Caveolin-1 (CAV-1) is a structural components of caveolae, which are 50-100 nm invaginations of the plasma membrane that play various physiological roles. CAV1 associates with cavin-1 and is confined to the plasma membrane []. CAV-1 has several functions, for instance, it regulates TCR signal strength and regulatory T cell differentiation into alloreactive T cells []. It can also affect the permeability of blood-tumour barrier (BTB) by regulating the expression of tight junction-associated proteins []. Caveolae are 50-100 nm invaginations located at the plasma membrane of many cell types and are known to transport molecules across endothelial cells []. Caveolae require the caveolin proteinfor formation. Caveolins may act as scaffolding proteins within caveolar membranes by compartmentalizing and concentrating signalling molecules. Mammals have three caveolin proteins:caveolin-1 (Cav-1, or VIP21), caveolin-2 and caveolin-3 (or M-caveolin). Various classes of signalling molecules, including G-protein subunits, receptor and non-receptor tyrosine kinases, endothelial nitric oxide synthase (eNOS), and small GTPases, bind Cav-1 through its 'caveolin-scaffolding domain' [].Caveolins are proteins of about 20 Kd, they form high molecular mass homo-oligomers. Structurally they seem to have N-terminal and C-terminal hydrophilic segments and a long central transmembrane domain that probably forms a hairpin in the membrane. Both extremities are known to face the cytoplasm. Caveolae are enriched with cholesterol and Cav-1 is one of the few proteins that binds cholesterol tightly and specifically.
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.This entry represents all types of alpha-1 subunit.
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.L-type calcium channels are formed from alpha-1S, alpha-1C, alpha-1D, and alpha-1F subunits. They are widely distributed and are well characterised in the heart, smooth and skeletal muscle, and some neurons. Their primary functions are in both excitation-contraction and excitation-secretion coupling. In skeletal muscle, the L-type calcium channels act as a voltage sensor for excitation-contraction coupling, and in cardiac muscle, they provide a pathway for calcium influx. Mutations affecting L-type channel subunits result in three diseases: (1) muscular dystrophy, which is characterised by a lack of functional skeletal muscle; (2) hypokalaemic periodic paralysis, which is characterised by episodic attacks of skeletal muscle weakness; and (3) malignant hyperthermia, which is the main cause of death due to anaesthesia. 1,4-dihydropyridines act as antagonists of these channels [, ].
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.N-type calcium channels are composed from alpha-1B subunits. Experiments employing CTX have demonstrated the physiological importance of the N-type calcium channels in the nervous system, where they have a significant developmental role in the migration of immature neurons before the establishment of their synaptic circuit. In peripheral neurons, such as autonomic neurons, motor neurons and spinal cord neurons, they have also been shown to be critically involved in the release of neurotransmitters, including glutamate [], gamma-amino-butyric acid, acetylcholine, dopamine []and norepinephrine []. N-type calcium channels are promising targets for the development of drugs to relieve chronic and neuropathic pain []. Recently, it has been shown to be specifically blocked by ziconotide [].
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.Several genes encoding alpha-1 subunits have been identified, each forming a distinct electrophysiological channel. P- and Q-type channels are formed from alpha-1A subunits and function in transmitter release []. P-type channels are prevalent in cerebellar Purkinje cells, but are also expressed in many central and peripheral neurons, such as the spinal cord and visual cortex. By contrast, Q-type channels are found in cerebellar granule neurones and the hippocampus. Different mutations in the alpha-1A subunit can produce the following human diseases: episodic ataxia type-2familial hemiplegic migrainespinocerebellar ataxia type-6All 3 diseases result in cerebellar atrophy, but they differ in the extent and rate of progression of neuronal degeneration.
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.R-type calcium channels are composed of alpha-1E subunits and are found in a variety of neuronal populations, such as cerebellar granule neurons and dendrites of hippocampal pyrimidal neurons []. They are believed to play an important role in the body's natural communication network, where they contribute to the regulation of brain function by synaptic integration []. Their hypolarised inactivation range and rapid kinetics of inactivation make R-type channels more suited to providing a transient surge of Ca2+ influx [].
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.T-type calcium channels exhibit unique voltage-dependent kinetics, small single channel conductance, rapid inactivation, slow deactivation and a relatively high permeability to calcium []. They are primarily responsible for rebound burst firing in central neurons and are implicated in normal brain functions, such as slow wave sleep, and in diseased states, such as epilepsy []. They also play an important role in hormone secretion []and smooth muscle excitability [].
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.L-type calcium channels are formed from alpha-1S, alpha-1C, alpha-1D, and alpha-1F subunits. They are widely distributed and are well characterised in the heart, smooth and skeletal muscle, and some neurons. Their primary functions are in both excitation-contraction and excitation-secretion coupling. In skeletal muscle, the L-type calcium channels act as a voltage sensor for excitation-contraction coupling, and in cardiac muscle, they provide a pathway for calcium influx. Mutations affecting L-type channel subunits result in three diseases: (1) muscular dystrophy, which is characterised by a lack of functional skeletal muscle; (2) hypokalaemic periodic paralysis, which is characterised by episodic attacks of skeletal muscle weakness; and (3) malignant hyperthermia, which is the main cause of death due to anaesthesia. 1,4-dihydropyridines act as antagonists of these channels [, ].Alpha-1C subunits can be found in a number of excitable tissues, as well as the heart and lungs []. The variability in the C-terminal region of the alpha-1C subunit generated by alternative splicing influences the kinetics, calcium- and voltage-dependence of L-type channels []. The N terminus is also a site of significant structural diversity [].
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.L-type calcium channels are formed from alpha-1S, alpha-1C, alpha-1D, and alpha-1F subunits. They are widely distributed and are well characterised in the heart, smooth and skeletal muscle, and some neurons. Their primary functions are in both excitation-contraction and excitation-secretion coupling. In skeletal muscle, the L-type calcium channels act as a voltage sensor for excitation-contraction coupling, and in cardiac muscle, they provide a pathway for calcium influx. Mutations affecting L-type channel subunits result in three diseases: (1) muscular dystrophy, which is characterised by a lack of functional skeletal muscle; (2) hypokalaemic periodic paralysis, which is characterised by episodic attacks of skeletal muscle weakness; and (3) malignant hyperthermia, which is the main cause of death due to anaesthesia. 1,4-dihydropyridines act as antagonists of these channels [, ].Alpha-1D subunits allow cells to slowly inactivate voltage-gated Ca2+ influx to weak depolarisations []. This property allows them to participate in important physiological functions, such as tonic neurotransmitter release in cochlear inner hair cells []. In addition, these properties make them ideally suited to contribute to subthreshold Ca2+ signalling, for example in hippocampal pyramidal cells []. Mutations in this channel have been associated with autism spectrum disorders and epilepsy [].
Ca2+ ions are unique in that they not only carry charge but they are also the most widely used of diffusible second messengers. Voltage-dependent Ca2+ channels (VDCC) are a family of molecules that allow cells to couple electrical activity to intracellular Ca2+ signalling. The opening and closing of these channels by depolarizing stimuli, such as action potentials, allows Ca2+ ions to enter neurons down a steep electrochemical gradient, producing transient intracellular Ca2+ signals. Many of the processes that occur in neurons, including transmitter release, gene transcription and metabolism are controlled by Ca2+ influx occurring simultaneously at different cellular locales. The pore is formed by the alpha-1 subunit which incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins []. The activity of this pore is modulated by four tightly-coupled subunits: an intracellular beta subunit; a transmembrane gamma subunit; and a disulphide-linked complex of alpha-2 and delta subunits, which are proteolytically cleaved from the same gene product. Properties of the protein including gating voltage-dependence, G protein modulation and kinase susceptibility can be influenced by these subunits.Voltage-gated calcium channels are classified as T, L, N, P, Q and R, and are distinguished by their sensitivity to pharmacological blocks, single-channel conductance kinetics, and voltage-dependence. On the basis of their voltage activation properties, the voltage-gated calcium classes can be further divided into two broad groups: the low (T-type) and high (L, N, P, Q and R-type) threshold-activated channels.The alpha-1 subunit forms the pore for the import of extracellular calcium ions and, though regulated by the other subunits, is primarily responsible for the pharmacological properties of the channel []. It shares sequence characteristics with all voltage-dependent cation channels, and exploits the same 6-helix bundle structural motif - in both sodium and calcium channels, this motif is repeated 4 times within the sequence to give a 24-helix bundle. Within each of these repeats, 5 of the transmembrane (TM) segments (S1, S2, S3, S5, S6) are hydrophobic, while the other (S4) is positively charged and serves as the voltage-sensor. Several genes encoding alpha-1 subunits have been identified and can be divided into three functionally distinct families based on sequence homology - Cav1, Cav2 and Cav3 []. The Cav1 family forms channels mediating L-type calcium currents, the Cav2 family mediates P/Q-, N-, and R-type calcium currents, while the Cav3 family mediates T-type calcium currents.L-type calcium channels are formed from alpha-1S, alpha-1C, alpha-1D, and alpha-1F subunits. They are widely distributed and are well characterised in the heart, smooth and skeletal muscle, and some neurons. Their primary functions are in both excitation-contraction and excitation-secretion coupling. In skeletal muscle, the L-type calcium channels act as a voltage sensor for excitation-contraction coupling, and in cardiac muscle, they provide a pathway for calcium influx. Mutations affecting L-type channel subunits result in three diseases: (1) muscular dystrophy, which is characterised by a lack of functional skeletal muscle; (2) hypokalaemic periodic paralysis, which is characterised by episodic attacks of skeletal muscle weakness; and (3) malignant hyperthermia, which is the main cause of death due to anaesthesia. 1,4-dihydropyridines act as antagonists of these channels [, ].The alpha-1S subunit is present in skeletal muscle and has also been detected in kidney and brain []. In the skeletal muscle, it is present in two size variants, a full-length, minor (~5%) form of ~212kDa, and a major (~95%) species of ~190kDa, derived from the longer protein by post-translational cleavage.
