides of the pore, implying a Ca2+ relay mechanism. Nine modes of nifedipine binding to SCP were simulated by the MCM calculations. In the energetically optimal mode, the ligand fits snugly in the pore. The complex is stabilized by Ca2+ bound between two Asp-17 residues and hydrophilic groups of the ligand. The latter substitute water molecules adjacent to Ca2+ in the ligand-free pore and thus do not obstruct Ca2+ relay. The ligand-binding site is proximal to a hydrophobic bracelet of Ile-10 residues whose rotation is sterically hindered. In some conformations, the bracelet is narrow enough to block the permeation of the hydrated Ca2+ ions. The bracelet may thus act as a "gate" in SCP. Nifedipine and (R)-Bay K 8644, which act as blockers of the SCP, extend a side-chain hydrophobic moiety toward the Ile-10 residues. This would stabilize the pore-closing conformation of the gate. In contrast, the channel activator (S)-Bay K 8644 exposes a hydrophilic moiety toward the Ile-10 residues, thus destabilizing the pore-closing conformation of the gate.

Atrial natriuretic peptide (ANP)-induced inhibition of glucagon secretion: mechanism of action in isolated rat pancreatic islets

ANP increases insulin levels in vivo. Because in vitro an ANP-induced increase in cGMP levels of islets of Langerhans was observed but no simultaneous increase in insulin release, secreted glucagon may be a candidate for this second messenger affected by ANP. The inhibitory effect of glucose on glucagon secretion was pronounced by 1.0 nM ANP at 3.0 mM glucose as well as at 5.6 and 8.3 mM glucose. Because in other tissues cGMP (the specific second messenger of ANP1 inhibits Ca2+ channels, the uptake of 45Ca2+ was investigated. ANP (1.0 nM) inhibited 45Ca2+ uptake, which was nearly completely abolished by a pertussis toxin (PT) pretreatment. The inhibition of 45Ca2+ uptake fits to inhibitory ANP effects on glucagon secretion but does not fit to insulin secretion. The glucagon secretion coupling cascade affected by ANP probably involves an increase in cGMP because 8-Br-cGMP (a membrane-permeable cGMP analogue) also decreased glucagon secretion. ANP(4-23), a truncated form of ANP, which is selective for the ANP clearance receptor, also inhibited glucagon secretion. HS-42-1, a guanylate cyclase receptor antagonist, tended to reverse the effect of ANP on glucagon release. The data indicate that in the presence of ANP, the in vivo homeostasis of glucose, though plasma insulin levels are increased, is not due to an ANP-mediated increase in glucagon secretion; ANP has a complex inhibitory effect on glucagon release. The data further indicate that the ANP-induced inhibition of glucagon secretion probably involves the cGMP system, an inhibition of Ca2+ uptake and the involvement of PT-sensitive G-proteins. Moreover, an involvement of the clearance receptor seems to be likely. ANP is a valuable tool for investigating glucagon secretion from pancreatic islets because paracrine effects of insulin can be excluded.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.

Structural and functional diversity of voltage-activated calcium channels

Data gathered from the expression of cDNAs that encode the subunits of voltage-dependent Ca2+ channels have demonstrated important structural and functional similarities among these channels. Despite these convergences, there are also significant differences in the nature and functional importance of subunit-subunit and protein-Ca2+ channel interactions. There is evidence demonstrating that the functional differences between Ca2+ channel subtypes is due to several factors, including the expression of distinct alpha 1 subunit proteins, the selective association of structural subunits and modulatory proteins, and differences in posttranslational processing and cell regulation. We summarize several avenues of research that should provide significant clues about the structural features involved in the biophysical and functional diversity of voltage-dependent Ca2+ channels.

The expression of neuronal voltage-dependent calcium channels in human cerebellum

Little is known about the comparative distribution of voltage-dependent calcium channel subtypes in normal human brain. Previous studies in experimental animals have predominantly focused on the regional expression of single alpha 1 genes. We describe the preparation of riboprobes and antisera specific for human alpha 1A, alpha 1B and alpha 1E subunits and their application in comprehensive mapping studies of the human cerebellum. Within the cerebellar cortex, these pore forming proteins were found to have differential localisations when examined in adjacent sections. The alpha 1A and alpha 1B subunits broadly colocalised and were both present, though at apparently different levels, in the molecular, Purkinje and granule cell layers whilst alpha 1E was predominantly expressed in Purkinje cells. In the dentate nucleus, an area which has received little attention in previous studies, alpha 1A was highly expressed in regions in which Purkinje cell nerve terminals form synapses with deep cerebellar neurones.

Activation of nicotinic acetylcholine receptors expressed in quail fibroblasts: effects on intracellular calcium

1. The aim of these experiments was to determine the ability of the muscle-type nicotinic acetylcholine receptor (AChR) stably expressed in quail fibroblasts (QF18 cells) to elevate intracellular calcium ([Ca2+]i) upon activation. Ratiometric confocal microscopy, with the calcium-sensitive fluorescent dye Indo-1 was used. 2. Application of the nicotine agonist, suberyldicholine (SDC), to the transfected QF18 cells caused an increase in [Ca2+]i. Control [Ca2+]i levels in QF18 cells were found to be 164 +/- 22 nM (mean +/- s.e. mean; n = 40 cells) rising to 600 +/- 81 nM on addition of SDC (10 microM; n = 15 cells), whereas no increase in [Ca2+]i was seen in non-transfected control QT6 fibroblasts (before: 128 +/- 9 nM, n = 40; after; 113 +/- 13 nM, n = 15). 3. The increase in [Ca2+]i caused by application of SDC was dose-dependent, with an EC50 value of 12.7 +/- 5.9 microM (n = 14). 4. The responses to SDC in QF18 cells were blocked by prior application of alpha-bungarotoxin (200 nM), by the addition of Ca2+ (100 microM), by removal of Na+ ions from the extracellular solution, or by the voltage-sensitive calcium channel blockers nifedipine and omega-conotoxin, which act with IC50 values of 100 nM and 100 pM respectively. 5. We conclude that activation of the nicotinic AChRs leads to a Na(+)-dependent depolarization and hence activation of endogenous voltage-sensitive Ca2+ channels in the plasma membrane and an increase in [Ca2+]i. There is no significant entry of Ca2+ through the nicotinic receptor itself.

Distinct omega-agatoxin-sensitive calcium currents in somata and axon terminals of rat supraoptic neurones

1. Voltage-dependent calcium currents were measured at room temperature using whole-cell patch clamp in acutely isolated somata and axon terminals of the magnocellular neurosecretory cells (MNCs) from the rat supraoptic nucleus. 2. Administration of omega-agatoxin IVA (omega-Aga IVA) blocked a high-threshold non-inactivating current. This current has an IC50 for omega-Aga IVA of 3 nM; no other types of currents were blocked at doses of up to 500 nM. 3. In the axon terminals omega-Aga IVA blocked a high-threshold current that inactivates markedly (tau approximately 448 ms), and has a much lower sensitivity to the toxin, with an IC50 of 270 nM. Unlike the somatic current, the effect of omega-Aga IVA in the terminals is largely prevented by omega-conotoxin GVIA (omega-CgTX). 4. These data suggest that MNC somata express a single type of omega-Aga IVA-sensitive calcium current similar to the P-type calcium current described in other cells. However, the omega-Aga IVA-sensitive current in axon terminals differs from both the P-type and the recently identified Q-type current in that it is also sensitive to omega-CgTX. The distinct biophysical properties of the currents in somata and axon terminals may have important physiological implications.

Models of ion pores in N-type voltage-gated calcium channels

Two computer models of the outer vestibule of the pore of the N-type voltage-gated Ca2+ channel are predicted. The models are constructed from beta-hairpin peptide segments in the S5-S6 loops of each of the four domains that produce the channel. These hairpins together are modeled to form a short eight-stranded beta barrel. The models contain a ring of glutamates at the base of the barrel, which have been shown by mutagenesis experiments to function as a selectivity filter. These filters are suggested by the models to be of the correct dimensions to allow the permeation of a hydrated calcium ion, where the filter glutamates may substitute for molecules of water from the hydration shell of the ion. The models also suggest that a ring of threonines and an aspartate might be present between the mouth of the pore and the filter, and hence the models may prove useful in suggesting future mutagenesis experiments.

Pharmacological characterization of presynaptic calcium channels using subsecond biochemical measurements of synaptosomal neurosecretion

The recent development of peptide antagonists that selectively block subtypes of neuronal calcium channel has provided tools to study the role of presynaptic calcium channels in triggering exocytosis. A variety of methods have consistently demonstrated that multiple channel types participate in exocytosis. We have studied the subsecond kinetics of [3H]glutamate release from rat cortical synaptosomes as an assay for presynaptic calcium channel activity. The system has been characterized over a broad range of conditions in an effort to compare biochemical measurements of transmitter release with electrophysiological measurements of synaptic currents. The efficacies of omega-agatoxin IVA and omega-conotoxins GVIA and MVIIC were increased when Ca2+ influx was decreased by: (1) decreasing the KCl concentration to diminish the extent of depolarization, (2) decreasing the Ca2+ concentration, or (3) partially blocking Ca2+ influx with one of the other antagonists. By using these toxins in combination, we found that at least three types of pharmacologically distinct channel participate in exocytosis. The largest fraction of glutamate release is blocked by omega-agatoxin IVA (IC50 = 12.2 nM) and by omega-conotoxin MVIIC (IC50 = 35 nM), consistent with the pharmacology of a P type channel. The effects of saturating concentrations (1 microM) of omega-agatoxin IVA or omega-conotoxin MVIIC occlude each other, suggesting that these peptides overlap completely. The specific N type antagonist omega-conotoxin GVIA inhibits a significant portion of release (IC50 less than 1 nM) but only under conditions of reduced Ca2+ concentration. These results suggest that the N type channel in nerve terminals is distinct from that found in hippocampal somata, since it appears to be resistant to by omega-conotoxin MVIIC. The combination of omega-conotoxin GVIA (100 nM) and either omega-agatoxin IVA or omega-conotoxin MVIIC (1 microM each) blocked approx 90% of release when the Ca2+ concentration was reduced (0.46 mM or less), but 30-40% of release remained when the concentration of Ca2+ in the stimulus buffer was 1 mM or greater, indicating that a resistant channel type(s) also participates in exocytosis. Specific inhibitors of this resistant phenotype will be useful for further refinement of our understanding of the role of presynaptic calcium channels in mediating neurosecretion.