Parallel activation of Ca(2+)-induced survival and death pathways in cardiomyocytes by sorbitol-induced hyperosmotic stress.

Hyperosmotic stress promotes rapid and pronounced apoptosis in cultured cardiomyocytes. Here, we investigated if Ca(2+) signals contribute to this response. Exposure of cardiomyocytes to sorbitol [600 mosmol (kg water)(-1)] elicited large and oscillatory intracellular Ca(2+) concentration increases. These Ca(2+) signals were inhibited by nifedipine, Cd(2+), U73122, xestospongin C and ryanodine, suggesting contributions from both Ca(2+) influx through voltage dependent L-type Ca(2+) channels plus Ca(2+) release from intracellular stores mediated by IP(3) receptors and ryanodine receptors. Hyperosmotic stress also increased mitochondrial Ca(2+) levels, promoted mitochondrial depolarization, reduced intracellular ATP content, and activated the transcriptional factor cyclic AMP responsive element binding protein (CREB), determined by increased CREB phosphorylation and electrophoretic mobility shift assays. Incubation with 1 mM EGTA to decrease extracellular [Ca(2+)] prevented cardiomyocyte apoptosis induced by hyperosmotic stress, while overexpression of an adenoviral dominant negative form of CREB abolished the cardioprotection provided by 1 mM EGTA. These results suggest that hyperosmotic stress induced by sorbitol, by increasing Ca(2+) influx and raising intracellular Ca(2+) concentration, activates Ca(2+) release from stores and causes cell death through mitochondrial function collapse. In addition, the present results suggest that the Ca(2+) increase induced by hyperosmotic stress promotes cell survival by recruiting CREB-mediated signaling. Thus, the fate of cardiomyocytes under hyperosmotic stress will depend on the balance between Ca(2+)-induced survival and death pathways.

Quaternary indole alkaloids from the stem bark of Strychnos guianensis.

Two new quaternary alkaloids, 9-methoxy-Nb-methylgeissoschizol and guiachrysine together with the known compounds C-alkaloid O, fluorocurine, mavacurine, macusine B and C-profluorocurine, were isolated from Strychnos guianensis stembark. The structures of the compounds were elucidated on the basis of spectroscopic studies.

A review on conotoxins targeting ion channels and acetylcholine receptors of the vertebrate neuromuscular junction.

In this article we present an overview of some peptides extracted and purified from the venom of marine snails of the genus Conus. These active peptides named conotoxins can be used as research tools to target voltage-gated ion channels as well as ligand-gated receptors. Because of their relatively small size, conotoxins can be chemically synthesized and made widely available. In this review we focus on conotoxins that target voltage-sensitive sodium channels, voltage-dependent calcium channels and nicotinic acetylcholine receptors of the vertebrate neuromuscular junction. Emphasis is given on summarizing our current knowledge of their primary structure and their specific pharmacological actions at the pre- and the post-synaptic level of the neuromuscular junction. Evidence is presented for conotoxins that discriminate between pre- and post-synaptic voltage-gated sodium channels. Among these peptides, the mu-conotoxin family is well characterized by its ability to block selectively sodium channels in skeletal muscle fibres without affecting axonal and nerve terminal Na+ channels. Furthermore, new conotoxins like Conus consors toxin (CcTx) and conotoxin EVIA selectively target Na+ channels in axons and nerve terminals without affecting skeletal muscle fibres. omega-conotoxins known as highly potent and selective blockers of voltage-sensitive calcium channels have proven to be valuable in determining the roles of the various subtypes of channels involved in acetylcholine release from motor nerve endings. Finally, Conus peptides which act at muscle nicotinic acetylcholine receptors constitute the most extensive characterized family of conopeptides that exhibit sequence similarity, different structural motifs and surprising diversity in their competitive and non-competitive actions.

A review on conotoxins targeting ion channels and acetylcholine receptors of the vertebrate neuromuscular junction.

In this article we present an overview of some peptides extracted and purified from the venom of marine snails of the genus Conus. These active peptides named conotoxins can be used as research tools to target voltage-gated ion channels as well as ligand-gated receptors. Because of their relatively small size, conotoxins can be chemically synthesized and made widely available. In this review we focus on conotoxins that target voltage-sensitive sodium channels, voltage-dependent calcium channels and nicotinic acetylcholine receptors of the vertebrate neuromuscular junction. Emphasis is given on summarizing our current knowledge of their primary structure and their specific pharmacological actions at the pre- and the post-synaptic level of the neuromuscular junction. Evidence is presented for conotoxins that discriminate between pre- and post-synaptic voltage-gated sodium channels. Among these peptides, the mu-conotoxin family is well characterized by its ability to block selectively sodium channels in skeletal muscle fibres without affecting axonal and nerve terminal Na+ channels. Furthermore, new conotoxins like Conus consors toxin (CcTx) and conotoxin EVIA selectively target Na+ channels in axons and nerve terminals without affecting skeletal muscle fibres. omega-conotoxins known as highly potent and selective blockers of voltage-sensitive calcium channels have proven to be valuable in determining the roles of the various subtypes of channels involved in acetylcholine release from motor nerve endings. Finally, Conus peptides which act at muscle nicotinic acetylcholine receptors constitute the most extensive characterized family of conopeptides that exhibit sequence similarity, different structural motifs and surprising diversity in their competitive and non-competitive actions.

A review on conotoxins targeting ion channels and acetylcholine receptors of the vertebrate neuromuscular junction

In this article we present an overview of some peptides extracted and purified from the venom of marine snails of the genus Conus. These active peptides named conotoxins can be used as research tools to target voltage-gated ion channels as well as ligand-gated receptors. Because of their relatively small size, conotoxins can be chemically synthesized and made widely available. In this review we focus on conotoxins that target voltage-sensitive sodium channels, voltage-dependent calcium channels and nicotinic acetylcholine receptors of the vertebrate neuromuscular junction. Emphasis is given on summarizing our current knowledge of their primary structure and their specific pharmacological actions at the pre- and the post-synaptic level of the neuromuscular junction. Evidence is presented for conotoxins that discriminate between pre- and post-synaptic voltage-gated sodium channels. Among these peptides, the mu-conotoxin family is well characterized by its ability to block selectively sodium channels in skeletal muscle fibres without affecting axonal and nerve terminal Na+ channels. Furthermore, new conotoxins like Conus consors toxin (CcTx) and conotoxin EVIA selectively target Na+ channels in axons and nerve terminals without affecting skeletal muscle fibres. omega-conotoxins known as highly potent and selective blockers of voltage-sensitive calcium channels have proven to be valuable in determining the roles of the various subtypes of channels involved in acetylcholine release from motor nerve endings. Finally, Conus peptides which act at muscle nicotinic acetylcholine receptors constitute the most extensive characterized family of conopeptides that exhibit sequence similarity, different structural motifs and surprising diversity in their competitive and non-competitive actions.

A review on drugs and toxins affecting presynaptic K+ currents and phasic quantal transmitter release at motor nerve terminals.

This review assembles available information concerning drugs and toxins which block the different types of presynaptic K+ currents and discusses the relative importance of these currents in controlling phasic quantal transmitter release. Drugs and toxins which block the fast voltage-dependent potassium current (IKf), enhance phasic acetylcholine release evoked by nerve impulses. This effect is due to increased Ca2+ influx during prolonged presynaptic membrane depolarization. Selective blockade of the Ca(2+)-dependent K+ current (IK(Ca)) does not induce any change in phasic transmitter release indicating that, under physiological conditions, IK(Ca) has no significant role in presynaptic membrane repolarization. The contribution of the slow voltage-dependent K+ current (IKs) to the regulation of phasic acetylcholine release remains to be clarified. In conclusion, IKf, IK(Ca) and IKs can modulate the entry of Ca2+ into motor nerve terminals. However, under physiological conditions only IKf plays a key role in controlling the transient Ca2+ influx which is responsible for the phasic transmitter release.

A review on drugs and toxins affecting presynaptic K+ currents and phasic quantal transmitter release at motor nerve terminals.

This review assembles available information concerning drugs and toxins which block the different types of presynaptic K+ currents and discusses the relative importance of these currents in controlling phasic quantal transmitter release. Drugs and toxins which block the fast voltage-dependent potassium current (IKf), enhance phasic acetylcholine release evoked by nerve impulses. This effect is due to increased Ca2+ influx during prolonged presynaptic membrane depolarization. Selective blockade of the Ca(2+)-dependent K+ current (IK(Ca)) does not induce any change in phasic transmitter release indicating that, under physiological conditions, IK(Ca) has no significant role in presynaptic membrane repolarization. The contribution of the slow voltage-dependent K+ current (IKs) to the regulation of phasic acetylcholine release remains to be clarified. In conclusion, IKf, IK(Ca) and IKs can modulate the entry of Ca2+ into motor nerve terminals. However, under physiological conditions only IKf plays a key role in controlling the transient Ca2+ influx which is responsible for the phasic transmitter release.

Increase in transmitter release from motor nerve terminals induced by some pyridine derivatives.

The effects of 4-nitropyridine (4-NP), 4-aminopyridine-N-oxide (4-AP-N-O), 4-hydroxypyridine (4-HP), 2,6-diaminopyridine (2,6-DAP), 2,4-dihydroxypyridine (2,4-DHP) and pyridine on isolated sciatic nerve-sartorius muscle preparations were studied by means of intracellular and extracellular recording techniques. In junctions treated with (+) tubocurarine, 4-NP, 4-AP-N-O, 4-HP and 2,6-DAP reversibly increased the amplitude and the latency of end-plate potentials (EPPs) and induced repetitive EPPs in response to single nerve impulses. As shown by extracellular focal recordings, the increase in latency of EPPs was due to a prolongation of the minimum synaptic delay, while the appearance of repetitive EPPs was the result of repetitive firing of motor nerve terminals. 4-NP, 4-AP-N-O, 4-HP and 2,6-DAP increased dose-dependently the quantal content of EPPs, while 2,4-DHP and pyridine were found to be inactive. Comparison of the apparent rank order of potency in increasing quantal transmitter release indicates that the relative activity of the different pyridine derivatives studied is unrelated to their pK values. Spontaneous quantal transmitter release in resting junctions was unaffected by 4-NP, 4-AP-N-O, 4-HP and 2,6-DAP when applied at concentrations that enhanced evoked transmitter release. 4-NP differed from the other pyridine derivatives by producing in high concentrations a time-dependent increase in miniature end-plate potential frequency and a depolarization of the muscle fibres. In addition, 4-AP-N-O, 4-HP and 2,6-DAP were found to have no effect on MEPP frequency accelerated by increasing the external K+ concentration. In conclusion the data presented strongly suggest that 4-NP, 4-HP, 4-AP-N-O and 2,6-DAP facilitate evoked transmitter release from motor nerve terminals by a presynaptic action that seems related to an increased calcium influx secondary to the blockade of potassium channels in the nerve terminal.

Increase in transmitter release from motor nerve terminals induced by some pyridine derivatives.

The effects of 4-nitropyridine (4-NP), 4-aminopyridine-N-oxide (4-AP-N-O), 4-hydroxypyridine (4-HP), 2,6-diaminopyridine (2,6-DAP), 2,4-dihydroxypyridine (2,4-DHP) and pyridine on isolated sciatic nerve-sartorius muscle preparations were studied by means of intracellular and extracellular recording techniques. In junctions treated with (+) tubocurarine, 4-NP, 4-AP-N-O, 4-HP and 2,6-DAP reversibly increased the amplitude and the latency of end-plate potentials (EPPs) and induced repetitive EPPs in response to single nerve impulses. As shown by extracellular focal recordings, the increase in latency of EPPs was due to a prolongation of the minimum synaptic delay, while the appearance of repetitive EPPs was the result of repetitive firing of motor nerve terminals. 4-NP, 4-AP-N-O, 4-HP and 2,6-DAP increased dose-dependently the quantal content of EPPs, while 2,4-DHP and pyridine were found to be inactive. Comparison of the apparent rank order of potency in increasing quantal transmitter release indicates that the relative activity of the different pyridine derivatives studied is unrelated to their pK values. Spontaneous quantal transmitter release in resting junctions was unaffected by 4-NP, 4-AP-N-O, 4-HP and 2,6-DAP when applied at concentrations that enhanced evoked transmitter release. 4-NP differed from the other pyridine derivatives by producing in high concentrations a time-dependent increase in miniature end-plate potential frequency and a depolarization of the muscle fibres. In addition, 4-AP-N-O, 4-HP and 2,6-DAP were found to have no effect on MEPP frequency accelerated by increasing the external K+ concentration. In conclusion the data presented strongly suggest that 4-NP, 4-HP, 4-AP-N-O and 2,6-DAP facilitate evoked transmitter release from motor nerve terminals by a presynaptic action that seems related to an increased calcium influx secondary to the blockade of potassium channels in the nerve terminal.