Slow calcium and potassium currents in frog skeletal muscle: their relationship and pharmacologic properties

Calcium Homeostasis, Transporters, and Blockers in Health and Diseases of the Cardiovascular System

Calcium is a highly positively charged ionic species. It regulates all cell types' functions and is an important second messenger that controls and triggers several mechanisms, including membrane stabilization, permeability, contraction, secretion, mitosis, intercellular communications, and in the activation of kinases and gene expression. Therefore, controlling calcium transport and its intracellular homeostasis in physiology leads to the healthy functioning of the biological system. However, abnormal extracellular and intracellular calcium homeostasis leads to cardiovascular, skeletal, immune, secretory diseases, and cancer. Therefore, the pharmacological control of calcium influx directly via calcium channels and exchangers and its outflow via calcium pumps and uptake by the ER/SR are crucial in treating calcium transport remodeling in pathology. Here, we mainly focused on selective calcium transporters and blockers in the cardiovascular system.

Central Nervous System-Toxic Lidocaine Concentrations Unmask L-Type Ca²⁺ Current-Mediated Action Potentials in Rat Thalamocortical Neurons: An In Vitro Mechanism of Action Study

BACKGROUND

High systemic lidocaine concentrations exert well-known toxic effects on the central nervous system (CNS), including seizures, coma, and death. The underlying mechanisms are still largely obscure, and the actions of lidocaine on supraspinal neurons have received comparatively little study. We recently found that lidocaine at clinically neurotoxic concentrations increases excitability mediated by Na-independent, high-threshold (HT) action potential spikes in rat thalamocortical neurons. Our goal in this study was to characterize these spikes and test the hypothesis that they are generated by HT Ca currents, previously implicated in neurotoxicity. We also sought to identify and isolate the specific underlying subtype of Ca current.

METHODS

We investigated the actions of lidocaine in the CNS-toxic concentration range (100 μM-1 mM) on ventrobasal thalamocortical neurons in rat brain slices in vitro, using whole-cell patch-clamp recordings aided by differential interference contrast infrared videomicroscopy. Drugs were bath applied; action potentials were generated using current clamp protocols, and underlying currents were identified and isolated with ion channel blockers and electrolyte substitution.

RESULTS

Lidocaine (100 μM-1 mM) abolished Na-dependent tonic firing in all neurons tested (n = 46). However, in 39 of 46 (85%) neurons, lidocaine unmasked evoked HT action potentials with lower amplitudes and rates of de-/repolarization compared with control. These HT action potentials remained during the application of tetrodotoxin (600 nM), were blocked by Cd (50 μM), and disappeared after superfusion with an extracellular solution deprived of Ca. These features implied that the unmasked potentials were generated by high-voltage-activated Ca channels and not by Na channels. Application of the L-type Ca channel blocker, nifedipine (5 μM), completely blocked the HT potentials, whereas the N-type Ca channel blocker, ω-conotoxin GVIA (1 μM), had little effect.

CONCLUSIONS

At clinically CNS-toxic concentrations, lidocaine unmasked in thalamocortical neurons evoked HT action potentials mediated by the L-type Ca current while substantially suppressing Na-dependent excitability. On the basis of the known role of an increase in intracellular Ca in the pathogenesis of local anesthetic neurotoxicity, this novel action represents a plausible contributing candidate mechanism for lidocaine's CNS toxicity in vivo.

A malignant hyperthermia-inducing mutation in RYR1 (R163C): alterations in Ca2+ entry, release, and retrograde signaling to the DHPR

Bidirectional signaling between the sarcolemmal L-type Ca(2+) channel (1,4-dihydropyridine receptor [DHPR]) and the sarcoplasmic reticulum (SR) Ca(2+) release channel (type 1 ryanodine receptor [RYR1]) of skeletal muscle is essential for excitation-contraction coupling (ECC) and is a well-understood prototype of conformational coupling. Mutations in either channel alter coupling fidelity and with an added pharmacologic stimulus or stress can trigger malignant hyperthermia (MH). In this study, we measured the response of wild-type (WT), heterozygous (Het), or homozygous (Hom) RYR1-R163C knock-in mouse myotubes to maintained K(+) depolarization. The new findings are: (a) For all three genotypes, Ca(2+) transients decay during prolonged depolarization, and this decay is not a consequence of SR depletion or RYR1 inactivation. (b) The R163C mutation retards the decay rate with a rank order WT > Het > Hom. (c) The removal of external Ca(2+) or the addition of Ca(2+) entry blockers (nifedipine, SKF96365, and Ni(2+)) enhanced the rate of decay in all genotypes. (d) When Ca(2+) entry is blocked, the decay rates are slower for Hom and Het than WT, indicating that the rate of inactivation of ECC is affected by the R163C mutation and is genotype dependent (WT > Het > Hom). (e) Reduced ECC inactivation in Het and Hom myotubes was shown directly using two identical K(+) depolarizations separated by varying time intervals. These data suggest that conformational changes induced by the R163C MH mutation alter the retrograde signal that is sent from RYR1 to the DHPR, delaying the inactivation of the DHPR voltage sensor.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.

A multiply charged tetracaine derivative blocks cyclic nucleotide-gated channels at subnanomolar concentrations

Cyclic nucleotide-gated (CNG) ion channels are central participants in sensory transduction, generating the electrical response to light in retinal photoreceptors and to odorants in olfactory receptors. They are expressed in many other tissues where their specific roles in signaling remain unclear. As is true for many other ion channels, there is a paucity of specific blockers needed to dissect the contributions of these channels to cell signaling. CNG channels are members of the superfamily of voltage-gated ion channels, and the local anesthetic tetracaine is known to block CNG channels in a manner that resembles the block of voltage-gated Na(+) channels. The amine in local anesthetics interacts with the charged selectivity filter of Na(+) channels, while the aromatic ring gets stuck in the inner cavity and has hydrophobic interactions with the residues lining that region. Here we have synthesized a derivative of tetracaine, 3-[(aminopropyl)amino]-N,N-dimethyl-N-(2-[[4-(butylamino)benzoyl]oxy]ethyl)propan-1-aminium acetate (APPA-tetracaine), that contains three positively charged amines at physiological pH instead of one. This compound blocked several different CNG channels in the picomolar to nanomolar concentration range at positive membrane potentials, making it several orders of magnitude more potent than tetracaine. In contrast, significant block of Na(+) channels by APPA-tetracaine required concentrations of hundreds of nanomolar. The results suggest that the highly charged moiety of APPA-tetracaine interacts strongly with the negative charge cluster in the selectivity filter of CNG channels. We propose that a variety of potent and specific ion channel blockers could be generated by expanding on traditional blocker structures to target the selectivity filters of other channels.

Biphasic effects of oxethazaine, a topical anesthetic, on the intracellular Ca(2+) concentration of PC12 cells

There have been few reports on the mechanism(s) of action of oxethazaine (OXZ) despite its potent local anesthetic action. Generally, local anesthetics (LAs) not only inhibit Na(+) channels but also affect various membrane functions. In the present study, using PC12 cells as a nerve cell model, the effects of OXZ on intracellular Ca(2+) concentration ([Ca(2+)](i)) were examined in relation to cytotoxicity and dopamine release. [Ca(2+)](i) was determined by the quin2 method. In resting cells, (6-10)x10(-5)M OXZ produced lactate dehydrogenase leakage, which was Ca(2+)-dependent, inhibited by metal Ca(2+) channel blockers, and preceded by a marked increase in [Ca(2+)](i). Some other LAs showed no cytotoxicity at these concentrations. In K(+)-depolarized cells, however, lower concentrations of OXZ (10(-6)-10(-7)M), that had no effect on resting [Ca(2+)](i), inhibited both the dopamine release and the increase of [Ca(2+)](i) in parallel. The inhibitory potency against the [Ca(2+)](i) increase was in the order of nifedipine>OXZ approximately verapamil>diltiazem, and OXZ acted additively on the Ca(2+) channel blockers. OXZ showed the least effect on K(+)-depolarization as determined by bisoxonol uptake. OXZ also inhibited the increase in [Ca(2+)](i) induced by S(-)-BAY K 8644, a Ca(2+) channel agonist. These observations suggested that low concentrations of OXZ interact with L-type Ca(2+) channels. The biphasic effects of OXZ on Ca(2+) movement may be due to a unique chemical structure, and may participate in and complicate the understanding of the potent pharmacological and toxicological actions of OXZ.

Antinociceptive interaction between spinal clonidine and lidocaine in the rat formalin test: an isobolographic analysis

Clinical and basic science studies suggest that spinal alpha-2-adrenergic receptor agonists and local anesthetics produce analgesia, but interaction between alpha-2-adrenergic receptor agonists and local anesthetics in the persistent pain model has not been examined. In the present study, using isobolographic analysis, we investigated the antinociceptive interaction of intrathecal clonidine and lidocaine in the rat formalin test. Sprague-Dawley rats were implanted with chronic lumbar intrathecal catheters, and were tested for paw flinch by formalin injection. Biphasic painful behavior was counted. Intrathecal clonidine (3-12 nmol) was administered 15 min before formalin, and intrathecal lidocaine (375-1850 nmol) was administered 5 min before formalin. To examine the interaction of intrathecal clonidine and lidocaine, an isobolographic design was used. Spinal administration of clonidine produced dose-dependent suppression of the biphasic responses in the formalin test. Spinal lidocaine resulted in dose-dependent transient motor dysfunction and the motor dysfunction recovered to normal at 10-15 min after administration. Spinal lidocaine produced dose-dependent suppression of phase-2 activity in the formalin test. Isobolographic analysis showed that the combination of intrathecal clonidine and lidocaine synergistically reduced Phase-2 activity. We conclude that intrathecal clonidine synergistically interacts with lidocaine in reducing the nociceptive response in the formalin test.

IMPLICATIONS

Preformalin administration of intrathecal clonidine and lidocaine dose-dependently produced antinociception in the formalin test. The combination of clonidine and lidocaine, synergistically produced suppression of nociceptive response in the persistent pain model.

Isobolographic analysis of interaction between spinal endomorphin-1, a newly isolated endogenous opioid peptide, and lidocaine in the rat formalin test

Endomorphin-1, a newly isolated endogenous opioid ligand, has a potential affinity with mu-opioid receptor. We investigated antinociception of intrathecal endomorphin-1 and lidocaine in the rat formalin test and examined the interaction between the two agents using isobolographic analysis. Intrathecal endomorphin-1 caused dose-dependent suppression of the formalin-induced biphasic behavioral response. Intrathecal lidocaine produced dose-dependent inhibition of phase-2 behavioral response. Isobolographic analysis confirmed that combination of intrathecal endomorphin-1 and lidocaine, given at a fixed dose ratio, produced synergistic suppression of phase-2 behavioral response. These data demonstrate that spinal endomorphin-1 synergistically interacts with local anesthetic lidocaine in producing antinociception in the formalin test.

Inhibition by local anesthetics of Ca2+ channels in rat anterior pituitary cells

The characteristics of local anesthetic inhibition of voltage-dependent Ca2+ channels in a rat pituitary clonal cell line were investigated by whole-cell voltage clamp and compared with inhibition by the dihydropyridine Ca2+ channel antagonist, nicardipine. With extracellular Ba2+ (10 mM) as the current carrier, depolarization above -40 mV evoked a slowly inactivating I(Ba). Extracellularly applied lidocaine inhibited I(Ba) without changing the activation threshold, the voltage of peak current, or the reversal potential. Inhibition was greater at a holding potential of -60 mV (IC50 = 1.2 mM) than at -80 mV (IC50 = 2.6 mM). This depolarization-induced potentiation in I(Ba) inhibition developed over 0.1-10 s after membrane depolarization began. Nicardipine also dose-dependently inhibited I(Ba) with an IC50 = 90 nM (at a holding potential = -80 mV). Both lidocaine and nicardipine shifted the I(Ba) steady-state inactivation (availability) curves to the left. Double-pulse protocols revealed that lidocaine (1 mM) accelerated the depolarization-induced inhibition (inactivation) of I(Ba) over the rate in drug-free solutions, but had no effect on the hyperpolarization-induced removal of channel inactivation. Nicardipine also accelerated the depolarization-induced inactivation of I(Ba) but, in addition, it slowed the hyperpolarization-induced inactivation removal. The relative inhibitory action of lidocaine in suppressing I(Ba) was unchanged in the presence of nicardipine. These results suggest that lidocaine has a direct action on membrane Ca2+ channels, similar to the voltage-dependent action of dihydropyridine, but acting at a separate and independent site.

Calcium release by diltiazem from isolated sarcoplasmic reticulum of rabbit skeletal muscle

1. The effect of diltiazem on isolated sarcoplasmic reticulum (SR) from rabbit skeletal muscle was studied. To observe calcium movement into and out of the SR, a fluorescent chelate probe technique with chlortetracycline (CTC) as a reagent was employed. 2. Tris-ATP-induced calcium accumulation by the isolated SR was associated with a rise in the CTC fluorescence. The effect of ATP was dose dependent. 3. Diltiazem (6 x 10(-4)M, 2 x 10(-3)M) prevented ATP-induced calcium accumulation by the SR. 4. Addition of EGTA to the media chelates external calcium and caused calcium release that can be reversed by further addition of calcium chloride. Similarly diltiazem caused a rapid release of accumulated calcium from the SR, which is not reversed by the addition of calcium chloride. 5. It seems that the effect of diltiazem may be related to SR membrane-bound calcium being available for release.

Pharmacological analysis of heartbeat in Drosophila

Analysis of the mechanisms underlying cardiac excitability can be facilitated greatly by mutations that disrupt ion channels and receptors involved in this excitability. With an extensive repertoire of such mutations, Drosophila provides the best available genetic model for these studies. However, the use of Drosophila for this purpose has been severely handicapped by lack of a suitable preparation of heart and a complete lack of knowledge about the ionic currents that underlie its excitability. We describe a simple preparation to measure heartbeat in Drosophila. This preparation was used to ask if heartbeat in Drosophila is myogenic in origin, and to determine the types of ion channels involved in influencing the heart rate. Tetrodotoxin, even at a high concentration of 40 microM, did not affect heart rate, indicating that heartbeat may be myogenic in origin and that it may not be determined by Na+ channels. Heart rate was affected by PN200-110, verapamil, and diltiazem, which block vertebrate L-type Ca2+ channels. Thus, L-type channels, which contribute to the prolonged plateau of action potentials in vertebrate heart, may play a role in Drosophila cardiac excitability. It also suggests that Drosophila heart is subject to a similar intervention by organic Ca2+ channel blockers as the vertebrate heart. A role for K+ currents in the function of Drosophila heart was suggested by an effect of tetraethylammonium, which blocks all the four identified K+ currents in the larval body wall muscles, and quinidine, which blocks the delayed rectifier K+ current in these muscles. The preparation described here also provides an extremely simple method for identifying mutations that affect heart rate. Such mutations and pharmacological agents will be very useful for analyzing molecular components of cardiac excitability in Drosophila.

Effects of verapamil on spinal anesthesia with local anesthetics

The primary mode of action of local anesthetics is through sodium channel and axonal conduction blockade. Local anesthetics also have extensive effects on presynaptic calcium channels that must function to stimulate the release of neurotransmitters. Thus, interference with calcium channel conductance may enhance spinal anesthesia with local anesthetics. The present study was designed to investigate the effects of the intrathecal calcium channel blocker, verapamil, on the spinal anesthesia from lidocaine and tetracaine. Male Sprague-Dawley rats were chronically implanted with lumbar intrathecal catheters. Tail-flick (TF) and mechanical paw pressure (MPP) tests were used to assess thermal and mechanical nociceptive threshold, respectively. Motor function was assessed using a modified Langerman's scale. Intrathecal lidocaine or tetracaine alone showed the prolongation of TF latency, the increase of MPP threshold, and the increase in motor function scale in a time- and dose-dependent manner. Although intrathecal verapamil alone demonstrated neither sensory nor motor block at the doses used (50-200 micrograms), the combination of lidocaine (20, 50, 100, or 200 micrograms) or tetracaine (10, 20, 50, or 100 micrograms) and verapamil (50 micrograms) produced the more potent and prolonged antinociception and motor block when compared with local anesthetics alone. We interpreted these results to indicate that the intrathecal calcium channel blocker, verapamil, potentiates spinal anesthesia with local anesthetics.

Regulation of resting ionic conductances in frog skeletal muscle

The membrane electrical properties and resting ionic conductances of frog semitendinosus muscle fibres were studied in vitro at 25 degrees C with the two-micro-electrode cable technique, in the presence of an activator or inhibitor of protein kinase C (PKC) or in the presence of an activator of adenylate cyclase. The PKC activator, 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB), reduced chloride conductance (GCl) at concentrations greater than 1 microM and did not affect potassium conductance (GK). At 150 microM, the maximum concentration of 4 beta-PDB tested, GCl was reduced by 42%. The "inactive" phorbol ester 4 alpha-phorbol 12,13-dibutyrate did not affect GCl or GK. The inhibitory effect of 4 beta-PDB on GCl was prevented by pretreatment of the muscle preparation with the PKC inhibitor staurosporine. The adenylate cyclase activator forskolin (1.5-8 microM) significantly increased the GK of the fibres, without affecting GCl. Thus, we conclude that frog skeletal muscle GCl, unlike rat muscle GCl, is relatively insensitive to activators of PKC. Moreover, in frog muscle, protein kinase A is a likely modulator of GK, but not GCl.

Involvement of sarcoplasmic reticulum 'Ca2+ release channels' in excitation-contraction coupling in vertebrate skeletal muscle

1. Pharmacological blockers of calcium-induced calcium release from isolated skeletal sarcoplasmic reticulum (SR) vesicles have been introduced into frog skeletal muscle fibres to determine their effects on excitation-contraction coupling. 2. Among the blockers tested, Ruthenium Red, neomycin, gentamicin and 9-aminoacridine inhibited the SR Ca2+ release associated with excitation-contraction (E-C) coupling as much as they inhibited caffeine potentiation of that release. Protamine, certain of its derivatives, and spermine were ineffective in both in situ tests. 3. Alternative sites of polyamine action on the contractile proteins, SR Ca2+ uptake or charge movements were ruled out. 4. All polyamines tested required considerably higher concentrations to inhibit excitation-contraction coupling than to block Ca2+ release from isolated SR vesicles. 5. The quantitative pharmacological difference in sensitivity between isolated and intact systems serves as a reminder that results on isolated systems cannot generally be used to predict results of the same substances on more physiological systems. 6. Since caffeine is known to open the SR 'Ca2+ release channels' (the ryanodine receptors that mediate Ca(2+)-induced Ca2+ release), the equal effectiveness of these blockers at inhibiting excitation-contraction (E-C) coupling and its potentiation by caffeine suggests that the SR 'Ca2+ release channels' are indeed involved in excitation-concentration coupling in skeletal muscle, although the results do not indicate how the channel is gated open during E-C coupling.

The devapamil-binding site of the purified skeletal muscle receptor for organic-calcium channel blockers is modulated by micromolar and millimolar concentrations of Ca2+

The interaction of 2,7-dimethyl-3-(3,4-dimethoxyphenyl)-3-cyan-7-aza-9-(3- methoxyphenyl) nonahydrochloride (devapamil), a stereospecific analog of (3-[2-(3,4-dimethoxyphenyl)ethyl]- methylaminopropyl-3,4-dimethoxy-(1-methylethyl)benzeneacetonitr ile (verapamil), with the purified skeletal muscle receptor for calcium channel blockers (CaCB) was studied at 4 degrees C and 30 degrees C in the absence and presence of calcium. The purified CaCB receptor bound 0.9 mol devapamil/mol calcium-channel alpha 1 subunit, with an apparent Kd of 13 +/- 2.6 nM at 4 degrees C in the presence of 0.4 microM Ca2+. The affinity, and not the density, of the devapamil-binding site was decreased by lowering the pH from 8.5-6.5, or by increasing the Ca2+ concentration from 0.4 microM to 100 mM. The same results were obtained at 30 degrees C, although the ligand-receptor complex was not stable at Ca2+ concentrations below 10 microM. These binding data were confirmed by kinetic experiments. The rate constants calculated for a pseudo-first-order and a second-order reactions were identical and yielded fourfold lower k-1/k+1 (KD) values than the equilibrium experiments performed using 1 nM and 0.4 microM Ca2+, but the same values using 1 mM Ca2+. 1 mM Ca2+ increased the k-1/k+1 (KD) by decreasing 10-fold the association rate at 4 degrees C. The dissociation rate was increased about 10-fold by 5 microM devapamil or 100 microM D-cis-diltiazem, suggesting that the high affinity site is negatively regulated allosterically by millimolar Ca2+ concentrations and by the occupation of a second low-affinity site. Incubation of the CaCB receptors in the absence of Ca2+ and devapamil at 30 degrees C, but not at 4 degrees C, resulted in an apparent loss of devapamil-binding sites. The decrease in binding sites was caused by a reduced affinity. This apparent loss of binding sites was prevented by the addition of Ca2+ with an apparent median effective concentration of 0.4 microM. The apparent half-maximal inactivation times of the devapamil-binding site were 90 s and 12 min in the presence of 1 nM and 0.4 microM Ca2+, respectively. These results show that micromolar Ca2+ concentrations stabilize the CaCB receptor in a conformation which allows high-affinity binding of phenylalkylamines. Millimolar Ca2+ concentrations induce a low-affinity state of the devapamil-binding site on a stable CaCB receptor.

A molecular blueprint for the pore-forming structure of voltage-gated calcium channels

A protein that imitates the sequence of a highly conserved segment predicted to line the pore of dihydropyridine-sensitive L-type calcium channels was designed and synthesized. Single-channel conductance properties were studied in planar lipid bilayers. The synthetic protein emulates the ionic conductance, ionic selectivity, and pharmacological properties of the authentic calcium channel, including the stereospecific action of agonist and antagonist enantiomers of the dihydropyridine BayK 8644. The identified sequence is identical in L-type calcium channels from skeletal muscle and isoforms of cardiac muscle, brain, and aorta. It is plausible that this structural motif represents the molecular blueprint for the pore-forming structure of voltage-gated calcium channels.

Selective potentiation of subtetanic and tetanic contractions by the calcium-channel antagonist nifedipine in the rat diaphragm preparation

1. Nifedipine (1.5-3.0 x 10(-5) M) potentiated the (sub)tetanic tension during 10-50 Hz indirect or direct stimulation of the rat diaphragm preparation; the twitch contractions were not potentiated. 2. The effect was antagonized in high Ca2+ (5-10 x normal) solutions. 3. A comparison with the twitch potentiators caffeine (1.0 x 10(-3) M), quinine (1.4 x 10(-5) M) and phenytoin (2.0 x 10(-5) M), showed that only phenytoin, a putative Ca-antagonist, caused a nifedipine-like frequency-dependent potentiation, indicating a Ca-antagonistic rather than an unspecific effect. 4. A similar (sub)tetanic potentiation was found in a K(+)-free solution. 5. The slow development of the potentiation during repetitive stimulation is in accordance with an effect on the slow Ca channels known to be present in mammalian skeletal muscle. 6. A delay of the fatigue-inducing accumulation of K+ in the T tubules, which may occur during a nifedipine-induced reduction of a Ca2(+)-stimulated K+ efflux, as well as in a K(+)-free solution, may explain the effect.

Inositol 1,4,5-trisphosphate may regulate rat brain Cai++ by inhibiting membrane bound Na(+)-Ca++ exchanger

The role of inositol 1,4,5-trisphosphate (1,4,5-IP3) in regulating cytosolic Ca++ by stimulating Ca++ release from intracellular organelles is well established. However, other modes of intracellular Ca++ regulation by 1,4,5-IP3 have not been determined. To determine if 1,4,5-IP3 may regulate cell cytosolic Ca++ by acting on plasma membrane bound Na(+)-Ca++ exchanger, we investigated Ca++ transport in synaptosomes using 45Ca++ as tracer. In the presence of either an inhibitor of voltage gated Na+ channels (tetrodotoxin) or the K+ ionophore (valinomycin), Ca++ uptake was significantly inhibited (P less than 0.05) by 1,4,5-IP3 in a concentration dependent manner, with half-maximal inhibition occurring at submicromolar concentrations between 10(-9) M and 10(-10) M 1,4,5-IP3. Similarly, Ca++ efflux by the exchanger was significantly inhibited 40% by 1,4,5-IP3. The inhibitory effect of 1,4,5-IP3 on the Na(+)-Ca++ exchanger was observed in the presence of Ca++ channel blockers, and in vesicles pretreated with caffeine to deplete the 1,4,5-IP3-sensitive stores of Ca++. These results suggest that during signal transduction in brain, 1,4,5-IP3 may increase cytosolic [Ca++] in part by inhibiting the Na(+)-Ca++ exchanger and thus, Ca++ efflux from cell.

Internal and external effects of dihydropyridines in the calcium channel of skeletal muscle

The agonist effect of the dihydropyridine (DHP) (-)Bay K 8644 and the inhibitory effects of nine antagonist DHPs were studied at a constant membrane potential of 0 mV in Ca channels of skeletal muscle transverse tubules incorporated into planar lipid bilayers. Four phenylalkylamines (verapamil, D600, D575, and D890) and d-cis-diltiazem were also tested. In Ca channels activated by 1 microM Bay K 8644, the antagonists nifedipine, nitrendipine, PN200-110, nimodipine, and pure enantiomer antagonists (+)nimodipine, (-)nimodipine, (+)Bay K 8644, inhibited activity in the concentration range of 10 nM to 10 microM. Effective doses (ED50) were 2 to 10 times higher when HDPs were added to the internal side than when added to the external side. This sidedness arises from different structure-activity relationships for DHPs on both sides of the Ca channel since the ranking potency of DHPs is PN200-110 greater than (-)nimodipine greater than nifedipine approximately S207-180 on the external side while PN200-110 greater than S207-180 greater than nifedipine approximately (-)nimodipine on the internal side. A comparison of ED50's for inhibition of single channels by DHPs added to the external side and ED50's for displacement of [3H]PN200-110 bound to the DHP receptor, revealed a good quantitative agreement. However, internal ED50's of channels were consistently higher than radioligand binding affinities by up to two orders of magnitude. Evidently, Ca channels of skeletal muscle are functionally coupled to two DHP receptor sites on opposite sides of the membrane.

Competitive action of divalent cations and D600 in frog slow muscle fibers

Single, slow muscle fibers from Rana temporaria were equilibrated in normal Ringer's. 95 mmol/liter K(+)-solution containing various concentrations of Ca2+, Ni2+, Mn2+ or Mg2+ was applied, and the ensuing contractures were recorded isometrically. While peak tension (Fmax) was little affected, maintained tension (measured 1 min after onset of contracture) strongly depended on the concentration and species of divalent cations. Tension was maintained at its peak value in the presence of all species of divalent cations provided their concentrations were adequately increased. Dose-response curves were hyperbolic; Lineweaver-Burk plots revealed straight lines with different slopes intersecting near 1/Fmax, and indicating the following order of efficiency: Ni2+ greater than Ca2+ greater than Mn2+ much greater than Mg2+. Hill plots for these cations resulted in straight lines with slopes near 1. Qualitatively similar relationships were obtained with contracture solutions containing D600 (3-12 mumol/liter). However, under these conditions higher concentrations of Ca2+ or Ni2+ were required in order to fully maintain tension. After a step concentration change in the medium during contracture, the effects of Ca2+ or D600 were detectable only after a delay of 9 and 18 sec, respectively. It is concluded that divalent cations and D600 compete for the same binding site according to a 1:1 reaction. This site is presumably located inside the transverse tubular system and controls inactivation of the contractile force.

Activation of purified calcium channels by stoichiometric protein phosphorylation

Purified dihydropyridine-sensitive calcium channels from rabbit skeletal muscle were reconstituted into phosphatidylcholine vesicles to evaluate the effect of phosphorylation by cyclic AMP-dependent protein kinase (PK-A) on their function. Both the rate and extent of 45Ca2+ uptake into vesicles containing reconstituted calcium channels were increased severalfold after incubation with ATP and PK-A. The degree of stimulation of 45Ca2+ uptake was linearly proportional to the extent of phosphorylation of the alpha 1 and beta subunits of the calcium channel up to a stoichiometry of approximately 1 mol of phosphate incorporated into each subunit. The calcium channels activated by phosphorylation were determined to be incorporated into the reconstituted vesicles in the inside-out orientation and were completely inhibited by low concentrations of dihydropyridines, phenylalkylamines, Cd2+, Ni2+, and Mg2+. The results demonstrate a direct relationship between PK-A-catalyzed phosphorylation of the alpha 1 and beta subunits of the purified calcium channel and activation of the ion conductance activity of the dihydropyridine-sensitive calcium channels.

Dual action (stimulation, inhibition) of D600 on contractility and calcium channels in guinea-pig and cat heart cells

1. We examined the effects of D600 (0.2-40 microM, generally 2 microM) on the following (i) developed tension in guinea-pig papillary muscles, (ii) calcium current (Ica) and tension in cat ventricular muscle strands, (iii) Ica in guinea-pig and cat ventricular myocytes, (iv) single Ca2+ channel currents carried by Ba2+ in cell-attached membrane patches of guinea-pig ventricular myocytes, and (v) Ba2+ currents through dihydropyridine (DHP)-binding sites (skeletal muscle) reconstituted into single functional Ca2+ channels in lipid bilayers. 2. In 27 of 140 preparations studied, D600 elicited a transient stimulation that preceded marked inhibition. The stimulation was normally of short duration (less than 5 min) and moderate strength (less than 50% increase). 3. D600 had no effect on the unit conductance of single cardiac Ca2+ channels. Stimulation was characterized by a decrease in the number of records with no openings (blanks) and an increase in the open-state probability of non-blanks (longer open times, shorter closed times). Inhibition began with an increase in the number of blanks and later included a curtailment of open times and a prolongation of closed times. The net effect after 9 min D600 was a 75% reduction in average current amplitude. 4. A similar pattern of changes in channel open and closed times produced enhancement and then depression of time-averaged open-state probability in single reconstituted channels. 5. Single Ca2+ channel current that was stimulated by adrenaline was only slightly depressed after 2 microM-D600 for 30 min. It may be that channel phosphorylation or Gs-protein activation following beta-receptor stimulation reduces channel affinity for D600. 6. Short-lived binding of D600 to a single inhibitory site may enhance association/activation of Gs-protein and thereby cause transient up-regulation prior to increased drug occupancy and inhibition. Alternatively, there may be separate stimulatory and inhibitory sites. One aspect of inhibition, the increased frequency of blanks, is attributed to a stabilization of the inactivated state; the other aspect, changes in fast kinetics, seems to require a different explanation.

The effect of D600 on potassium contractures of slow muscle fibres of Rana temporaria

(1) The effect of 30 microM D600 on the amplitude and time course of isometric contractures was studied in single slow fibres of Rana temporaria. (2) D600 only slightly reduced the amplitude of contractures evoked with 30 or 95 mM K-Ringer's. Maintenance of tension was strongly impaired by D600 only during exposure to 95 mM K. The caffeine contracture was not affected. (3) Addition of 10 mM Ca2+ or other divalent cations to the medium strongly counteracted the effect of D600 on maintained tension. The order of efficiency was Ca2+ = Ni2+ greater than Co2+ greater than Mn2+ much greater than Mg2+. (4) During 2 min exposure to 95 mM K-Ringer's the slow fibres inactivated to a variable degree; recovery from inactivation in normal Ringer's proceeded with a half time of the order of 1 min, while in the presence of D600 recovery was prolonged 3.3 to 27 times. (5) It is concluded that the effect of D600 on the contractile behaviour of slow fibres from Rana temporaria is predominantly due to a prolongation of the inactivated state. It is suggested that D600 binds to a site at the outer membrane surface which also binds divalent cations and determines the degree of contractile inactivation during exposure to potassium. Blocking of Ca2+ channels is unlikely to be the mechanism of this D600-effect.

Rabbit skeletal muscle microsomes contain two distinct phenylalkylamine-binding sites

Lu49888, a photoaffinity analog of verapamil, was used to identify specific binding sites for phenylalkylamines of calcium channels present in rabbit skeletal muscle microsomes. Direct binding equilibrium measurements and displacement curves of Lu49888 by its non-radioactive analog yielded an apparent single class of binding sites with Kd and Bmax values of 16.5 nM and 7.5 pmol/mg respectively. Lu49888 was specifically incorporated into three proteins of apparently 165 kDa, and 33 kDa. Incorporation into the 55-kDa protein was blocked by 10--50-fold higher concentrations of unlabeled phenylalkylamines compared to incorporation into the 165-kDa protein, suggesting that the 165-kDa and 55-kDa proteins contain a high and a low-affinity verapamil-binding site respectively. The photoaffinity-labeled proteins were solubilized by 1% digitonin or 1% Chaps in roughly equal amounts. The 165-kDa protein bound to wheat-germ-agglutinin(WGA)--Sepharose and sedimented in sucrose density gradients with the same constant as the purified dihydropyridine receptor, which has been reconstituted to a functional calcium channel. The 55-kDa membrane protein did not bind to the WGA-Sepharose column and sedimented in sucrose density gradients with a lower s value than the 165-kDa protein. The 165-kDa but not the 55-kDa membrane protein was specifically labeled by azidopine, the photoaffinity analogue of dihydropyridines. The 55-kDa protein of the purified dihydropyridine receptor was not significantly labeled by Lu49888 showing that the 55-kDa protein of the membrane is unrelated to the purified high-affinity dihydropyridine receptor.

Heterogeneity of calcium channels from a purified dihydropyridine receptor preparation

Dihydropyridine receptors were purified from rabbit skeletal muscle transverse tubule membranes and incorporated into planar lipid bilayers. Calcium channels from both the purified dihydropyridine receptor preparation and the intact transverse tubule membranes exhibited two sizes of unitary currents, corresponding to conductances of 7 +/- 1 pS and 16 +/- 3 pS in 80 mM BaCl2. Both conductance levels were selective for divalent cations over monovalent cations and anions. Cadmium, an inorganic calcium channel blocker, reduced the single channel conductance of calcium channels from the purified preparation. The organic calcium channel antagonist nifedipine reduced the probability of a single channel being open with little effect on the single channel conductance. The presence of two conductance levels in both the intact transverse tubule membranes and the purified dihydropyridine receptor preparation suggests that the calcium channel may have multiple conductance levels or that multiple types of calcium channels with closely related structures are present in transverse tubule membranes.

Bay K 8644 enhances slow inward and outward currents in voltage-clamped frog skeletal muscle fibres

In isolated frog skeletal muscle fibre slow inward calcium current and slow outward potassium current were recorded by means of a double mannitol-gap device. Bay K 8644, the so-called Ca-channel activator, shifted the activation threshold of the slow inward calcium current (recorded in Cl-free, Ca-rich solution), towards negative potential by 15 mV. It increased the peak current amplitude in a dose-dependent manner (from 10(-11) to 10(-7) M; EC50 approximately equal to 10(-9) M). Apamin, the bee venom toxin which is known to specifically block a class of calcium-dependent potassium channels, failed to block the slow inward calcium current and slowed down its declining phase. This effect exhibited a potential dependence: the more the membrane was depolarized, the more the current decay was slowed down. Bay K 8644 (10(-7) M) transiently decreased the slow outward potassium current, which then progressively increased to stabilize at 135% of the control value. This effect seemed to be more pronounced at potentials above the reversal potential for inward ICa. The results suggest that the increase of the slow outward current is due to a direct action of Bay K 8644 on the slow K channel, rather than an indirect action via potentiation of slow inward calcium current. Moreover, results obtained with apamin indicated that the slow outward potassium current is unlikely to flow through Ca-channels.

Purification of a functional receptor for calcium-channel blockers from rabbit skeletal-muscle microsomes

The dihydropyridine receptor was purified from rabbit skeletal muscle microsomes in the presence of [3H]nitrendipine plus diltiazem or [3H](+)PN 200-110 to an apparent density of 1.5-2 nmol binding sites/mg protein. Sodium dodecyl sulfate gel electrophoresis in the absence of reducing agents yielded three peptide bands of 142, 56 and 30 kDa in a relative ratio of 11:1:1.3, whereas in the presence of 40 mM dithiothreitol bands of 142, 122, 56, 31, 26 and 22 kDa were obtained in a relative ratio of 5.5:2.2:1:0.9:14:0.09. This gel pattern was observed regardless of whether the receptor was purified as a complex with nitrendipine plus diltiazem or with (+)PN 200-110. cAMP-dependent protein kinase phosphorylated preferentially the 142-kDa band up to a stoichiometry of 0.82 +/- 0.07 (15) mol phosphate/mol peptide. The 56-kDa band was phosphorylated only in substoichiometric amounts. [3H]PN 200-110 bound at 4 degrees C to one site with apparent Kd and Bmax values of 9.3 +/- 1.7 nM and 2.2 +/- 0.3 (3) nmol/mg protein, respectively. The binding was stereospecific and was not observed in the presence of 1 mM EGTA. Desmethoxyverapamil interfered with the binding of [3H]PN 200-110 in an apparent allosteric manner. (-)Desmethoxyverapamil inhibited the binding of [3H]PN 200-110 at 37 degrees C and stimulated it at 18 degrees C. In agreement with these results, (-)desmethoxyverapamil increased the dissociation rate of [3H]PN 200-110 from 0.29 min-1 to 0.38 min-1 at 37 degrees C and decreased it threefold from 0.046 min-1 to 0.017 min-1 at 18 degrees C. The (+)isomer of desmethoxyverapamil inhibited PN 200-110 binding at all temperatures tested. d-cis-Diltiazem stimulated the binding of [3H]PN 200-110 at 37 degrees C with an apparent EC50 of 1.4 microM and decreased the dissociation rate from 0.29 min-1 to 0.11 min-1. The stimulatory effect of d-cis-diltiazem was temperature-dependent and was seen only at temperatures above 18 degrees C. These results suggest that the purified dihydropyridine receptor retains the basic properties of the membrane-bound receptor and contains separate sites for at least dihydropyridines and phenylalkylamines.

Purified dihydropyridine-binding site from skeletal muscle t-tubules is a functional calcium channel

Many excitable cells contain at least two different voltage-dependent Ca channels (L- and T-type). The cardiac, slow, L-type Ca channel is further modulated by cyclic AMP-dependent phosphorylation, which increases the probability of it being open, and is readily blocked by Ca channel blockers including dihydropyridines and phenylalkylamines. The tritiated congeners of these blockers bind in vitro to sites which have the same pharmacological characteristics as those observed in vivo, that is, stereospecific and allosteric interaction between distinct sites. The dihydropyridine-binding site purified from skeletal muscle t-tubules contains three peptides of relative molecular mass (Mr) 142,000 (142K), 56K and 31K. The cAMP kinase incorporates one mol phosphate per mol of the 142K peptide and binding of (+)PN-200/110, a potent Ca antagonist, is allosterically affected by D-cis-diltiazem and verapamil. The purified dihydropyridine-receptor complex has also been incorporated into phospholipid bilayer membranes. Here, we show for the first time that the complex can be reconstituted to form a functional 20-pS Ca channel that retains the principal regulatory, biochemical and pharmacological properties of membrane-bound L-type Ca channels.

The apamin-sensitive potassium current in frog skeletal muscle: its dependence on the extracellular calcium and sensitivity to calcium channel blockers

Slow outward potassium currents were recorded in isolated frog skeletal muscle fibres using the double mannitol-gap voltage-clamp technique. Detubulated fibres failed to generate a slow outward current, and apamin had no effect on the remaining current. The maximum blocking effect of organic and inorganic Ca2+-channel blockers on the slow outward channels of intact fibres was larger than that of apamin. Apamin failed to induce an additional block when applied after Ca2+-channel blockers. In a low-Ca2+ solution (OCa, EGTA 1 mM) the slow outward current was slightly increased and the blocking effect of apamin was enhanced. A Ca2+-rich solution (Ca2+ X 10) increased the slow outward current and the blocking effect of apamin was drastically reduced. It is concluded that the apamin-sensitive current which is a component of the slow outward K+ current is located in the tubular membrane. Its activation seems barely dependent on the Ca2+ influx via the slow inward Ca2+ current. Apamin-receptor binding appears to be dependent on the extracellular Ca2+ concentration. Blockade of slow outward current by Ca2+-channel blockers is likely to be the result of a direct action on the slow K+ permeability rather than a consequence of Ca2+ channel inhibition.