Selective block of calcium current by lanthanum in single bullfrog atrial cells

A unique inventory of ion transporters poises the Venus flytrap to fast-propagating action potentials and calcium waves

Since the 19^th century, it has been known that the carnivorous Venus flytrap is electrically excitable. Nevertheless, the mechanism and the molecular entities of the flytrap action potential (AP) remain unknown. When entering the electrically excitable stage, the trap expressed a characteristic inventory of ion transporters, among which the increase in glutamate receptor GLR3.6 RNA was most pronounced. Trigger hair stimulation or glutamate application evoked an AP and a cytoplasmic Ca²⁺ transient that both propagated at the same speed from the site of induction along the entire trap lobe surface. A priming Ca²⁺ moiety entering the cytoplasm in the context of the AP was further potentiated by an organelle-localized calcium-induced calcium release (CICR)-like system prolonging the Ca²⁺ signal. While the Ca²⁺ transient persisted, SKOR K⁺ channels and AHA H⁺-ATPases repolarized the AP already. By counting the number of APs and long-lasting Ca²⁺ transients, the trap directs the different steps in the carnivorous plant's hunting cycle. VIDEO ABSTRACT.

The Pathogenic A116V Mutation Enhances Ion-Selective Channel Formation by Prion Protein in Membranes

Prion diseases are a group of fatal neurodegenerative disorders that afflict mammals. Misfolded and aggregated forms of the prion protein (PrP(Sc)) have been associated with many prion diseases. A transmembrane form of PrP favored by the pathogenic mutation A116V is associated with Gerstmann-Sträussler-Scheinker syndrome, but no accumulation of PrP(Sc) is detected. However, the role of the transmembrane form of PrP in pathological processes leading to neuronal death remains unclear. This study reports that the full-length mouse PrP (moPrP) significantly increases the permeability of living cells to K(+), and forms K(+)- and Ca(2+)-selective channels in lipid membranes. Importantly, the pathogenic mutation A116V greatly increases the channel-forming capability of moPrP. The channels thus formed are impermeable to sodium and chloride ions, and are blocked by blockers of voltage-gated ion channels. Hydrogen-deuterium exchange studies coupled with mass spectrometry (HDX-MS) show that upon interaction with lipid, the central hydrophobic region (109-132) of the protein is protected against exchange, making it a good candidate for inserting into the membrane and lining the channel. HDX-MS also shows a dramatic increase in the protein-lipid stoichiometry for A116V moPrP, providing a rationale for its increased channel-forming capability. The results suggest that ion channel formation may be a possible mechanism of PrP-mediated neurodegeneration by the transmembrane forms of PrP.

Live Cell Imaging with R-GECO1 Sheds Light on flg22- and Chitin-Induced Transient [Ca(2+)]cyt Patterns in Arabidopsis

Intracellular Ca(2+) transients are an integral part of the signaling cascade during pathogen-associated molecular pattern (PAMP)-triggered immunity in plants. Yet, our knowledge about the spatial distribution of PAMP-induced Ca(2+) signals is limited. Investigation of cell- and tissue-specific properties of Ca(2+)-dependent signaling processes requires versatile Ca(2+) reporters that are able to extract spatial information from cellular and subcellular structures, as well as from whole tissues over time periods from seconds to hours. Fluorescence-based reporters cover both a broad spatial and temporal range, which makes them ideally suited to study Ca(2+) signaling in living cells. In this study, we compared two fluorescence-based Ca(2+) sensors: the Förster resonance energy transfer (FRET)-based reporter yellow cameleon NES-YC3.6 and the intensity-based sensor R-GECO1. We demonstrate that R-GECO1 exhibits a significantly increased signal change compared with ratiometric NES-YC3.6 in response to several stimuli. Due to its superior sensitivity, R-GECO1 is able to report flg22- and chitin-induced Ca(2+) signals on a cellular scale, which allowed identification of defined [Ca(2+)]cyt oscillations in epidermal and guard cells in response to the fungal elicitor chitin. Moreover, we discovered that flg22- and chitin-induced Ca(2+) signals in the root initiate from the elongation zone.

Fluorescent imaging reports an extracellular alkalinization induced by glutamatergic activation of isolated retinal horizontal cells

Extracellular acidification induced by retinal horizontal cells has been hypothesized to underlie lateral feedback inhibition onto vertebrate photoreceptors. To test this hypothesis, the H(+)-sensitive fluorophore 5-hexadecanoylaminofluorescein (HAF) was used to measure changes in H(+) from horizontal cells isolated from the retina of the catfish. HAF staining conditions were modified to minimize intracellular accumulation of HAF and maximize membrane-associated staining, and ratiometric fluorescent imaging of cells displaying primarily membrane-associated HAF fluorescence was conducted. Challenge of such HAF-labeled cells with glutamate or the ionotropic glutamate receptor agonist kainate produced an increase in the fluorescence ratio, consistent with an alkalinization response of +0.12 pH units and +0.23 pH units, respectively. This alkalinization was blocked by the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the L-type calcium channel blocker nifedipine, and lanthanum. The alkalinization reported by HAF was consistent with extracellular alkalinizations detected in previous studies using self-referencing H(+)-selective microelectrodes. The spatial distribution of the kainate-induced changes in extracellular H(+) was also examined. An overall global alkalinization around the cell was observed, with no obvious signs of discrete centers of acidification. Taken together, these data argue against the hypothesis that glutamatergic-induced efflux of protons from horizontal cells mediates lateral feedback inhibition in the outer retina.

Extracellular pH dynamics of retinal horizontal cells examined using electrochemical and fluorometric methods

Extracellular H(+) has been hypothesized to mediate feedback inhibition from horizontal cells onto vertebrate photoreceptors. According to this hypothesis, depolarization of horizontal cells should induce extracellular acidification adjacent to the cell membrane. Experiments testing this hypothesis have produced conflicting results. Studies examining carp and goldfish horizontal cells loaded with the pH-sensitive dye 5-hexadecanoylaminofluorescein (HAF) reported an extracellular acidification on depolarization by glutamate or potassium. However, investigations using H(+)-selective microelectrodes report an extracellular alkalinization on depolarization of skate and catfish horizontal cells. These studies differed in the species and extracellular pH buffer used and the presence or absence of cobalt. We used both techniques to examine H(+) changes from isolated catfish horizontal cells under identical experimental conditions (1 mM HEPES, no cobalt). HAF fluorescence indicated an acidification response to high extracellular potassium or glutamate. However, a clear extracellular alkalinization was found using H(+)-selective microelectrodes under the same conditions. Confocal microscopy revealed that HAF was not localized exclusively to the extracellular surface, but rather was detected throughout the intracellular compartment. A high degree of colocalization between HAF and the mitochondrion-specific dye MitoTracker was observed. When HAF fluorescence was monitored from optical sections from the center of a cell, glutamate produced an intracellular acidification. These results are consistent with a model in which depolarization allows calcium influx, followed by activation of a Ca(2+)/H(+) plasma membrane ATPase. Our results suggest that HAF is reporting intracellular pH changes and that depolarization of horizontal cells induces an extracellular alkalinization, which may relieve H(+)-mediated inhibition of photoreceptor synaptic transmission.

Effects of Potential Therapeutic Agents on Copper Accumulations in Gill of Crassostrea virginica

Copper is an essential trace element for organisms, but when in excess, copper's redox potential enhances oxyradical formation and increases cellular oxidative stress. Copper is a major pollutant in Jamaica Bay and other aquatic areas. Bivalves are filter feeders that accumulate heavy metals and other pollutants from their environment. Previously it was determined that seed from the bivalve Crassostrea virginica, transplanted from an oyster farm to Jamaica Bay readily accumulated copper and other pollutants into their tissues. In the present study we utilized Atomic Absorption Spectrometry to measure the uptake of copper into C. virginica gill in the presence and absence of three potential copper -blocking agents: diltiazem, lanthanum, and p-aminosalicyclic acid. Diltiazem and lanthanum are known calcium-channel blockers and p-aminosalicylic acid is an anti-infammarory agent with possible metal chelating properties. We also used the DMAB-Rhodanine histochemistry staining technique to confirm that copper was entering gill cells. Our result showed that diltiazem and p-aminosalicyclic acid reduced copper accumulations in the gill, while lanthanum did not. DMAB-Rhodanine histochemistry showed enhanced cellular copper staining in copper-treated samples and further demonstrated that diltiazem was able to reduce copper uptake. The accumulation of copper into oyster gill and its potential toxic effects could be of physiological significance to the growth and long term health of oysters and other marine animals living in a copper polluted environment. Identifying agents that block cellular copper uptake will further the understanding of metal transport mechanisms and may be beneficial in the therapeutic treatment of copper toxicity in humans.

The Effects of Copper and Copper Blocking Agents on Gill Mitochondrial O(2) Utilization of Crassostrea virginica

While essential in trace amounts, excess copper is toxic to cells and tissues. Copper is a major aquatic pollutant. Previously, our lab demonstrated that tissues of the bivalve mollusc Crassostrea virginica readily accumulated copper and other metals from their surrounding environment. In this study we showed that O(2) consumption in isolated gill mitochondria of C. virginica was impaired by in vitro copper additions and that copper's deleterious effects on mitochondria respiration could be blocked by the presence of the membrane channel blocker Diltiazam.

Ion flow regulates left-right asymmetry in sea urchin development

The degree of conservation among phyla of early mechanisms that pattern the left-right (LR) axis is poorly understood. Larvae of sea urchins exhibit consistently oriented LR asymmetry. The main part of the adult rudiment is formed from the left coelomic sac of larvae, the left hydrocoel. Although this left preference is conserved among all echinoderm larvae, its mechanism is largely not understood. Using two marker genes, HpNot and HpFoxFQ-like, which are asymmetrically expressed during larval development of the sea urchin Hemicentrotus pulcherrimus, we examined in this study the possibility that the recently discovered ion flux mechanism controls asymmetry in sea urchins as it does in several vertebrate species. Several ion-transporter inhibitors were screened for the ability to alter the expression of the asymmetric marker genes. Blockers of the H(+)/K(+)-ATPase (omeprazole, lansoprazole and SCH28080), as well as a calcium ionophore (A23187), significantly altered the normal sidedness of asymmetric gene expression. Exposure to omeprazole disrupted the consistent asymmetry of adult rudiment formation in larvae. Immuno-detection revealed that H(+)/K(+)-ATPase-like antigens in sea urchin embryos were present through blastula stage and exhibited a striking asymmetry being present in a single blastomere in 32-cell embryos. These results suggest that, as in vertebrates, endogenous spatially-regulated early transport of H(+) and/or K(+), and also of Ca(2+), functions in the establishment of LR asymmetry in sea urchin development.

Induction of contracture and extracellular Ca2+ influx in cardiac muscle by sanguinarine: a study on cardiotoxicity of sanguinarine

In this study, the toxic effect of sanguinarine (SANG) on heart was studied with isolated cardiac muscle strip isolated from Wistar rat. SANG induced positive inotropic action followed by contracture on the left ventricle and both atria strips. In addition, SANG dose-dependently inhibited spontaneous beat of the right atrium. SANG-induced contracture was completely suppressed by pretreatment with La3+ or in a Ca2+ free Tyrode solution containing 2.5 mM EGTA. Incubating isolated cardiomyocytes with SANG enhanced the 45Ca2+ influx, which could be inhibited by pretreatment with La3+. However, the SANG-induced 45Ca2+ influx could not be inhibited by pretreatment with other Ca2+ channel blockers, such as nifedipine, verapamil, diltiazem, nickel and manganese, and amiloride. Although antioxidants can inhibit the SANG-induced lipid peroxidation, they could not prevent the SANG-induced contracture. N-acetylcysteine and dithiothreitol, the sulfhydryl reducing agents, were shown to be effective in preventing the SANG-induced contracture. These data suggested that the SANG-induced contracture is caused by the influx of extracellular Ca2+ through a La3+-sensitive Ca2+ channel.

Relationship between age of menopause and cell-mediated immune hypersensitivity in right- and left-handed women

The aim of this work was to study the relationship between age of menopause and cell-mediated immune hypersensitivity in right- (N = 32) and left-handed (N = 15) women who had experienced menopause after age of 34 at least one year before interview. Age of menopause was higher in right-handed than left-handed women. Cell-mediated immunity was higher in left-handers than right-handers. Hand-preference correlated with age of menopause, but inversely correlated with tuberculin reaction and percentages of CD4+ and CD8+ lymphocytes; age of menopause correlated with cell-mediated immunity. The results suggested that early menopause in left-handed women may be due to a more active immune system, especially cell-mediated immunity.

Blocking effect of lanthanum on delayed-rectifier K+ current in Drosophila neurons

The delayed-rectifier potassium current (IKDR) is important in regulating neuronal excitability. The authors characterized the neurotoxic effect of lanthanum on IKDR. The conventional whole-cell patch-clamp technique was applied to cultured Drosophila neurons derived from embryonic neuroblasts. IKDR was measured from neurons before and after application of 0.1 mM lanthanum to the external saline. IKDR was smaller in the lanthanum-containing saline (441 +/- 57 pA) than in the control saline (680 +/- 35 pA) (p <.001). Activation and inactivation of IKDR were unchanged by lanthanum. Because these results suggest that lanthanum acts as a potent blocker of IKDR, neuronal excitability may be altered during lanthanum neurotoxicity.

Inhibition of neuronal nicotinic acetylcholine receptors by La(3+)

A study was made of the effects of La(3+) on neuronal alpha 2 beta 4 nicotinic acetylcholine receptors expressed in Xenopus oocytes. La(3+) by itself (up to 10 microM) did not elicit significant membrane currents. However, La(3+) reversibly inhibited the ionic currents induced by acetylcholine (IC(50)=13.5+/-4.3 microM). When La(3+) and acetylcholine were simultaneously applied onto an oocyte, the level of inhibition of the acetylcholine response was the same as when the oocyte was first preincubated with La(3+) and then exposed to acetylcholine plus La(3+). In the presence of La(3+), the EC(50) decreased from 43.8+/-6.4 to 26.5+/-5.1 microM, suggesting a small increase in the affinity of acetylcholine for the receptors through a noncompetitive mechanism. The inhibition of acetylcholine response was independent of the membrane potential. From these results we conclude that La(3+) regulates nicotinic receptors, reversibly and noncompetitively, presumably by inhibiting allosterically the receptor through interactions at an external domain of the receptor complex.

Extracellular Ca2+ increases cytosolic free Ca2+ in freshly isolated rat odontoblasts

Recent evidence suggests that extracellular Ca2+ may modulate cell function in mineralized tissue. To determine whether dentinogenic cells, in particular, are sensitive to extracellular Ca2+, fura-2 microfluorometry was used to monitor intracellular calcium levels in odontoblasts freshly isolated from rat incisor. In response to applications of 0.5-4.0 mM extracellular calcium (CaCl2), most odontoblasts (84%; 107/128) showed an increase in intracellular calcium. For the majority of these cells (70%; 75/107), the typical response was biphasic; there was an initial, transient increase in intracellular calcium which reached peak levels within 30-50 s and decayed rapidly, followed by a slower (> 300 s) recovery toward basal levels. In general, the response of these cells to calcium was repeatable and the mean calcium concentration for the half-maximal response was approximately 1.3 mM. This effect could be partially blocked by either 200 microM lanthanum, a nonspecific blocker of Ca2+ channels, or 20 microM dantrolene, a potent inhibitor of Ca2+ release from internal stores. Used in combination, lanthanum, and dantrolene nearly abolished the calcium response completely. In addition, this response was sensitive to the dihydropyridine-sensitive calcium channel blocking agent nicardipine (60 microM), indicating a role for voltage-gated calcium channels during these events. These results show that odontoblasts respond to external calcium through mechanisms involving both influx of external calcium as well as release of calcium from internal stores and suggest a role for extracellular calcium in regulating the function of these cells.

Surface charge and lanthanum block of calcium current in bullfrog sympathetic neurons

The density of surface charge associated with the calcium channel pore was estimated from the effect of extracellular ionic strength on block by La3+. Currents carried by 2 mM Ba2+ were recorded from isolated frog sympathetic neurons by the whole-cell patch-clamp technique. In normal ionic strength (120 mM N-methyl-D-glucamine, NMG), La3+ blocked the current with high affinity (IC50 = 22 nM at 0 mV). La3+ block was relieved by strong depolarization in a time- and voltage-dependent manner. After unblocking, open channels reblocked rapidly at 0 mV, allowing estimation of association and dissociation rates for La3+: k(on) = (7.2 +/- 0.7) x 10(8) M(-1) s(-1), k(off) = 10.0 +/- 0.5 s(-1). To assess surface charge effects, La3+ block was also measured in low ionic strength (12.5 mM NMG) and high ionic strength (250 mM NMG). La3+ block was higher affinity and faster by two- to threefold in 12.5 mM NMG, with little effect of 250 mM NMG. The data could be described by Gouy-Chapman theory with a surface charge density of approximately 1 e-/3000-4000 A2. These results indicate that there is a small but detectable surface charge associated with the pore of voltage-dependent calcium channels.

Different effects of gadolinium on I(KR), I(KS) and I(K1) in guinea-pig isolated ventricular myocytes

1. Using the whole cell configuration of the patch clamp technique, we studied the potential blocking effects of gadolinium (1 microM to 1 mM) on potassium currents: I(KR), I(KS) and I(K1). The study was performed on guinea-pig isolated ventricular myocytes. 2. The background current, I(K1) was insensitive to Gd3+. Thus, we found that no obvious screening of surface charges was visible with concentrations of Gd3+ up to 100 microM. 3. By use of three different protocols: tail currents fitting, analysis of envelope of tails and electrophysiological dissection, we found that I(KR) was the only component of IK that was sensitive to Gd3+. The sensitivity was apparently different depending on the protocol used. 4. Comparison of the results obtained with the different protocols revealed that the rapid component of I(KR) is more sensitive to Gd3+ than the slow one. 5. Of the different protocols used to distinguish between I(KR) and I(KS), the electrophysiological dissection seems to be the more accurate.

The reaction of lanthanide ions with n-doxyl stearic acids and its utilization for the ESR study on the permeability of lipid-bilayer of erythrocyte membrane to gadolinium ions

The reaction of lanthanide ions with n-doxyl stearic acid (nDS) spin labels (n = 5,7,12,16) was investigated by the electron spin resonance technique in aqueous solution. Among the lanthanides, the Gd3+, Tb3+, Tm3+ and Ce3+ ions strongly quenched the ESR signal of spin labels, but the effects of La3+, Eu3+ and Lu3+ are very weak. The quenching effects are featured by: (1) the dependence on the concentration of lanthanide ions; (2) no obvious changes of the ESR line shape in the presence of lanthanide ions; (3) the quenching constant decreases in the order: Gd3+ > Tb3+ > Tm3+ > Ce3+; (4) the quenching effects of lanthanide ions are found to strikingly correlate with their magnetic properties. These findings indicate that the interaction of lanthanide ions with nitroxide oxygen leading to the reduction of ESR signal amplitude is dominated by their magnetic characteristics rather than the coordination effect. By labeling erythrocyte membrane with nDS, n = 5,7,12,16 at different depths, we studied the diffusion of Gd3+ into the lipid-bilayer of erythrocyte membrane by monitoring the reduction processes of the ESR signals of nitroxide spin labels located at different depths of membrane lipid-bilayer after addition of Gd3+. These results revealed that the Gd3+ ions can penetrate into the lipid-bilayer, though the entry rate is slow. It was shown that the Gd3+ ions bind to the membrane and enhance the permeability of extracellular ascorbate into erythrocyte membrane. The transport mechanism of Gd3+ ions through the lipid-bilayer might be involved in the Gd3+ cation-induced pore formation in the surface of membrane.

Substrate effects on sarcolemmal permeability in the normoxic and hypoxic perfused rat heart

OBJECTIVES

Based on the hypothesis that provision of glucose is good and fatty acids are bad for the ischaemic myocardium, the aims of this study were to determine i) the effects of different substrates on sarcolemmal permeability during normoxia, low-flow hypoxia (HLF) and reperfusion, ii) whether increased membrane permeability is associated with ultrastructural damage and increased influx of Ca2+ into cells and iii) whether changes in membrane permeability correlate with myocardial function and high energy phosphate metabolism.

METHODS

The isolated rat heart subjected to HLF was used as model of global ischaemia, and sarcolemmal permeability assessed by release of LDH from and influx of lanthanum and Ca2+ into myocardial tissue. Myocyte structural injury was also evaluated quantitatively, and mechanical activity was monitored throughout the experimental protocol.

RESULTS

Regardless of the substrate used, HLF caused a 80-90% and 20-40% reduction in myocardial oxygen uptake and coronary flow rate, respectively. Palmitate (0.5 mM conjugated to 0.1 mM albumin) or substrate-free perfusion caused ultrastructural damage and loss of normal sarcolemmal integrity during both normoxia and HLF. Although reperfusion reversed injury in some cells, in general, myocytes exhibited myofibrillar contracture, while membrane integrity recovered to some extent, as indicated by reduced lanthanum influx. Intracellular Ca2+ increased significantly upon reperfusion. Mechanical function as well as tissue high energy phosphates were significantly depressed during both HLF and reperfusion. Glucose, on the other hand, protected against ischaemia-induced structural damage and loss of sarcolemmal integrity. Reperfusion in these experiments resulted in almost complete recovery of normal morphology, ultrastructure and sarcolemmal integrity, while intracellular Ca2+ remained unchanged. Mechanical function and tissue high energy phosphates were significantly higher in glucose-perfused hearts than in palmitate-perfused or substrate-free hearts. Glucose was also able to attenuate the harmful effects of palmitate on myocardial ultrastructure, membrane integrity, mechanical function, energy metabolism and prevented Ca2+ overloading during reperfusion.

CONCLUSION

The results provide new evidence for the protective role of glucose during myocardial ischaemia and reperfusion. Although the exact mechanism of the beneficial actions of glucose remains to be established, the results suggest that glycolytic flux and thus glycolytically derived ATP protect against ischaemic damage via preservation of membrane integrity.

Actions of aluminum on voltage-activated calcium channel currents

1. Extracellular and intracellular effects of aluminum (Al) on voltage-activated calcium channel currents (VACCCs) of cultured rat dorsal root ganglion (DRG) neurons were investigated. Al (0.54 to 5.4 micrograms/ml = 20-200 microM) applied extracellularly reduces VACCCs in a concentration-dependent manner. The IC50 was calculated to be 2.3 micrograms/ml (83 microM). All types of VACCCs were similarly reduced by Al treatment. A slight shift of the current-voltage relation to depolarized potentials was observed for higher Al concentrations (> 2 micrograms/ml). The action of Al was found to be use dependent, with little recovery (max. 20%) after wash. 2. The effect of Al was highly pH dependent in the investigated range (pH 6.4 to 7.8). We observed a rightward shift of the concentration-response curve at pH 7.7 (IC50:3.1 micrograms/ml) and a leftward shift at pH 6.4 (IC50:0.56 microgram/ml) compared to the concentration-response curve at pH 7.3. 3. The VACCC declined when 2.7 micrograms/ml Al was added to the internal solution. A steady state was reached within a few minutes. Additional extracellular application of the same concentration lead to an additional decrease of the current. These observations strongly suggest the existence of both intracellular and extracellular accessible binding sites for Al on voltage-activated calcium channels (VACCs). 4. The special characteristics of the action of Al on VACCCs, i.e., the irreversibility, use dependence, and pH dependence, as well as the additional internal binding site may contribute to its neurotoxicity.

Pb2+ reduces voltage- and N-methyl-D-aspartate (NMDA)-activated calcium channel currents

1. While intracellular calcium concentrations are closely regulated, two types of ion channels in neurons allow calcium influx: both voltage-activated and NMDA-activated channels are significantly permeable to calcium. In this study we compare the effects of lead (Pb2+) on currents carried through voltage-activated calcium channels and NMDA-activated channels. 2. Pb2+ reduces voltage-activated calcium channel currents elicited by a voltage jump from -80 to 0 mV at 0.1 to 1 microM, with an IC50 of 0.64 microM and a Hill slope of 1.22. This effect was partially reversible and not voltage dependent. Sodium and potassium currents were relatively unaffected at Pb2+ concentrations sufficient to block calcium channel currents by more than 80%. Pb2+ is, thus, a potent, reversible and selective blocker of voltage-dependent calcium channel currents. 3. A fast reversible and slow irreversible blocking action of Pb2+ was found on NMDA-activated currents. When Pb2+ was applied simultaneously with aspartate and glycine (Asp/Gly), the inward currents were rapidly and reversibly reduced in a dose-dependent manner with a minimum effective concentration below 2 microM and a total blockade (> 80%) with 100 microM Pb2+. The IC50 was approximately 45 microM and the Hill coefficient 1.1. Preincubation with 50 microM Pb2+ resulted in a greater reduction in the response to Asp/Gly/Pb2+. This effect was reversed within 2 to 5 sec of wash. The lack of voltage dependence suggests that Pb2+ does not block the channel but rather alters the binding of agonists. Prolonged superfusion of a cell with the Asp/Gly/Pb(2+)-containing external solution resulted in a slow and irreversible decrease in the Asp/Gly activated current.(ABSTRACT TRUNCATED AT 250 WORDS)

Imipramine blocks rapidly activating and delays slowly activating K+ current activation in guinea pig ventricular myocytes

Imipramine is a tricyclic antidepressant drug that also exhibits antiarrhythmic effects and whose clinical spectrum of activity is similar to that of quinidine. It has been previously demonstrated that imipramine inhibits the aggregate time-dependent outward K+ current (IK). IK is composed of at least two components: a slowly activating La(3+)-resistant delayed rectifying current (IK,s) and a rapidly activating La(3+)-sensitive current (IK,r). To assess the effects of imipramine on IK,r and IK,s, single guinea pig ventricular myocytes were studied using the nystatin-perforated patch-clamp technique in the absence and in the presence of La3+. Imipramine inhibited IK,r and IK,s in a concentration-dependent manner. The effects of imipramine on the aggregate time-dependent outward current were more marked than those on IK,s alone. Thus, 1 mumol/L imipramine decreased the tail currents elicited on return to -30 mV after long depolarizing pulses (5 seconds, from -40 to +50 mV) in the absence and in the presence of La3+ by 27 +/- 4% and 15 +/- 3% (n = 6), respectively. Moreover, the inhibition induced by imipramine was greater after short (0.5-second) pulses than after 5-second depolarizing pulses, both in the absence and in the presence of La3+ (53 +/- 3% and 30 +/- 5%, respectively; n = 6; P < .05). Imipramine did not significantly modify either the activation midpoint or the slope factor of the aggregate IK and IK,s activation curves. The reduction of IK,s by imipramine was voltage dependent and was more marked at negative membrane potentials. In the presence of 1 mumol/L imipramine, the ratio of tail current to time-dependent current remained constant at 0.37 +/- 0.03, regardless of the test pulse duration at +50 mV. Thus, the envelope-of-tails test was satisfied in the presence of 1 mumol/L imipramine, which indicates that imipramine, at this concentration, blocks IK,r. Imipramine (1, 5, and 10 mumol/L) had no effect on the kinetics of the later phase of IK activation but delayed the beginning of the activation of IK,s by 62 +/- 22, 74 +/- 23, and 155 +/- 53 milliseconds in the presence of 1, 5, and 10 mumol/L imipramine, respectively. These results suggest that imipramine preferentially blocks rapidly activating K+ channels. In addition, experiments performed in the presence of 30 mumol/L La3+ suggest that the drug preferentially binds, but maybe not exclusively, to a closed state of the slowly activating K+ channel.

Calcium regulation of skeletal myogenesis. II. Extracellular and cell surface effects

The process of myoblast fusion during skeletal myogenesis is calcium regulated. Suppression of fusion is obtained by lowering medium [Ca2+] and re-initiated by raising medium [Ca2+]. Previously, we showed that such changes in medium [Ca2+] produced concomitant changes in myoblast [Ca2+] and that a critical cellular concentration of calcium must be present in myoblasts for fusion to occur. In this study, we report on further investigations on the relationship between myoblast [Ca2+] and fusion and also present data which suggest that an outer cell surface pool of calcium is involved in the fusion process. Cellular [Ca2+] must reach greater than 0.8 pmoles/cell and the medium [Ca2+] must be greater than 0.2-0.4 mM for myoblast fusion to occur. These conditions do not have a trigger effect on the entire myoblast population; instead, myoblast fusion was a dose-dependent linear response. If medium [Ca2+] was not maintained at 0.9 mM then cellular [Ca2+] decreased below a critical 0.8 pmoles/cell nucleus and fusion ceased. The cell surface pool of calcium was detected with the calcium antagonist lanthanum. A defined culture medium with 0.9 mM Ca2+ was used to maintain cell viability, and to prevent precipitation of medium components and changes in medium pH with La3+ (0.1 mM). La3+ did not enter the myoblasts as detected by electron microscopy, did not inhibit Ca2+ movement into the cells and the cellular [Ca2+] was sufficient to promote myoblast fusion. Under these conditions, myoblast fusion was inhibited. Morphologically, the fusion-suppressed myoblasts resembled those suppressed by lowered medium [Ca2+]. After removal of the fusion-block by washing, the myotubes that formed were equivalent to those present in unaltered cultures. These results suggest that a La3+ displaceable Ca2+ pool exists at the surface of myoblasts which is involved in myoblast fusion. Thus, it appears that myoblast fusion is dependent on the continuous presence of cell surface calcium and an adequate intracellular Ca2+. An influx of calcium alone is not sufficient to promote myotube formation.

Correlations between changes in membrane capacitance induced by changes in ionic environment and the conductance of channels incorporated into bilayer lipid membranes

The action of metal polycations and pH on ionic channels produced in bilayer lipid membranes (BLM) by three different toxins was studied by measuring membrane capacitance and channel conductance. Here, we show that critical concentrations of Cd2+, La3+ or Tb3+ induce complex changes in membrane capacitance. The time course of capacitance changes is similar to the time course of channel blocking by these ions at low concentration. No changes in BLM capacitance or conductance were observed in the range of pH 5.8-9.0. A pH shift from 7.4 to 3-4 or 11-12 induced large changes in BLM capacitance and channel conductance. For all studied channel-forming proteins, the initial capacitance increase preceded the conductance decrease caused by addition of polycations or by a change in pH. A close relationship between membrane lipid packing and ion channel protein is suggested.

Species variants of the IsK protein: differences in kinetics, voltage dependence, and La3+ block of the currents expressed in Xenopus oocytes

We have compared the slowly activating K+ currents (IsK) resulting from the expression of the human, mouse, or rat IsK proteins in Xenopus oocytes, utilizing natural, species-dependent sequence variations to initiate structure-function studies of this channel. Differences were found between the human and rodent currents in their voltage dependence, kinetics, and sensitivity to external La3+. The current/voltage relationships of the human and rat IsK currents differed significantly, with greater depolarizations required for activation of the human channel. The first 30 s of activation during depolarizations to potentials between -10 and +40 mV was best described by a triexponential function for each of the three species variants. The activation rates were, however, significantly faster for the human current than for either of the rodent forms. Similarly, deactivation kinetics were best described as a biexponential decay for each of the species variants but the human currents deactivated more rapidly than the rodent currents. The human and the rodent forms of IsK were also differentially affected by external La3+. Low concentrations (10, 50 microM) rapidly and reversibly reduced the magnitude of the mouse and rat currents during a test depolarization and increased the deactivation rates of the tail currents. In contrast, the magnitude and deactivation rates of the human IsK currents were unaffected by 50 microM La3+.

Voltage gated calcium channel currents of rat dorsal root ganglion (DRG) cells are blocked by Al3+

The effects of the trivalent cation aluminum (Al3+) on voltage activated calcium channel currents were examined. Al3+ blocks sustained and transient components of voltage activated calcium channel currents of cultured rat dorsal root ganglion (DRG) cells. Currents were elicited by voltage jumps from -80 to 0 mV. The channel block was use dependent. Steady state blockade occurred after 1 to 5 min, when opening the channel every 10 s. There was little or no recovery after washing. Threshold concentration was about 20 microM Al3+ and total blockade (> 80%) was obtained at 200 microM Al3+; the IC50 was 83 microM and the Hill number was around 3. The degree of blockade was pH dependent, and increased with H+ concentration. The current-voltage relation frequently shifted to depolarised voltages after applying Al3+. The degree of the shift was a function of Al3+ concentration, but the magnitude differed from cell to cell. In the effective concentration range (< 200 microM Al3+) the effect was quite specific to voltage activated calcium channel currents. Voltage activated potassium and sodium channels were reduced less than 15% by 200 microM Al3+. We conclude that Al3+ is a potent and irreversible blocker of voltage activated calcium channel currents in mammalian neurons.

The role of calcium in the Chlamydomonas reinhardtii mating reaction

The mating reaction of Chlamydomonas reinhardtii entails a rapid series of cell-cell interactions leading to cell fusion. We have demonstrated (Pasquale, S. M., and U. Goodenough. 1987. J. Cell Biol. 105:2279-2293) that cAMP plays a key role in this process: gametic flagellar adhesion elicits a sharp increase in intracellular cAMP, and presentation of dibutyryl-cAMP to unmated gametes elicits all known mating responses. The present study evaluates the role of Ca2+ in this system. We document that the mating-induced increase in cAMP, and hence the mating responses themselves, are blocked by a variety of drugs known to interfere with Ca(2+)-sensitive processes. These data suggest that Ca(2+)-mediated events may couple adhesion to the generation of cAMP. Such events, however, appear to be localized to the flagellar membrane; we find no evidence for the mating-related increase in cytosolic free Ca2+ that has been postulated by others. Indeed, by monitoring the length of the Ca(2+)-sensitive centrin-containing nucleus-basal body connector, we show that cytosolic free Ca2+ levels, if anything, decrease in response to cAMP signaling. We confirm a previous report that Ca2+ levels increase in the mating medium, but document that this represents a response to augmented cAMP levels and not a prelude. Finally, we show that IP3 levels remain constant throughout the mating reaction. These results are discussed in terms of the various signal transduction systems that have now been identified in Chlamydomonas.

Extracellular divalent and trivalent cation effects on sodium current kinetics in single canine cardiac Purkinje cells

1. The effects of the extracellular divalent cations barium, calcium, cadmium, cobalt, magnesium, manganese, nickel and zinc and the trivalent cation lanthanum on macroscopic sodium current (INa) were characterized in enzymatically isolated single canine cardiac Purkinje cells under voltage clamp at 9-14 degrees C. 2. All di(tri)valent cations produced depolarizing shifts in the conductance-voltage relationship. The order of efficacy, taken as the concentration required to produce a 5 mV shift in the mid-point of peak INa conductance, from least to most effective was (mM): Ca2+ (2.97) approximately Mg2+ (2.67) approximately Ba2+ (1.93) > CO2+ (1.02) approximately Mn2+ (0.88) > Ni2+ (0.54) > La3+ (0.095) approximately Cd2+ (0.083) approximately Zn2+ (0.076). 3. Addition of di(tri)valent cations also produced depolarizing shifts in voltage-dependent availability. The order of efficacy from the least to most effective was (mM): Cd2+ (7.70) approximately Mg2+ (6.86) approximately Ba2+ (4.50) > Ca2+ (2.47) approximately CO2+ (1.87) approximately Mn2+ (1.24) approximately Ni2+ (1.20) > Zn2+ (0.300) > La3+ (0.060). 4. The Gouy-Chapman-Stern equations were used to evaluate di(tri)valent cation efficacy in binding to surface charges. Surface charge density was estimated as 0.72 sites nm-2, and it was assumed that Mg2+, the divalent cation that produced the smallest shift, screened but did not bind to surface charges. Based on voltage-dependent availability, KD from lowest to highest affinity were (mM): Ba2+ (2500) > CO2+ (1670) approximately Mn2+ (1430) approximately Ca2+ = Cd2+ = Ni2+ (1200) > Zn2+ (250) > La3+ (30). 5. All di(tri)valent cations also produced a concentration-dependent acceleration of INa tail current relaxation. The addition of Ca2+ and La3+ produced acceleration of tail current relaxations that could be accounted for by the surface charge effects predicted from the shift in voltage-dependent availability. Cd2+, which produced almost no change in voltage-dependent availability, dramatically accelerated tail current relaxation. Zn2+, Ni2+, Mn2+ and CO2+ also produced greater acceleration of tail current relaxation than could be accounted for by surface charge effects. 6. Di(tri)valent cations delayed time to peak INa in a concentration-dependent manner. The time to peak INa-voltage relationship was well described by an exponential plus a constant, and di(tri)valent cations did not affect the slope factor or constant but shifted the relationship in the depolarizing direction. Similar to their effect on tail currents, addition of some di(tri)valent cations produced larger effects on time to peak INa than expected from the shift of voltage-dependent availability.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of extracellular divalent and trivalent cation block of the sodium current in canine cardiac Purkinje cells

1. Single canine cardiac Purkinje cells were internally perfused and voltage clamped with a large-bore perfusion pipette for measurement of sodium ionic current (INa) in the absence and presence of extracellular group IIA divalent cations (Mg2+, Ba2+ and Ca2+), transition divalent cations (CO2+, Mn2+ and Ni2+), group IIB divalent cations (Cd2+ and Zn2+), and the trivalent cation La3+. 2. Open channel block of cardiac INa by external Ca2+, assessed from instantaneous INa-voltage (I-V) relationships, has been well described by a two-barrier, one-well model with a dissociation constant at 0 mV, KB(0), of 37 mM and an electrical distance, z' = delta, of 0.34. At the most negative test potentials there was less block of INa than predicted by the model, but correction of INa for the contribution of Na+ channel gating current (Ig) to the peak current improved the fit by the model. 3. The divalent cations Ba2+, Mg2+, CO2+ and Mn2+ produced voltage-dependent, open channel block of INa, which by the two-barrier, one-well model predicted a similar z' about one-third into the membrane field. The relative efficacy for voltage-dependent block was CO2+ > Mn2+ > Ca2+ > Mg2+ > Ba2+ with respective KB(0)s of 11, 13, 37, 43 and 61 mM. 4. Cd2+, Zn2+ and La3+ produced block of INa at low concentrations that was nearly voltage independent with z' < or = 0.13. Fits of single-site binding curves to peak INa in response to step depolarizations at positive test potentials gave the following apparent KD values: Zn2+ 0.14 mM, Cd2+ 0.27 mM and La3+ 0.50 mM. 5. In the presence of Cd2+, INa tail current relaxations were much faster than could be accounted for by Cd2+ binding to and/or screening of extracellular surface charges. Fits of the data to a model that assumed voltage-dependent open channel block during the tail current relaxations estimated the KB(0) for Cd2+ to be 0.80 mM. 6. Both z' and KB(0) for Ni2+ from fits of the two-barrier, one-well model to instantaneous I-V relationships varied as a function of [Ni2+], consistent with the hypothesis that Ni2+ blocked with similar affinity at a voltage-dependent and a voltage-independent site. At [Ni2+] > or = 5 mM, KB(0) was 7.6 mM and z' was 0.21.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium is necessary for light excitation in barnacle photoreceptors

Illumination of barnacle (Balanus amphitrite) photoreceptors is known to increase the membrane permeability to sodium and Ca2+ ions resulting in a depolarizing receptor potential. In this report, we show that lanthanum (La3+), a known inhibitor of Ca-binding proteins, reversibly eliminates the receptor potential of barnacle photoreceptors when applied to the extracellular space. Similar reversible elimination of the light response was obtained by removing extracellular Ca2+ by application of the calcium chelating agent EGTA. Iontophoretic injection of Ca2+, but not K+ into the cells protected both the transient and the steady-state phases of the receptor potential from elimination by EGTA while only the transient phase was protected in the presence of La3+. The EGTA experiments suggest that internal Ca2+ is necessary for light excitation of barnacle photoreceptors while the La3+ experiments suggest that La(3+)-sensitive inward current is necessary to maintain excitation during prolonged light.

Activation of K+ channels by lanthanum contributes to the block of transmitter release in chick and rat sympathetic neurons

We studied the effects of lanthanum (La3+) on the release of 3H-norepinephrine (3H-NE), intracellular Ca2+ concentration, and voltage clamped Ca2+ and K+ currents in cultured sympathetic neurons. La3+ (0.1 to 10 microM) produced concentration-dependent inhibition of depolarization induced Ca2+ influx and 3H-NE release. La3+ was more potent and more efficacious in blocking 3H-NE release than the Ca(2+)-channel blockers cadmium and verapamil, which never blocked more than 70% of the release. At 3 microM, La3+ produced a complete block of the electrically stimulated rise in intracellular free Ca2+ ([Ca2+]i) in the cell body and the growth cone. The stimulation-evoked release of 3H-NE was also completely blocked by 3 microM La3+. However, 3 microM La3+ produced only a partial block of voltage clamped Ca2+ current (ICa). Following La3+ (10 microM) treatment 3H-NE release could be evoked by high K+ stimulation of neurons which were refractory to electrical stimulation. La3+ (1 microM) increased the hyperpolarization activated, 4-aminopyridine (4-AP) sensitive, transient K+ current (IA) with little effect on the late outward current elicited from depolarized holding potentials. We conclude that the effective block of electrically stimulated 3H-NE release is a result of the unique ability of La3+ to activate a stabilizing, outward K+ current at the same concentration that it blocks inward Ca2+ current.

Lanthanum actions on excitatory amino acid-gated currents and voltage-gated calcium currents in rat dorsal horn neurons

1. The effects of lanthanum ions (La3+) on voltage-gated calcium currents (VGCCs) and excitatory amino acid (EAA)-evoked currents were characterized using cultured or acutely dissociated neurons from the dorsal horn of the rat spinal cord. 2. VGCCs evoked by depolarizing voltage steps were reversibly blocked by La3+ with an apparent log dissociation constant Kd of 163 nM. 3. La3+ antagonism of currents evoked by NMDA was less potent, with an EC50 (half-maximal effective concentration) of 2 microM. The block of NMDA-evoked currents was voltage independent and non-competitive with respect to activation of the NMDA receptor. 4. La3+ had both enhancing and blocking actions on currents evoked by kainate or by quisqualate; concentrations of La3+ between 1 and 100 microM enhanced kainate- and quisqualate-evoked currents, while the currents were blocked by concentrations of La3+ greater than 100 microM. Both the blocking and the enhancing actions of La3+ were independent of membrane potential. 5. An enhancing dose of La3+ shifted the dose-response curve for kainate to lower concentrations of agonist without changing the maximum evoked current, and a similar leftward shift of the quisqualate dose-response curve occurred at non-saturating concentrations of quisqualate. This enhancement might occur either due to increased affinity of the receptor for ligand, or by increased concentration of ligand at the membrane surface; the latter effect could result from a reduction in the membrane surface charge. 6. The divalent cation Zn(2+)-mimicked the effects of La3+ on excitatory amino acid-evoked currents in dorsal horn neurons, but was less potent both as a blocker and as an enhancer. This suggests that La3+ and Zn2+ could act with different potencies at the same site or sites, and that La3+ may be a useful probe for the mechanisms of Zn2+ effects. 7. Since La3+ enhances kainate- and quisqualate-evoked responses at the same concentrations at which it suppresses VGCCs (and NMDA-gated currents), it can be a useful probe for separating VGCC activation from kainate- and quisqualate-induced depolarizations in experiments where voltage clamp is impractical.

Membrane photomodification of cardiac myocytes: potassium and leakage currents

Cardiac myocytes were isolated from the atria of frogs (Rana pipiens) and whole cell potassium (IK) and "leakage" (Ileak) currents were monitored using the patch clamp technique. Cells were photosensitized by exposure to Rose Bengal (0.125-0.5 microM). Illumination produced an exponential decrease in IK, and an increase in Ileak. Current modifications varied with light intensity and sensitizer concentration. IK stabilized when illumination ceased, while Ileak continued to increase at a slower rate after illumination ended. The exponential nature of IK modification suggests that potassium channels are photomodified with single hit kinetics. The stabilization of IK following illumination suggests (1) that the photomodification of the potassium channel does not involve long lasting (minutes) radical chain reactions and (2) that this photomodification is not repaired in the course of a few minutes.

Ultrastructural correlates of ischaemic contracture during global subtotal ischaemia in the rat heart

The development of left ventricular ischaemic contracture and its correlation with ultrastructural and sarcolemmal permeability defects were studied in isolated rat hearts during global subtotal ischaemia. With acetate as substrate the hearts exhibited a rise in diastolic tension after 8-10 min at which time small foci of contracted myocytes were scattered throughout the myocardium. In hearts with 5% of the maximum diastolic tension (termed 5% contracture), the foci were situated predominantly in the subendocardium and papillary muscle. Contracted myocytes in these foci were capable of excluding ionic lanthanum thus demonstrating retention of normal sarcolemmal permeability properties. With 30% contracture ultrastructural damage had spread to the subepicardium and with further contracture there was an associated increase in the number and size of foci in all regions. In these foci, swelling of the tubular sarcolemmal system and occasionally of the sarcoplasmic reticulum appeared to precede myofibrillar contraction. At 50% contracture lanthanum influx into contracted cells became more frequent. Hearts developed full contracture by 15-18 min at which time most myocytes were contracted and retained lanthanum intracellularly. The heterogeneity of the response at a cellular level may offer a possible explanation for the lack of correlation between contracture and tissue ATP. A possible sequence of structural injury leading to impaired calcium homeostasis is also suggested.

Lead inhibits the voltage-activated calcium current of Aplysia neurons

Lead is a potent and reversible inhibitor of the voltage-dependent calcium current of Aplysia neurons in a concentration range (1-1000 microM) similar to that which elicits toxic effects in man. The threshold for inhibition is 1 microM and the dissociation constant about 90 microM. The inhibition is due to reduction of the peak current amplitude, and not due to alteration of the voltage dependence of the activation or inactivation. The effect of lead is specific to the calcium current as the delayed rectifier potassium and the sodium currents are not affected by concentrations of lead (200 microM) which give near-maximal inhibition. The calcium current of Aplysia neurons appears similar to the current flowing through the mammalian L-type calcium channel. This suggests that inhibition of the investigated calcium channel may contribute to the toxic effects of lead in mammals.

Lanthanum mimicks the trp photoreceptor mutant of Drosophila in the blowfly Calliphora

The effect of lanthanum on the light response of blowfly (Calliphora erythrocephala) photoreceptors was studied. The electrophysiological behaviour of the photoreceptors in the presence of La can be summarized as follows: 1. Upon long stimulation the photoreceptors responded with a 'transient receptor potential', i.e. the cells depolarized at the onset of the stimulus and then repolarized to (or below) the resting potential. This effect was dependent on stimulus intensity and occurred only at high intensities. During illumination membrane noise was reduced. 2. The light-induced changes in membrane potential were paralleled by changes in membrane resistance. 3. The time course of the receptor response was slowed down. 4. Light adaptation led to an increase in response latency. 5. The recovery of the receptor response after light adaptation was slowed down. 6. The sensitivity of the receptor cells measured by the response to short light stimuli was reduced. In summary, the electrophysiological behaviour of Calliphora photoreceptors in the presence of La was very similar to that of the photoreceptors of the trp (transient receptor potential) mutant of Drosophila melanogaster. This result suggests that La and trp mutation affect the same cellular processes in the photoreceptors.

Delayed K+ current and external K+ in single cardiac Purkinje cells

The effect of external K+ on the delayed K+ current was investigated in rabbit single Purkinje cells. Whole cell voltage clamp and intracellular dialysis were used. At K+ concentrations less than 1 mM the kinetics of the delayed K+ current were not changed, but the conductance was markedly reduced. This effect was due to a direct change at an extracellular site and not due to secondary changes in intracellular Na+ or Ca2+ concentrations. A rise in intracellular Na+ or Ca2+ rather increased the delayed K+ current. The decrease in the delayed K+ current in low external K+ was absent when the experiments were done in Na+-free solution. It is concluded that external Na+ exerts an inhibitory effect on the conductance of the delayed K+ current.

Existence of two calcium currents recorded at normal calcium concentrations in single frog atrial cells

A low threshold calcium current (ICALT) was found in Cs+-loaded frog atrial cells in addition to the classical (high threshold) calcium current (ICaHT), and was investigated at physiological Ca2+ concentrations using the whole-cell patch-clamp technique. The threshold potentials were approximately -60 mV for ICaLT and -40 mV for ICaHT. The amplitude and time course of ICaLT were almost unaffected by exchanging Ca2+ for Ba2+ or Sr2+, while those of ICaLT were modified. ICaLT was inhibited by Ni2+ (40 x 10(-6) M) but was not affected by Cd2+ (20 x 10(-6) M) while ICaHT was inhibited by Cd2+ and only slightly reduced by Ni2+ at the same concentrations. Co2+ (10(-3) M) inhibited both types of calcium currents while La3+ (5 x 10(-6) M) had a greater blocking effect on ICaHT. ICaLT was neither modified by dihydropyridines (nisoldipine, Bay K) nor by adrenergic agonists (adrenaline, noradrenaline, isoprenaline), in contrast with the effects of these agents on ICaHT. Angiotensin II (40 x 10(-9) M) increased and atrial natriuretic factor (0.1 x 10(-6) M) decreased ICaLT while ICaHT, was not modified by these two substances.

Modulation of the delayed rectifier K+ current by isoprenaline in bull-frog atrial myocytes

1. The effects of isoprenaline (ISO) on the calcium current (ICa) and delayed rectifier K+ current (IK) were examined using a tight-seal whole-cell voltage-clamp technique in single cells from bull-frog atrium to examine the ionic mechanism(s) of catecholamine-induced action potential shape changes. 2. The effects of ISO on the action potential were dose-dependent. Very low doses (5 x 10(-9) M) prolonged the action potential. Higher doses (10(-6) M) of ISO increased the plateau height, but shortened the action potential by accelerating the early repolarization phase. 3. ISO increased IK and ICa in a dose-dependent fashion. Both of these effects were blocked by a beta-receptor antagonist, propranolol (3 x 10(-7) M). In contrast IK1, the inwardly rectifying K+ current, was not changed significantly by ISO. 4. The ISO-induced increase in IK was observed in the presence of CdCl2 (3 x 10(-4) M), indicating that this effect is not due to a Ca2(+)-activated potassium current. 5. The reversal potential of IK in normal Ringer solution (-83 +/- 2 mV) was not significantly changed by ISO. Thus, stimulation of the Na(+)-K+ pump and a consequent hyperpolarizing shift in EK are not responsible for the increase in IK. 6. In the presence of ISO (10(-6) M) the steady-state activation curve (n infinity) for IK was consistently shifted to more negative values (by approximately 10 mV). The activation and deactivation kinetics of IK were also changed by ISO: activation was accelerated, deactivation was slowed. These ISO-induced changes in IK result in an increase in IK at voltages corresponding to the plateau of the action potential. 7. ISO (10(-6) M) increased ICa dramatically, approximately 6-fold at 0 mV. At the same time, the time constant of ICa inactivation decreased significantly (34 +/- 4 ms control; 23 +/- 4 ms ISO). 8. These results confirm that low doses of sympathetic agonists acting via beta-receptors increase ICa. Relatively high doses of beta-receptor agonists increase both ICa and IK, but these two effects appear to be generated by different biophysical mechanisms. 9. These dose-dependent changes in ICa and IK can explain the observed ISO-induced changes in action potential shape. At doses of approximately 10(-8) M ICa is increased, resulting in a more depolarized plateau and small lengthening of the action potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of La3+ influx in ventricular cells by indo-1 fluorescence

We exposed indo-1-loaded cultured embryonic chick ventricular cells to 0.03-1.0 mM extracellular lanthanum concentration ([La3+]o) and simultaneously measured cell contractile motion and the 410/480 nm fluorescence intensity ratio. After exposure to La3+, ventricular cells stopped contracting and relaxed within seconds, and the 410/480 fluorescence ratio increased. The increase in the 410/480 signal was related to [La3+]o but was not affected by short exposures to zero extracellular calcium concentration ([Ca2+]o) or caffeine, suggesting that the fluorescence was not caused by a La3+-induced increase in intracellular calcium concentration ([Ca2+]i) but rather to increased intracellular lanthanum concentration ([La3+]i). In vitro studies confirmed that indo-1 fluorescence was sensitive to La3+. The increase in [La3+]i in 0.1 mM [La3+]o was directly related to intracellular sodium concentration ([Na+]i), suggesting that La3+ entered cells via Na+-La3+ exchange. In contrast to ventricular cells, which have a functionally distinct Na+-Ca2+ exchange system, exposure of indo-1-loaded cultured bovine endothelial cells to La3+ failed to produce an increase in [La3+]i. These results indicate that exposure of ventricular cells to 0.1-1.0 mM [La3+]o results in a [La3+]i greater than 250 nM within 1 min. Therefore, changes in myocardial 45Ca2+ fluxes and contents induced by La3+ cannot be ascribed solely to extracellular La3+ effects.