Reversal of current through calcium channels in dialysed single heart cells

Stevia Nonsweetener Fraction Displays an Insulinotropic Effect Involving Neurotransmission in Pancreatic Islets

Stevia rebaudiana (Bert.) Bertoni besides being a source of noncaloric sweeteners is also an important source of bioactive molecules. Many plant extracts, mostly obtained with ethyl acetate solvent, are rich in polyphenol compounds that present insulinotropic effects. To investigate whether the nonsweetener fraction, which is rich in phenolic compounds isolated from Stevia rebaudiana with the solvent ethyl acetate (EAF), has an insulinotropic effect, including interference at the terminals of the autonomic nervous system of the pancreatic islets of rats. Pancreatic islets were isolated from Wistar rats and incubated with EAF and inhibitory or stimulatory substances of insulin secretion, including cholinergic and adrenergic agonists and antagonists. EAF potentiates glucose-stimulated insulin secretion (GSIS) only in the presence of high glucose and calcium-dependent concentrations. EAF increased muscarinic insulinotropic effects in pancreatic islets, interfering with the muscarinic receptor subfamily M3. Adrenergic inhibitory effects on GSIS were attenuated in the presence of EAF, which interfered with the adrenergic α2 receptor. Results suggest that EAF isolated from stevia leaves is a potential therapy for treating type 2 diabetes mellitus by stimulating insulin secretion only in high glucose concentrations, enhancing parasympathetic signal transduction and inhibiting sympathetic signal transduction in beta cells.

Modeling the response of ON and OFF retinal bipolar cells during electric stimulation

Retinal implants allowing blind people suffering from diseases like retinitis pigmentosa and macular degeneration to regain rudimentary vision are struggling with several obstacles. One of the main problems during external electric stimulation is the co-activation of the ON and OFF pathways which results in mutual impairment. In this study the response of ON and OFF cone retinal bipolar cells during extracellular electric stimulation from the subretinal space was examined. To gain deeper insight into the behavior of these cells sustained L-type and transient T-type calcium channels were integrated in the synaptic terminals of reconstructed 3D morphologies of ON and OFF cone bipolar cells. Intracellular calcium concentration in the synaptic regions of the model neurons was investigated as well since calcium influx is a crucial parameter for cell-to-cell activity between bipolar cells and retinal ganglion cells. It was shown that monophasic stimulation results in significant different calcium concentrations in the synaptic terminals of ON and OFF bipolar cells. Intracellular calcium increased to values up to fourfold higher in the OFF bipolar model neuron in comparison to the ON bipolar cell. Furthermore, geometric properties strongly influence the activation of bipolar cells. Monophasic, biphasic, single and repetitive pulses with similar lengths, amplitudes and polarities were applied to the two model neurons.

Physiological modulation of voltage-dependent inactivation in the cardiac muscle L-type calcium channel: A modelling study

The inactivation of the L-type Ca2+ current is composed of voltage-dependent and calcium-dependent mechanisms. The relative contribution of these processes is still under dispute and the idea that the voltage-dependent inactivation could be subject to further modulation by other physiological processes had been ignored. This study sought to model physiological modulation of inactivation of the current in cardiac ventricular myocytes, based upon the recent detailed experimental data that separated total and voltage-dependent inactivation (VDI) by replacing extracellular Ca2+ with Mg2+ and monitoring L-type Ca2+ channel behaviour by outward K+ current flowing through the channel in the absence of inward current flow. Calcium-dependent inactivation (CDI) was based upon Ca2+ influx and formulated from data that was recorded during beta-adrenergic stimulation of the myocytes. Ca2+ influx and its competition with non-selective monovalent cation permeation were also incorporated into channel permeation in the model. The constructed model could closely reproduce the experimental Ba2+ and Ca2+ current results under basal condition where no beta-stimulation was added after a slight reduction of the development of fast voltage-dependent inactivation with depolarization. The model also predicted that under beta-adrenergic stimulation voltage-dependent inactivation is lost and calcium-dependent inactivation largely compensates it. The developed model thus will be useful to estimate the respective roles of VDI and CDI of L-type Ca2+ channels in various physiological and pathological conditions of the heart which would otherwise be difficult to show experimentally.

Aplysia cardioactive peptide (NdWFamide) enhances the L-type Ca2+ current of Aplysia ventricular myocytes

NdWFamide is a D-amino acid containing tripeptide purified from Aplysia heart. Although the cardioexcitatory action of NdWFamide is well established, little is known about how the excitatory action is induced. To examine the action of the peptide on the ion channels expressed in the Aplysia heart muscles, we carried out whole cell clamp experiments in the isolated Aplysia ventricular myocytes. We found that the high voltage-activated (HVA) Ca(2+) current of Aplysia ventricular myocytes is mostly a nifedipine-sensitive L-type current, and that the current was enhanced by NdWFamide via the activation of G proteins.

Differential effects of organic calcium-channel blockers on diastolic SR calcium-handling in the frog heart

1. Gradual loss of sarcoplasmic reticular (SR) calcium during a rest-period is responsible for the rest-induced decay (RID) of force in mammalian myocardium. Effect of verapamil and diltiazem on a similar RID in the frog myocardium suggests a new mechanism of action of these drugs. 2. Strips of frog-ventricle were paced at 0.2 Hz and the rhythm was interrupted by varying rest-periods ranging from 10 to 180 s. In control conditions, the amplitude of the post-rest beat was significantly lower than that of the pre-rest beat for rest-periods more than 40 s (RID). 3. Verapamil and diltiazem (which are organic calcium-channel blockers (OCCB)) changed the pattern of RID in the control solution to a 'rest-induced potentiation' (RIP) in the same preparation while another OCCB nifedipine and the inorganic calcium-channel blocker cadmium did not alter the post-rest phenomenon. 4. We propose that verapamil and diltiazem produce an RIP due to either blockade of SR calcium-leak during rest or enhancement of SR calcium-uptake during rest.

Ca2+ influx via the L-type Ca2+ channel during tail current and above current reversal potential in ferret ventricular myocytes

1. Current through L-type Ca2+ channels (ICa) was measured electrophysiologically at the same time as Ca2+ influx was measured by trapping entering Ca2+ with a high concentration of indo-1 (> 1 mM) in ferret ventricular myocytes. 2. Na+-free conditions prevented Na+-Ca2+ exchange and K+ currents were blocked by Cs+ and TEA. Thapsigargin (5 microM) prevented Ca2+ uptake and release by the sarcoplasmic reticulum. ICa was pre-activated by brief pulses to +120 mV (the equilibrium potential for Ca2+, ECa), followed by steps to different membrane potentials (Em, -80 to +100 mV), in some cases in the presence of the Ca2+ channel agonist FPL-64176. 3. Integrated ICa ( 82 ICa) was linearly related to the change in the concentration of Ca2+ bound to indo-1, which was assessed by the fluorescence difference signal DeltaFd (Fd = F500 - F400). This created an internal calibration of DeltaFd as a measure of Ca2+ influx. 4. The DeltaFd/ 82 ICadt relationship was virtually unchanged at all measurable inward ICa (at Em from -80 to +50 mV). This indicates that the fractional current carried by Ca2+ and channel selectivity are unchanged over this Em range, and also that the selectivity for Ca2+ is very high. 5. Ca2+ influx was readily detected by DeltaFd beyond the ICa reversal potential (+65 to +100 mV) and was not abolished until Em was +120 mV (i.e. ECa). This is explained by the fact that inward Ca2+ flux at the ICa reversal potential is exactly balanced by outward Cs+ current through the Ca2+ channels and can be described by classic Goldman flux analysis with a Ca2+/Cs+ selectivity of the order of 5000. 6. This result also emphasizes that net Ca2+ influx via Ca2+ channels occurs over a voltage range where the net channel current is outward.

Location of the permeation pathway in the recombinant type 1 inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) forms ligand-regulated intracellular Ca(2+) release channels in the endoplasmic reticulum of all mammalian cells. The InsP(3)R has been suggested to have six transmembrane regions (TMRs) near its carboxyl terminus. A TMR-deletion mutation strategy was applied to define the location of the InsP(3)R pore. Mutant InsP(3)Rs were expressed in COS-1 cells and single channel function was defined in planar lipid bilayers. Mutants having the fifth and sixth TMR (and the interceding lumenal loop), but missing all other TMRs, formed channels with permeation properties similar to wild-type channels (gCs = 284; gCa = 60 pS; P(Ca)/P(Cs) = 6.3). These mutant channels bound InsP(3), but ligand occupancy did not regulate the constitutively open pore (P(o) > 0.80). We propose that a region of 191 amino acids (including the fifth and sixth TMR, residues 2398-2589) near the COOH terminus of the protein forms the InsP(3)R pore. Further, we have produced a constitutively open InsP(3)R pore mutant that is ideal for future site-directed mutagenesis studies of the structure-function relationships that define Ca(2+) permeation through the InsP(3)R channel.

Cs+ inhibits spontaneous Ca2+ release from sarcoplasmic reticulum of skinned cardiac myocytes

The effect of Cs+ on the function of the cardiac sarcoplasmic reticulum (SR) has been investigated in skinned cardiac myocytes. Isolated rat ventricular myocytes were permeabilized using saponin and then perfused with a solution containing 150 nmol/l Ca2+ and 10 micromol/l fura 2. Fura 2 fluorescence from the skinned cell was monitored to assess SR Ca2+ release. The frequency of spontaneous Ca2+ release from the SR decreased when K+ in the bathing solution was completely replaced with Cs+. Cs+ had little effect on the amplitude of spontaneous release but prolonged both the rise time and decay time. The SR Ca2+ content, assessed by application of caffeine, was reduced in the Cs+ solution. Cyclopiazonic acid produced effects similar to those of Cs+. Extracellular Cs+ (20 mmol/l) increased the amplitude of the Ca2+ transient and the SR Ca2+ content in intact field-stimulated cells but had little effect on the Ca2+ transient when the amplitude and duration of depolarization were kept constant using voltage clamp. These data suggest that Cs+ slows Ca2+ movement across the SR membrane, possibly by blocking the SR K+ channel, but has additional effects in intact cells that overcome its inhibitory effects on the SR.

Electrical properties of a cockroach motor neuron soma depend on different characteristics of individual Ca components

Electrical properties of a cockroach motor neuron soma depend on different characteristics of individual Ca components. J. Neurophysiol. 78: 2455-2466, 1997. The "fast" coxal depressor motor neuron (Df) of the cockroach is among the most extensively studied of insect neurons. It has been shown that the cell body of this neuron can exhibit active electrical properties, which may change over time or with chemical modulation. To further understand these electrical events and their modulation, inward currents in Df have been characterized under conditions in which outward currents have been suppressed. The inward current activated at potentials positive to -60 mV and peaked between -10 and 0 mV when measured in barium saline and between 0 and +10 mV when measured in calcium saline. The inward current was insensitive to Ni2+ (100 mu M) but reduced by verapamil (50 mu M) and abolished by Cd2+ (1 mM). Two components of ICa were identified by their sensitivity to either 50 mu M nifedipine or micromolar Cd2+. The nifedipine-sensitive component activated positive to -60 mV and peaked between -10 and 0 mV, whereas the Cd2+-sensitive component activated positive to -40 mV and peaked between +10 and +20 mV. Immediately after dissection, depolarization of Df evoked plateau potentials, whereas 1-4 h after dissection, depolarization evoked action potentials. The plateau potentials were insensitive to 100 mu M Cd2+ but blocked by 50 mu M nifedipine, whereas the spikes required a combination of nifedipine (50 mu M) and Cd2+ (100 mu M) for complete suppression, indicating that only one component of ICa contributes to the plateau potential, whereas both components contribute to action potentials. Currents measured in calcium saline decayed faster than currents measured in barium saline. The inactivation characteristics were investigated with the use of double-pulse voltage-clamp experiments. ICa showed a greater degree of inactivation and slower recovery from inactivation than did IBa. Current decay and the extent of inactivation were reduced after injection of the calcium-chelator 1,2-bis(2-aminophenoxy)ethane-N,N, N',N'-tetraacetic acid (BAPTA). This suggests that the calcium current of this neuron displays calcium-dependent inactivation. An additional mechanism, most probably voltage-dependent inactivation, also occurs because IBa, even in neurons injected with BAPTA, displayed some inactivation. The inactivation characteristics may be important in determining activity displayed by Df. Indirect evidence suggests that intracellular calcium is high immediately after dissection. At this time, the calcium current may therefore be reduced due to calcium-dependent inactivation. This may, at least partly, explain why the cell does not spike shortly after dissection.

Action potentials and underlying voltage-dependent currents studied in cultured spiral ganglion neurons of the postnatal gerbil

The excitability of cultured spiral ganglion (SG) neurons from early postnatal gerbil (P0-P1) was examined with the whole-cell patch-clamp technique. The role of voltage-gated currents in shaping the kinetics of action potentials (APs) was analyzed. Cultured SG neurons displayed spontaneous APs with a low rate (< 0.1 Hz). The kinetics of APs were studied by injecting neurons with current pulses of various frequencies and duration. A single depolarizing pulse of long duration elicited only one AP in most SG neurons. When excited by a train of short current pulses given at rates greater than 50 Hz, the firing pattern displayed an adaptive mechanism with the result that successive APs fired with lower amplitude, broader duration and delayed peak time. Pulse trains of higher frequencies had higher failure rates in initiating APs. Current pulses given at 20 Hz or lower elicited APs that had very similar amplitudes. However, the width of the APs gradually broadened. Duration of APs was also found to be affected by the membrane potential of neurons. Between -75 mV and -55 mV, AP duration was broadened at a rate of about 33% per 10 mV depolarization. Voltage-gated currents that underlie the generation of APs were examined under voltage-clamp conditions. Tetrodotoxin-sensitive sodium currents and dihydropyridine-sensitive L-type calcium currents were found. More importantly, inactivation properties of the potassium current provided a direct explanation for the cumulative broadening of APs. This work demonstrated that SG neurons were able to fire APs long before hearing commences in gerbil. Possible roles of spontaneous APs in the development of the cochlea and the role of voltage-gated currents in the function of SG neurons under normal and pathological conditions are discussed.

Differential sensitivity to intracellular pH among high- and low-threshold Ca2+ currents in isolated rat CA1 neurons

The effects of intracellular pH (pHi) on high-threshold (HVA) and low-threshold (LVA) calcium currents were examined in acutely dissociated rat hippocampal Ca1 neurons with the use of the whole cell patch-clamp technique (21-23 degrees C). Internal pH was manipulated by external exposure to the weak base NH4Cl or in some cases to the weak acid Na-acetate (20 mM) at constant extracellular pH (7.4). Confocal fluorescence measurements using the pH-sensitive dye SNARF-1 in both dialyzed and intact cells confirmed that NH4Cl caused a reversible alkaline shift. However, the external TEA-Cl concentration used during ICa recording was sufficient to abolish cellular acidification upon NH4Cl wash out. With 10 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) in the pipette, NH4Cl exposure reversibly enhanced HVA currents by 29%, whereas exposure to Na-acetate markedly and reversibly depressed HVA Ca currents by 62%. The degree to which NH4Cl enhanced HVA currents was inversely related to the internal HEPES concentration but was unaffected when internal ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) was replaced by equimolar bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). When depolarizing test pulses were applied shortly after break-in (Vh = -100 mV), NH4Cl caused a proportionally greater increase in the sustained current relative to the peak. The dihydropyridine Ca channel antagonist nifedipine (5 microM) blocked nearly all of this sustained current. A slowly inactivating nifedipine-sensitive (L-type) HVA current could be evoked from a depolarized holding potential of -50 mV; NH4Cl enhanced this current by 40 +/- 3% (mean +/- SE) and reversibly shifted the tail-current activation curve by +6-8 mV. L-type currents exhibited more rapid rundown than N-type currents; HVA currents remaining after prolonged cell dialysis, or in the presence of nifedipine, inactivated rapidly and were depressed by omega-conotoxin (GVIA). NH4Cl enhanced these N-type currents by 76 +/- 9%. LVA Ca currents were observed in 32% of the cells and exhibited little if any rundown. These amiloride-sensitive currents activated at voltages negative to -50 mV, were enhanced by extracellular alkalosis and depressed by extracellular acidosis, but were unaffected by exposure to either NH4Cl or NaAC. These results demonstrate that HVA Ca currents in hippocampal CA1 neurons are bidirectionally modulated by internal pH shifts, and that N-type currents are more sensitive to alkaline shifts than are L- or T-type (N > L > T). Our findings strengthen the idea that distinct cellular processes governed by different Ca channels may be subject to selective modulation by uniform shifts in cytosolic pH.

Coupling between charge movement and pore opening in vertebrate neuronal alpha 1E calcium channels

1. Neuronal alpha 1E Ca2+ channels were expressed alone and in combination with the beta 2a subunit in Xenopus laevis oocytes. 2. The properties of ionic and gating currents of alpha 1E were investigated: ionic currents were measured in 10 mM external Ba2+; gating currents were isolated in 2 mM external Co2+. 3. Charge movement preceded channel opening. The charge movement voltage curve (Q(V)) preceded the ionic conductance voltage dependence (G(V)) by approximately 20 mV. 4. Coexpression of alpha 1E with the beta 2a subunit did not modify the voltage dependence of charge movement but shifted the G(V) curve to more negative potentials. The voltage gap between Q(V) and G(V) curves was reduced by the beta 2a subunit and both curves overlapped at potentials near 0 mV. 5. The coupling efficiency between the charge movement and pore opening was estimated by the ration between limiting conductance and maximum charge movement (Gmax/Qmax). Coexpression of the beta 2a subunit increased the Gmax/Qmax ratio from 9.2 x 10(5) +/- 1.4 x 10(5) to 21.9 x 10(5) +/- 2.8 X 10(5) S C-1 for alpha 1E and alpha 1E + beta 2a, respectively. 6. We conclude that in the neuronal alpha 1E the charge movement is tightly coupled with the pore opening and that the beta 2a subunit coexpression further improves this coupling.

Molecular pharmacology of voltage-dependent calcium channels

Voltage-dependent Ca2+ channels serve as the only link to transduce membrane depolarization into cellular Ca(2+)-dependent reactions. A wide variety of chemical substances that have the ability to modulate Ca2+ channels have been demonstrated both for their clinic utility and for importance in elucidating the molecular basis of various biological responses. Recently, introduction of molecular biology to pharmacology has brought a great deal of information about the molecular basis of drug action in Ca2+ channels. In this review, we attempt to overview recent progress in understanding the interactions between Ca2+ channels and their blockers, namely Ca2+ antagonists, from a molecular and structural point of view.

Glutamate receptor-mediated calcium entry in neurons derived from P19 embryonal carcinoma cells

We have examined the control of calcium elevation by glutamate in neurons derived from the mouse P19 embryonal carcinoma cell line. Following transient exposure to retinoic acid, P19 cells differentiate into neurons that express both NMDA and non-NMDA glutamate receptor subtypes. Fluorescence videomicroscopy using the indicator fura-2 revealed concentration-dependent elevation in cytosolic calcium levels with exposure to NMDA or kainate. Replacement of extracellular sodium with N-methylglucamine significantly reduced the action of kainate. Exposure to high K+ medium also elicited an elevation of cytosolic calcium in P19 cells, which was partially inhibited by the calcium channel antagonist nimodipine. These experiments suggest that the elevation in calcium produced by kainate involves the activation of voltage-gated calcium channels as a consequence of membrane depolarization, in contrast to direct calcium entry through NMDA receptor channels. Whole-cell recordings revealed that P19 NMDA receptors were highly permeable to calcium (PCa/PNa = 5.6 +/- 0.2). In most cells, channels gated by kainate displayed low permeability to calcium; the median permeability ratio, PCa/PNa, was 0.053 (range 0.045 to 0.132). Activation of peak currents by NMDA, glycine, and kainate was half-maximal at 24 microM, 240 nM, and 81 microM, respectively. In addition, cadmium-sensitive currents through voltage-gated calcium channels were recorded in P19 cells bathed in barium/TEA chloride. Staining with antibodies directed against AMPA receptor subunits revealed wide-spread immunoreactivity for anti-GluR-B/C and anti-GluR-B/D. About half of the P19 cells were stained with antibodies selective for GluR-D but there was little or no immunoreactivity for the GluR-A subunit.

Depletion of intracellular calcium stores activates an outward potassium current in mast and RBL-1 cells that is correlated with CRAC channel activation

Highly Ca2+ selective Ca2+ channels activated by store depletion have been recently described in several cell types and have been termed CRAC channels (for calcium release-activated calcium). The present study shows that following store depletion in mast and RBL-1 cells, monovalent outward currents could be recorded if the internal solution contained K+ but not Cs+. The activation of the outward K+ current correlated with the activation of ICRAC, in both time and amplitude, suggesting that the K+ current might be carried by CRAC channels. The amplitude of the outward current was increased if external Ca2+ was reduced or replaced by external Ba2+. The outward K+ conductance might have a physiological role in maintaining the driving force for Ca2+ entry during the activation of CRAC channels.

Voltage-gated calcium currents in whole-cell patch-clamped bullfrog dorsal root ganglion cells: effects of cell size and intracellular solutions

Acutely dissociated bullfrog dorsal root ganglion (DRG) cells could be divided into two classes by measurement of cell capacitance. A bimodal distribution of cell capacitance was found and a value of 75 pF was used to divide frog DRG cells into 'small' and 'large' types. Two distinct voltage-activated Ca2+ currents were evoked in both classes of cells: a rapidly inactivating, low-voltage-activated current and a slowly-inactivating, high-voltage-activated current. When the recording pipette contained CsCl, greater peak inward current values and densities were seen in large cells compared to small cells. No significant differences were observed in the distribution of low-and high-voltage-activated currents in small and large cells. Replacement of pipette solutions containing CsCl with solutions containing equimolar concentrations of Cs glutamate, L-arginine Cl, or N-methyl-D-glucamine significantly increased both the reversal potential and the maximum amplitude of the Ca2+ currents in both small and large DRG cells. These increases indicate that internal substitutions with organic ions suppresses outward currents more effectively than does CsCl. In contrast to findings with CsCl, when organic ions were used in the pipette solution a significantly higher proportion of low-threshold Ca2+ channels was observed in small cells compared to large cells. These observations indicate that when organic solutions were used internally, significant differences in the proportion of low-threshold to high-threshold Ca2+ channels were observed in small and large cells. The composition of the internal solution is a critical variable when determining the type and amount of inward Ca2+ current in different types of neurons.

Actions of perezone on rat smooth muscle

1. Perezone (PZN) on the in vitro intestinal smooth muscle of the rat relaxes the basal tonus of the smooth muscle, interrupts spontaneous activity and also blocks the contractile response induced by ACh, K+, and Ba2+; these actions are dose dependent. 2. Although in presence of small doses of PZN, the isometric contractile response to ACh was increased. 3. In calcium free intestinal smooth muscle preparation, the addition of PZN in low dose before Ca2+ increased the contractile effect of added calcium to the bath, but in presence of high doses of PZN the response to calcium was depressed. 4. PZN in calcium free preparations antagonized the contraction caused by adding barium. 5. These findings suggested that with small doses of PZN more availability of intracellular calcium free exist and/or an increase in excitability and/or an inhibition of AChase could coexist. 6. The responses of the intestine to high doses of PZN were possibly in part by blocking calcium entry. 7. The smooth muscle responses to PZN suggest that it has a membranal effect and/or an action on the internal calcium stores possibly increasing the intracellular calcium concentration. It is likely to be the expression of an increase in the intracellular calcium concentration above the levels normally reached that would be responsible for uncoupling of the smooth muscle, which would occur if the [Ca2+]i rises excessively.

Description of a nonselective cation current in human atrium

Ion currents were examined in isolated human atrial myocytes by using the whole-cell patch-clamp technique. When currents were recorded with a K(+)-containing pipette solution, depolarizing voltage pulses elicited a rapidly activating outward current that decayed to an apparent steady state. Exposure of cells to 10 mmol/L 4-aminopyridine markedly reduced current amplitude; however, a rapidly activating current that was approximately 30% of the steady state current amplitude remained. When pipette K+ was replaced with Cs+, a similar rapidly activating current that reversed polarity at approximately 0 mV was recorded. This current was seen in 100% of the cells tested from 17 different hearts (n = 142), and its amplitude was approximately 40% of the amplitude of the steady state current recorded in the presence of pipette K+. The current amplitude was not significantly different in cells isolated from adult (6.31 +/- 1.35 pA/pF, n = 8) and pediatric (5.54 +/- 1.04 pA/pF, n = 9) hearts. Studies designed to determine the charge-carrying species indicated that changes in bath Cl- concentration had no effect on either the amplitude or the reversal potential of this current, whereas removal of pipette Cs+ and bath Na+ dramatically reduced this current. In addition, this current was not modulated by either isoproterenol (1 mumol/L, 22 degrees C) or cell swelling. This study provides the first description of a nonselective cation current in human atrial myocytes, which may play an important role in repolarization in human atria.

Inositol (1,4,5)-trisphosphate (InsP3)-gated Ca channels from cerebellum: conduction properties for divalent cations and regulation by intraluminal calcium

The conduction properties of inositol (1,4,5)-trisphosphate (InsP3)-gated calcium (Ca) channels (InsP3R) from canine cerebellum for divalent cations and the regulation of the channels by intraluminal Ca were studied using channels reconstituted into planar lipid bilayers. Analysis of single-channel recordings performed with different divalent cations present at 55 mM on the trans (intraluminal) side of the membrane revealed that the current amplitude at 0 mV and the single-channel slope conductance fell in the sequence: Ba (2.2 pA, 85 pS) > Sr (2.0 pA, 77 pS) > Ca (1.4 pA, 53 pS) > Mg (1.1 pA, 42 pS). The mean open time of the InsP3R recorded with Ca (2.9 ms) was significantly shorter than with other divalent cations (approximately 5.5 ms). The "anomalous mole fraction effect" was not observed in mixtures of divalent cations (Mg and Ba), suggesting that these channels are single-ion pores. Measurements of InsP3R activity at different intraluminal Ca levels demonstrated that Ca in the submillimolar range did not potentiate channel activity, and that very high levels of intraluminal Ca (> or = 10 mM) decreased channel open probability 5-10-fold. When InsP3R were measured with Ba as a current carrier in the presence of 110 mM cis potassium, a PBa/PK of 6.3 was estimated from the extrapolated value for the reversal potential. When the unitary current through the InsP3R at 0 mV was measured as a function of the permeant ion (Ba) concentration, the half-maximal current occurred at 10 mM trans Ba. The following conclusions are drawn from these data: (a) the conduction properties of InsP3R are similar to the properties of the ryanodine receptor, another intracellular Ca channel, and differ dramatically from the properties of voltage-gated Ca channels of the plasma membrane. (b) The estimated size of the Ca current through the InsP3R under physiological conditions is 0.5 pA, approximately four times less than the Ca current through the ryanodine receptor. (c) The potentiation of InsP3R by intraluminal Ca in the submillimolar range remains controversial. (d) A quantitative model that explains the inhibitory effects of high trans Ca on InsP3R activity was developed and the kinetic parameters of InsP3R gating were determined.

Calcium homeostasis in identified rat gonadotrophs

1. Whole-cell voltage clamp was used in conjunction with the fluorescent Ca2+ indicator indo-1 to measure extracellular Ca2+ entry and intracellular Ca2+ concentrations ([Ca2+]i) in rat gonadotrophs identified with the reverse haemolytic plaque assay. 2. Depolarizations to potentials more positive than -40 mV elicited inward Ca2+ current (ICa) and transient elevations of [Ca2+]i. 3. The relationship between [Ca2+]i elevations and Ca2+ entry with different Ca2+ buffer concentrations in the pipette showed that endogenous Ca2+ buffers normally bind approximately 99% of the Ca2+ entering the cell. 4. With [Ca2+]i elevations less than 500 nM, decay of [Ca2+]i could be approximated by an exponential whose time constant increased with the concentration of exogenous Ca2+ buffers. 5. Inhibitors of intracellular Ca(2+)-ATPases, thapsigargin, cyclopiazonic acid (CPA) and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (BHQ), caused [Ca2+]i to rise. Application of BHQ during [Ca2+]i oscillations induced by gonadotrophin-releasing hormone (GnRH) terminated the oscillation in a slowly decaying elevation. BHQ slowed the decay of depolarization-induced [Ca2+]i elevations about 3-fold. 6. Taking into account the Ca2+ buffering properties of the cytoplasm permitted estimation of the fluxes and rate constants for Ca2+ movements in gonadotrophs. The intracellular store is a major determinant of Ca2+ homeostasis in gonadotrophs.

Role of L-type calcium channel window current in generating current-induced early afterdepolarizations

INTRODUCTION

Early afterdepolarizations (EADs) can give rise to triggered activity and thereby produce cardiac arrhythmias. We used the whole-cell patch clamp technique to examine the relationship between L-type Ca2+ channel window current and the generation of EADs in single ventricular myocytes isolated from guinea pig hearts.

METHODS AND RESULTS

With a high concentration of EGTA in the internal solution and Na(+)-containing physiologic external solution, EADs were induced in unclamped cells by injecting intracellular depolarizing current pulses. During voltage clamp protocols designed to simulate action potentials interrupted by EADs, we recorded an inward shift in total current up to 0.7 pA/pF over 400 msec at test steps in the range of the take-off potential for EADs. Cd2+ (0.2 mM) blocked most of the inward shift of current during the test steps and abolished EADs. When the same voltage clamp protocol was used following perfusion with an Na(+)-free, K(+)-free external solution, the Cd(2+)-sensitive inward currents recorded during the test steps were similar to those obtained in physiologic external solution. The overlapping range of potentials for partial activation of the d and f variables of L-type Ca2+ current ("window" region) measured in Na(+)-free, K(+)-free external solution was virtually the same as the voltage range of the Cd(2+)-sensitive inward currents.

CONCLUSION

Our experiments suggest that: (1) EADs can arise under conditions of high EGTA buffering of intracellular [Ca2+]; and (2) under these conditions, L-type Ca2+ channel window current plays a major role in the initiation of EADs.

Ethanol inhibition of L-type Ca2+ channels in PC12 cells: role of permeant ions

The effects of acute ethanol exposure on voltage-activated Ca2+ channels in undifferentiated pheochromocytoma (PC12) cells were investigated using whole-cell patch clamp techniques. Concentrations of ethanol (5, 25 and 50 mM), at or below blood alcohol levels which constitute legal intoxication significantly reduced Ca2+ currents evoked from a holding potential of -30 mV. Ethanol-induced inhibition of current was voltage-dependent in some cases, but this was not consistently observed. Inhibition of currents was reversible and was not due to an osmotic effect. The non-inactivating nature of the current, the inhibition of the current by nifedipine, and the lack of inhibition by omega-conotoxin, indicated that the current was carried through high-voltage activated, L-type Ca2+ channels. Since intracellular Ca2+ levels were highly buffered by exchanged with the contents of the patch pipet, ethanol-induced inhibition of currents in PC12 cells is not likely to involve either a change in driving force due to a change in intracellular Ca2+ levels or potentiation of Ca(2+)-dependent Ca2+ channel inactivation by the influx of Ca2+. The degree of inhibition by 25 mM ethanol was the same when either Ca2+, Ba2+ or Na+ was used as the current-carrying ion. This equivalency suggest that the channel's ion selectivity filter is not a site of action for ethanol.

Distinctive biophysical and pharmacological properties of class A (BI) calcium channel alpha 1 subunits

Transcripts for the class A Ca2+ channel alpha 1 subunit (also known as BI) are present at high levels in many parts of the mammalian CNS and are widely assumed to encode the P-type Ca2+ channel. To characterize the biophysical and pharmacological properties of alpha 1A channels, macroscopic and single-channel recordings were made in Xenopus oocytes injected with alpha 1A cRNA. alpha 1-specific properties were identified by making systematic comparisons with the more familiar class C alpha 1 subunit under the condition of a standard ancillary subunit (alpha 2/delta + beta) makeup. alpha 1A currents activate and inactivate more rapidly and display steeper voltage dependence of gating than alpha 1C currents. Unlike alpha 1C, alpha 1A channels are largely insensitive to dihydropyridines and FPL 64176, but respond to the cone snail peptide omega-CTx-MVIIC(SNX-230), a potent and fairly selective inhibitor. In comparison with P-type Ca2+ channels in rat cerebellar Purkinje cells, alpha 1A channels in oocytes are approximately 10(2)-fold less sensitive to omega-Aga-IVA and approximately 10-fold more sensitive to omega-CTx-MVIIC. alpha 1A channels are not inhibited by Bay K 8644 and inactivate much more rapidly than P-type Ca2+ channels. Thus, alpha 1A is capable of generating a Ca2+ channel phenotype quite different from P-type current.

Manganese induces spreading and process outgrowth in rat pheochromocytoma (PC12) cells

Mn2+ has been shown to promote cell-substrate adhesion and cell spreading in many cell culture systems. In this study, we present data demonstrating that Mn2+ not only promotes spreading, but also induces process outgrowth in rat pheochromocytoma (PC12) cells. In the presence of 1.0 mM MnCl2, cell spreading was apparent by 6 hr, and nearly 50% of the exposed cells extended neurite-like processes. These morphological effects of Mn2+ were both time- and dose-dependent. In the presence of cycloheximide, a protein synthesis inhibitor, both Mn(2+)-induced spreading and neurite outgrowth were prevented, indicating that de novo protein synthesis is required for the effects of Mn2+ to take place. Of the other divalent cations tested, Mg2+, Cd2+, Cu2+, Ni2+, and Zn2+ were ineffective, and only Co2+ partially mimicked the effects of Mn2+. Although Mn(2+)-induced cell adhesion and spreading have been extensively studied, this is the first report that this divalent cation can cause neurite outgrowth. The neurite outgrowth-promoting effects of Mn2+ were distinct from those of nerve growth factor in that the response to Mn2+ was considerably more rapid, but apparently lacked the ability to sustain continuous outgrowth and networking of neurites. Mn2+ also induced the levels of GAP-43 and peripherin, two proteins associated with neuronal differentiation of PC-12 cells. In cells grown in serum-free defined medium, Mn2+ was capable of promoting neurite outgrowth when the cells were plated on surfaces pretreated with normal growth medium, vitronectin, or fibronectin, while it failed to cause these morphological changes in cells plated on untreated or poly-D-lysine-coated substrata. Similarly, Mn2+ also promoted neurite outgrowth from rat sympathetic neurons attached to laminin-treated substrate, but had no effect on neurons maintained on substrate with polylysine only. The pentapeptide Gly-Arg-Gly-Asp-Ser nearly completely prevented the morphological effects of Mn2+ on PC12 cells. These findings are consistent with a hypothesis that Mn(2+)-mediated alteration of an RGD-dependent extracellular matrix-integrin interaction is responsible for the neuritogenic effects.

Currents in the presynaptic terminal arbors of barnacle photoreceptors

We have described the currents flowing across the presynaptic membranes of the four median photoreceptors of the giant barnacle, Balanus nubilus, using a quasi-voltage clamp arrangement. Membrane potential, measured in the terminal region of one photoreceptor, was controlled in all four terminals by feedback current supplied through the nerve containing the photoreceptors' axons. The [Ca2+]o in the saline was reduced to decrease the Ca2+ current, enabling better voltage control, and tetraethylammonium ion (TEA, 20 mM) was added to block a fast voltage-dependent K+ conductance. Depolarizing voltage steps from the resting potential in the dark (-60 mV) evoked slow, inward Ca(2+)-dependent currents which could be blocked by Co2+, Mg2+, or Cd2+. The Ca2+ currents were followed by large outward currents that persisted for many seconds after the offset of moderate or large pulses. These tail currents increased in magnitude and duration with pulse duration and reversed at about -80 mV, consistent with previous evidence for a Ca(2+)-activated K+ conductance in this membrane. When the Ca(2+)-activated outward current was reduced to zero by increasing the [K+]o so as to set EK at -20 mV, and then stepping the voltage to this value, the step evoked a steady inward Ca2+ current. Thus, the Ca2+ current did not show voltage- or Ca(2+)-dependent inactivation. When Ba2+ was substituted for Ca2+, 500-ms depolarizing steps evoked steady inward currents but no outward currents. In any given experiment, the activation voltage of the Ca2+ or Ba2+ current did not depend on holding potential. At the barnacle photoreceptor's synapse, the postsynaptic cell adapts to maintained presynaptic voltage by a mechanism that is not understood. We conclude that neither Ca2+ current inactivation nor a shift in activation voltage with holding potential can account for this adaptation.

Modulation of contraction by intracellular Na+ via Na(+)-Ca2+ exchange in single shark (Squalus acanthias) ventricular myocytes

1. The effect of direct alteration of intracellular Na+ concentration on contractile properties of whole-cell clamped shark ventricular myocytes was studied using an array of 256 photodiodes to monitor the length of the isolated myocytes. 2. In myocytes dialysed with Na(+)-free solution, the voltage dependence of Ca2+ current (ICa) and contraction were similar and bell shaped. Contractions activated at all voltages were completely suppressed by nifedipine (5 microM), and failed to show significant tonic components, suggesting dependence of the contraction on Ca2+ influx through the L-type Ca2+ channel. 3. In myocytes dialysed with 60 mM Na+, a ICa-dependent and a ICa-independent component of contraction could be identified. The Ca2+ current-dependent component was prominent in voltages between -30 to +10 mV. The ICa-independent contractions were maintained for the duration of depolarization, increased with increasing depolarization between +10 to +100 mV, and were insensitive to nifedipine. 4. In such myocytes, repolarization produced slowly decaying inward tail currents closely related to the time course of relaxation and the degree of shortening prior to repolarization. 5. With 60 mM Na+ in the pipette solution, positive clamp potentials activated decaying outward currents which correlated to the size of contraction. These outward currents appeared to be generated by the Na(+)-Ca(2+)-exchanger since they depended on the presence of intracellular Na+, and were neither suppressed by nifedipine nor by K+ channel blockers. 6. The results suggest that in shark (Squalus acanthias) ventricular myocytes, which lack functionally relevant Ca2+ release pools, both Ca2+ channel and the Na(+)-Ca2+ exchanger deliver sufficient Ca2+ to activate contraction, though the effectiveness of the latter mechanism was highly dependent on the [Na+]i.

Effects of propofol and enflurane on action potentials, membrane currents and contraction of guinea-pig isolated ventricular myocytes

1. The effects of two general anaesthetics, propofol and enflurane, on electrical activity and contractions were investigated in single myocytes isolated from guinea-pig ventricles. 2. Propofol and enflurane depressed the plateau and shortened the duration of action potentials. 3. Under voltage-clamp conditions, propofol and enflurane reduced the amplitude of inward calcium current and of additional inward current activated by cytosolic calcium. 4. Contractions (measured with an optical technique) accompanying either action potentials or second inward currents (in response to depolarizations to 0 mV) were reduced by both anaesthetics. The mechanisms for calcium entry during contractions accompanying pulses to positive potentials such as +60 mV are thought to differ from those accompanying second inward currents which are evoked by pulses from -40 to 0 mV. Enflurane enhanced the amplitudes of contractions accompanying pulses to positive potentials; in contrast these contractions were depressed by propofol. 5. In experiments where recovery processes were investigated by use of pairs of voltage-clamp pulses with a variable interval between them, enflurane but not propofol slowed the recovery of contractions and calcium-activated 'tail' currents. These observations are consistent with the hypothesis that enflurane may impair calcium handling by the sarcoplasmic reticulum whereas propofol has little, if any, effect at this site. 6. In conclusion, the actions of propofol and enflurane on second inward currents contribute to their effects on action potentials and contraction. The negative inotropic effect of both anaesthetics may result partly from reduced calcium influx to trigger contraction, and for enflurane, partly from an impairment of calcium handling by the sarcoplasmic reticulum.

Permeability and Mg2+ blockade of histamine-operated cation channel in endothelial cells of rat intrapulmonary artery

1. In the cell-attached and inside-out patch-clamp experiments using undispersed endothelial cells of the rat intrapulmonary artery, the majority of channels were cation selective. 2. Under physiological ionic conditions, the I-V relationship for the inward currents fell to -80 mV and the slope conductance was 22.5 pS. There was an inward rectification and the outward currents were smaller than the inward currents. 3. Under symmetric high-K+ conditions, the slope conductance for the inward currents was 26.4 pS and the inward rectification was observed when the high-K+ solution contained 1 mM-Mg2+. The channel activity was weakly voltage dependent at negative membrane potentials, while it was much enhanced at positive potentials. 4. The channel activity did not depend on intracellular Ca2+ concentrations. 5. Mg2+ was not only impermeant, it also blocked this channel in a voltage-dependent manner and rectifications appeared in the I-V relationship. Mg2+ blocked the channel from both sides of the membrane. 6. Ca2+ permeated this channel and the permeability ratios calculated from the reversal potentials using the constant-field theory were; PK:PNa:PCa = 1:1:15.7. 7. Histamine but not acetylcholine applied to the pipette activated this channel. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) applied to the intracellular surface of the patch did not mimic the effect of histamine. 8. Thus, in the endothelial cell membrane of the rat intrapulmonary artery, there exists a cation channel which is selective to Ca2+ but also permeable to Na+ and K+. This channel has inward rectifying properties, possibly due to intracellular Mg2+. Histamine, but not acetylcholine, activates this cation channel to elevate endothelial [Ca2+]i.

Properties of the calcium-activated chloride current in heart

We used the whole cell patch clamp technique to study transient outward currents of single rabbit atrial cells. A large transient current, IA, was blocked by 4-aminopyridine (4AP) and/or by depolarized holding potentials. After block of IA, a smaller transient current remained. It was completely blocked by nisoldipine, cadmium, ryanodine, or caffeine, which indicates that all of the 4AP-resistant current is activated by the calcium transient that causes contraction. Neither calcium-activated potassium current nor calcium-activated nonspecific cation current appeared to contribute to the 4AP-resistant transient current. The transient current disappeared when ECl was made equal to the pulse potential; it was present in potassium-free internal and external solutions. It was blocked by the anion transport blockers SITS and DIDS, and the reversal potential of instantaneous current-voltage relations varied with extracellular chloride as predicted for a chloride-selective conductance. We concluded that the 4AP-resistant transient outward current of atrial cells is produced by a calcium-activated chloride current like the current ICl(Ca) of ventricular cells (1991. Circulation Research. 68:424-437). ICl(Ca) in atrial cells demonstrated outward rectification, even when intracellular chloride concentration was higher than extracellular. When ICa was inactivated or allowed to recover from inactivation, amplitudes of ICl(Ca) and ICa were closely correlated. The results were consistent with the view that ICl(Ca) does not undergo independent inactivation. Tentatively, we propose that ICl(Ca) is transient because it is activated by an intracellular calcium transient. Lowering extracellular sodium increased the peak outward transient current. The current was insensitive to the choice of sodium substitute. Because a recently identified time-independent, adrenergically activated chloride current in heart is reduced in low sodium, these data suggest that the two chloride currents are produced by different populations of channels.

Modeling the dynamic features of the electrogenic Na,K pump of cardiac cells

The purpose of this paper is to examine the dynamic features of the electrogenic Na,K pump of cardiac cells, based on a comparative analysis of a mechanistic model and an ad hoc mathematical description of the Na,K pump. Both representations are incorporated into a modified version of the Beeler-Reuter model for the ventricular membrane, and the resulting action potential models are studied under conditions of repetitive stimulation at steady rates between 0 and 3 Hz. The two Na,K pump representations have nearly identical steady-state characteristics of sensitivity to internal Na+ concentration, external K+ concentration, and membrane potential. Rapid voltage-dependent transient pump currents are present in the mechanistic model, while they are absent in the ad hoc mathematical description we used. The stimulation results show that a sizable peak of pump current caused by the action potential upstroke in the mechanistic model affects phase 1 repolarization, and that this effect is relatively independent of the stimulation rate. The pump current generated by our ad hoc mathematical description is constant during the action potential and does not affect directly the repolarization time course. While the two Na,K pump models show similar pumping efficiency at low stimulation rates, the mechanistic pump is more efficient at high rates of activity. In essence, the distinctive features of the mechanistic model are due to an energy barrier expressing the voltage dependence of the translocation step of the mechanism, and to the redistribution of the intermediates of the biochemical reactions during activity. In comparison, the ad hoc mathematical description exhibits a fixed dependence of the pump current on voltage and ionic concentrations.

The effect of strophanthidin on action potential, calcium current and contraction in isolated guinea-pig ventricular myocytes

1. A method is described for producing high yields of calcium-tolerant ventricular myocytes from guinea-pig hearts (73.4% rod-shaped cells, n = 19). Their action potential (AP) and membrane currents were recorded using conventional microelectrodes and cell shortening was measured optically using a linear photodiode array. 2. The sensitivity of the guinea-pig Na(+)-K+ pump to strophanthidin (a rapidly acting digitalis analogue) was determined by measuring the inhibition of outward pump current by different doses. The pump was found to have a dissociation constant (KD) for strophanthidin of 1.11 x 10(-5) M, and 5 x 10(-4) M-strophanthidin inhibited the pump maximally. 3. Exposure to strophanthidin resulted in an initial lengthening followed by a shortening of the AP, and an increased contraction. Initial AP lengthening was associated with a more positive AP plateau which became more negative as the AP shortened. 4. There was a reversible reduction of Ca2+ current (ICa) during exposure to strophanthidin. ICa changed reciprocally with contraction and with a similar time course. 5. Strophanthidin exposure caused a reduction of ICa at all activating voltages, suggesting that it resulted in a reduction of Ca2+ conductance with little change of its voltage dependence. 6. The role of an increase of intracellular calcium (Cai2+) was investigated by impaling myocytes with microelectrodes containing BAPTA 1,2-bis (2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid, a calcium chelator) to increase Cai2+ buffering. Strophanthidin still shortened the AP when BAPTA was present, suggesting that a rise of Cai2+ is not a major cause of AP shortening. 7. Although AP shortening was little affected, the decline of ICa with strophanthidin was markedly reduced when BAPTA was present, suggesting that a rise of Cai2+ was the cause of the ICa decline with strophanthidin. 8. When barium ions carried the current through Ca2+ channels, strophanthidin did not reduce Ca2+ channel current, suggesting that this compound does not have a direct inhibitory effect on the channel. 9. The results suggest that strophanthidin causes a reduction of ICa by increasing Cai2+, via the mechanism of Cai(2+)-dependent inactivation of ICa. The reduction of ICa at least partially explains the AP shortening and more negative plateau with strophanthidin. 10. The shortening of the AP, more negative plateau and reduced ICa have negative inotropic effects which oppose the direct positive inotropic effect of strophanthidin.

Whole cell current analyses of pancreatic acinar AR42J cells. I. Voltage- and Ca(2+)-activated currents

Voltage- and Ca(2+)-activated whole cell currents were studied in AR42J cells, a clonal cell line derived from rat pancreatic acinar cells, using a patch electrode voltage-clamp technique. Four kinds of ionic currents were identified by their ionic dependencies, pharmacological properties, and kinetic parameters: 1) an outward current flow due mainly to a voltage-dependent K(+)-conductance increase, 2) an initial transient inward current due to an Na(+)-conductance increase, 3) transient and long-duration inward current due to a Ca(2+)-conductance increase, and 4) a slowly activating inward current that persists over the duration of the depolarizing pulse and deactivates slowly upon repolarization, producing a slow inward tail current. The slow inward tail current was particularly robust and was interpreted as due to a Ca(2+)-activated Cl(-)-conductance increase, since 1) the generation of this current was blocked by removing the extracellular Ca2+, applying Ca(2+)-channel blockers (Cd2+, nifedipine), or by lowering the intracellular Ca2+ concentration [( Ca2+]i) with EGTA; and 2) the reversal potential (Erev) of the slow inward tail current was close to 0 mV in the control condition (152 mM [Cl-]o/154 mM [Cl-]i), and changes of the [Cl-]o/[Cl )i ratio shifted the Erev toward the predicted Cl- equilibrium potential.

Role of Ca2+ channel in cardiac excitation-contraction coupling in the rat: evidence from Ca2+ transients and contraction

1. Optical methods were used to measure simultaneously unloaded cell shortening and intracellular Ca2+ transients in whole-cell voltage clamped rat ventricular myocytes. Red light (greater than 670 nm) was used to measure cell shortening with a linear photodiode array. The dyes Fura-2 (Kd = 140 nM) and Mag-Fura-2 (Kd = 44 microM) were used as Ca2+ indicators with fluorescence excitation at 340 and 410 nm and emission at 510 nm. 2. Repeated measurements at 6 s intervals as 0.4 mM-Fura-2 diffused into the cell from the tip of the voltage clamp pipette showed no decrease in the rate of rise and peak value of the intracellular Ca2+ transient and only a small suppression of cell shortening, suggesting that the molecular mechanisms regulating the Ca2+ release were not significantly altered by the buffering capacity of the Fura-2. 3. Experiments in which the sarcoplasmic reticulum (SR) was depleted of Ca2+ either by exposure to caffeine or by repeated brief (20 ms) voltage clamp depolarizations confirm that the SR is the major source of activator Ca2+. 4. Mag-Fura-2 (1 or 5 mM) was used to register the initial rapid development of the [Ca2+]i transient but the later time course of the Ca2+ transients measured with this dye was obscured by motion artifacts resulting from cell shortening. 5. Both Fura-2 and Mag-Fura-2 showed that depolarization to 0 mV from a holding potential of -80 mV resulted in a [Ca2+]i transient which developed with a delay of 3-9 ms and approached its peak value in an additional 8-19 ms. Both Ca2+ indicators also showed that the Ca2+ transient approached its peak value more slowly as the clamped membrane potential was made increasingly more positive. 6. The voltage dependencies of the Ca2+ signal (Fura-2) and cell shortening were both bell-shaped and were qualitatively similar to the voltage dependence of Ca2+ current simultaneously measured. This was observed with holding potentials of both -40 and -80 mV. 7. Comparison of the temporal relation of the Ca2+ current, ICa, and intracellular Ca2+ transient (Fura-2) and cell shortening at different membrane potentials showed that Ca2+ transient measured 25 ms into the depolarization correlated closely to the integral of the Ca2+ current measured prior to this time. Cell shortening, on the other hand, peaked about 100 ms later and correlated with measurements of the Ca2+ activity at the later time.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium channel currents in isolated smooth muscle cells from the basilar artery of the guinea pig

Ca2+ channel currents were studied in smooth muscle cells from the basilar artery of the guinea pig using the whole-cell patch-clamp technique. 10 mM Ba2+ as the charge carrier and strong buffering of intracellular Ca2+ with EGTA (pCai = 8). Cell capacitance was 18.8 +/- 6.6 pF (n = 96) and maximum current density at +10 to +20 mV (holding potential less than -55 mV), measured early in dialysis, was -14.8 +/- 4.9 pA/pF (n = 83). Currents reversed at approximately +95 mV and, at more positive potentials, outward Cs+ currents were recorded that were blocked by either external Cd2+ or Ca2+. One component of current was identified that had properties consistent with L-type channels. On the basis of measurements of tail currents, its threshold for activation was -15 mV, its voltage dependence of activation was steep and it was half-activated at +8.5 mV. It inactivated very slowly at +15 mV (2787 +/- 511 ms) and it deactivated rapidly (251 +/- 55 microseconds) at -55 mV. It was quickly lost during dialysis and was largely blocked by 1 nM nifedipine (1-s pulses, holding potential = -55 mV). A second component, termed B-type current, was identified that had properties inconsistent with those of T-type channels. On the basis of tail currents, its threshold for activation was -30 mV, its voltage dependence of activation was less steep and it was half-activated at +33.7 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

High- and low-threshold calcium currents in neurons acutely isolated from rat sensorimotor cortex

Neurons were isolated by papain treatment and trituration of the frontoparietal cortex of 14 to 28-day-old rats. Whole cell voltage clamp revealed a slowly inactivating high-threshold Ca2+ current, activated positive to -45 mV, and a transient low-threshold Ca2+ current, activated positive to -65 mV. The high-threshold current was more sensitive to block by Cd2+ and the low-threshold current was more sensitive to block by Ni2+. Replacement of Ca2+ by Ba2+ increased the high-threshold current and reduced the low-threshold current. The high-threshold current was enhanced by Bay K 8644 and reduced by nimodipine and omega-conotoxin. The low-threshold current was also reduced by nimodipine but was insensitive to Bay K 8644 and omega-conotoxin. The properties of the currents were consistent with different underlying Ca2+ channel types.

Inhibition of calcium currents by noradrenaline, somatostatin and opioids in guinea-pig submucosal neurones

1. Whole-cell recordings were made from submucosal neurones acutely dissociated from guinea-pigs. The actions of noradrenaline, somatostatin and [Met5]enkephalin on currents carried by calcium ions were studied. 2. On depolarization from a holding potential of -70 mV, an inward current activated at -40 mV, reached its peak amplitude at 10 mV and reversed to outward at 72 mV (with external calcium of 5 mM and internal caesium of 160 mM). 3. Cadmium, nickel and cobalt reversibly blocked the calcium current; concentrations causing 50% block were 2.5, 500 and 2000 microM respectively. The calcium current (holding at -70 or -30 mV) was reversibly blocked by omega-conotoxin (100 nM), and unaffected by Bay K 8644 (0.1-10 microM) and nifedipine (1 microM). Cadmium caused an outward shift in holding current at -30 mV, implying that there was a persistent inward calcium current at this potential. 4. Noradrenaline, somatostatin and [Met5]enkephalin decreased the calcium current. The maximal inhibition observed with any one agonist, or with a combination of two agonists, did not exceed 50%; concentrations giving half-maximal inhibition were 5.5 microM for noradrenaline, 4 nM for somatostatin and 1 microM for [Met5]enkephalin. The inhibition was independent of membrane potential. All three agonists also reduced the persistent calcium current at -30 mV. 5. Inhibition of the calcium current by noradrenaline occurred with a latency of not less than 175 ms; cadmium applied by the same method depressed the current within 5-45 ms. 6. Experiments with selective agonists and antagonists indicated that the receptor types involved in calcium current inhibition were alpha 2-adrenoceptors and delta-opioid receptors. Somatostatin acted at a distinct receptor. 7. Calcium currents were also inhibited by intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S). Agonists were ineffective in cells pre-treated with pertussis toxin, but their action was restored when purified GTP-binding proteins (Go or Gi) were included in the intracellular recording solution. 8. It is concluded that noradrenaline, somatostatin and [Met5]enkephalin act at their respective receptors on guinea-pig submucosal neurones to inhibit a voltage-dependent calcium current. Activation of the same receptors also increases a potassium conductance in these cells: in both cases a pertussis-sensitive G protein is involved.

Sustained subthreshold-for-twitch depolarization in rat single ventricular myocytes causes sustained calcium channel activation and sarcoplasmic reticulum calcium release

Single rat ventricular myocytes, voltage-clamped at -50 to -40 mV, were depolarized in small steps in order to define the mechanisms that govern the increase in cytosolic [Ca2+] (Cai) and contraction, measured as a reduction in myocyte length. Small (3-5 mV), sustained (seconds) depolarizations that caused a small inward or no detectable change in current were followed after a delay by small (less than 2% of the resting length), steady reductions in cell length measured via a photodiode array, and small, steady increases in Cai measured by changes in Indo-1 fluorescence. Larger (greater than -30 and less than -20 mV), sustained depolarizations produced phasic Ca2+ currents, Cai transients, and twitch contractions, followed by a steady current and a steady increase in Cai and contraction. Nitrendipine (or Cd, verapamil, or Ni) abolished the steady contraction and always produced an outward shift in steady current. The steady, nitrendipine-sensitive current and sustained increase in Cai and contraction exhibited a similar voltage dependence over the voltage range between -40 and -20 mV. 2 microM ryanodine in the presence of intact Ca2+ channel activity also abolished the steady increase in Cai and contraction over this voltage range. We conclude that when a sustained depolarization does not exceed about -20 mV, the resultant steady, graded contraction is due to SR Ca2+ release graded by a steady ("window") Ca2+ current. The existence of appreciable, sustained, graded Ca2+ release in response to Ca2+ current generated by arbitrarily small depolarizations is not compatible with any model of Ca2(+)-induced Ca2+ release in which the releasing effect of the Ca2+ channel current is mediated solely by Ca2+ entry into a common cytosolic pool. Our results therefore imply a distinction between the triggering and released Ca2+ pools.

Electrophysiology of parasympathetic neurones isolated from the interatrial septum of bull-frog heart

1. Whole-cell voltage-clamp techniques were used to study the voltage-dependent membrane conductances in parasympathetic neurones enzymatically isolated from the interatrial septum of bull-frog heart and maintained in short-term (1-10 day) tissue culture. 2. The resting potential of the isolated neurones averaged -55.4 +/- 1.1 mV (+/- S.E.M., n = 11). Action potentials evoked in the isolated cells by brief (1-2 ms) current injections were similar to those recorded from neurones in the 'intact' septum. The amplitude of action potentials of isolated neurones averaged about 113 mV, with a peak depolarization of +32.8 +/- 2.8 mV and after-hyperpolarization of -80.0 +/- 2.8 mV. 3. The pattern of membrane currents recorded using voltage clamp with 'normal' external (containing 110 mM-Na+) and internal (110 mM-K+) solutions consisted of a rapidly activating and inactivating inward current followed by a slower, sustained outward current. 4. The inward components of current were isolated by using an internal solution in which Cs+ and TEA+ (tetrathylammonium) ions replaced K+. Depolarizations from holding potentials of -50 to -70 mV produced inward currents which had an initial transient phase followed by a maintained, or very slowly inactivating, component. The current-voltage relation for the initial transient phase reached a peak at membrane potentials near 0 mV, while the maintained phase, measured, for example, at the end of 50 ms voltage-clamp steps, had its peak near +10 mV. 5. The transient component of inward current was carried primarily by Na+ ions, as replacement of Na+ by TEA+ in the external solution abolished the transient. This current was thus identified as a voltage-dependent Na+ current, INa. The maintained component was greatly attenuated by removing 80-90% of the external Ca2+ ions, and it was abolished by divalent cations such as Cd2+ (0.2-0.4 mM), Ni2+ (0.5 mM) and La3+ (10-100 microM). This maintained component was thus a voltage-dependent calcium current, ICa. 6. About 80% of INa recorded in the presence of low (0.2-0.5 mM) external Ca2+ and 2 microM-LaCl3 was blocked by tetrodotoxin (TTX) with an apparent Kd of about 8 nM. The remaining 20% of INa was resistant to block by 2-10 microM-TTX. However, the 'TTX-resistant' component of INa was blocked by Cd2+ (0.2-0.4 mM). 7. The voltage-dependent calcium current, ICa, measured in saline in which Na+ was replaced by N-methyl-D-glucamine, activated near -40 mV and reached a peak near +10 to +15 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Inward current generated by Na-Ca exchange during the action potential in single atrial cells of the rabbit

To investigate the underlying ionic mechanism of the late plateau phase of the action potential in rabbit atrium the whole-cell patch-clamp technique with intracellular perfusion was used. We recorded the inward current during repolarizations following a brief 2 ms depolarizing pulse to +40 mV from a holding potential of between -70 and -80 mV. The development of this current coincides with the onset of the late plateau phase of the action potential. Peak activation of the current occurs about 10 ms from the beginning of the depolarizing pulse, and it decays spontaneously with a slow timecourse. Its voltage dependency from -40 mV to +40 mV shows very steep activation (-40 to -20 mV) and shows almost the same maximum magnitude between -10 mV and +40 mV. This behaviour is quite different from that of the calcium current. The inward current and the late plateau phase of the action potential were both abolished by the application of 5 mM EGTA, 1 microM ryanodine and by reducing the Na+ gradient. The fully activated current-voltage relation of the inward current was plotted as the difference current before and after treatment with Ryanodine, Diltiazem, 20 mM Na+ inside or 30% Na+ outside and shows an exponential voltage dependence with the largest magnitude of the current occurring at negative potentials. The current-voltage (I-V) curve was well fitted by the Na-Ca exchange equation, i = A exp (-(1 - r)EF/RT). The results suggest that the inward current contributes to the generation of the late plateau phase of the rabbit atrial action potential, and is activated by intracellular calcium released from the sarcoplasmic reticulum. Sarcoplasmic reticulum calcium release appears to be triggered both by the membrane voltage and by the calcium current. It is concluded that the inward current is generated by Na-Ca exchange.

Macroscopic and single-channel studies of two Ca2+ channel types in oocytes of the ascidian Ciona intestinalis

Whole-cell and single-channel patch-clamp experiments were performed on unfertilized oocytes of the ascidian Ciona intestinalis to investigate the properties of two voltage-dependent Ca2+ currents found in this cell. The peak of the low threshold current (channel I) occurred at -20 mV, the peak of the high-threshold current (channel II) at +20 mV. The two currents could be distinguished by voltage dependence, kinetics of inactivation and ion selectivity. During large depolarizing voltage pulses, a transient outward current was recorded which appeared to be due to potassium efflux through channel II. When the external concentrations of Ca2+ and Mg2+ were reduced sufficiently, large inward Na currents flowed through both channels I and II. Using divalent-free solutions in cell-attached patch recordings, single-channel currents representing Na influx through channels I and II were recorded. The two types of unitary events could be distinguished on the basis of open time (channel I longer) and conductance (channel I smaller). Blocking events during channel I openings were recorded when micromolar concentrations of Ca2+ or Mg2+ were added to the patch pipette solutions. Slopes of the blocking rate constant vs. concentration gave binding constants of 6.4 X 10(6) M-1 sec-1 for Mg2+ and 4.5 X 10(8) M-1 sec-1 for Ca2+. The Ca2+ block was somewhat relieved at negative potentials, whereas the Mg2+ block was not, suggesting that Ca2+, but not Mg2+, can exit from the binding site toward the cell interior.

Cardiac-type excitation-contraction coupling in dysgenic skeletal muscle injected with cardiac dihydropyridine receptor cDNA

There are dihydropyridine (DHP)-sensitive calcium currents in both skeletal and cardiac muscle cells, although the properties of these currents are very different in the two cell types (for simplicity, we refer to currents in both tissues as L-type). The mechanisms of depolarization-contraction coupling also differ. As the predominant voltage-dependent calcium current of cardiac cells, the L-type current represents a major pathway for entry of extracellular calcium. This entry triggers the subsequent large release of calcium from the sarcoplasmic reticulum (SR). In contrast, depolarization of skeletal muscle releases calcium from the SR without the requirement for entry of extracellular calcium through L-type calcium channels. To investigate the molecular basis for these differences in calcium currents and in excitation-contraction (E-C) coupling, we expressed complementary DNAs for the DHP receptors from skeletal and cardiac muscle in dysgenic skeletal muscle. We compared the properties of the L-type channels produced and showed that expression of a cardiac calcium channel in skeletal muscle cells results in E-C coupling resembling that of cardiac muscle.

Somatostatin decreases the calcium inward current in guinea-pig atria

1. The effects of somatostatin on mechanical and electrophysiological responses were studied in guinea-pig atrial muscle preparations and single cells. 2. Somatostatin (greater than or equal to 10(-8) M) decreased the twitch contraction in a concentration-dependent manner in electrically driven left atria and spontaneously beating right atria. However, the beating rate was not affected. 3. The negative inotropic effect of somatostatin was transient. Desensitization to this agent developed slowly during continuous exposure to the peptide. 4. In single atrial cells, somatostatin significantly shortened the action potential duration, but the resting potential and action potential amplitude were not affected. 5. Under whole cell voltage-clamp conditions, somatostatin decreased the calcium inward current without affecting the sodium and potassium currents. 6. These results suggest that somatostatin selectively acts on the calcium channel of guinea-pig atrial cells to reduce the calcium inward current, which in turn gives rise to the negative inotropic effect.

A model study of the contribution of active Na-K transport to membrane repolarization in cardiac cells

A biochemical model of active Na-K transport in cardiac cells was studied in conjunction with a representation of the passive membrane currents and ion concentration changes. The active transport model is based on the thermodynamic and kinetic properties of a six-step reaction scheme for the Na,K-ATPase. It has a fixed Na:K stoechiometry of 3:2, and its activation is governed by three parameters: membrane potential intracellular Na+ concentration, and interstitial K+ concentration. The Na-K pump current is directly proportional to the density of Na,K-ATPase molecules. The passive membrane currents and ion concentration changes involve only Na+ and K+ ions, and no attempt was made to provide a precise representation of Ca2+ currents or Ca2+ concentration changes. The surface-to-volume ratio of the interstitial compartment is 55 times larger than that of the intracellular compartment. The flux balance conditions are such that the original equilibrium concentration values are re-established at each stimulation cycle. The underlying assumptions of the model were checked against experimental measurements on Na-K pump activity in a variety of preparations. In addition, the qualitative validation of the model was carried out by comparing its behavior following sudden frequency shifts to corresponding experimental observations. The overall behavior of the model is quite satisfactory and it is used to provide the following indications: (1) when the intracellular and interstitial volumes are relatively large, the ion concentration transients are small and the pumping rate depends essentially on average concentration levels. (2) An increase in internal Na+ concentration potentiates the response of the Na-K pump to rapid membrane depolarizations. (3) When the internal Na+ concentration is large enough, the Na-K pump current transient plays an important role in shaping the plateau and repolarization phase of the action potential. (4) A rapid increase in external K+ concentration during voltage clamp in multicellular preparations could saturate the Na-K pump response and lead to a fairly linear dependence of the pump activity on the internal Na+ concentration.