Mechanism of ion permeation through calcium channels

The use of Xenopus oocytes for the study of ion channels

Recently, in addition to the "traditional" research on meiotic reinitiation and fertilization mechanisms, the oocytes of the African frog Xenopus laevis have been exploited for the study of numerous aspects of ion channel function and regulation, such as the properties of several endogenous voltage-dependent channels and the involvement of second messengers in mediation of neurotransmitter-evoked membrane responses. In addition, injection of these cells with exogenous messenger RNA results in production and functional expression of foreign membranal proteins, including various voltage- and neurotransmitter-operated ion channels originating from brain, heart, and other excitable tissues. This method provides unique opportunities for the study of the structure, function, and regulation of these channels. A multidisciplinary approach is required, involving molecular biology, electrophysiology, biochemistry, pharmacology, and cytology.

Calcium dependence of presynaptic calcium current and post-synaptic response at the squid giant synapse

1. Neurotransmitter release has a non-linear dependence upon the external Ca concentration, [Ca]o. This may be due to a 'co-operative' action of Ca in triggering release. The dependence of presynaptic Ca currents and post-synaptic currents (p.s.c.s) upon [Ca]o was examined at voltage-clamped 'giant' synapses of squid to determine whether this 'co-operativity' occurs during or after influx of Ca into the presynaptic terminal. 2. Presynaptic Ca current was proportional to [( Ca]o/(1 + [Ca]o/KD]n, where n, the order of the function, was roughly 1 and KD, the apparent dissociation constant for Ca, was approximately 80 mM. 3. P.s.c.s also could be described by the same function, but had an n of 3-4 and a lower KD. 4. These results suggest that the 'co-operative' action of Ca occurs at a step or steps beyond entry of Ca into the presynaptic terminal. 5. Synaptic transfer curves relating presynaptic Ca currents, elicited by depolarizations to different potentials, to resultant p.s.c.s were power functions whose exponent depended upon [Ca]o. Maximum exponents were as high as 4 at [Ca]o of 3 mM. The dependence of these curves upon [Ca]o helps to explain why previous determinations, which were performed at a variety of [Ca]o levels, yielded a variety of transfer curve exponent values. 6. Transfer curves generated from responses to constant presynaptic depolarizations, with Ca current varied by [Ca]o changes, also were power functions with exponents of approximately 4. Thus p.s.c.s were high-exponent power functions of Ca current regardless of whether Ca current was modified by changes in membrane potential or in [Ca]o.

Multi-ion conduction and selectivity in the high-conductance Ca++-activated K+ channel from skeletal muscle

Open-channel ion permeation properties were investigated for Ca++-activated K+ (CaK) channels in solutions of K+ and its analogues T1+, Rb+, and NH4+. Single CaK channels were inserted into planar lipid bilayers composed of neutral phospholipids, and open-channel current-voltage (I-V) relations were measured in symmetrical and asymmetrical solutions of each of these individual ions. For all concentrations studied, the zero-voltage conductance falls in the sequence K+ greater than T1+ greater than NH4+ greater than Rb+. The shape of the I-V curve in symmetrical solutions of a single permeant ion is non-ohmic and is species-dependent. The I-V shape is sublinear for K+ and T1+ and superlinear for Rb+ and NH4+. As judged by reversal potentials under bi-ionic conditions with K+ on one side of the bilayer and the test cation on the other, the permeability sequence is T1+ greater than K+ greater than Rb+ greater than NH4+ at 300 mM, which differs from the conductance sequence. Symmetrical mixtures of K+ or NH4+ with Rb+ show a striking anomalous mole fraction behavior, i.e., a minimum in single-channel conductance when the composition of a two-ion mixture is varied at constant total ion concentration. This result is incompatible with present models that consider the CaK channel a single-ion pore. In total, the results show that the CaK channel finely discriminates among K+-like ions, exhibiting different energy profiles among these species, and that several such ions can reside simultaneously within the conduction pathway.

Effects of Ca2+/Mg2+ removal on aiNa, aiK, and tension in cardiac Purkinje fibers

Sheep cardiac Purkinje fibers were exposed to solutions free of divalent cations for hour-long periods, while intracellular Na+ and K+ activities were measured using ion-sensitive microelectrodes. Intracellular Na+ activity (aiNa) increased to 50.1 +/- 8.1 mM, and intracellular K+ activity (aiK) decreased to 76.7 +/- 3.5 mM. These ionic changes could be blocked by the presence of Mg2+ or the Ca2+ channel blocking agents D 600 and nifedipine. The rise in aiNa and the fall in aiK was accentuated by the inhibition of the Na+-K+ pump with acetylstrophanthidin or by removal of extracellular K+. These results demonstrate that in cardiac Purkinje fibers removal of divalent cations produces intracellular loading of Na+ by Na+ entry through the Ca2+ channel. On reexposure to Ca2+-containing solutions, the cells become loaded with Ca2+, and the fibers exhibit large contractures. These observations implicate Na+-Ca2+ exchange in the entry of Ca2+ into these cells during Ca2+ repletion and in the etiology of the calcium paradox.

Different properties of two voltage-dependent inward currents of the ciliate Stylonychia mytilus

1. Membrane currents and membrane potentials of the fresh-water ciliate Stylonychia mytilus were investigated by voltage-clamp and constant-current injection techniques. 2. The Ca-dependent action potential of Stylonychia in a solution containing 0.1 mM-CaCl2 was prolonged by the addition of Mg or Na ions. 3. In a nominally Ca-free solution, containing 2 mM-MgCl2, the cells generated repetitive, spontaneous action potentials of relatively small amplitude (17 mV). The addition of 0.5 microgram concanavalin A/ml completely inhibited these action potentials in 2 mM-Mg. 4. In voltage-clamp experiments in standard solution, the inward current-voltage relationship has two maxima, confirming the existence of two different voltage-dependent Ca currents in Stylonychia: inward current I and II. In a nominally Ca-free, Mg-containing solution, the remaining inward current was inhibited by concanavalin A, a specific inhibitor of inward current I. No residual second inward current (current II) was detected in a solution containing Mg and concanavalin A. 5. Experiments, with altered ratio of Ca and Mg ions and constant concentration of divalent cations (mole-fraction experiments), showed that Mg and Ca do not inhibit each other's passage through channel I. Calculations assuming a Ca-channel model with one cation-binding site per ion channel I showed good correlation with the experimental data. 6. A similar inward current was seen after replacement of Mg by Na in nominally Ca-free solution.

Binding of omega-conotoxin to receptor sites associated with the voltage-sensitive calcium channel

The binding of radioiodinated omega-conotoxin GVIA, a probable Ca channel antagonist, to synaptic plasma membranes of rat brain was examined. Two kinds of specific binding sites were found with apparent dissociation constants of 10 pM and 0.5 nM and maximum binding capacities of 0.5 and 3.4 pmol/mg prot., respectively. The binding of the toxin was not affected by high concentrations of Ca antagonists or an agonist, indicating distinct binding sites of the toxin from those of these drugs. Divalent and trivalent metal ions strongly inhibited the binding. The order of their inhibitory potencies was similar to that for inhibition of the Ca current through certain Ca channels. These results suggest that the binding sites of omega-conotoxin GVIA are functionally related to the Ca2+-binding site postulated to be in the pore of the Ca channel.

Channels produced by spider venoms in bilayer lipid membrane: mechanisms of ion transport and toxic action

The selectivity of ion channels produced by latrotoxin obtained from a black widow spider venom and by venom from the spider Steatoda paykulliana in bilayer phospholipid membrane was studied. Experimental current-voltage curves of these channels were used for the estimation of parameters of a two barrier model of their energy profiles. Selectivities of both types of channels are similar. Alkaline earth cations are permeable, the permeability increasing in the order Mg2+ less than Ca2+ less than Sr2+ less than Ba2+. In contrast transition metal cations block the channel, their efficiency decreases in the order: Cd2+ greater than or equal to Ni2+ greater than Zn2+ greater than Co2+ greater than Mn2+ (Steatoda paykulliana spider venom) and Cd2+ greater than Co2+ greater than Ni2+ greater than Zn2+ greater than Mn2+ (latrotoxin). Amplitudes of current carried by corresponding ions are mainly determined by the depth of the potential well for this ion, i.e., by its affinity to the cation binding site in the channel. The channels are also permeable to monovalent cations but they do not bind them. Selectivity for monovalent cations depends on Ca2+ concentration at the cis-side of membrane in the micromolar range. However, the addition of Ca2+ to the trans-side up to 10 mM does not affect currents carried by monovalent ions. It is suggested that venom-induced calcium channels have two conformational states with different selectivities which interconvert upon binding one calcium ion. Possible general schemes for the organisation of calcium channels in excitable membranes are also discussed. Finally, using a mathematical model of synaptic transmission, possible mechanisms of toxic action of spider venoms are considered.

Sodium conductance in calcium channels of guinea-pig ventricular cells induced by removal of external calcium ions

An inward current characterized by a slow inactivation, was induced when the extracellular Ca2+ concentration was reduced by EGTA. It was suppressed by replacing external Na+ with Tris+ or by D-600, increased by epinephrine, and was not affected by TTX. These findings suggest that this current is carried by Na+ ions through the Ca channels. The Na current decreased in amplitude as the concentration of external divalent cations was elevated. Blocking the Na current by divalent cations could be approximated by a bimolecular interaction between divalent cation and channel, with a dissociation constant of 1.2 microM for Ca2+ and 60 microM for Mg2+. Single channel currents were recorded in the cell-attached configuration. With a pipette solution of pCa = 7.5 or pCa greater than 8, the single channel I - V relationship was linear and the slope conductance was 70-75 pS. For 40 mV depolarizations from the resting potential, unitary currents were smaller at pCa = 6 than at pCa = 7.5. However, single channel events, which were observed after the repolarizing step to the resting potential, were much the same amplitude. The open time histogram was fitted with a single exponential having a time constant of 1.9 ms at around -40 mV (pCa greater than 8, with 5 microM Bay K 8644 in the bath solution), which was decreased with increasing the Ca2+ concentration in the pipette solution. Noise power spectra of patch currents at pCa = 6 revealed a high-frequency component at around 1500 Hz. These results suggest that Ca binding to the sites with a high affinity for Ca2+ blocks the Na conductance in Ca channels. Reduction of the unitary current at higher concentrations of Ca2+ might be attributed to a rapid block by Ca2+.

Barium modulates c-fos expression and post-translational modification

The addition of exogenous barium ions to PC12 rat pheochromocytoma cells elicits a transient induction of the c-fos gene. Induction is antagonized by extracellular calcium and the dihydropyridine calcium channel blockers, and it is attenuated in the presence of calmodulin inhibitors. Thus, barium appears to enter the cell through a voltage-dependent calcium channel and, either directly or indirectly, interacts with calmodulin to stimulate c-fos expression. This property of barium is not shared by a range of di- and trivalent cations examined. Agents that induce the c-fos gene in PC12 cells may be divided into two broad categories. The first comprises polypeptide growth factors and phorbol esters, which give rise to a c-fos protein that undergoes extensive post-translational modification. The second, which comprises depolarizing agents and barium ions, acts via calcium channels and yields a c-fos protein that undergoes less extensive post-translational modification. These different forms of c-fos protein can be distinguished on the basis of their apparent molecular weights on sodium dodecyl sulfate/polyacrylamide gels.

Charge movement and depolarization-contraction coupling in arthropod vs. vertebrate skeletal muscle

Voltage-dependent charge movement has been characterized in arthropod skeletal muscle. Charge movement in scorpion (Centuroides sculpturatus) muscle is distinguishable from that in vertebrate skeletal muscle by criteria of kinetics, voltage dependence, and pharmacology. The function of scorpion charge movement is gating of calcium channels in the sarcolemma, and depolarization-contraction coupling relies on calcium influx through these channels.

Two distinct calcium-activated potassium currents in larval muscle fibres of Drosophila melanogaster

The non-synaptic membrane currents of muscle fibres have been studied in late embryogenesis of Drosophila melanogaster using the voltage-clamp technique in wild-type and Shaker mutant third instar larvae. Five currents were found in the wild type muscle membrane at this embryonic stage: one fast inward Ca current (ICa), two fast outward K currents (IA and IAcd) and two slow outward K currents (IK and IC). IAcd and IC are Ca-dependent. Several procedures were used to separate IAcd from IA: IAcd is present in Shaker mutants which are characterized by the absence of IA (Salkoff and Wyman 1981); IAcd, but not IA, is suppressed by Co2+ (10 mM) or La3+ (1 mM); IAcd shows steady-state inactivation at more positive potentials than IA; IAcd, unlike IA, is 3,4-diaminopyridine (3,4-DAP) resistant. Furthermore, tetraethylammonium (TEA, 20 mM) which is known to be uneffective on IA, blocks IAcd. IAcd could not be triggered by using strontium or barium as calcium substitutes. By partial substitution of Ca by Ba or Sr ions, it was found that Ba, but not Sr, blocks the IAcd channel. A non-inactivating, TEA sensitive, Ca-dependent K current (IC), which gave N-shaped I-V plots, could be separated from IK by using Ca-channel blockers. IC and IK activate at membrane potentials of about -25 mV and -10 mV, respectively. The participation of IAcd and IC to membrane electrophysiology is discussed.

Ionic mechanisms underlying the firing properties of rat neonatal motoneurons studied in vitro

Ionic mechanisms underlying the firing properties of spinal motoneurons of neonatal rats (postnatal days 3-10) have been investigated using a hemisected, in vitro spinal cord preparation. These results demonstrate the presence of a high-threshold voltage-dependent calcium response and partial sodium-dependent spikes. The calcium current is evident during the falling phase of the action potential and is the major component of the after-depolarizing potential. The subsequent increase in intracellular calcium concentration activates a calcium-dependent potassium conductance (gK-Ca), the major component of the after-hyperpolarizing potential. The gCa, by activating gK-Ca, is the primary determinant of firing rate in neonatal motoneurons. For, when gCa was blocked by Cd2+, the interspike interval decreased, the maximum firing rate and the slope of the firing frequency-injected current relation increased. The calcium current is particularly robust during the first few postnatal days; during this period, tetrodotoxin resistant action potentials can be elicited by direct stimulation under control conditions. In animals older than 5 days such calcium spikes could be elicited only after decreasing gK with intracellular Cs+ or extracellular tetraethylammonium. This was the case even when 1 mM of the bath CaCl2 was replaced with BaCl2. The rising phases of calcium spikes recorded from neurons in both age groups demonstrate several components suggesting the calcium spikes comprise several discrete events, which probably originate across the dendritic membrane. When gK was decreased by bath application of tetraethylammonium+ and Cs+, neonatal motoneurons generated prolonged Ca-dependent spikes lasting for up to 6 s. Repolarization of Ca spikes occurred in two stages, the first was rapid (-2.11 +/- 0.8 V/s, n = 6) but incomplete. The second, was slower (-0.01 +/- 0.003 V/s, n = 5) and returned the membrane potential to the resting level after about 1-2 s. It is suggested that accumulation of extracellular potassium may contribute to the slow phase of repolarization. Motoneurons from the younger age group (3-5 days old) demonstrate all-or-none partial spikes rising from the after-depolarization of directly elicited sodium-dependent action potentials. Similar partial spikes were elicited from neurons from older animals during intracellular Cs+ loading. The partial spikes had faster rates of rise than the tetrodotoxin-resistant spikes and were not seen after tetrodotoxin treatment, suggesting that they are sodium-dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

Step reductions in extracellular Ca2+ activate a transient inward current in chick dorsal root ganglion cells

We investigated whether transient step reductions in divalent cations would produce detectable changes in neuronal excitability similar to those reported in the total absence of divalent cations. Using cultured chick dorsal root ganglion cells as a model system, our results indicate that a step reduction in divalent cations induces a transient inward current. This response is mediated by a tetrodotoxin-resistant, Na+-permeable, cation channel that is blocked by cadmium. This, and our observation that the response is abolished by verapamil, suggests that the current passes through calcium channels. This transient inward current was estimated to be activated by decreases in extracellular calcium ([Ca2+]o) as small as 0.5-0.8 mM and thus represents a different response from the one previously observed when steady-state [Ca2+]o levels were reduced to micromolar levels.

Membrane currents recorded from a fragment of rabbit intestinal smooth muscle cell

Properties of ionic currents in smooth muscle membranes of the longitudinal muscle layer of the rabbit ileum were investigated using the single electrode voltage clamp method. In the present experiments, this method was applicable only to the smooth muscle ball (fragment) and not for the dispersed whole cell, because of incompleteness of the voltage clamping. A voltage step elicited a transient inward current followed by an outward current. This outward current was partly inhibited by Mn2+ or nisoldipine or by a reduction in the extracellular [Ca2+] ([Ca2+]o). Tetraethylammonium (TEA) reduced the delayed outward current in a dose-dependent manner, but 50 mM TEA did not produce a complete block of a residual current. When the pipette contained K+-free (Cs+ with TEA+) solution, the residual outward current was abolished. The inward current was elicited at -30 mV (holding potential of -60 mV) and reached the maximal value at +10 mV; the polarity was reversed at +60 mV. This inward current depended on the [Ca2+]o and was blocked by Mn2+ or nisoldipine. Ba2+ also permeated the membrane, and the inward current evoked by Ba2+ was also blocked by Mn2+ or nisoldipine. Reduction of [Na+]o in a solution containing 2.4 mM Ca2+ neither modified the current-voltage relation nor the decay of the inward current, but when [Ca2+]o was reduced to below 1 microM, Na+ permeated the membrane and was blocked by nisoldipine. In conclusion, ionic currents were recordable from the fragmented ball of the longitudinal muscle of rabbit ileum. There were at least two K+ currents as the outward current (Ca2+-dependent K+ and delayed K+ currents) and a Ca2+ current as the inward current. The property of the Ca2+ channel was similar to that observed with other preparations.

Maximal Ca2+-activated force elicited by tetanization of ferret papillary muscle and whole heart: mechanism and characteristics of steady contractile activation in intact myocardium

Rapid (8-12 Hz) stimulation of intact heart muscle treated with ryanodine results in steady contractile activation known as tetanus, the amplitude of which can be graded by changing extracellular Ca2+ concentration ([Ca2+]o). The mechanism of the sustained force generation was explored in ferret papillary muscles by measuring membrane potential and by determining the responsiveness of force and intracellular free Ca2+ concentration ([Ca2+]i, estimated with aequorin) to dihydropyridine Ca channel ligands. Membrane potential during tetani ranged from -25 to -60 mV, suggesting that fast or slow Ca channels, or Na-Ca exchange, might be mediating Ca2+ entry. Dihydropyridine effects indicated that slow Ca channels play a predominant role: The agonist Bay K 8644 (0.3-1 microM) increased force and aequorin luminescence, whereas the antagonist nitrendipine (1-30 microM) abolished the tetanus. Under conditions analogous to those in the papillary muscle experiments, tetani were produced in whole Langendorff-perfused ferret hearts following exposure to ryanodine. Contraction saturated as a function of [Ca2+]o in both papillary muscles and whole hearts; i.e., as [Ca2+]o was increased above 10 mM, no further increase in force or pressure generation occurred. In contrast, aequorin luminescence measured in the papillary muscles showed no such saturation. Thus, maximal Ca2+-activated force (or pressure) was achieved during tetani at [Ca2+]o greater than or equal to 10 mM. Calculations of wall stress during tetani in whole heart (15 mM [Ca2+]o) agree well with direct measurements of maximal tension in papillary muscles (5.84 g/mm2 vs. 6.41 g/mm2, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent modulation of Ca channel current in heart cells by Bay K8644

We have investigated the voltage-dependent effects of the dihydropyridine Bay K8644 on Ca channel currents in calf Purkinje fibers and enzymatically dispersed rat ventricular myocytes. Bay K8644 increases the apparent rate of inactivation of these currents, measured during depolarizing voltage pulses, and shifts both channel activation and inactivation in the hyperpolarizing direction. Consequently, currents measured after hyperpolarizing conditioning pulses are larger in the presence of drug compared with control conditions, but are smaller than control if they are measured after positive conditioning pulses. Most of our experimental observations on macroscopic currents can be explained by a single drug-induced change in one rate constant of a simple kinetic model. The rate constant change is consistent with results obtained by others with single channel recordings.

On the time reversal of noise signals

This paper addresses the question of whether, and under what conditions, a noise trace changes its statistical properties when the time axis is reversed in direction. The autocorrelation function of the noise or its power spectrum cannot be used to identify the directionality of time in a noise signal since both are always the same for the signal and for its time reverse, regardless of the characteristics of the noise. However, the autocorrelation function can be generalized to represent the average of the products of powers of the signal at pairs of time instances separated by a given interval. If the powers are not the same for the first and second time instances, the generalized autocorrelation function can detect whether the statistical properties of a noise signal change upon the reversal of the direction of the time axis. We show that noise generated by systems that obey microscopic reversibility, i.e., that are at thermodynamic equilibrium, show the same statistical properties when evaluated forward and backward in time. A noise signal that does not demonstrate such time-reversal behavior discloses that the system that generates it is not at thermodynamic equilibrium. Several model examples are presented for illustration.

Sodium-conducting channels in cardiac membranes in low calcium

With no Ca in the patch electrode, two kinds of channels conduct Na in spontaneously beating embryonic chick heart cells. One channel conducts Na primarily during the upstroke of the action potential and is blocked by tetrodotoxin (TTX). The other channel conducts Na primarily during the late plateau and early repolarization phase of the action potential, but only in Ca concentrations below 10(-6) M. This second channel is TTX-insensitive and has a conductance of 50 to 90 pS, depending upon the interpretation of open-channel flickering. These two Na-conducting channels correspond to the channels that normally carry the fast Na current (INa) and the slow Ca current (Isi).

Effect of prostaglandin E2 on ACTH and beta-endorphin release from rat adenohypophysis in vitro after secretagogues which can mimic various first or second messengers

The purpose of the present study was to further characterize the inhibition by prostaglandin E2 (PGE2) of adrenocorticotropin (ACTH) and beta-endorphin release from rat anterior pituitary fragments in vitro. Peptide hormone release was induced by vasopressin, which initiates secretion via cell surface receptors, or by secretagogues which can mimic various post-receptor mechanisms and the effect of PGE2 was examined. Concentration-response curves of the effect of vasopressin on the release of beta-endorphin-like (beta-End-IR) and ACTH-like immunoreactivity (ACTH-IR) were constructed in the absence or presence of a fixed concentration of PGE2. The concentration-response curve of vasopressin was shifted to the right about 8-fold by PGE2 (1 mumol/l) without altering the maximum effect. PGE2 (60 nmol/l-1 mumol/l) markedly reduced beta-End-IR release induced by 8-bromoadenosine-3',5'-cyclic-monophosphate (8Br-cAMP) (1 mmol/l). Omission of Ca2+ from the incubation medium did not prevent PGE2-induced inhibition of 8Br-cAMP-evoked secretion. 4 beta-Phorbol, 12 beta-myristate, 13 alpha-acetate (PMA) stimulated beta-End-IR and ACTH-IR release in a concentration-dependent manner. This effect was not blocked by indometacin or eicosatetraynoic acid. PG E2 (greater than 100 nmol/l) reduced PMA (100 nmol/l)-elicited secretion by about 50%. PG E2 (1 mumol/l) almost halved beta-End-IR release caused by K+ (30 mmol/l). After pre-incubation in Ca2+-free medium, re-introduction of Ca2+ (1.3 mmol/l) elicited beta-End-IR release. This response was abolished by PG E2 (1 mumol/l). The addition of Ba2+ (10 mmol/l) to a Ca2+-free medium markedly enhanced beta-End-IR release.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulation of the conduction pathway of muscle transverse tubule calcium channels from the surface charge of bilayer phospholipid

Functional calcium channels present in purified skeletal muscle transverse tubules were inserted into planar phospholipid bilayers composed of the neutral lipid phosphatidylethanolamine (PE), the negatively charged lipid phosphatidylserine (PS), and mixtures of both. The lengthening of the mean open time and stabilization of single channel fluctuations under constant holding potentials was accomplished by the use of the agonist Bay K8644. It was found that the barium current carried through the channel saturates as a function of the BaCl2 concentration at a maximum current of 0.6 pA (at a holding potential of 0 mV) and a half-saturation value of 40 mM. Under saturation, the slope conductance of the channel is 20 pS at voltages more negative than -50 mV and 13 pS at a holding potential of 0 mV. At barium concentrations above and below the half-saturation point, the open channel currents were independent of the bilayer mole fraction of PS from XPS = 0 (pure PE) to XPS = 1.0 (pure PS). It is shown that in the absence of barium, the calcium channel transports sodium or potassium ions (P Na/PK = 1.4) at saturating rates higher than those for barium alone. The sodium conductance in pure PE bilayers saturates as a function of NaCl concentration, following a curve that can be described as a rectangular hyperbola with a half-saturation value of 200 mM and a maximum conductance of 68 pS (slope conductance at a holding potential of 0 mV). In pure PS bilayers, the sodium conductance is about twice that measured in PE at concentrations below 100 mM NaCl. The maximum channel conductance at high ionic strength is unaffected by the lipid charge. This effect at low ionic strength was analyzed according to J. Bell and C. Miller (1984. Biophysical Journal. 45:279-287) and interpreted as if the conduction pathway of the calcium channel were separated from the bilayer lipid by approximately 20 A. This distance thereby effectively insulates the ion entry to the channel from the bulk of the bilayer lipid surface charge. Current vs. voltage curves measured in NaCl in pure PE and pure PS show that similarly small surface charge effects are present in both inward and outward currents. This suggests that the same conduction insulation is present at both ends of the calcium channel.

Single cyclic GMP-activated channel activity in excised patches of rod outer segment membrane

The plasma membrane of retinal rod outer segments contains a cyclic GMP-activated conductance which appears to be the light-sensitive conductance involved in phototransduction. Recently, it has been found that this conductance is partially blocked by Mg2+ and Ca2+ at physiological concentrations, thus possibly accounting for the absence of observable single-channel activity in excised membrane patches and for the unusually small apparent unit conductance deduced from noise measurements on intact cells. We now report that, as expected from this idea, single cGMP-activated channel activity can be detected from an excised rod membrane patch in the absence of divalent cations. The most prominent unitary current had a mean conductance of approximately 25 pS. Both individual channel openings (mean open time approximately 1 ms) and short bursts of openings (mean burst duration of about a few milliseconds) were observed. In addition, there were smaller events which probably represented other states of the conductance. The mean current increased with the third power of cGMP concentration, suggesting that there are at least three cGMP-binding sites on the channel molecule. With 0.2 mM Mg2+ in the cGMP-containing solution, a flickering block of the open channel was observed; the effect of Ca2+ was similar. The results resolve a puzzle about the light-sensitive conductance by demonstrating that it is an aqueous pore rather than a carrier.

Voltage-dependent properties of macroscopic and elementary calcium channel currents in guinea pig ventricular myocytes

Whole-cell Ca channel currents were recorded from guinea pig ventricular myocytes that were internally perfused with Cs solution and bathed in solutions containing 3.6 mM Ca, 3.6 mM Ba or 90 mM Ba (34 degrees C). Single Ca channel currents were recorded from cell-attached membrane patches of similar myocytes; the patch pipettes contained a 90 mM Ba solution. 1. Although the shape of the whole-cell I-V relation was independent of the bathing solution, this was not the case with the location of the inward current maximum (Vpeak); Vpeak in 90 mM Ba was about 30 mV positive to Vpeak in 3.6 mM Ba. 2. The activation and inactivation of whole-cell currents were voltage dependent. Compared to the voltage dependencies in 3.6 mM Ba, those in 90 mM Ba were shifted by about 30 mV to the right, suggesting a neutralization of surface charges. 3. Observations compatible with the ion permeation model proposed by Hess and Tsien (1984) included (a) a depression of current during Ca/Ba solution exchange, (b) a high divalent to monovalent ion permeability, and (c) rectification of the outward limb of the I-V relation. 4. Estimated current densities at Vpeak were similar for myocytes in 3.6 mM Ca and 3.6 mM Ba, and about 10 times larger in 90 mM Ba. 5. Average currents (I) calculated from ensembles of records of single Ca channel current had voltage-dependent time courses resembling those of whole-cell IBa (90 mM). 6. Single-channel I-V relations were superimposable on whole-cell I-V curves suggesting that voltage-dependent single-channel parameters (probability of opening, elementary current amplitude) can be related to the voltage-dependent macroscopic current parameters (activation, instantaneous I-V relation) when scaled by channel number. 7. The density of Ca channels in myocytes was calculated from whole-cell IBa (90 mM) and average current through single channels. The outcome, 3-5 channels/micron 2, agrees with two other recent estimates (Tsien et al. 1983; Lux and Brown 1984). However, it is difficult to reconcile with the much lower density that one would forecast from the frequency of functional channel observation in myocyte membrane patches (Pelzer et al. 1985c).

Arsenazo III transients and calcium current in a normally non-spiking neuronal soma of crayfish

Arsenazo III was used to investigate Ca2+ transients in the normally non-excitable soma of the motor giant neurones of the crayfish Procambarus clarkii. Two kinds of regenerative potentials could be obtained depending on membrane potential conditioning: a fast spike after a pre-hyperpolarization to -90 mV and a slow action potential after a pre-depolarization to -50 mV. Only the second of these was accompanied by an Arsenazo III transient. In voltage-clamped, somata injected, with tetraethylammonium chloride, an absorbance change could be obtained by pulsing the membrane potential above -44 mV. The relationship between absorbance change and potential peaked between 0 and +10 mV then fell off to zero at ca. +150 mV. Changes in light absorbance studied using double-pulse protocols suggested that the inactivation of Ca2+ entry was predominantly mediated by the intracellular free Ca2+ concentration. External application of 1 mM-CdCl2 abolished both the absorbance changes and the (Ca2+) inward current. The voltage dependence of this current was similar to that of the absorbance change. For positive membrane potential the current-voltage relationship showed a voltage-dependent conductance property, the origin of which is discussed.

Lanthanum as a surrogate for calcium in transmitter release at mouse motor nerve terminals

The mechanism by which lanthanum (La3+) causes an increased frequency of miniature end-plate potentials (m.e.p.p.s) was studied at the mouse neuromuscular junction. At concentrations as low as 0.25 microM, La3+ caused a progressive rise in m.e.p.p. frequency, to a maximum of several hundred per second. 'Washing' with solution containing EDTA arrested the rise, but did not substantially reduce the raised m.e.p.p. frequency. At partially 'lanthanized' junctions high frequencies of m.e.p.p.s were maintained indefinitely, even in 0 Ca2+/EDTA solutions. The rate of development of high m.e.p.p. frequency was increased by repetitive nerve stimulation or by depolarization of the nerve terminal (high K+ or focally applied current), and appeared to be proportional to the concentration of La3+ over the range of 0.25-5 microM. At low concentrations of La3+ the rise of m.e.p.p. frequency depended upon the co-presence of a small amount of Ca2+ (greater than 10 microM) and was slowed and partially blocked by Cd2+, or by Ca2+ at about 10 microM. The quantal content of end-plate potentials was usually reduced in the presence of La3+, but was increased over control values after removal of La3+ by 'washing' with solution containing EDTA, once a raised m.e.p.p. frequency had developed. At partially lanthanized junctions the absolute increases in m.e.p.p. frequency produced by Ca2+ (in raised K+), ethanol, or by nerve stimulation in the presence of Ba2+, were greater than at control junctions, but in each case the increases in the logarithm of m.e.p.p. frequency were less than at control junctions. It is concluded that La3+ causes transmitter release only after entry into the nerve terminal via voltage-sensitive channels, probably those that normally admit Ca2+, that La3+ and Ca2+ may co-operate at internal sites to induce transmitter release, and that these ions both co-operate and compete at external sites that regulate their entry into the nerve terminal.

Calcium channels in planar lipid bilayers: insights into mechanisms of ion permeation and gating

Electrophysiological recordings were used to analyze single calcium channels in planar lipid bilayers after membranes from bovine cardiac sarcolemmal vesicles had been incorporated into the bilayer. In these cell-free conditions, channels in the bilayer showed unitary barium or calcium conductances, gating kinetics, and pharmacological responses that were similar to dihydropyridine-sensitive calcium channels in intact cells. The open channel current varied in a nonlinear manner with voltage under asymmetric (that is, physiological) ionic conditions. However, with identical solutions on both sides of the bilayer, the current-voltage relation was linear. In matched experiments, calcium channels from skeletal muscle T-tubules differed significantly from cardiac calcium channels in their conductance properties and gating kinetics.

Fast and slow gating behaviour of single calcium channels in cardiac cells. Relation to activation and inactivation of calcium-channel current

The Ca-channel gating behaviour during steady and stepwise depolarization was examined in recordings of single Ca-channel activity from cell-attached membrane patches of single ventricular cells isolated enzymatically from hearts of adult guinea pigs. The single-channel recordings were performed by means of the improved patch-clamp technique (Hamill et al. 1981) with 90 mM Ba in the pipettes. Upon step depolarization, two types of current records were regularly observed in the ensembles: (1) traces with Ca-channel activity (in the form of closely-spaced brief pulses of inward current with a unitary amplitude) of various length, and (2) blank sweeps without any detectable single-channel opening. The records with Ca-channel activity show a distinct tendency for openings to occur towards the beginning of the clamp pulse, followed by long periods of silence. The blank sweeps seem to reflect a condition or conditions where the Ca channel is unavailable for opening. The corresponding ensemble mean current I(t) displayed a rapid rising phase to its peak followed by a slow decay. During steady depolarization, kinetic analysis of the distributions of all open and shut lifetimes revealed a monoexponential probability density distribution function of all open times. By contrast, more than two exponential terms were required for an accurate description of the frequency distribution of all shut lifetimes. Corresponding to the two well-separated fast closed time components, individual Ca-channel openings were grouped into bursts of openings. The bursting behaviour reflected fast gating transitions and was related to the fluctuations of the Ca channel between two short-lived closed states and one open state. This fast gating was terminated by the entrance of the Ca channel into at least one long-lived closed state, exit from which was slow in comparison to the rapid cycling. As consequence, bursts of openings were further grouped together in clusters of bursts, the cluster behaviour being related to slow gating transitions in the kinetics of the Ca channel. The biphasic frequency distribution of the first latencies (resulting from the transit through the two short-lived shut states, before the open state is entered) superimposed on the first time derivative of the rising phase of the ensemble mean current, I(t), upon step depolarization. The time constant of the monoexponential distribution function of all cluster lifetimes matched the declining phase of I(t) during maintained depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+-channel current and its modification by the dihydropyridine agonist BAY k 8644 in isolated smooth muscle cells

The electrophysiological properties of single smooth muscle cells isolated from the longitudinal layer of the guinea-pig ileum were studied with the whole-cell patch-clamp technique. The finding of resting potentials between -45 and -50 mV and the occurrence of spontaneous electrical activity when K+ was the predominant intracellular cation indicated that the cells were not leaky or hyperpermeable. The existence of an inward Ca2+ current overlapping in time with an outward rectifying K+ current was demonstrated. The latter could be selectively blocked by replacing internal K+ with Cs+ and external Ca2+ with Ba2+. Depolarizations to potentials between -40 and +50 mV evoked time-dependent inward currents, with a maximum peak value between -20 and 0 mV. For depolarizations beyond +50 mV time-dependent outward currents appeared. These currents were inhibited by 0.1 mM CdCl2. The activation of the inward current showed a sigmoidal time course, and the rate of onset of the current increased at more positive potentials. Inactivation could be described by two exponentials. The threshold for activation was about -40 mV, and full activation was reached at 0 mV. Inactivation was complete near 0 mV, whereas the channels were fully available at -80 mV. The fully-activated Ca2+-channel current was strongly voltage dependent. The conductance decreased for potentials close to the reversal potential, and showed rectification for hyperpolarizing potentials. The Ca2+ agonist BAY k 8644 enhanced the Ca2+-channel current without a significant effect on its kinetics. The fully-activated current and the steady-state activation were enhanced in a rather voltage-independent way.

Pharmacologic levels of nitrendipine do not affect actin-myosin interaction in the human uterus and placenta

Because of the potential of dihydropyridine calcium channel blockers in the management of premature labor, we have studied the direct effects of nitrendipine on actomyosin in the pregnant and nonpregnant uterus and in the term human placenta. Actomyosin adenosinetriphosphatase in the three tissues and another model of actin-myosin interaction, superprecipitation of placental actomyosin, were inhibited by nitrendipine. The inhibition was not diminished by high concentrations of calcium. To identify the mechanism, placental myosin was phosphorylated in the absence and presence of 0.8 X 10(-4) mol/L of nitrendipine. The myosin phosphorylated in the presence of nitrendipine had lower actin-activated adenosinetriphosphatase, which is consistent with the inhibition of myosin light chain phosphorylation. However, nitrendipine did not affect the adenosinetriphosphatase activity of myosin nor did further reduce the adenosinetriphosphatase of the already phosphorylated placental actomyosin. Thus nitrendipine inhibition is directed to the phosphorylation reaction but not to the adenosinetriphosphatase site of myosin. Myometrial relaxation in vivo or in vitro occurs at the pharmacologic nitrendipine levels of 10(-9) to 10(-8) mol/L, which is at least 10,000 times lower than that of the concentration of 50% inhibition of myosin light chain phosphorylation (0.0026 +/- 0.00015 mol/L of nitrendipine, mean +/- SEM) demonstrated in the present work. Because of this difference, the direct intracellular actions of dihydropyridine calcium channel blockers are not expected to cause adverse effects in the uteroplacental system when these drugs are used in the prevention or treatment of premature labor.

Effects of the external pH on Ca channels: experimental studies and theoretical considerations using a two-site, two-ion model

Some effects of the external pH on Ca channels were studied in a hybridoma cell line (mAb-7B), by using the whole-cell configuration of the patch-clamp technique. As the pH was lowered, both the activation and the inactivation curves shifted toward less negative membrane potentials, suggesting a pH-induced decrease of an external negative surface potential, sensed by the mechanism of gating. The potential for half-activation, V1/2, and that for half-inactivation, Vh, were related by a straight line with a slope of one. The inward current varied exponentially with V1/2, as would be expected if the field inside the channel and the Ca2+ concentration at the entrance were sensitive to the surface potential. However, the reversal potential and the outward current were unaltered by changes in the pH. Under the hypothesis that the channel senses the surface potential, all these results, as well as the nernstian behavior of the reversal potential with respect to Ca2+, observed in previous studies, are accounted for by a three-barrier, two-ion model for a channel, provided it is assumed that the potential in the channel drops almost entirely across the barrier adjacent to the external solution.

Interactions of divalent cations with single calcium channels from rat brain synaptosomes

Voltage-dependent calcium channels from a rat brain membrane preparation ("synaptosomes") were incorporated into planar lipid bilayers. The effects of calcium, barium, strontium, manganese, and cadmium ions on the amplitudes and kinetics of single channel currents were examined. The order of single channel conductances was gBa greater than gSr greater than gMn, which was the inverse of the order of the mean channel open times: TMn greater than TCa = TSr greater than TBa. In contrast, the identity of the charge carrier had little or no effect on the mean closed times of the channel. Manganese, in the absence of other permeant ions, can pass through single channels (gMn = 4 pS). However, when added to a solution that contained another type of permeant divalent cation, manganese reduced the single channel current in a voltage-dependent manner. Cadmium, a potent blocker of macroscopic "ensemble" calcium currents in many preparations, reduced the current through an open channel in a manner consistent with Cd ions both not being measurably permeant and interacting with a single site. The permeant ions competed with cadmium for this site with the following order: Mn greater than Sr = Ca greater than Ba. These results are consistent with the existence of no less than one divalent cation binding site in the channel that regulates ion permeation.

Some limitations of the cell-attached patch clamp technique: a two-electrode analysis

With two independent patch electrodes sealed to small clusters of electrically coupled chick embryo cardiac cells, we have measured four parameters: true seal and patch resistance, channel conductance, and membrane potential. One electrode was in the cell-attached mode, and recorded current flowing in parallel through the membrane patch and seal. The second electrode, sealed on a different cell in the cluster, was in the whole cell recording configuration, and served to record or control the membrane potential of the cluster. We fit the four measured parameters to a simple electrical model to reveal errors not usually recognized in the patch-clamp technique. Among these are the following: (1) The apparent seal resistance, determined by changing the potential in a patch electrode, may be a poor estimate of true seal resistance, since it includes the parallel combination of seal- and patch-resistance. (2) Patch resistance may be influenced by the electrode filling solution, and is often much lower than is usually assumed. (3) With a small cell preparation that has an input resistance in the gigaohm range, measurements of single-channel conductance using a cell-attached patch electrode may be inaccurate because cell membrane potential does not remain constant as electrode potential is varied.

Kainic acid responses and toxicity show pronounced Ca2+ dependence

Responses of pyramidal neurons to ionophoretic kainate, quisqualate and N-methyl aspartate were studied in a submerged rat piriform cortex slice as a function of Ca2+ and Mg2+ concentrations. The results suggest that the channel activated by kainate is unusually influenced by Ca2+, excitotoxicity is Ca2+-dependent and a function of Ca2+ concentration, and the excitotoxic actions of various amino acid agonists are correlated with the Ca2+ dependence of their responses.

The development of calcium and potassium currents during oogenesis in the starfish, Leptasterias hexactis

The development of membrane electrical properties of oocytes of the starfish Leptasterias hexactis during oogenesis was studied using voltage- and current-clamp techniques. Two voltage-dependent K currents--the fast transient and inwardly rectifying--are present early in oogenesis, before the rapid growth phase, and are maintained throughout oogenesis at the same current density and kinetics. The inward current, which is composed of a Ca current and a slower Ca-dependent inward sodium current, is also present early in oogenesis, but at very low current density. Late in oogenesis, after the oocyte has grown to full size, the inward current increases in amplitude by about fivefold, and undergoes major changes in kinetics. These changes are closely associated with the migration of the germinal vesicle to the cell periphery. The relationship of these events to electrophysiological changes during subsequent maturation and fertilization of the oocytes is discussed.

Calcium currents of cesium loaded isolated smooth muscle cells (urinary bladder of the guinea pig)

Single smooth muscle cells isolated from the urinary bladder of the guinea-pig were studied at 35 degrees C in a solution composed of 150 mM NaCl, 3.6 mM CaCl2, 1.2 mM MgCl2, 5.4 mM KCl, 20 mM TEA-Cl, 5 mM glucose, 10 mM HEPES/NaOH (pH 7.4). Whole cells were clamped with a single patch electrode. The clamp settled a step from -65 to -5 mV within 260 microseconds, and afterwards the voltage inhomogeneities were less than 2 mV (measured at the cell edge with a second electrode). The calcium inward current iCa was dissected from net currents by blocking potassium outward currents by means of patch electrodes filled with 130 mM CsCl (Klöckner and Isenberg 1985 a). Pyruvate, succinate and oxalacetate in the patch electrode stabilized iCa and prevented its "run down". 140 ms long clamp steps from -65 to -5 mV evoked a net inward current which could be reversibly blocked by 5 mM NiCl2. The "Ni-sensitive" difference current iCa peaked within 2-4 ms to about 1 nA per cell. Afterwards it completely inactivated; the inactivation could be fitted with three exponentials (time constants of 4, 30, and 250 ms, respectively). The half decay time of 16 ms suggests that most of the inactivation resulted from the fast exponential process. The reference current in the presence of Ni was nearly time independent and almost zero; therefore, iCa could be approximated from the net inward current using the zero current as a reference line.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac calcium channels and their control by neurotransmitters and drugs

Calcium channels, which play a primary role in the control of the calcium influx into cardiac cells, were initially studied by recording macroscopic currents in multicellular preparations. More recently, channel research has combined studies of whole cell calcium currents and elementary currents through single calcium channels, both measured in isolated cardiac cells. These studies provide insight into the mechanism of opening and closing of single calcium channels and enable inferences to be made about the whole cell calcium current from the average gating behavior of single channels. In addition, they promise a more complete understanding of the relation between the biophysical properties and molecular structure of the calcium channel. New information has also been obtained on the modulation of calcium channel gating by neurotransmitters and drugs.

Ruthenium red-insensitive Ca2+ uptake and release by mitochondria

Calcium uptake by rat liver mitochondria driven by an artificial pH gradient is ruthenium red insensitive, electrically neutral, and inhibited by the local anesthetic, nupercaine. This pH-driven Ca2+ transport is also inhibited by NH3, Pi, and acetate. Direct measurements of Pi indicate it is not translocated with Ca2+ during pH-driven Ca2+ uptake. Calcium is therefore not transported by a Ca2+-Pi symport mechanism. Ruthenium red-insensitive Ca2+ efflux is similar in its inhibition by nupercaine and its kinetics, and is also electroneutral. This suggests that the Ca2+ uptake described here occurs via reversal of the principal pathway of mitochondrial Ca2+ release. From the available data, pH-driven Ca2+ uptake (and presumably Ca2+ efflux) is hypothesized to occur by Ca2+ symport with unidentified anions. Protons may move counter to Ca2+ or reversibly dissociate from cotransported anions, which therefore couples Ca2+ transport to the pH gradient.

Voltage-dependent ion channels in T-lymphocytes

The gigaohm seal 'patch-clamp' technique has recently enabled exploration of the electrical properties of cells of the immune system. In this paper we review progress made to date in cataloguing the ion channels present in the cell membranes of T-lymphocytes and present new data on the types of ion channels present in a number of human and murine T-cell-derived cell lines. The ion channels thus far described in these cells are strikingly similar to those found in nerve and muscle cells. Voltage-gated potassium channels resembling delayed rectifier potassium channels in excitable cells are present in most T-lymphocytes, T-lymphocyte-derived cell lines and macrophages. Sodium channels indistinguishable from those in excitable cells are present in a small fraction of T-cells and T-cell lines, and in some natural killer cells. Calcium channels have been reported in B-lymphocyte-derived cell lines, but have not been found in T-lymphocytes or in any T-cell-derived cell line. Potassium channels are required for activation of T-lymphocytes by mitogen, allogeneic cells, or by antigen, for lysis of target cells by natural killer cells, and may be involved in the triggering mechanism for activation of T-cells. The prevailing conception of early events in T-lymphocyte activation, the 'calcium hypothesis', involves an elevation of cytoplasmic free calcium levels as the proposed 'second messenger' in activation, giving rise to a cascade of subsequent events resulting eventually in cell division. A major focus of this paper is to evaluate specific mechanisms which have been proposed to account for experimental evidence, both in the literature and also presented here, pertaining to the calcium hypothesis. One such mechanism involves calcium channels, which have been postulated to account for the early calcium influx in activated T-lymphocytes. Since calcium channels have not been detected in T-cells, we explore the possibility that existing data can be accounted for without calcium channels. In particular, we show that many of the effects of 'calcium channel antagonists' such as verapamil, nifedipine, diltiazem and some polyvalent cations, can be accounted for by their blocking of voltage-gated potassium channels.

Characterization of the Ca2+ coordination site regulating binding of Ca2+ channel inhibitors d-cis-diltiazem, (+/-)bepridil and (-)desmethoxyverapamil to their receptor site in skeletal muscle transverse tubule membranes

Ca2+ inhibits (-)[3H]desmethoxyverapamil, d-cis-[3H]diltiazem and (+/-)[3H]bepridil binding to skeletal muscle transverse-tubule membranes with a half-maximum inhibition constant, K0.5 = 5 +/- 1 microM. This value is close to that of the high affinity Ca2+ binding site which controls the ionic selectivity of the Ca2+ channel found in electrophysiological experiments suggesting that the Ca2+ coordination site which regulates the ionic selectivity is also the one which alters binding of the Ca2+ channel inhibitors investigated here. Ca2+ and (-)D888 bind to distinct sites. Occupation of the Ca2+ coordination site decreases the affinity of (-)D888 for its receptor by a factor of 5. Other divalent cations have the same type of inhibition behavior with the rank order of potency Ca2+ (K0.5 = 5 microM) greater than Sr2+ (K0.5 = 25 microM) greater than Ba2+ (K0.5 = 50 microM) greater than Mg2+ (K0.5 = 170 microM).

Potassium channels in cultured bovine adrenal chromaffin cells

K channels of bovine adrenal chromaffin cells were studied using patch-clamp techniques. Whole-cell K currents measured near +10 mV were much larger in 1 mM-external Ca than in Ca-free saline. Noise analysis suggested that this Ca-dependent current was carried by a large unitary conductance channel, called BK channel, which was previously described in inside-out patches (Marty, 1981). The Ca-dependent K current near +10 mV declined with time due to 'run-down' of Ca channels. At the same time, a fraction of the outward current observed above +50 mV was also eliminated. This outward current component probably represents K efflux through Ca channels. Whole-cell Ca-dependent K currents were studied using various Ca buffers. EGTA buffers were surprisingly inefficient: in order to block the current entirely, it was necessary to use an isotonic EGTA solution and to increase internal pH. 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was at least five times more efficient than EGTA. In isolated patches three types of single-channel K currents were observed. Under normal ionic conditions (140 mM-K inside, 140 mM-Na outside), the unitary conductances measured between -20 and +40 mV were 96 pS, 18 pS and 8 pS. The 96 pS channels are the Ca-dependent BK channels. 18 pS and 8 pS channels were both activated and then inactivated by membrane depolarization. Both displayed complex kinetics; single-channel currents were grouped in bursts. Activation and inactivation kinetics were faster for the 18 pS channel (therefore termed FK channel, for fast K channel) than for the 8 pS channel (SK channel, for slow or small amplitude channel). The voltage dependence of opening probability was steeper for the FK channel as compared to the SK channel.

The Ca channel in skeletal muscle is a large pore

The permeability of Ca channels to various foreign cations has been investigated in the absence of external Ca2+. All physiological metal cations are clearly permeant, including Mg2+. The large organic cation n-butylamine+ is sparingly permeant or impermeant, but its larger derivative 1,4-diaminobutane2+ is highly permeant. Among the cations of the methylated ammonium series, permeability diminishes in a graded fashion as ion size increases. Tetramethylammonium, the largest cation found to be permeant, has a diameter of about 6 A; hence, the aqueous pore of the Ca channel at its narrowest point can be no smaller. That the pore is so large strengthens our view that, under physiologic conditions, the high selectivity of Ca channels is due to selective binding of Ca2+ rather than to rejection of other cations by, for example, a sieving mechanism.

Kinetic properties and selectivity of calcium-permeable single channels in Aplysia neurones

Two kinds of single channels, carrying inward currents even above the Na and Cl ion equilibrium potentials, were observed in outside-out patches from Aplysia neurones bathed in K-free internal and external solutions. The channel carrying the larger elementary current has been studied in detail. When the internal solution contained mainly CsCl, this channel usually inactivated during the first minutes following isolation of the membrane patch. However, when the internal solution contained NaCl instead of CsCl, the channel remained functional during several hours, thus allowing the present study. Na-Tris, NaCl-mannitol and Ca-Ba external substitution experiments showed that the channel studied is much more permeable to divalent cations than to sodium ions. Mono-exponential open-time distributions obtained under identical conditions from different membrane patches indicated either slow (in the order of 100 ms at 0 mV) or rapid (a few milliseconds at 0 mV) mean open-times. Biphasic open-time distributions could be obtained from other membrane patches under the same conditions. These results suggest the existence of two different gating modes. Both the open-time distribution and the closed-time distribution are voltage sensitive: membrane depolarization activates the channel by lengthening the openings and shortening the closures. The threshold of activation if any, is very low and the inactivation, if present, is never complete. Ca-Ba, Ca-Sr and Ca-Mg external substitution experiments showed that the elementary current amplitude is not very sensitive to the nature of the external divalent cation. The elementary current can be slightly larger when carried by Ba ions rather than by Ca ions, but is nearly identical whether carried by Ca, Sr or even Mg ions, which leads to the elementary conductance sequence: Ba greater than or equal to Ca = Sr congruent to Mg. In contrast, the mean open-time of the channel is very sensitive to the nature of the external permeant ion. The longest mean open-time is observed in the presence of Ca ions, and the mean open-time sequence is: Ca greater than Sr greater than Ba greater than Mg. The closed-time distribution is also affected by the nature of the external divalent cation. The above results show that the nature of the permeant ion affects the kinetic properties of the channel much more than its elementary current.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+-sensitive, spontaneously fluctuating, cation channels in the apical membrane of the adult frog skin epithelium

The fluctuations in transepithelial current through the abdominal skin of bullfrogs (Rana catesbeiana) were analysed while the transepithelial voltage was clamped to zero. A Lorentzian component in the power spectrum was recorded when the skin was bathed with Ca2+ free NaCl Ringer's on both sides. After replacement of all mucosal Na+ by choline the Lorentzian component disappeared. The application of mucosa positive potentials enhanced the plateau of the relaxation noise component while it was depressed by mucosa negative potentials. These observations showed that the current associated with the relaxation noise, was carried by Na+ moving in the inward direction. Divalent cations added to the mucosal solution in micromolar concentrations depressed the relaxation noise immediately, which is indicative for an apical localization of the fluctuating channels. The relaxation noise depended strongly on the pH of the mucosal medium: alkalinization enhanced the relaxation noise while acidification depressed the fluctuations. Micromolar concentrations of the diuretic amiloride, which is known to block the Na+ entry into the cellular compartment, enhanced the Na+-dependent relaxation noise while at higher concentrations an inhibitory effect was observed. From these observations it was concluded that the relaxation noise is caused by inward Na+ movement through fluctuating channels which are localized in the apical membrane. These channels seem to constitute a pathway in parallel with the amiloride-blockable channels. Ionic substitution of Na+ by other monovalent cations showed that these channels are also permeable for K+, Rb+, NH4+, Cs+ and Tl+, but not for Li+. Divalent cations in micromolar concentrations completely occlude these fluctuating channels. Therefore, this pathway will be blocked for monovalent cations when normal Ca2+ containing Ringer's are used as mucosal bathing medium.

Light-evoked depolarizations in the retina of Strombus: role of calcium and other divalent cations

Previous studies indicate that overlapping inward sodium and outward potassium currents play a role in generating the waveform of light-evoked depolarizations (LEDs) in one type of retinal neuron in Strombus luhuanus, a marine gastropod [Chinn, K. S., and Gillary, H. L. (1985). Comp. Biochem. Physiol. 80A:233-245]. This paper concerns the effects of divalent cations on the LED. The LED can exhibit a distinct early phase of depolarization (DE). Increasing the [Ca2+] in the artificial seawater (ASW) bathing medium reduced the amplitude of the entire LED, and omitting Ca2+ increased it. Adding 10 mM Sr2+ or 10 mM Mn2+ to either normal ASW or 0-Ca2+ ASW decreased the LED amplitude. Adding 10 mM Ba2+ to 0-Ca2+ ASW also decreased the LED amplitude, but adding Ba2+ to normal ASW selectively increased DE. Cd2+ (100 microM) selectively reduced DE when added to normal ASW but not when added to 0-Ca2+ ASW. The results show that a variety of divalent cations can alter the currents that underlie the LED. They also suggest that an inward Ca2+ current occurs during DE.

The K+ channel of sarcoplasmic reticulum. A new look at Cs+ block

K+-selective ion channels from mammalian sarcoplasmic reticulum were inserted into planar phospholipid bilayers, and single-channel currents measured in solutions containing Cs+. Current through this channel can be observed in symmetrical solutions containing only Cs+ salts. At zero voltage, the Cs+ conductance is approximately 15-fold lower than the corresponding K+ conductance. The open channel rectifies strongly in symmetrical Cs+ solutions, and the Cs+ currents are independent of Cs+ concentration in the range 18-600 mM. Biionic (Cs+/K+) reversal potentials are only 10 mV, showing that Cs+ is nearly as permeant as K+, though much less conductive. Addition of Cs+ to symmetrical K+ solutions reduces current through the channel in a voltage-dependent way. The results can be explained by a free energy profile in which the channel's selectivity filter acts in two ways: to provide binding sites for the conducting ions and to serve as a major rate-determining structure. According to this picture, the main difference between high-conductance K+ and low-conductance Cs+ is that Cs+ binds to an asymmetrically positioned site approximately 20-fold more tightly than does K+.

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

Slow Ca and K currents across frog skeletal muscle membrane were recorded with the Vaseline gap voltage clamp in order to investigate block by divalent cations and various organic compounds. Cd2+, Ni2+, Co2+, Mn2+, Mg2+ all block Ca currents, as do barbiturates, D-600 and nifedipine. Local anesthetics also block Ca currents, with the impermeant quaternary lidocaine derivative, OX-314, being more than an order of magnitude less potent than its permeant parent compound. Surprisingly, all agents that blocked Ca currents also blocked the slow K currents. To explain this pharmacologic parallel, one could suggest that K current is activated by Ca2+ appearing in the myoplasm due to the combination of Ca current and release from internal stores. While possibly correct for intact fibres, this hypothesis appears not to apply in our case where the myoplasm contained the Ca chelator EGTA at high concentration. Instead, K currents seem to be activated by a decrease in external [Ca2+]. In the transverse tubules, Ca current is known to cause [Ca2+] to decline to submicromolar concentrations, and evidence is presented that K currents are activated by Ca depletion from a restricted extracellular space. It is suggested that K currents flow through Ca channels that have become capable of passing monovalent cations after the tubules have become depleted of Ca2+.