Cardiac calcium channels in planar lipid bilayers. L-type channels and calcium-permeable channels open at negative membrane potentials

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

Cav2.3 channel function and Zn2+-induced modulation: potential mechanisms and (patho)physiological relevance

Voltage-gated calcium channels (VGCCs) are critical for Ca²⁺ influx into all types of excitable cells, but their exact function is still poorly understood. Recent reconstruction of homology models for all human VGCCs at atomic resolution provides the opportunity for a structure-based discussion of VGCC function and novel insights into the mechanisms underlying Ca²⁺ selective flux through these channels. In the present review, we use these data as a basis to examine the structure, function, and Zn²⁺-induced modulation of Ca_v2.3 VGCCs, which mediate native R-type currents and belong to the most enigmatic members of the family. Their unique sensitivity to Zn²⁺ and the existence of multiple mechanisms of Zn²⁺ action strongly argue for a role of these channels in the modulatory action of endogenous loosely bound Zn²⁺, pools of which have been detected in a number of neuronal, endocrine, and reproductive tissues. Following a description of the different mechanisms by which Zn²⁺ has been shown or is thought to alter the function of these channels, we discuss their potential (patho)physiological relevance, taking into account what is known about the magnitude and function of extracellular Zn²⁺ signals in different tissues. While still far from complete, the picture that emerges is one where Ca_v2.3 channel expression parallels the occurrence of loosely bound Zn²⁺ pools in different tissues and where these channels may serve to translate physiological Zn²⁺ signals into changes of electrical activity and/or intracellular Ca²⁺ levels.

Therapeutic Effects of Wenxin Keli in Cardiovascular Diseases: An Experimental and Mechanism Overview

Cardiovascular diseases (CVDs) are the major public health problem and a leading cause of morbidity and mortality on a global basis. Wenxin Keli (WXKL), a formally classical Chinese patent medicine with obvious efficacy and favorable safety, plays a great role in the management of patients with CVDs. Accumulating evidence from various animal and cell studies has showed that WXKL could protect myocardium and anti-arrhythmia against CVDs. WXKL exhibited its cardioprotective roles by inhibiting inflammatory reaction, decreasing oxidative stress, regulating vasomotor disorders, lowering cell apoptosis, and protection against endothelial injure, myocardial ischemia, cardiac fibrosis, and cardiac hypertrophy. Besides, WXKL could effectively shorten the QRS and Q-T intervals, decrease the incidence of atrial/ventricular fibrillation and the number of ventricular tachycardia episodes, improve the severity of arrhythmias by regulating various ion channels with different potencies, mainly comprising peak sodium current (INa), late sodium current (INaL), transient outward potassium current (Ito), L-type calcium current (ICaL), and pacemaker current (If).

The cardioprotective and antiarrhythmic effects of Nardostachys chinensis in animal and cell experiments

Background

Cardiovascular disease (CVD) is the leading cause of premature death throughout the world. An estimated 17.5 million people died from CVD in 2012, representing 31% of all global deaths. Nardostachys chinensis (NC), a typical traditional Chinese medicine (TCM), plays a crucial role in the management of patients with CVD, especially for those with cardiac arrhythmia. The purpose of this study was to evaluate the cardioprotective and antiarrhythmic effects of NC in animal and cell experiments.

Methods

To review the cardioprotective and antiarrhythmic effects of NC, studies of NC on cardiovascular diseases in animal and cell experiments were identified from five databases through April 2016. Two investigators independently conducted the literature search, study selection, and data extraction.

Results

A total of 16 studies were identified, including five animal experiments and eleven cell experiments. Four studies showed significant effects of NC on myocardial protection by inhibiting myocardial apoptosis, inflammation and oxidative stress. Twelve studies indicated significant beneficial effects of NC in cardiac arrhythmia primarily through the modulation of ion channels (I_k, I_k1, I_Na, I_Ca-L, I_to).

Conclusion

The above findings showed the possible efficacy of NC via its cardioprotective and antiarrhythmic effects, but the results should be interpreted with caution due to the limitations and the deficiencies in the studies.

A rendezvous with the queen of ion channels: Three decades of ion channel research by David T Yue and his Calcium Signals Laboratory

David T. Yue was a renowned biophysicist who dedicated his life to the study of Ca(2+) signaling in cells. In the wake of his passing, we are left not only with a feeling of great loss, but with a tremendous and impactful body of work contributed by a remarkable man. David's research spanned the spectrum from atomic structure to organ systems, with a quantitative rigor aimed at understanding the fundamental mechanisms underlying biological function. Along the way he developed new tools and approaches, enabling not only his own research but that of his contemporaries and those who will come after him. While we cannot hope to replicate the eloquence and style we are accustomed to in David's writing, we nonetheless undertake a review of David's chosen field of study with a focus on many of his contributions to the calcium channel field.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Temperature-sensitive Cav1.2 calcium channels support intrinsic firing of pyramidal neurons and provide a target for the treatment of febrile seizures

Febrile seizures are associated with increased brain temperature and are often resistant to treatments with antiepileptic drugs, such as carbamazepine and phenytoin, which are sodium channel blockers. Although they are clearly correlated with the hyperthermic condition, the precise cellular mechanisms of febrile seizures remain unclear. We performed patch-clamp recordings from pyramidal cells in acute rat brain slices at temperatures up to 40°C and found that, at ≥37°C, L-type calcium channels are active at unexpectedly hyperpolarized potentials and drive intrinsic firing, which is also supported by a temperature-dependent, gadolinium-sensitive sodium conductance. Pharmacological data, RT-PCR, and the current persistence in Cav1.3 knock-out mice suggested a critical contribution of Cav1.2 subunits to the temperature-dependent intrinsic firing, which was blocked by nimodipine. Because intrinsic firing may play a critical role in febrile seizures, we tested the effect of nimodipine in an in vivo model of febrile seizures and found that this drug dramatically reduces both the incidence and duration of febrile seizures in rat pups, suggesting new possibilities of intervention for this important pathological condition.

Ca(v)1.3 channels produce persistent calcium sparklets, but Ca(v)1.2 channels are responsible for sparklets in mouse arterial smooth muscle

Ca(2+) sparklets are local elevations in intracellular Ca(2+) produced by the opening of a single or a cluster of L-type Ca(2+) channels. In arterial myocytes, Ca(2+) sparklets regulate local and global intracellular Ca(2+). At present, the molecular identity of the L-type Ca(2+) channels underlying Ca(2+) sparklets in these cells is undetermined. Here, we tested the hypotheses that voltage-gated calcium channel-alpha 1.3 subunit (Ca(v)1.3) can produce Ca(2+) sparklets and that Ca(v)1.2 and/or Ca(v)1.3 channels are responsible for Ca(2+) sparklets in mouse arterial myocytes. First, we investigated the functional properties of single Ca(v)1.3 channels in tsA201 cells. With 110 mM Ba(2+) as the charge carrier, Ca(v)1.3 channels had a conductance of 20 pS. This value is similar to that of Ca(v)1.2 and native L-type Ca(2+) channels. As previously shown for Ca(v)1.2 channels, Ca(v)1.3 channels can operate in two gating modes characterized by short and long open times. Expressed Ca(v)1.3 channels also produced Ca(2+) sparklets. Ca(v)1.3 sparklets had properties similar to those produced by Ca(v)1.2 and native L-type channels, including quantal amplitude, dihydropyridine sensitivity, bimodal gating, and dual-event duration times. However, the voltage dependencies of conductance and steady-state inactivation of the Ca(2+) current (I(Ca)) in arterial myocytes were similar to those recorded from cells expressing Ca(v)1.2 but not Ca(v)1.3 channels. Furthermore, nifedipine (10 microM) eliminated Ca(2+) sparklets in wild-type myocytes but not in myocytes expressing dihydropyridine-insensitive Ca(v)1.2 channels. Accordingly, Ca(v)1.3 transcript and protein were not detected in isolated arterial myocytes. We conclude that although Ca(v)1.3 channels can produce Ca(2+) sparklets, Ca(v)1.2 channels underlie I(Ca), Ca(2+) sparklets, and hence dihydropyridine-sensitive Ca(2+) influx in mouse arterial myocytes.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

Large-conductance calcium-activated potassium channels facilitate transmitter release in salamander rod synapse

Large-conductance calcium-activated potassium (BK) channels are colocalized with calcium channels at sites of exocytosis at the presynaptic terminals throughout the nervous system. It is expected that their activation would provide negative feedback to transmitter release, but the opposite is sometimes observed. Attempts to resolve this apparent paradox based on alterations in action potential waveform have been ambiguous. In an alternative approach, we investigated the influence of this channel on neurotransmitter release in a nonspiking neuron, the salamander rod photoreceptors. Surprisingly, the BK channel facilitates calcium-mediated transmitter release from rods. The two presynaptic channels form a positive coupled loop. Calcium influx activates the BK channel current, leading to potassium efflux that increases the calcium current. The normal physiological voltage range of the rod is well matched to the dynamics of this positive loop. When the rod is further depolarized, then the hyperpolarizing BK channel current exceeds its facilitatory effect, causing truncation of transmitter release. Thus, the calcium channel-BK channel linkage performs two functions at the synapse: nonlinear potentiator and safety brake.

Sustained Ca2+ entry elicits transient postsynaptic currents at a retinal ribbon synapse

Night (scotopic) vision is mediated by a distinct retinal circuit in which the light responses of rod-driven neurons are faster than those of the rods themselves. To investigate the dynamics of synaptic transmission at the second synapse in the rod pathway, we made paired voltage-clamp recordings from rod bipolar cells (RBCs) and postsynaptic AII and A17 amacrine cells in rat retinal slices. Depolarization of RBCs from -60 mV elicited sustained Ca2+ currents and evoked AMPA receptor (AMPAR)-mediated EPSCs in synaptically coupled amacrine cells that exhibited large, rapidly rising initial peaks that decayed rapidly to smaller, steady-state levels. The transient component persisted in the absence of feedback inhibition to the RBC terminal and when postsynaptic AMPA receptor desensitization was blocked with cyclothiazide, indicating that it reflects a time-dependent decrease in the rate of exocytosis from the presynaptic terminal. The EPSC waveform was similar when RBCs were recorded in perforated-patch or whole-cell configurations, but asynchronous release from RBCs was enhanced when the intraterminal Ca2+ buffer capacity was reduced. When RBCs were depolarized from -100 mV, inactivating, low voltage-activated (T-type channel-mediated) Ca2+ currents were evident. Although Ca2+ influx through T-type channels boosted vesicle release, as reflected by larger EPSCs, it did not make the EPSCs faster, indicating that activation of T-type channels is not necessary to generate a transient phase of exocytosis. We conclude that the time course of vesicle release from RBCs is inherently transient and, together with the fast kinetics of postsynaptic AMPARs, speeds transmission at this synapse.

Sustained Ca2+ entry elicits transient postsynaptic currents at a retinal ribbon synapse

Night (scotopic) vision is mediated by a distinct retinal circuit in which the light responses of rod-driven neurons are faster than those of the rods themselves. To investigate the dynamics of synaptic transmission at the second synapse in the rod pathway, we made paired voltage-clamp recordings from rod bipolar cells (RBCs) and postsynaptic AII and A17 amacrine cells in rat retinal slices. Depolarization of RBCs from -60 mV elicited sustained Ca2+ currents and evoked AMPA receptor (AMPAR)-mediated EPSCs in synaptically coupled amacrine cells that exhibited large, rapidly rising initial peaks that decayed rapidly to smaller, steady-state levels. The transient component persisted in the absence of feedback inhibition to the RBC terminal and when postsynaptic AMPA receptor desensitization was blocked with cyclothiazide, indicating that it reflects a time-dependent decrease in the rate of exocytosis from the presynaptic terminal. The EPSC waveform was similar when RBCs were recorded in perforated-patch or whole-cell configurations, but asynchronous release from RBCs was enhanced when the intraterminal Ca2+ buffer capacity was reduced. When RBCs were depolarized from -100 mV, inactivating, low voltage-activated (T-type channel-mediated) Ca2+ currents were evident. Although Ca2+ influx through T-type channels boosted vesicle release, as reflected by larger EPSCs, it did not make the EPSCs faster, indicating that activation of T-type channels is not necessary to generate a transient phase of exocytosis. We conclude that the time course of vesicle release from RBCs is inherently transient and, together with the fast kinetics of postsynaptic AMPARs, speeds transmission at this synapse.

Adult rat cardiomyocytes exhibit capacitative calcium entry

Capacitative Ca(2+) entry (CCE) refers to the influx of Ca(2+) through plasma membrane channels activated on depletion of endoplasmic-sarcoplasmic reticulum Ca(2+) stores. We utilized two Ca(2+)-sensitive dyes (one monitoring cytoplasmic free Ca(2+) and the other free Ca(2+) within the sarcoplasmic reticulum) to determine whether adult rat ventricular myocytes exhibit CCE. Treatments with inhibitors of the sarcoplasmic endoplasmic reticulum Ca(2+)-ATPases were not efficient in releasing Ca(2+) from stores. However, when these inhibitors were coupled with either Ca(2+) ionophores or angiotensin II (an agonist generating inositol 1,4,5 trisphosphate), depletion of stores was observed. This depletion was accompanied by a significant influx of extracellular Ca(2+) characteristic of CCE. CCE was also observed when stores were depleted with caffeine. This influx of Ca(2+) was sensitive to four inhibitors of CCE (glucosamine, lanthanum, gadolinium, and SKF-96365) but not to inhibitors of L-type channels or the Na(+)/Ca(2+) exchanger. In the whole cell configuration, an inward current of approximately 0.7 pA/pF at -90 mV was activated when a Ca(2+) chelator or inositol (1,4,5)-trisphosphate was included in the pipette or when Ca(2+) stores were depleted with a Ca(2+)-ATPase inhibitor and ionophore. The current was maximal at hyperpolarizing voltages and inwardly rectified. The channel was relatively permeant to Ca(2+) and Ba(2+) but only poorly to Mg(2+) or Mn(2+). Taken together, these data support the existence of CCE in adult cardiomyocytes, a finding with likely implications to physiological responses to phospholipase C-generating agonists.

Stimulation of cardiac L-type calcium channels by extracellular ATP

The co-release of ATP with norepinephrine from sympathetic nerve terminals in the heart may augment adrenergic stimulation of cardiac Ca(2+) channel activity. To test for a possible direct effect of extracellular ATP on L-type Ca(2+) channels, single channels were reconstituted from porcine sarcolemma into planar lipid bilayers so that intracellular signaling pathways could be controlled. Extracellular ATP (2-100 microM) increased the open probability of the reconstituted channels, with a maximal increase of approximately 2.6-fold and an EC(50) of 3.9 microM. The increase in open probability was due to an increase in channel availability and a decrease in channel inactivation rate. Other nucleotides displayed a rank order of effectiveness of ATP > alpha,beta-methylene-ATP > 2-methylthio-ATP > UTP > adenosine 5'-O-(3-thiotriphosphate) >> ADP; adenosine had no effect. Several antagonists of P2 receptors had no impact on the ATP-dependent increase in open probability, indicating that receptor activation was not required. These results suggest that extracellular ATP and other nucleotides can stimulate the activity of cardiac L-type Ca(2+) channels via a direct interaction with the channels.

Facilitation of spontaneous acetylcholine release induced by activation of cAMP in rat neuromuscular junctions

Regulation of neurotransmitter release is thought to involve modulation of the release probability by protein phosphorylation. Activation of the cAMP-protein kinase A (PKA) pathway has been shown to facilitate synaptic transmission in mammalian neuromuscular synapses, although the relevant phosphorylation targets are mostly unknown. We found that the inhibitor of the phosphodiesterase aminophylline (1 mM AMIN), the membrane-permeable analog of cAMP, 8-Br-cAMP (5 mM) and, the direct adenylate cyclase activator, forskolin (20 microM), induced an increase of miniature end-plate potentials (MEPPs) frequency in rat neuromuscular junctions. We investigated the possible involvement of the voltage-dependent calcium channels (VDCC), since these proteins are known to be phosphorylated by PKA. But this possibility was ruled out, since the increase in MEPPs frequency was not attenuated by the VDCC blocker Cd2+ (100 microM) and it was observed when AMIN was studied on hyperosmotic response, which is independent of [Ca2+]o and of Ca2+ influx through the VDCC. The lack of action of AMIN on MEPPs frequency when [Ca2+]i was diminished by exposing the preparations to zero Ca2+-EGTA solution (isotonic condition) or when nerve terminals were loaded with a permeant Ca2+ chelator (BAPTA-AM) (hypertonic condition), indicate that cAMP-mediated presynaptic facilitation is a function of nerve terminal Ca2+ concentration. We also found that AMIN exerted a comparable increase in MEPPs frequency in control and high K+ (10 and 15 mM), suggesting a single mechanism of action for spontaneous and K+-induced secretion.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Biophysical properties of scorpion alpha-toxin-sensitive background sodium channel contributing to the pacemaker activity in insect neurosecretory cells (DUM neurons)

A scorpion alpha-toxin-sensitive background sodium channel was characterized in short-term cultured adult cockroach dorsal unpaired median (DUM) neurons using the cell-attached patch-clamp configuration. Under control conditions, spontaneous sodium currents were recorded at different steady-state holding potentials, including the range of normal resting membrane potential. At -50 mV, the sodium current was observed as unclustered, single openings. For potentials more negative than -70 mV, investigated patches contained large unitary current steps appearing generally in bursts. These background channels were blocked by tetrodotoxin (TTX, 100 nm), and replacing sodium with TMA-Cl led to a complete loss of channel activity. The current-voltage relationship has a slope conductance of 36 pS. At -50 mV, the mean open time constant was 0.22 +/- 0.05 ms (n = 5). The curve of the open probability versus holding potentials was bell-shaped, with its maximum (0.008 +/- 0.004; n = 5) at -50 mV. LqhalphaIT (10-8 m) altered the background channel activity in a time-dependent manner. At -50 mV, the channel activity appeared in bursts. The linear current-voltage relationship of the LqhalphaIT-modified sodium current determined for the first three well-resolved open states gave three conductance levels: 34, 69 and 104 pS, and reversed at the same extrapolated reversal potential (+52 mV). LqhalphaIT increased the open probability but did not affect either the bell-shaped voltage dependence or the open time constant. Mammal toxin AaHII induced very similar effects on background sodium channels but at a concentration 100 x higher than LqhalphaIT. At 10-7 m, LqhalphaIT produced longer silence periods interrupted by bursts of increased channel activity. Whole-cell experiments suggested that background sodium channels can provide the depolarizing drive for DUM neurons essential to maintain beating pacemaker activity, and revealed that 10-7 m LqhalphaIT transformed a beating pacemaker activity into a rhythmic bursting.

Inhibition of cardiac L-type calcium channels by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs), products of the cytochrome P-450 monooxygenase metabolism of arachidonic acid, can regulate the activity of ion channels. We examined the effects of EETs on cardiac L-type Ca2+ channels that play important roles in regulating cardiac contractility, controlling heart rate, and mediating slow conduction in normal nodal cells and ischemic myocardium. Our experimental approach was to reconstitute porcine L-type Ca2+ channels into planar lipid bilayers where we could control the aqueous and lipid environments of the channels and the regulatory pathways that change channel properties. We found that 20 to 125 nM EETs inhibited the open probability of reconstituted L-type Ca2+ channels, accelerated the inactivation of the channels, and reduced the unitary current amplitude of open channels. There was no selectivity among different EET regioisomers or stereoisomers. When 11,12-EET was esterified to the sn-2 position of phosphatidylcholine, restricting it to the hydrophobic phase of the planar lipid bilayer, the reconstituted channels were similarly inhibited, suggesting that the EET interacts directly with Ca2+ channels through the lipid phase. The inhibitory effects of EET persisted in the presence of microcystin, an inhibitor of protein phosphatases 1 and 2A, suggesting that dephosphorylation was not the mechanism through which these eicosanoids down-regulate channel activity. This inhibition may be an important protective mechanism in the setting of cardiac ischemia where arachidonic acid levels are dramatically increased and EETs have been shown to manifest preconditioning-like effects.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

Multiple channels mediate calcium leakage in the A7r5 smooth muscle-derived cell line

Ca2+ entry under resting conditions may be important for contraction of vascular smooth muscle, but little is known about the mechanisms involved. Ca2+ leakage was studied in the A7r5 smooth muscle-derived cell line by patch-clamp techniques. Two channels that could mediate calcium influx at resting membrane potentials were characterized. In 110 mM Ba2+, one channel had a slope conductance of 6.0 +/- 0.6 pS and an extrapolated reversal potential of +41 +/- 13 mV (mean +/- SD, n = 8). The current rectified strongly, with no detectable outward current, even at +90 mV. Channel gating was voltage independent. A second type of channel had a linear current-voltage relationship, a slope conductance of 17.0 +/- 3.2 pS, and a reversal potential of +7 +/- 4 mV (n = 9). The open probability increased e-fold per 44 +/- 10 mV depolarization (n = 5). Both channels were also observed in 110 mM Ca2+. Noise analysis of whole-cell currents indicates that approximately 100 6-pS channels and 30 17-pS channels are open per cell. These 6-pS and 17-pS channels may contribute to resting calcium entry in vascular smooth muscle cells.

Pore size of the malaria parasite's nutrient channel

The malaria parasite, Plasmodium falciparum, requires large amounts of nutrients to sustain its rapid growth within the human red blood cell. A recently identified ion channel on the surface of the intraerythrocytic parasite may provide direct access to these nutrients in the red blood cell cytosol. Evidence supporting this role was obtained by incorporating this channel into planar lipid bilayers. In bilayers, this channel has conductance and gating properties identical to the in situ channel, passes soluble macromolecules of up to 1400 Da, and functions as a high capacity, low affinity molecular sieve. These properties, remarkably similar to those of a pore on Toxoplasma gondii (another protozoan parasite causing human disease), suggest a novel class of channels used by these intracellular parasites to acquire nutrients from host cytosol.

Single L-type calcium channel conductance with physiological levels of calcium in chick ciliary ganglion neurons

1. Single L-type calcium channels in chick ciliary ganglion neurons were studied at high current resolution in cell-attached patch recordings using quartz-glass micropipettes. 2. A single open-channel current amplitude was observed when Ba2+ was the charge carrier with a conductance of 26 pS at 110 mM barium. However, with 110 mM calcium two current fluctuation amplitudes were observed. These were termed low and high fluctuation amplitudes, CaL and CaH, and had conductances of 8.8 and 12 pS, respectively. These two levels probably reflect two different channel species. CaL was identified as an L-type calcium channel on the basis of resistance to inactivation, conductance, and dihydropyridine sensitivity. 3. Single-channel current fluctuations could be detected with calcium concentrations as low as 1.0 mM. Although the unitary conductance (gamma) was much greater with barium than calcium at every concentration tested, the concentration dependence of conductance was similar for gamma Ba, gamma CaH and gamma CaL. Fitting the concentration dependencies of these conductances with a Langmuir isotherm gave KD estimates of 4.7, 5.6 and 5.0 mM for barium, CaL and CaH, respectively 4. The single-channel conductance of the L-type channel (gamma L) can be described by the relation: conductance (in pS) = 9.2/(1 + 5.6/[Ca]) where [Ca] is the external calcium concentration in the 1.0-110 mM range. Thus, at a physiological external calcium concentration of 2 mM the conductance is 2.4 pS. 5. Ca2+ transport through the L-type calcium channel is particularly sensitive to changes in external calcium concentration in the physiological range but approaches saturation at about 10 mM. this characteristic may optimize the responsiveness of the cell to small changes in ambient calcium concentrations while limiting excess entry in the presence of abnormally high calcium levels.

A capacitative calcium current in cultured skeletal muscle cells is mediated by the calcium-specific leak channel and inhibited by dihydropyridine compounds

Calcium stores from cultured skeletal muscle cells were depleted using cyclopiazonic acid (CPA), a reversible inhibitor of Ca2+-ATPases at the sarcoplasmic reticulum. Store depletion led to activation of the calcium-specific leak channel, as assayed using single-channel patch clamp analysis and rates of manganese influx and quenching of fura-2 fluorescence. Two novel dihydropyridine compounds inhibited this single-channel leak channel activity, the resting and depletion-induced manganese influx, and refilling of the CPA-depleted intracellular calcium store. These compounds represent the first antagonists for a calcium leak channel and for a channel that mediates a capacitative current. The development of the skeletal muscle capacitative current was inhibited by genistein, a tyrosine kinase inhibitor, but was not affected by okadaic acid, a phosphatase inhibitor, or econazole. Thus, the capacitative current in cultured skeletal muscle cells was mediated by the calcium leak channel and was inhibited by pharmacological antagonists and may provide a model system for uncovering the complete set of signals leading from store depletion to channel activation.

Characterization of a chloride channel reconstituted from cardiac sarcoplasmic reticulum

We have characterized a voltage-sensitive chloride channel from cardiac sarcoplasmic reticulum (SR) following reconstitution of porcine heart SR into planar lipid bilayers. In 250 mM KCl, the channel had a main conductance level of 130 pS and exhibited two substrates of 61 and 154 pS. The channel was very selective for Cl- over K+ or Na+ (PK+/PCl- = 0.012 and PNa+/PCl- approximately 0.040). It was permeable to several anions and displayed the following sequence of anion permeability: SCN- > I- > NO3- approximately Br- > Cl- > F- > HCOO-. Single-channel conductance saturated with increasing Cl- concentrations (Km = 900 mM and gamma max = 488 pS). Channel activity was voltage dependent, with an open probability ranging from approximately 1.0 around 0 mV to approximately 0.5 at +80 mV. From -20 to +80 mV, channel gating was time-independent. However, at voltages below -40 mV the channel entered a long-lasting closed state. Mean open times varied with voltage, from approximately 340 msec at -20 mV to approximately 6 msec at +80 mV, whereas closed times were unaffected. The channel was not Ca(2+)-dependent. Channel activity was blocked by disulfonic stilbenes, arylaminobenzoates, zinc, and cadmium. Single-channel conductance was sensitive to trans pH, ranging from approximately 190 pS at pH 5.5 to approximately 60 pS at pH 9.0. These characteristics are different from those previously described for Cl- channels from skeletal or cardiac muscle SR.

Divalent cation channels activated by phenothiazines in membrane of rat ventricular myocytes

Phenothiazines (PTZ) such as chlorpromazine (CPZ) or trifluoperazine (TPZ) induced a sustained divalent cation-permeable channel activity when applied on either side of inside-out patches or on external side of cell-attached patches of adult rat ventricular myocytes. The percentage of active patches was approximately 20%. In the case of CPZ, the Kd of the dose-response curve was 160 microM. CPZ-activated channels were potential-independent in the physiological range of membrane potential and were permeable to several divalent ions (Ba2+, Ca2+, Mg2+, Mn2+). At least three levels of currents were usually detected with conductances of 23, 50 and 80 pS in symmetrical 96 mM Ba2+ solution and 17, 36 and 61 pS in symmetrical 96 mM Ca2+ solution. Saturation curves corresponding to the three main conductances determined in Ba2+ symmetrical solutions (tonicity compensated with choline-Cl) gave maximum conductances of 36, 81 and 116 pS (with corresponding half-saturating concentration constants of 31.5, 38 and 34.5 mM). The corresponding conductance values were estimated to 1.7, 3.3 and 5.2 pS in symmetrical 1.8 mM Ba2+ and to 1.1, 2.4 and 3.7 pS in symmetrical 1.8 mM Ca2+ (the value in normal Tyrode solution). Channels were poorly permeable to monovalent cations, such as Na, with a PBa/PNa ratio of 10. A PTZ-induced channel activity similar to that described in cardiac cells was also observed in cultured rat aortic smooth muscle cells but not in cultured neuroblastoma cells. PTZ-activated channels described in cardiac cells appear very similar to the sporadically active divalent ion permeable channels described in a previous paper (Coulombe et al., 1989). Surprisingly, when 100 microM CPZ were applied to myocytes studied in the whole-cell configuration, and maintained at a holding potential of -80 mV in the presence of 24 mM external Ca2+ or Ba2+, no detectable macroscopic inward current could be observed, whereas the L-type Ca2+ current triggered by depolarizing pulses was markedly and reversibly reduced. The possible reasons are discussed.

Boar sperm plasma membrane Ca(2+)-selective channels in planar lipid bilayers

Entry of Ca2+ through Ca2+ channels is thought to trigger the acrosome reaction of spermatozoa during fertilization. Antagonists of the L-type Ca2+ channel are known to prevent the intracellular Ca2+ (Ca2+) increase and inhibit acrosomal exocytosis in mammalian sperm. Planar bilayer recordings were used to study Ca2- channels incorporated from partially purified boar sperm plasma membranes. With symmetrical 50 mM NaCl and 100 mM BaCl2 on the cis side, single-channel events consistent with Ba2+ flux from cis to trans were observed. These channels were activated by the dihydropyridine agonist (+/-)BAY K 8644 and blocked by the antagonist nitrendipine. Sperm Ca2- channels did not require depolarization for activation and did not inactivate. The (+/-)BAY K 8644 and (S-)BAY K 8644 enantiomers increased apparent open time in a dose-dependent [half-maximal activity constant (K0.5) = 0.9 and 0.3 microM, respectively] manner. Dihydropyridine antagonists nitrendipine (K0.5 = 0.5 microM) and (R+)BAY K 8644 (K0.5 = 2.8 microM) decreased apparent open times. The channels described in this report share some properties with brain, cardiac, and skeletal muscle t tubule Ca2+ channels and may be involved in increasing Cai2+ before the acrosome reaction.

Surface charge and calcium channel saturation in bullfrog sympathetic neurons

Currents carried by Ba2+ through calcium channels were recorded in the whole-cell configuration in isolated frog sympathetic neurons. The effect of surface charge on the apparent saturation of the channel with Ba2+ was examined by varying [Ba2+]o and ionic strength. The current increased with [Ba2+]o, and the I-V relation and the activation curve shifted to more positive voltages. The shift of activation could be described by Gouy-Chapman theory, with a surface charge density of 1 e-/140 A2, calculated from the Grahame equation. Changes in ionic strength (replacing N-methyl-D-glucamine with sucrose) shifted the activation curve as expected for a surface charge density of 1 e-/85 A2, in reasonable agreement with the value from changing [Ba2+]o. The instantaneous I-V for fully activated channels also changed with ionic strength, which could be described either by a low surface charge density (less than 1 e-/1,500 A2), or by block by NMG with Kd approximately 300 mM (assuming no surface charge). We conclude that the channel permeation mechanism sees much less surface charge than the gating mechanism. The peak inward current saturated with an apparent Kd = 11.6 mM for Ba2+, while the instantaneous I-V saturated with an apparent Kd = 23.5 mM at 0 mV. This discrepancy can be explained by a lower surface charge near the pore, compared to the voltage sensor. After correction for a surface charge near the pore of 1 e-/1,500 A2, the instantaneous I-V saturated as a function of local [Ba2+]o, with Kd = 65 mM. These results suggest that the channel pore does bind Ba2+ in a saturable manner, but the current-[Ba2+]o relationship may be significantly affected by surface charge.

Voltage-dependent inactivation in a cardiac-skeletal chimeric calcium channel

The loci for inactivation in calcium channel proteins are unknown. Mechanisms for inactivation may be distributed across Ca2+ channel subunits and appear to be complex, multiple and interacting. We took advantage of the properties of chimeras, constructed between cardiac (H4) and skeletal muscle (Sk4) calcium channel alpha 1 subunits to study the molecular mechanism of inactivation in L-type calcium channels. Sk1H3, a chimeric construct of these two L-type calcium channels, was expressed in Xenopus oocytes in the absence of auxiliary subunits. Sk1H3 incorporated repeat I from skeletal muscle alpha 1 and repeats II, III, IV from heart alpha 1 subunit. Sk1H3 inactivated faster (tau = 300 ms) and more fully than the wild-type H4 with Ba2+ ions as the charge carrier. Thus, inactivation of Sk1H3 was 90% complete after a 5-s conditioning pulse at +20 mV while inactivation of H4 was only 37% complete. Sk1H3 inactivation also developed at more negative potentials with E0.5 = -15 mV as compared to E0.5 = -5 mV for H4. In the presence of external calcium ions, the extent of inactivation significantly increased from 37 to 83% for H4 while inactivation of Sk1H3 was only slightly increased. Inactivation with Ba2+ as the charge carrier was confirmed at the single- channel level where averaged single-channel ensembles showed a similar rate of inactivation. Collectively, these observations demonstrate that Sk1H3 inactivation appears to have a prominent voltage-dependent component. Whether Sk1H3 inactivation involves interactions within repeat I alone or interactions between repeat I and site(s) located in the three other repeats of the alpha 1 subunit has yet to be determined.

Inactivation of L-type Ca channels in embryonic chick ventricle cells: dependence on the cytoskeletal agents colchicine and taxol

This article shows that colchicine and taxol strongly influence the kinetics of L-type Ca channels in intact cardiac cells, and it suggests a mechanism for this action. It is known that colchicine disassociates microtubules into tubulin, and that taxol stabilizes microtubules. We have found that colchicine increases the probability that Ca channels are in the closed state and that taxol increases the probability they are in the open state. Moreover, taxol lengthens the mean open time of Ca channels. In this regard, taxol is similar to Bay-K 8644; however, Bay K works on inside-out patches, but taxol does not. Neither colchicine nor taxol alters the number of Ca channels in a patch. We have quantified these results as follows. It is known that L-type channels in embryonic chick heart ventricle cells have voltage- and current-dependent inactivation. In 10 mM Ba, channel conductance is linear in the range -10 to 20 mV. The conductance is 12 +/- 1 pS, and the extrapolated reversal potential is 42 +/- 2 mV (n = 3). In cell-attached patches, inactivation depends on the number of channels. One channel (holding at -80 mV and stepping to 0 mV for 500 ms) shows virtually no inactivation. However, three channels inactivate with a time constant of 360 +/- 20 ms (n = 6). In similar patches, colchicine (80 microM for 15 min) decreases the inactivation time constant to 162 +/- 33 ms (n = 4) and taxol (50 microM for 10 min) virtually abolishes inactivation (time constant 812 +/- 265 ms (n = 4)). We suggest that colchicine and taxol affect Ca channels through their action on the cytoskeleton, which in turn regulates the effective concentration of inactivating ions near the mouths of channels. An alternate explanation is that free tubulin interacts directly with Ca channels.

Dual modulation of unitary L-type Ca2+ channel currents by [Ca2+]i in fura-2-loaded guinea-pig ventricular myocytes

1. Single-channel studies were performed to clarify how tonic changes in intracellular Ca2+ concentrations ([Ca2+]i) modulate cardiac L-type Ca2+ channels. Currents were recorded from fura-2-loaded guinea-pig ventricular myocytes in the cell-attached configuration. Fura-2 fluorescence signals were recorded simultaneously during pulses to elicit channel activity. 2. The myocyte [Ca2+]i was altered through changes in bath Ca2+ concentration during K+ depolarization. When [Ca2+]i exceeded approximately 2 times the resting level (estimated [Ca2+]i around 180-400 nM), the activity of Ca2+ channels was reversibly potentiated without changes in unitary current amplitudes. 3. Increased channel open probability during Ca(2+)-dependent potentiation resulted from increased availability and increased open probability during non-blank sweeps. Closed time analysis revealed a distribution best fitted with two exponentials. Increased [Ca2+]i reduced the longer time constant, but had no effect on the shorter time constant. The open time constant was unchanged in most cases. Current records occasionally included sweeps with long openings (approximately 10 ms or more), whose appearance increased during potentiation. 4. When [Ca2+]i was increased after cAMP-dependent upregulation of Ca2+ channels, the change in channel activity was diminished. Similar results were observed when Ca(2+)-dependent potentiation was examined in myocytes exposed to a membrane-permeant protein kinase inhibitor, H-89. This suggests that channel phosphorylation may be responsible for Ca(2+)-dependent potentiation. 5. When [Ca2+]i was further increased, but remained below the threshold for contraction (estimated [Ca2+]i above 600 nM), Ca2+ channel activity was suppressed. 6. Our results demonstrate directly at the single-channel level that [Ca2+]i modulates the activity of cardiac L-type Ca2+ channels, enhancing it with modest [Ca2+]i increases and decreasing it with greater [Ca2+]i increases.

Epithelial Ca2+ channels sensitive to dihydropyridines and activated by hyperpolarizing voltages

Parathyroid hormone (PTH) increases transcellular Ca2+ absorption in renal cortical thick ascending limbs and distal convoluted tubules (DCT). In cells isolated from these nephron segments, PTH increases Ca2+ uptake by a pathway that is sensitive to dihydropyridine-type agonists and antagonists (B. J. Bacskai and P. A. Friedman. Nature Lond. 347: 388-391, 1990). Patch-clamp techniques were used to identify Ca(2+)-permeable channels in DCT cells. Channel activity was detectable in cell-attached patches only in cells pretreated with PTH. Ca2+ channels exhibited prolonged open times (seconds), had a low single-channel conductance (2.1 pS), and open channel probability increased at hyperpolarizing voltages (-50 to -90 mV). Channel activity was sensitive to dihydropyridine-type compounds, nifedipine, and BAY K8644, as was Ca2+ uptake. However, Ca2+ entry was insensitive to verapamil or omega-conotoxin. These results demonstrate that these channels mediate PTH-stimulated apical membrane Ca2+ entry in DCT cells, which are the principal Ca(2+)-transporting cells of the kidney.

Reduced calcium currents in subendocardial Purkinje myocytes that survive in the 24- and 48-hour infarcted heart

BACKGROUND

The abnormal transmembrane action potentials of subendocardial Purkinje fibers that survive 24 to 48 hours after coronary artery occlusion can be a source of the multiform ventricular tachycardias that occur during this time. A change in the density or function of either or both the T-type and L-type cardiac Ca2+ channels may contribute to the altered electrical activity of these Purkinje myocytes.

METHODS AND RESULTS

The purpose of this study was to determine the function of the T- and L-type Ca2+ currents (iCat and iCaL, respectively) in Purkinje myocytes dispersed from the subendocardium of the left ventricle 24 and 48 hours after coronary artery occlusion (IZPC24 and IZPC48, respectively). To do this we compared whole-cell Ca2+ currents from Purkinje myocytes enzymatically dispersed from free-running fiber bundles (SPCs), from the subendocardium of the noninfarcted canine heart (NZPCs), and from IZPC24 and IZPC48. ICaL and iCat were recorded with Cs(+)- and EGTA-rich pipettes and in Na(+)-K(+)-free external solutions to eliminate overlapping currents. ICaL density was significantly reduced in IZPC48 compared with NZPC or IZPC24. This was not accompanied by a shift in the current-voltage relation or by a change in the time course of decay of iCaL. Replacement of Ca2+ with equimolar Ba2+ increased iCaL density in all cell types, but peak iBaL of IZPC48 remained reduced compared with control iBaL values. T-type Ca2+ currents were recorded in all SPCs and NZPCs. In IZPC24 and IZPC48 there was a reduction in peak iCat amplitudes and densities. This was not accompanied by a shift in the current-voltage relation or by a change in the time course of decay of peak iCat. However, there was a hyperpolarizing shift in the steady-state availability relations in both IZPC24 and IZPC48. In addition, the maximally available iCat in IZPC24 was not different from control, whereas it was significantly reduced in IZPC48.

CONCLUSIONS

The L-type ICa density in subendocardial Purkinje myocytes that survive in the infarcted heart is significantly decreased by 48 hours after the time of coronary artery occlusion. The peak T-type ICa density is decreased in subendocardial Purkinje myocytes that survive in the infarcted heart at 24 hours, but further reduction occurs in these myocytes by 48 hours. This loss in Ca2+ channel function could contribute to the abnormal transmembrane potentials of these myocytes surviving in the infarcted heart.

Neurohormone D induces ionic current changes in cockroach central neurones

The octapeptide neurohormone D (NHD), a member of the family of adipokinetic hormones (AKH-peptides), increases the frequency of spontaneous activity in dorsal unpaired median (DUM) neurones isolated from the terminal ganglion of the cockroach Periplaneta americana. The increase in spike frequency is accompanied by changes in the shape and the amplitude of the single action potentials, e.g. a more pronounced afterhyperpolarization. Effects of NHD on membrane currents were investigated in these DUM cells with whole-cell voltage-clamp measurements. A voltage-independent Ca2+ current flowing at the resting potential (ICa,R) was found. NHD, at nanomolar concentrations, enhanced this ICa,R in a concentration-dependent manner. 0.1 mM Cd2+ markedly reduced ICa,R and in this case ICa,R was hardly potentiated by NHD. In the presence of NHD a fast activating Ca(2+)-dependent K+ current sensitive to charybdotoxin and to low concentrations of tetraethylammonium was augmented. The enhanced afterhyperpolarization of action potentials can be accounted for by the increase in the Ca(2+)-dependent K+ current. The changes of the membrane currents induced by NHD are discussed with respect to further effects on the spike pattern and in relation to the previously described mode of action of AKH-peptides in other preparations.

Calcium-dependent inactivation of L-type calcium channels in planar lipid bilayers

Intracellular Ca2+ can inhibit the activity of voltage-gated Ca channels by modulating the rate of channel inactivation. Ca(2+)-dependent inactivation of these channels may be a common negative feedback process important for regulating Ca2+ entry under physiological and pathological conditions. This article demonstrates that the inactivation of cardiac L-type Ca channels, reconstituted into planar lipid bilayers and studied in the presence of a dihydropyridine agonist, is sensitive to Ca2+. The rates and extents of inactivation, determined from ensemble averages of unitary Ba2+ currents, decreased when the calcium concentration facing the intracellular surface of the channel ([Ca2+]i) was lowered from approximately 10 microM to 20 nM by the addition of Ca2+ chelators. The rates and extents of Ba2+ current inactivation could also be increased by subsequent addition of Ca2+ raising the [Ca2+]i to 15 microM, thus demonstrating that the Ca2+ dependence of inactivation could be reversibly regulated by changes in [Ca2+]i. In addition, reconstituted Ca channels inactivated more quickly when the inward current was carried by Ca2+ than when it was carried by Ba2+, suggesting that local increases in [Ca2+]i could activate Ca(2+)-dependent inactivation. These data support models in which Ca2+ binds to the channel itself or to closely associated regulatory proteins to control the rate of channel inactivation, and are inconsistent with purely enzymatic models for channel inactivation.

Design of a functional calcium channel protein: inferences about an ion channel-forming motif derived from the primary structure of voltage-gated calcium channels

To identify sequence-specific motifs associated with the formation of an ionic pore, we systematically evaluated the channel-forming activity of synthetic peptides with sequence of predicted transmembrane segments of the voltage-gated calcium channel. The amino acid sequence of voltage-gated, dihydropyridine (DHP)-sensitive calcium channels suggests the presence in each of four homologous repeats (I-IV) of six segments (S1-S6) predicted to form membrane-spanning, alpha-helical structures. Only peptides representing amphipathic segments S2 or S3 form channels in lipid bilayers. To generate a functional calcium channel based on a four-helix bundle motif, four-helix bundle proteins representing IVS2 (T4CaIVS2) or IVS3 (T4CaIVS3) were synthesized. Both proteins form cation-selective channels, but with distinct characteristics: the single-channel conductance in 50 mM BaCl2 is 3 pS and 10 pS. For T4CaIVS3, the conductance saturates with increasing concentration of divalent cation. The dissociation constants for Ba2+, Ca2+, and Sr2+ are 13.6 mM, 17.7 mM, and 15.0 mM, respectively. The conductance of T4CaIVS2 does not saturate up to 150 mM salt. Whereas T4CaIVS3 is blocked by microM Ca2+ and Cd2+, T4CaIVS2 is not blocked by divalent cations. Only T4CaIVS3 is modulated by enantiomers of the DHP derivative BayK 8644, demonstrating sequence requirement for specific drug action. Thus, only T4CaIVS3 exhibits pore properties characteristic also of authentic calcium channels. The designed functional calcium channel may provide insights into fundamental mechanisms of ionic permeation and drug action, information that may in turn further our understanding of molecular determinants underlying authentic pore structures.

Regulation of cardiac L-type Ca channels in planar lipid bilayers by G proteins and protein phosphorylation

We have studied the effects of activated G proteins (Gs alpha and Gi1 alpha), adenosine 3',5'-cyclic monophosphate-dependent protein kinase (PKA), and okadaic acid on L-type Ca channels incorporated from porcine ventricular sarcolemma into planar lipid bilayers. Channel activity evoked by membrane depolarizations diminished to extremely low levels within 2 min of incorporation (channel "rundown"). When Gs alpha [activated with guanosine 5'-O-(3-thiotriphosphate)] was present in the intracellular chamber, the initial level of channel activity was increased and rundown was delayed, so that channel activity was sustained for longer times after incorporation. The effect was specific for activated Gs alpha; activated Gi1 alpha, heat-denatured, activated Gs alpha, and unactivated Gs alpha did not augment channel activity. Activated Gi1 alpha inhibited the stimulation of Ca channel activity by Gs alpha. Treatment of the sarcolemmal membranes with PKA and Mg-ATP also increased the initial channel open probability and delayed their rundown. Addition of intracellular Gs alpha to PKA-treated channels increased the initial level of activity above that seen with PKA or Gs alpha alone, suggesting different nonocclusive pathways for the channel stimulation. This was also supported by the observation that activated Gi1 alpha had no effect on PKA-treated channels. Okadaic acid (100 nM) increased the level of Ca channel activity, suggesting that dephosphorylation by endogenous phosphatases participated in the downregulation of the channels in cell-free membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

A cytoskeletal mechanism for Ca2+ channel metabolic dependence and inactivation by intracellular Ca2+

Many different types of voltage-dependent Ca2+ channels inactivate when intracellular ATP declines or intracellular Ca2+ rises. An inside-out, patch-clamp technique was applied to the Ca2+ channels of Lymnaea neurons to determine the mechanism(s) underlying these two phenomena. Although no evidence was found for a phosphorylation mechanism, agents that act on the cytoskeleton were found to alter Ca2+ channel activity. The cytoskeletal disrupters colchicine and cytochalasin B were found to speed Ca2+ channel decline in ATP, whereas the cytoskeletal stabilizers taxol and phalloidin were found to prolong Ca2+ channel activity without ATP. In addition, cytoskeletal stabilizers reduced Ca(2+)-dependent channel inactivation, suggesting that both channel metabolic dependence and Ca(2+)-dependent inactivation result from a cytoskeletal interaction.

Permeation and gating properties of the L-type calcium channel in mouse pancreatic beta cells

Ba2+ currents through L-type Ca2+ channels were recorded from cell-attached patches on mouse pancreatic beta cells. In 10 mM Ba2+, single-channel currents were recorded at -70 mV, the beta cell resting membrane potential. This suggests that Ca2+ influx at negative membrane potentials may contribute to the resting intracellular Ca2+ concentration and thus to basal insulin release. Increasing external Ba2+ increased the single-channel current amplitude and shifted the current-voltage relation to more positive potentials. This voltage shift could be modeled by assuming that divalent cations both screen and bind to surface charges located at the channel mouth. The single-channel conductance was related to the bulk Ba2+ concentration by a Langmuir isotherm with a dissociation constant (Kd(gamma)) of 5.5 mM and a maximum single-channel conductance (gamma max) of 22 pS. A closer fit to the data was obtained when the barium concentration at the membrane surface was used (Kd(gamma) = 200 mM and gamma max = 47 pS), which suggests that saturation of the concentration-conductance curve may be due to saturation of the surface Ba2+ concentration. Increasing external Ba2+ also shifted the voltage dependence of ensemble currents to positive potentials, consistent with Ba2+ screening and binding to membrane surface charge associated with gating. Ensemble currents recorded with 10 mM Ca2+ activated at more positive potentials than in 10 mM Ba2+, suggesting that external Ca2+ binds more tightly to membrane surface charge associated with gating. The perforated-patch technique was used to record whole-cell currents flowing through L-type Ca2+ channels. Inward currents in 10 mM Ba2+ had a similar voltage dependence to those recorded at a physiological Ca2+ concentration (2.6 mM). BAY-K 8644 (1 microM) increased the amplitude of the ensemble and whole-cell currents but did not alter their voltage dependence. Our results suggest that the high divalent cation solutions usually used to record single L-type Ca2+ channel activity produce a positive shift in the voltage dependence of activation (approximately 32 mV in 100 mM Ba2+).

Procaine effects on single sarcoplasmic reticulum Ca2+ release channels

The effects of the Ca(2+)-induced Ca2+ release blocker procaine on individual sarcoplasmic reticulum Ca2+ release channels have been examined in planar lipid bilayers. Procaine did not reduce the single channel conductance nor appreciably shorten the mean open times of the channel; rather, it increased the longest closed time. These results indicated that procaine interacted selectively with a closed state of the channel rather than with an open state. Gating of the sarcoplasmic reticulum Ca2+ release channel was described by a modified scheme of Ashley and Williams (1990. J. Gen. Physiol. 95:981-1005), including an additional long-lived closed state. Computer simulations determined that procaine was more likely to interact with this long-lived Ca(2+)-bound closed state of the channel rather than with other states of the channel. Simulations with the same model were also able to reproduce a prominent Ca(2+)-sensitive transition between "random" and "bursting" forms of gating of the channel, variations of which may account for "gearshift" behavior reported in studies with this and other single channels.

Photo-released intracellular Ca2+ rapidly blocks Ba2+ current in Lymnaea neurons

1. The effect of intracellular Ca2+ on Ba2+ current flowing through voltage-dependent Ca2+ channels was studied using the whole-cell patch-clamp technique on isolated neurons from the snail Lymnaea stagnalis. Intracellular Ca2+ was increased by flash photolysis of the caged Ca2+ compound DM-nitrophen and measured with the optical indicator fluo-3. 2. After the highest intensity flashes, peak Ba2+ current was blocked by 42% with a time constant of 5 ms. The onset of the block followed a similar time course whether channels were activated or closed. The Ba2+ current surviving after the flash had the same voltage dependence of activation and rate of inactivation as did the total Ba2+ current before the flash. 3. Recovery of the Ba2+ current from block was nearly complete and occurred with a time constant of 16 s. Multiple episodes of photolysis-induced block could be studied in the same cell when 7-10 min were allowed between flashes. In some cells, recovery from block was accompanied by a transient enhancement of the current above the pre-block magnitude. 4. Neurons greatly reduced the ability of photolysis to increase Ca2+, both by unloading the DM-nitrophen before flashes were applied and by rapidly buffering the photolytically released Ca2+. Maximal flashes on extracellular droplets of the DM-Ca2+ solution created a Ca2+ jump from 110 nM to 40 microM. In contrast, the same flashes on DM-Ca(2+)-loaded neurons resulted in a Ca2+ transient starting from a baseline of 36 nM to a peak of 130 nM. This intracellular Ca2+ transient decayed with three time constants (120 ms, 2 s and 13 s). 5. Endogenous buffer(s) binds Ca2+ rapidly. When intracellular Ca2+ was monitored within 2 ms of the flash, no rapid Ca2+ spike due to binding of photo-released Ca2+ could be detected. Addition of dibromo-BAPTA to the intracellular solution reduced the block by one third, which is consistent with the measured reduction of intracellular Ca2+. This indicates that the endogenous buffer can bind Ca2+ as rapidly as dibromo-BAPTA and as fast as Ca2+ is released by photolysis. 6. The Ca2+ dependence of the block, obtained by varying flash intensity, indicates some saturation by 130 nM. A simple two-state model of the block consistent with both the time course of block and recovery and the concentration dependence gave a dissociation constant of approximately 50 nM and forward rate constant of 7 x 10(8) M-1 s-1.(ABSTRACT TRUNCATED AT 400 WORDS)

Drugs acting on the intracellular signalling system

Cellular response to extracellular messages is a basic process to maintain and to support cell life. Several signalling molecules important as sites of therapeutic drug action are involved in the response. Recent studies on life sciences have elucidated molecular properties of intracellular signalling factors and mechanisms of cascading. Novel drugs acting on signalling molecules and possessing new sites and mechanisms of action have been found. This article summarizes the properties (subtypes, structures, functions) of signalling factors (receptors, ion channels, GTP binding proteins, second messenger-generating enzymes, second messenger-metabolizing enzymes, second messengers protein kinases, protein phosphatases) and lists in Tables A-H drugs that act on signalling molecules and which should find clinical use.

Blockade of insulin sensitive steady-state R-type Ca2+ channel by PN 200-110 in heart and vascular smooth muscle

The effect of high K+ concentration, insulin and the L-type Ca2+ channel blocker PN 200-110 on cytosolic intracellular free calcium ([Ca2+]i) was studied in single ventricular myocytes of 10-day-old embryonic chick heart, 20-week-old human fetus and rabbit aorta (VSM) single cells using the Ca(2+)-sensitive fluorescent dye, Fura-2 microfluorometry and digital imaging technique. Depolarization of the cell membrane of both heart and VSM cells with continuous superfusion of 30 mM [K+]o induced a rapid transient increase of [Ca2+]i that was followed by a sustained component. The early transient increase of [Ca2+]i by high [K+]o was blocked by the L-type calcium channel antagonist nifedipine. However, the sustained component was found to be insensitive to this drug. PN 200-110 another L-type Ca2+ blocker was found to decrease both the early transient and the sustained increase of [Ca2+]i induced by depolarization of the cell membrane with high [K+]o. Insulin at a concentration of 40 to 80 microU/ml only produced a sustained increase of [Ca2+]i that was blocked by PN 200-110 or by lowering the extracellular Ca2+ concentration with EGTA. The sustained increase of [Ca2+]i induced by high [K+]o or insulin was insensitive to metabolic inhibitors such as KCN and ouabain as well to the fast Na+ channel blocker, tetrodotoxin and to the increase of intracellular concentrations of cyclic nucleotides. Using the patch clamp technique, insulin did not affect the L-type Ca2+ current and the delayed outward K+ current. These results suggest that the early increase of [Ca2+]i during depolarization of the cell membrane of heart and VSM cells with high [K+]o is due to the opening and decay of an L-type Ca2+ channel. However, the sustained increase of [Ca2+]i during a sustained depolarization is due to the activation of a resting (R) Ca2+ channel that is insensitive to lowering [ATP]i and sensitive to insulin.

Macroscopic and unitary properties of physiological ion flux through L-type Ca2+ channels in guinea-pig heart cells

1. We investigated the currents through L-type Ca2+ channels when Ca2+ (1-10 mM) was the charge carrier, as is the case physiologically. 2. Na+ was removed from both the external and internal solutions to eliminate currents through Na+ channels and Na(+)-Ca2+ exchange. 3. From a holding potential of -50 mV only L-type channels were available to open with depolarization. Macroscopic L-type currents were maximal during depolarizing pulses to +10 mV (peak current density of 4.7 +/- 0.3 nA nF-1). 4. During depolarizing steps as long as 180 ms, the decay of current through L-type channels was incomplete, in contrast to that of T-type current. 5. Unitary currents recorded with comparable ionic conditions and voltage protocols exhibited a single-channel conductance of 6.9 pS in 10 mM Ca2+. Ensemble average currents reproduced accurately the features of whole-cell L-type current, including the maintained component. 6. Convolution analysis was employed to clarify the single-channel basis of the complex current waveform of L-type channels. First openings underlie the peak, while the maintained pedestal is generated by multiple re-openings. As with T-type channels, single openings are brief and contribute little to the time course of the average current. 7. The prominent maintained component of macroscopic and ensemble average L-type current cannot be explained by simple Markov models in which current decay reflects the progressive entry of channels into an absorbing inactivated state. 8. We considered the possibility that the maintained component of current arises from the existence of multiple distinct gating patterns, one of which lacks inactivation. Individual sweeps were sorted among three patterns of gating (no openings, active-early and active-late). Patterns of activity are not randomly distributed; instead, they tend to cluster over time. 9. Most of the maintained current is attributable to the 'active-late' pattern of gating. Considered separately, this pattern can be well described by a simple Markov chain lacking an inactivated state. The 'active-early' gating pattern accounts entirely for the initial current transient, and for about one-third of the maintained component; thus, inactivation, even when present, must be reversible rather than absorbing. 10. The unitary current amplitudes and peak open probabilities measured for single L-type channels, when compared to the average macroscopic L-type current density, predict 170 functional channels per picofarad, or 28,000 L-type channels per typical ventricular myocyte.(ABSTRACT TRUNCATED AT 400 WORDS)

Reconstitution and regulation of an epithelial chloride channel

We have used a monoclonal antibody (MAb E12), one of several such antibodies raised against theophylline-treated Necturus gallbladder epithelial cells, to isolate a chloride channel protein by the use of an immunoaffinity column and FPLC. This protein (M(r) 219,000) has been reconstituted into a planar lipid bilayer, where it behaves as a chloride-selective channel (PCl/PNa = 20.2; PNa/PK = 1) whose unit conductance is 62.4 +/- 4.6 pS. Antibody added to the trans side (there is no effect from the cis side) causes channel open probability to drop to virtually zero, but has no effect on the conductance or the selectivity of single channels. To test the role of phosphorylation in the activity of the native channel, we studied the effects of the protein phosphatase inhibitor okadaic acid (OA) on intact gallbladders, and showed that channels opened by theophylline treatment and closed by antibody are reopened reversibly by OA (0.01-1.0 microM). Addition of the catalytic subunit of protein phosphatase 2A (PP-2A) to the cis side of a bilayer containing reconstituted chloride channels caused closure of the channels after a delay, and subsequent addition of ATP and the catalytic subunit of cAMP-dependent protein kinase (PKA) caused immediate reopening. These data indicate that (a) this chloride channel protein inserts in a directed way into the bilayer such that the cis side is 'intracellular', (b) the purified channel protein is phosphorylated, and (c) gating from the cellular side is controlled by the direct phosphorylation and dephosphorylation of the channel protein.

Background calcium permeable channels in glomerulosa cells from adrenal gland

The cell-attached recording mode of the patch-clamp technique was used to study Ca2+ permeable background currents of glomerulosa cells from rat and bovine adrenal gland. With a pipette filled with 110 mM BaCl2 or 90 mM CaCl2, three different types of unitary currents were detected. The B1 channel demonstrates a nonlinear I-V curve. The conductances are 4 and 7 pS at -40 and -70 mV, respectively. The curve of the opening probability vs. membrane potential is bell shaped with its maximum at -70 mV. The B2 channel has a conductance of 6 pS, while the B3 channel shows a nonlinear I-V relationship with conductances close to 17 and 10 pS at HPs of -60 and -20 mV. The three types of currents are insensitive to dihydropyridines. We suggest that these background currents could be responsible for the basal calcium influx and aldosterone secretion previously observed in nonstimulated glomerulosa cells.