Sodium-calcium exchange in heart: membrane currents and changes in [Ca2+]i

Recordings have been made of changes in intracellular calcium ion concentration ([Ca2+]i) that can be attributed to the operation of an electrogenic, voltage-dependent sodium-calcium (Na-Ca) exchanger in mammalian heart cells. Guinea pig ventricular myocytes under voltage clamp were perfused internally with fura-2, a fluorescent Ca2+-indicator, and changes in [Ca2+]i and membrane current that resulted from Na-Ca exchange were identified through the use of various organic channel blockers and impermeant ions. Depolarization of cells elicited slow increases in [Ca2+]i, with the maximum increase depending on internal [Na+], external [Ca2+], and membrane voltage. Repolarization was associated with net Ca2+ efflux and a decline in the inward current that developed instantaneously upon repolarization. The relation between [Ca2+]i and current was linear, and the slope was made steeper by hyperpolarization.     

Voltage-dependent calcium channels in glial cells

The electrophysiological properties of glial cells were examined in primary culture in the presence of tetraethylammonium and Ba2+, a treatment that reduces K+ permeability of the membrane and enhances currents through voltage-dependent Ca2+ channels. Under these conditions, glial cells showed both spontaneous action potentials and action potentials evoked by the injections of current. These responses appear to represent entry of Ba2+ through Ca2+ channels because they were resistant to tetrodotoxin but were blocked by Mn2+ or Cd2+.     

Cytoprotective role of Ca2+- activated K+ channels in the cardiac inner mitochondrial membrane

Ion channels on the mitochondrial inner membrane influence cell function in specific ways that can be detrimental or beneficial to cell survival. At least one type of potassium (K+) channel, the mitochondrial adenosine triphosphate-sensitive K+ channel (mitoKATP), is an important effector of protection against necrotic and apoptotic cell injury after ischemia. Here another channel with properties similar to the surface membrane calcium-activated K+ channel was found on the mitochondrial inner membrane (mitoKCa) of guinea pig ventricular cells. MitoKCa significantly contributed to mitochondrial K+ uptake of the myocyte, and an opener of mitoKCa protected hearts against infarction.     

Atrionatriuretic peptide transforms cardiac sodium channels into calcium-conducting channels

The atrionatriuretic peptide (ANP) is released from atrial cells in response to increased extracellular fluid volume and reduces sodium absorption by the kidney, thus reducing the blood volume. In this report, ANP suppressed the calcium and sodium currents in rat and guinea pig ventricular myocytes. The suppression of sodium current was caused by enhanced permeability of the sodium channel to calcium without significant changes in the kinetics or the tetrodotoxin sensitivity of the channel. Thus, ANP may regulate the sodium channel by altering its cationic selectivity site to calcium, thereby repressing the sodium current. The suppression of sodium and calcium channels and the resultant depressed excitability of the atrial cells may help to regulate ANP secretion.     

TRPM4 regulates calcium oscillations after T cell activation

TRPM4 has recently been described as a calcium-activated nonselective (CAN) cation channel that mediates membrane depolarization. However, the functional importance of TRPM4 in the context of calcium (Ca2+) signaling and its effect on cellular responses are not known. Here, the molecular inhibition of endogenous TRPM4 in T cells was shown to suppress TRPM4 currents, with a profound influence on receptor-mediated Ca2+ mobilization. Agonist-mediated oscillations in intracellular Ca2+ concentration ([Ca2+]i), which are driven by store-operated Ca2+ influx, were transformed into a sustained elevation in [Ca2+]i. This increase in Ca2+ influx enhanced interleukin-2 production. Thus, TRPM4-mediated depolarization modulates Ca2+ oscillations, with downstream effects on cytokine production in T lymphocytes.     

IgG from patients with Lambert-Eaton syndrome blocks voltage-dependent calcium channels

Lambert-Eaton syndrome, an autoimmune disorder frequently associated with small-cell carcinoma of the lung, is characterized by impaired evoked release of acetylcholine from the motor nerve terminal. Immunoglobulin G (IgG) antibodies from patients with the syndrome, applied to bovine adrenal chromaffin cells, reduced the voltage-dependent calcium channel currents by about 40 percent. When calcium was administered directly into the cytoplasm, however, the IgG-treated cells exhibited normal exocytotic secretion, as assayed by membrane capacitance measurement. Measurement with the fluorescent calcium indicator fura-2 indicated that the IgG treatment reduced potassium-stimulated increase in free intracellular calcium concentration. The pathogenic IgG modified neither kinetics of calcium channel activation nor elementary channel activity, suggesting that a reduction in the number of functional calcium channels underlies the IgG-induced effect. Therefore, Lambert-Eaton syndrome IgG reacts with voltage-dependent calcium channels and blocks their function, a phenomenon that can account for the presynaptic impairment characteristic of this disorder.     

Voltage-dependent sodium channels in an invertebrate striated muscle

Striated skeletal muscles from the planktonic arrowworm Sagitta elegans (phylum Chaetognatha) were voltage-clamped. The muscles displayed classical voltage-dependent sodium channels that (i) showed peak transient currents when the membrane was depolarized 90 millivolts from rest, (ii) opened rapidly with peak currents flowing within 0.4 milliseconds at 4 degrees C, (iii) showed voltage-dependent inactivation with 50 percent inactivation at +25 millivolts from rest, and (iv) were blocked by 500 nanomolar tetrodotoxin.     

Ca2+ and Ca2+-activated K+ currents in mammalian gastric smooth muscle cells

Inward movement of calcium through voltage-dependent channels in muscle is thought to initiate the action potential and trigger contraction. Calcium-activated potassium channels carry large outward potassium currents that may be responsible for membrane repolarization. Calcium and calcium-activated potassium currents were identified in enzymatically isolated mammalian gastric myocytes. These currents were blocked by cadmium and nifedipine but were not substantially affected by diltiazem or D600. No evidence for a tetrodotoxin-sensitive sodium current or an inwardly rectifying potassium current was found.     

Localization of calcium pump activity in smooth muscle

A microsomal fraction isolated from longitudinal smooth muscle of guinea pig ileum actively sequesters calcium ion in the presence of magnesium and adenosine triphosphate in a fashion previously described for microsomes of the rabbit aorta. This activity in guinea pig ileum appears to be associated primarily with the plasma membrane as is found in the red cell. By contrast the uptake of calcium in aortic smooth muscle appears to be associated to an appreciable extent with intracellular membranes, possibly analogous to the sarcoplasmic reticulum of skeletal muscle.     

Sequence of a probable potassium channel component encoded at Shaker locus of Drosophila

Potassium currents are crucial for the repolarization of electrically excitable membranes, a role that makes potassium channels a target for physiological modifications that alter synaptic efficacy. The Shaker locus of Drosophila is thought to encode a K+ channel. The sequence of two complementary DNA clones from the Shaker locus is reported here. The sequence predicts an integral membrane protein of 70,200 daltons containing seven potential membrane-spanning sequences. In addition, the predicted protein is homologous to the vertebrate sodium channel in a region previously proposed to be involved in the voltage-dependent activation of the Na+ channel. These results support the hypothesis that Shaker encodes a structural component of a voltage-dependent K+ channel and suggest a conserved mechanism for voltage activation.     

BKCa-Cav channel complexes mediate rapid and localized Ca2+-activated K+ signaling

Large-conductance calcium- and voltage-activated potassium channels (BKCa) are dually activated by membrane depolarization and elevation of cytosolic calcium ions (Ca2+). Under normal cellular conditions, BKCa channel activation requires Ca2+ concentrations that typically occur in close proximity to Ca2+ sources. We show that BKCa channels affinity-purified from rat brain are assembled into macromolecular complexes with the voltage-gated calcium channels Cav1.2 (L-type), Cav2.1 (P/Q-type), and Cav2.2 (N-type). Heterologously expressed BKCa-Cav complexes reconstitute a functional "Ca2+ nanodomain" where Ca2+ influx through the Cav channel activates BKCa in the physiological voltage range with submillisecond kinetics. Complex formation with distinct Cav channels enables BKCa-mediated membrane hyperpolarization that controls neuronal firing pattern and release of hormones and transmitters in the central nervous system.     

Relaxation of isolated ventricular cardiomyocytes by a voltage-dependent process

Cell contraction and relaxation were measured in single voltage-clamped guinea pig cardiomyocytes to investigate the contribution of sarcolemmal Na+-Ca2+ exchange to mechanical relaxation. Cells clamped from -80 to 0 millivolts displayed initial phasic and subsequent tonic contractions; caffeine reduced or abolished the phasic and enlarged the tonic contraction. The rate of relaxation from tonic contractions was steeply voltage-dependent and was significantly slowed in the absence of a sarcolemmal Na+ gradient. Tonic contractions elicited in the absence of a Na+ gradient promptly relaxed when external Na+ was applied, reflecting activation of Na+-Ca2+ exchange. It appears that a voltage-dependent Na+-Ca2+ exchange can rapidly mechanically relax mammalian heart muscle.     

Spatially resolved calcium dynamics of mammalian Purkinje cells in cerebellar slice

Microfluorometric imaging was used to study the correlation of intracellular calcium concentration with voltage-dependent electrical activity in guinea pig cerebellar Purkinje cells. The spatiotemporal dynamics of intracellular calcium concentration are demonstrated during spontaneous and evoked activity. The results are in agreement with hypotheses of dendritic segregation of calcium conductances suggested by electrophysiological experiments. These in vitro slice fluorescence imaging methods are applicable to a wide range of problems in central nervous system biochemical and electrophysiological functions.     

External calcium ions are required for potassium channel gating in squid neurons

The effects of calcium removal on the voltage-dependent potassium channels of isolated squid neurons were studied with whole cell patch-clamp techniques. When the calcium ion concentration was lowered from 10 to 0 millimolar (that is, no added calcium), potassium channel activity, identified from its characteristic time course, disappeared within a few seconds and there was a parallel increase in resting membrane conductance and in the holding current. The close temporal correlation of the changes in the three parameters suggests that potassium channels lose their ability to close in the absence of calcium and simultaneously lose their selectivity. If potassium channels were blocked by barium ion before calcium ion was removed, the increases in membrane conductance and holding current were delayed or prevented. Thus calcium is an essential cofactor in the gating of potassium channels in squid neurons.     

Cyclic AMP-modulated potassium channels in murine B cells and their precursors

A voltage-dependent potassium current (the delayed rectifier) has been found in murine B cells and their precursors with the whole-cell patch-clamp technique. The type of channel involved in the generation of this current appears to be present throughout all stages of pre-B-cell differentiation, since it is detected in pre-B cell lines infected with Abelson murine leukemia virus; these cell lines represent various phases of B-cell development. Thus, the presence of this channel is not obviously correlated with B-cell differentiation. Although blocked by Co2+, the channel, or channels, does not appear to be activated by Ca2+ entry. It is, however, inactivated by high intracellular Ca2+ concentrations. In addition, elevation of intracellular adenosine 3', 5'-monophosphate induces at all potentials a rapid decrease in the peak potassium conductance and increased rates of activation and inactivation. Therefore, potassium channels can be physiologically modulated by second messengers in lymphocytes.     

A G protein directly regulates mammalian cardiac calcium channels

A possible direct effect of guanine nucleotide binding (G) proteins on calcium channels was examined in membrane patches excised from guinea pig cardiac myocytes and bovine cardiac sarcolemmal vesicles incorporated into planar lipid bilayers. The guanosine triphosphate analog, GTP gamma S, prolonged the survival of excised calcium channels independently of the presence of adenosine 3',5'-monophosphate (cAMP), adenosine triphosphate, cAMP-activated protein kinase, and the protein kinase C activator tetradecanoyl phorbol acetate. A specific G protein, activated Gs, or its alpha subunit, purified from the plasma membranes of human erythrocytes, prolonged the survival of excised channels and stimulated the activity of incorporated channels. Thus, in addition to regulating calcium channels indirectly through activation of cytoplasmic kinases, G proteins can regulate calcium channels directly. Since they also directly regulate a subset of potassium channels, G proteins are now known to directly gate two classes of membrane ion channels.     

毛白杨花粉败育过程中Ca^2＋-ATPase的异常变化

为研究木本植物花粉败育过程中Ca2+ ATPase的影响,本文利用氯化铈沉淀和电镜细胞化学方法,对不同育性毛白杨花药在小孢子发生发育过程中Ca2+ ATPase进行了定位。结果表明:Ca2+ ATPase在可育花药小孢子母细胞时期大量分布于小孢子母细胞质膜、液泡膜及内膜系统,花药内壁细胞内膜系统和绒毡层细胞质膜,随后Ca2+ ATPase在上述细胞中均减少甚至消失,并于小孢子时期和成熟花粉时期分别在小孢子和花药内壁细胞中再次沉积;不育花药小孢子母细胞以及此时期绒毡层细胞质膜上并无明显Ca2+ ATPase,花药表皮、药室内壁以及中层的Ca2+ ATPase都高于同一时期的可育花药,其绒毡层细胞解体不完全。由上述结果推测:毛白杨不育花药内小孢子母细胞和药壁中Ca2+ ATPase分布异常,影响细胞内钙离子的主动转运,可能导致钙离子异常累积,进而通过影响小孢子母细胞和药室内壁细胞的正常代谢、绒毡层细胞的及时降解而导致花粉败育。      

Calcium and sodium channels in spontaneously contracting vascular muscle cells

Electrophysiological recordings of inward currents from whole cells showed that vascular muscle cells have one type of sodium channel and two types of calcium channels. One of the calcium channels, the transient calcium channel, was activated by small depolarizations but then rapidly inactivated. It was equally permeable to calcium and barium and was blocked by cadmium, but not by tetrodotoxin. The other type, the sustained calcium channel, was activated by larger depolarizations, but inactivated very little; it was more permeable to barium than calcium. The sustained calcium channel was more sensitive to block by cadmium than the transient channel, but also was not blocked by tetrodotoxin. The sodium channel inactivated 15 times more rapidly than the transient calcium channel and at more negative voltages. This sodium channel, which is unusual because it is only blocked by a very high (60 microM) tetrodotoxin concentration but not by cadmium, is the first to be characterized in vascular muscle, and together with the two calcium channels, provides a basis for different patterns of excitation in vascular muscles.     

Sequence and expression of mRNAs encoding the alpha 1 and alpha 2 subunits of a DHP-sensitive calcium channel

Complementary DNAs were isolated and used to deduce the primary structures of the alpha 1 and alpha 2 subunits of the dihydropyridine-sensitive, voltage-dependent calcium channel from rabbit skeletal muscle. The alpha 1 subunit, which contains putative binding sites for calcium antagonists, is a hydrophobic protein with a sequence that is consistent with multiple transmembrane domains and shows structural and sequence homology with other voltage-dependent ion channels. In contrast, the alpha 2 subunit is a hydrophilic protein without homology to other known protein sequences. Nucleic acid hybridization studies suggest that the alpha 1 and alpha 2 subunit mRNAs are expressed differentially in a tissue-specific manner and that there is a family of genes encoding additional calcium channel subtypes.     

How synaptotagmin promotes membrane fusion

Synaptic vesicles loaded with neurotransmitters are exocytosed in a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent manner after presynaptic depolarization induces calcium ion (Ca2+) influx. The Ca2+ sensor required for fast fusion is synaptotagmin-1. The activation energy of bilayer-bilayer fusion is very high (approximately 40 k(B)T). We found that, in response to Ca2+ binding, synaptotagmin-1 could promote SNARE-mediated fusion by lowering this activation barrier by inducing high positive curvature in target membranes on C2-domain membrane insertion. Thus, synaptotagmin-1 triggers the fusion of docked vesicles by local Ca2+-dependent buckling of the plasma membrane together with the zippering of SNAREs. This mechanism may be widely used in membrane fusion.