The selectivity of the delayed potassium conductance of frog skeletal muscle fibers

A physiologically based biokinetic model for cesium in the human body

A physiologically descriptive model of the biological behavior of cesium in the human body has been constructed around a detailed blood flow model. The rate of transfer from plasma into a tissue is determined by the blood perfusion rate and the tissue-specific extraction fraction of Cs during passage from arterial to venous plasma. Information on tissue-specific extraction of Cs is supplemented with information on the Cs analogues, K and Rb, and known patterns of discrimination between these metals by tissues. The rate of return from a tissue to plasma is estimated from the relative contents of Cs in plasma and the tissue at equilibrium as estimated from environmental studies. Transfers of Cs other than exchange between plasma and tissues (e.g. secretions into the gastrointestinal tract) are based on a combination of physiological considerations and empirical data on Cs or related elements. Model predictions are consistent with the sizable database on the time-dependent distribution and retention of radiocesium in the human body.

Properties of K+ conductances in cat retinal ganglion cells during the period of activity-mediated refinements in retinofugal pathways

During ontogeny retinal ganglion cells manifest pronounced changes in excitable membrane properties. To further our understanding of the ionic conductances underlying such functional changes, the whole-cell voltage-clamp variation of the patch-clamp technique was used to record potassium currents in 220 ganglion cells dissociated from cat retinas ranging in age from embryonic day 31 to postnatal day 10. Potassium currents were isolated by blocking voltage-gated Na+ and Ca2+ currents with tetrodoxin (TTX) and CoCl2 respectively and were characterized by their pharmacology, kinetics and voltage-dependence of activation and inactivation. In all cases, a combination of three currents accounted for the total outward calcium-independent K+ current: (i) a steady linear conductance; (ii) a voltage-gated transient current, IA, and (iii) a voltage-gated sustained current, IK. Both voltage-gated currents were affected by the application of 4-aminopyridine and tetraethylammonia (TEA): IA showed a greater sensitivity to 4-aminopyridine, while IK was more sensitive to TEA. Both voltage-gated currents were present throughout the developmental period examined; however, the percentage of retinal ganglion cells (RGCs) expressing IA showed a marked decline from 82% at E31 to 45% at postnatal ages. During this developmental period there was an increase in the density of the two voltage-gated and the linear conductance. Additionally, with maturation, significantly slower inactivation kinetics were observed for IK. These findings, and our previous results dealing with maturational changes in the TTX-sensitive voltage-gated Na current, are related to the generation of excitability in developing retinal ganglion cells. Furthermore, the presence of cells with and without transient K+ conductance throughout development suggests that the different spiking patterns observed in RGC classes may be partially due to differences in their membrane properties.

Characterisation of Ca(2+)-dependent inwardly rectifying K+ currents in HeLa cells

The whole-cell configuration of the patch-clamp technique was used to examine K+ currents in HeLa cells. Under quasi-physiological ionic gradients, using an intracellular solution containing 10(-7) mol/l free Ca2+, mainly outward currents were observed. Large inwardly rectifying currents were elicited in symmetrical 145 mmol/l KCl. Replacement of all extracellular K+ by isomolar Na+, greatly decreased inward currents and shifted the reversal potential as expected for K+ selectivity. The inwardly rectifying K+ currents exhibited little or no apparent voltage dependence within the range of from -120 mV to 120 mV. A square-root relationship between chord conductance and [K+] at negative potentials could be established. The inwardly rectifying nature of the currents was unaltered after removal of intracellular Mg2+ and chelation with ATP and ethylenediaminetetraacetic acid (EDTA). Permeability ratios for other monovalent cations relative to K+ were: K+ (1.0) > Rb+ (0.86) > Cs+ (0.12) > Li (0.08) > Na+ (0.03). Slope conductance ratios measured at -100 mV were: Rb+ (1.66) > K+ (1.0) > Na+ (0.09) > Li (0.08) > Cs+ (0.06). K+ conductance was highly sensitive to intracellular free Ca2+ concentration. The relationship between conductance at 0 mV and Ca2+ concentration was well described by a Hill expression with a dissociation constant, KD, of 70 nmol/l and a Hill coefficient, n, of 1.81. Extracellular Ba2+ blocked the currents in a concentration- and voltage-dependent manner. The dependence of the KD for the blockade was analysed using a Woodhull-type treatment, locating the ion interaction site at 19% of the distance across the electrical field of the membrane and a KD (0 mV) of 7 mmol/l. Tetraethylammonium and 4-aminopyridine were without effect whilst quinine and quinidine blocked the currents with concentrations for half-maximum effects equal to 7 mumol/l and 3.5 mumol/l, respectively. The unfractionated venom of the scorpion Leiurus quinquestriatus (LQV) blocked the K+ currents of HeLa cells. The toxins apamin and scyllatoxin had no detectable effect whilst charybdotoxin, a component of LQV, blocked in a voltage-dependent manner with half-maximal concentrations of 40 nmol/l at -120 mV and 189 nmol/l at 60 mV; blockade by charybdotoxin accounts for the effect of LQV. Application of ionomycin (5-10 mumol/l), histamine (1 mmol/l) or bradykinin (1-10 mumol/l) to cells dialysed with low-buffered intracellular solutions induced K+ currents showing inward rectification and a lack of voltage dependence.

Altered ion channel conductance and ionic selectivity induced by large imposed membrane potential pulse

The effects of large magnitude transmembrane potential pulses on voltage-gated Na and K channel behavior in frog skeletal muscle membrane were studied using a modified double vaseline-gap voltage clamp. The effects of electroconformational damage to ionic channels were separated from damage to lipid bilayer (electroporation). A 4 ms transmembrane potential pulse of -600 mV resulted in a reduction of both Na and K channel conductivities. The supraphysiologic pulses also reduced ionic selectivity of the K channels against Na+ ions, resulting in a depolarization of the membrane resting potential. However, TTX and TEA binding effects were unaltered. The kinetics of spontaneous reversal of the electroconformational damage of channel proteins was found to be dependent on the magnitude of imposed membrane potential pulse. These results suggest that muscle and nerve dysfunction after electrical shock may be in part caused by electroconformational damage to voltage-gated ion channels.

Specific blockage of squid axon resting potassium permeability by Haliclona viridis (Porifera: Haliclonidae) toxin (HvTX)

The action of partially purified HvTX, toxin of the marine sponge H. viridis, was explored on the giant axon of the tropical squids Doryteuthis plei and Sepioteuthis sepioidea. HvTX depolarizes the nerves dose dependently. The effect occurs after blocking sodium channels with tetrodoxin (1 microM), removing external Na+, blocking electrically excitable K+ channels with 3,4-diaminopyridine (10 mM) or internal and external application of tetraethylammonium (40 mM). Ouabain (up to 10 mM) does not modify HvTX effect. The action of HvTX occurs only when it is applied to the outer phase of the nerve membrane; microinjection of the toxin into the axons lacks depolarizing effects. HvTX reduces the dependence of membrane potential on external potassium concentration. The apparent 86Rb+ permeability (pi') was measured in axons of S. sepioidea. The value of pi' in normal artificial sea water was 80 (61,96) nm/sec (median and its 95% confidence interval, n = 8) and raised to 1030 (588, 2113) nm/sec (n = 7) when the axons were depolarized to 0 mV raising external K+ to 300 mM. In axons depolarized with HvTX (10 mM external K+) to 0 mV, pi' was 88 (55, 97) nm/sec (n = 8). HvTX could not prevent (P >> 0.05) the increase in pi' induced by 300 mM K+ when the ion concentration was raised before toxin application [pi' = 660 (354, 1876) nm/sec, n = 7]. Most of the 86Rb+ permeability increase in high K+ was prevented if HvTX was added before external K+ was raised [pi' = 298 (264, 337) nm/sec, n = 8]. All the measures of pi' were carried out in solutions containing 1 microM tetrodotoxin, 1 mM 3,4-diaminopyridine and 2 mM ouabain.

Activation of a slow outward current by the calcium released during contraction of cultured rat skeletal muscle cells

A slow outward current, activated during depolarization, which induced contraction in whole-cell patch-clamped rat skeletal muscle cells in primary culture [10], was extensively characterized in the present study. This current, Io, was simultaneously recorded with the contraction as a slow outward current during the test pulse, and a slow outward bell-shaped tail after repolarization. Io never appeared below the threshold potential for contraction, and the tail amplitude displayed a similar evolution with peak contraction amplitude as a function of membrane potential. This feature is consistent with the fact that Io was suppressed when contraction was blocked by 5 microM nifedipine [10], and it suggests that Io was dependent on calcium released during contraction. This was confirmed by the fact that the presence of 10 mM EGTA in the patch pipette prevented the development of both contraction and Io, and that Io could be activated during caffeine-induced contractures without applying depolarizations. Io could be carried by K+ or Cs+ ions, but not by Na+. The pharmacology of Io was different from that of Ca(2+)-dependent BK and SK channels, since it was resistant to tetraethylammonium (135 mM), charybdotoxin (25 nM) and apamin (50 nM). Io was also insensitive to 4-aminopyridine (1 mM) but blocked by 5 mM Ba2+ without change to contraction. It was concluded that rat cultured myoballs exhibit a Cs+ permeation through an atypical K+ channel type, which is activated by the calcium released during contraction.

Developmental changes in delayed rectifier K+ currents in the muscular- and neural-type blastomere of ascidian embryos

1. Developmental changes in the amplitude, kinetic properties, tetraethyl-ammonium (TEA) sensitivity, and ion selectivity of the delayed rectifier K+ currents were investigated in differentiating muscular-type (M) and neural-type (N) blastomeres isolated from the early cleavage-arrested ascidian embryos, using conventional two-microelectrode voltage clamp techniques. 2. No voltage-sensitive outward K+ currents were found in either type of blastomere during the first 35 h of development at 9 degrees C. Thereafter the delayed rectifier K+ current became apparent. The peak amplitude of the K+ current in the M-blastomere increased abruptly from 50 to 60 h and tended to plateau after 60 h, while in the N-blastomere it continued to increase after initial emergence at around 35 h. 3. The threshold potential level of the K+ current in the M-blastomere was initially about -10 mV in a standard external solution (1 mM-K+ solution), but shifted towards the hyperpolarized direction until it reached a steady level at 45 h after fertilization. At the fully differentiated stages, the threshold was around -32 mV and -26 mV in the M- and N-blastomeres, respectively. 4. Throughout development, the reversal potential of the tail current changed with the external K+ concentration in both M- and N-blastomeres as expected for a K(+)-electrode. There was no significant difference in the selectivity ratios for the K+ channel between the two types of blastomeres. The relative selectivities were K+ (1.000): Rb+ (0.774): NH4+ (0.122): Na+ (0.074) and K+ (1.000): Rb+ (0.724): NH4+ (0.155): Na+ (0.074) in the M- and N-blastomeres, respectively. 5. Modified Scatchard plots of TEA-sensitivity data indicated a one-to-one reaction between TEA and the K+ channel. These plots revealed the presence of TEA-resistant K+ channels in addition to TEA-sensitive K+ channels in the M-blastomere, but revealed only TEA-sensitive K+ channels in the N-blastomere. The dissociation constant (Ki) values of these three types of K+ channel did not change during development. In the M-blastomere, the Ki of the TEA-sensitive K+ channel was 1.29 +/- 0.05 mM (mean +/- S.E.M., n = 31) and that of the TEA-resistant K+ channel was 1.4 +/- 0.1 M (mean +/- S.E.M., n = 31) at a test potential of 45 mV. The Ki value of the neural-type K+ current was 1.38 +/- 0.03 mM (mean +/- S.E.M., n = 20) at 45 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Selectivity and gating of the type L potassium channel in mouse lymphocytes

Type l voltage-gated K+ channels in murine lymphocytes were studied under voltage clamp in cell-attached patches and in the whole-cell configuration. The kinetics of activation of whole-cell currents during depolarizing pulses could be fit by a single exponential after an initial delay. Deactivation upon repolarization of both macroscopic and microscopic currents was mono-exponential, except in Rb-Ringer or Cs-Ringer solution in which tail currents often displayed "hooks," wherein the current first increased or remained constant before decaying. In some cells type l currents were contaminated by a small component due to type n K+ channels, which deactivate approximately 10 times slower than type l channels. Both macroscopic and single channel currents could be dissected either kinetically or pharmacologically into these two K+ channel types. The ionic selectivity and conductance of type l channels were studied by varying the internal and external permeant ion. With 160 mM K+ in the cell, the relative permeability calculated from the reversal potential with the Goldman-Hodgkin-Katz equation was K+ (identical to 1.0) greater than Rb+ (0.76) greater than NH4+ = Cs+ (0.12) much greater than Na+ (less than 0.004). Measured 30 mV negative to the reversal potential, the relative conductance sequence was quite different: NH4+ (1.5) greater than K+ (identical to 1.0) greater than Rb+ (0.5) greater than Cs+ (0.06) much greater than Na+, Li+, TMA+ (unmeasurable). Single channel current rectification resembled that of the whole-cell instantaneous I-V relation. Anomalous mole-fraction dependence of the relative permeability PNH4/PK was observed in NH4(+)-K+ mixtures, indicating that the type l K+ channel is a multi-ion pore. Compared with other K+ channels, lymphocyte type l K+ channels are most similar to "g12" channels in myelinated nerve.

Rubidium ions and the gating of delayed rectifier potassium channels of frog skeletal muscle

1. Unitary currents were measured through delayed rectifier potassium channels of frog skeletal muscle, under conditions where either potassium or rubidium ions carried current. 2. Unitary currents were reduced in amplitude when Rb+ was the charge carrier, indicating that Rb+ permeated the channel less readily than did K+. On the other hand permeability ratios (PRb/PK) measured from the change in reversal potential upon ionic substitution were 0.92 for the external and 0.67 for the internal mouth of the channel. 3. Ensemble-averaged currents activated under depolarization along a similarly S-shaped time course whether K+ or Rb+ carried current, though slightly more slowly in Rb+. However, under repolarization to a negative level, tail currents were prolonged about tenfold in Rb+. 4. The duration of channel opening was substantially prolonged in Rb+. The distribution of open times was fitted by a single exponential whether K+ or Rb+ was the charge carrier, indicating a single open state. But the mean open time, averaged over all voltages investigated, was 2.65 times greater in Rb+. 5. The prolongation in Rb+ of tail currents under repolarization was associated with increases in the number of openings per burst and in the number of bursts during each tail. 6. The implications of these results for channel gating are discussed. It is argued that an early step in channel activation is more voltage dependent than later steps.

Rubidium and sodium permeability of the ATP-sensitive K+ channel in single rat pancreatic beta-cells

1. The patch-clamp method has been used to study the selectivity of single ATP-sensitive potassium channels in excised membrane patches from dissociated rat pancreatic beta-cells. 2. In symmetrical K+ concentrations the current-voltage relation of this channel showed slight inward rectification. The K+ permeability coefficient was 1.05 x 10(-13) cm3/s ([K+]o = 140 mM; 20 degrees C) in the inside-out patch and somewhat smaller when measured under the same conditions in the outside-out configuration (0.86 x 10(-13) cm3/s). 3. When intracellular Rb+ replaced K+, inward K+ currents were unaffected but the outward currents carried by Rb+ were substantially smaller. The extent of the reduction in the outward currents depended on the internal Rb+/K+ ratio and increased as [Rb+]i was raised. Both inward and outward Rb+ currents were blocked by 1 mM-ATP. No currents were measurable in symmetrical 140 mM-Rb+ solutions. 4. With 140 mM [K+]o and 140 mM [Rb+]i the single-channel current-voltage relation reversed at +8 mV and the potential at which the variance of the ATP-sensitive current was least was shifted by 6 mV to more positive potentials. These data suggest a PRb/PK ratio of around 0.7. 5. Outward Rb+ currents were reduced at all potentials in inside-out patches exposed to 107 mM-Rb+ solution intracellularly and 5 mM-K+ externally. 6. Partial replacement of external K+ with Rb+ substantially reduced the inward currents recorded from outside-out patches and also decreased outward K+ currents. 7. In outside-out patches, the addition of 1 mM-Rb+ to the external solution produced a block of inward K+ currents that initially increased and then decreased again with hyperpolarization. This suggests that the Rb+ block of K+ currents is voltage dependent and that Rb+ acts as a permeant blocker of K+ currents. 8. The sodium permeability of the channel, relative to that of potassium, was 0.39 for internal Na+ and 0.007 for external Na+ ions. 9. We conclude that Rb+ serves as an acceptable tracer for K+ in efflux studies when changes in K+ flux through ATP-sensitive K+ channels are of interest but that the magnitude of such fluxes will be considerably underestimated.

Potassium current activated by depolarization of dissociated neurons from adult guinea pig hippocampus

Currents were generated by depolarizing pulses in voltage-clamped, dissociated neurons from the CA1 region of adult guinea pig hippocampus in solutions containing 1 microm tetrodotoxin. When the extracellular potassium concentration was 100 mM, the currents reversed at -8.1 +/- 1.6 mV (n = 5), close to the calculated potassium equilibrium potential of -7 mV. The currents were depressed by 30 mM tetraethylammonium in the extracellular solution but were unaffected by 4-aminopyridine at concentrations of 0.5 or 1 mM. It was concluded that the currents were depolarization-activated potassium currents. Instantaneous current-voltage curves were nonlinear but could be fitted by a Goldman-Hodgkin-Katz equation with PNa/PK = 0.04. Conductance-voltage curves could be described by a Boltzmann-type equation: the average maximum conductance was 65.2 +/- 15.7 nS (n = 9) and the potential at which gK was half-maximal was -4.8 +/- 3.9 mV (mean +/- 1 SEM, n = 10). The relationship between the null potential and the extracellular potassium concentration was nonlinear and could be fitted by a Goldman-Hodgkin-Katz equation with PNa/PK = 0.04. The rising phase of potassium currents and the decay of tail currents could be fitted with exponentials with single time constants that varied with membrane potential. Potassium currents inactivated to a steady level with a time constant of approximately 450 ms that did not vary with potential. The currents were depressed by substituting cobalt or cadmium for extracellular calcium but similar effects were not obtained by substituting magnesium for calcium.

Inhibition of the electrogenic Na,K pump and Na,K-ATPase activity by tetraethylammonium, tetrabutylammonium, and apamin

The K+-induced hyperpolarization of Na-loaded mouse diaphragm muscle, enzymatic activity of Na,K-ATPase and 3H-ouabain binding to rat brain microsomes was measured in the presence of K+ channel blockers tetraethylammonium (TEA), tetrabutylammonium (TBA) and apamin. TBA, and to a lesser extent TEA in millimolar concentrations, inhibited the electrogenic effect of the Na,K pump, Na,K-ATPase activity, and 3H-ouabain binding. The inhibition of 3H-ouabain binding by TEA or TBA was more evident in the presence of ATP and Na+ ions. Apamin in nanomolar concentrations inhibited the electrogenic effect of Na,K pump and Na,K-ATPase but not the 3H-ouabain binding. The hyperpolarizing effects of insulin and NADH, but not that of noradrenaline, were also prevented by apamin. The inhibition of Na,K pump by TEA and TBA is apparently due to both competition with K+ for a binding site on the Na,K-ATPase and a reduction in the number of transporting sites. The site of action of apamin on Na,K-ATPase is different from that of tetra-alkylammonium compounds; it apparently decreases the turnover rate of the enzyme.

Recent advances in the characterization of epithelial ionic channels

Physiologists have long recognized the importance of channels in the functioning of neurons and excitable membranes. This brief review has been an attempt to illustrate how channel properties are also essential to an understanding of epithelial transport physiology. Among their more important functions, channels influence membrane potentials and serve as conduits for ion movements. As the need to understand the molecular basis for ion transport continues to develop, it is crucial to be able to distinguish between different channel properties. For example, apparent voltage-dependent properties can arise because of a voltage-dependent gating process, or alternatively, because of a rectification of channel conductance. Voltage-dependent effects can also be only indirect, mediated by changes in cell volume, intracellular ion levels, the levels of secondary intracellular messengers such as Ca2+ (perhaps through voltage-dependent membrane Ca2+ channels), or possibly even by morphological changes. An important area for future research is to differentiate mechanisms which modulate the activity of open channels. For example, a decrease in channel number, a reduction in open-channel conductance or a decline in the probability of channel opening can all underlie changes in macroscopic permeability. The factors which mediate hormonal activation of epithelial channels particularly need to be understood. Specifically, the mechanisms of aldosterone and anti-diuretic hormone activation of apical membrane Na+ channels need to be identified. In conclusion, we are witnessing a new era in epithelial electrophysiology which promises to resolve many issues concerning the cellular regulation of ion transport and open new, unanticipated avenues of inquiry.

Studies of the unitary properties of adenosine-5'-triphosphate-regulated potassium channels of frog skeletal muscle

1. Patch-clamp techniques were used to study adenosine-5'-triphosphate (ATP)-dependent K+ channels in sarcolemmal vesicles from frog skeletal muscle. In addition to its ATP dependence, opening of these channels was voltage dependent, the open-state probability (P open) increasing with depolarization. 2. The reversal potential of unitary currents changed with external K+ concentration, [K+]o, as expected if the Na-K permeability ration (pNa/pK) equals 0.015. Unitary conductance increased with increasing [K+]o from 14.8 +/- 0.5 pS (n = 5) in 2.5 mM-K+ to 42.3 +/- 1.0 pS (n = 8) in 60 mM-K+. This increase was less than that expected from independence. 3. Replacement of 60 mM-external K+ by 60 mM-external Rb+ shifted the reversal potential of unitary currents by -6.7 mV, suggesting that Rb+ enters channels nearly as easily as does K+ (Rb-K permeability ration, pRb/pK = 0.76). Unitary currents were much smaller in Rb+, consistent with Rb+ binding within the channel. 4. The ATP-regulated K+ channel was blocked by both internal and external tetraethylammonium ions (TEA+). 2 mM-TEA+, applied to the cytoplasmic face of membrane patches, interrupted channel openings. Higher concentrations reduced unitary current amplitude, suggesting an increase in the rapidity of TEA+ block. 5. The reduction in P open by ATP was consistent with 1:1 binding and a dissociation constant of 0.135 mM. ATP appeared not to be hydrolysed to close channels. Adenosine 5'-diphosphate (ADP) and adenosine 5'-monophosphate (AMP) were less effective than ATP, but retained channel closing properties. Substitution of adenine with other purines or with pyrimidine bases substantially reduced activity, as did substitution of ribose by 2'-deoxyribose or by ribose 2',3'-dialdehyde. 6. Sarcoplasmic Ca2+ did not influence P open. 7. Myotubes, grown from thigh muscles of new-born rats, appeared to lack ATP-dependent K+ channels. Adult frog muscle appeared to lack high-conductance Ca2+-dependent K+ channels, at least in the surface membrane. Such channels were found in myotube membranes. 8. Open- and closed-time histograms were constructed and were consistent with at least two open and at least three closed states. Channel openings were grouped in bursts. Open times, burst lengths and the number of openings per burst were reduced by ATP. 9. The effects of [K+]o on unitary conductance and of K+ replacement with Rb+ are discussed in terms of a simple Eyring rate theory formulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic channels: II. Voltage- and agonist-gated and agonist-modified channel properties and structure

This article reviews the different forms of ionic channels: voltage-gated, agonist-gated, and agonist- and second messenger-modified channels. The recent advances in our knowledge of the amino acid sequence of the sodium channel and the nicotinic acetylcholine receptor and the relationship of the primary structure to the channels' quarternary structure and function are discussed.

The dependence on external cation of sodium and potassium fluxes across the human red cell membrane at low temperatures

The fluxes of Na and K across the human red cell membrane have been studied as functions of temperature and external cation composition. In media containing any of a variety of organic compounds as the principal cation (choline, N-methyl-D-glucamine (NMDG), arginine, L-lysine and trimethyl-phenylammonium), the ouabain plus bumetanide-insensitive influxes and effluxes of Na and K displayed marked paradoxical temperature dependence such that the flux minimum, which normally occurs at about 8-10 degrees C in a NaCl medium, was shifted up to about 20 degrees C. Inhibitor and anion replacement studies excluded contributions by the major carrier-mediated systems evident at 37 degrees C. At 0 degrees C in NMDG, about 1 mM-external Na and 10 mM-external K were required half-maximally to inhibit the K and Na influxes respectively. When the K(86Rb) efflux in NMDG media at 0 degrees C was measured in the presence of low concentrations of a series of external inorganic ions and guanidine, the order of potency for reduction of the efflux was Li greater than Mg = Ca greater than Ba greater than Sr greater than Na greater than Rb = Cs = guanidine greater than K. The influxes of the neutral amino acid L-leucine and the cationic species L-lysine both showed simple monotonic temperature dependence in Na and NMDG media. These effects show that the permeability of the human red cell membrane to inorganic univalent cations at low temperatures is markedly dependent on the external ionic conditions. Low permeability is favoured by the presence of cations with a high charge density.

Ionic conductances in frog short skeletal muscle fibres with slow delayed rectifier currents

Short (0.8-1.6 mm) lumbricalis fibres of Rana pipiens were voltage clamped by a two-micro-electrode technique at 5 degrees C in sucrose hypertonic Ringer solution (SHR). Terminated linear cable analysis suggests that if the current electrode is placed near the centre of the fibre length and the voltage-sensing electrode is placed 0.19 times the fibre length from the current electrode, the fibre can be adequately voltage clamped and the conductance may be simply calculated as I/V for fibre length constants from 1.0 to 0.15 mm. In SHR solution lumbricalis fibres have action potentials with peak amplitudes of only +2 to 7 mV and a slow, gradual repolarization, distinct from the action potentials observed in sartorius muscle. In 60 mM-Na+ SHR the inward Na current could be adequately controlled over the fibre length, providing an estimated Na conductance (GNa) of 8.9 mS/cm2. The magnitude of GNa and GK (delayed rectifier) in lumbricalis fibres was approximately 20% of that reported for sartorius and semitendinosus, although the resting conductances were similar. Fibres demonstrated delayed rectifier currents with complex patterns of activation suggesting two components of conductance (fast, GK,f and slow, GK,s) which were combined together in varied amounts: (a) GK,f activated rapidly to a maximum within 80 ms at 0 mV as previously described (Adrian, Chandler & Hodgkin, 1970a); (b) GK,s activated gradually with depolarizations below -50 mV and achieving peak currents at about 400 ms at 0 mV. In about 10% of lumbricalis fibres studied, GK,s occurred in isolation with a peak magnitude of 1.4 +/- 0.4 mS/cm2 (+/- S.D.). GK,s activation kinetics and tail currents are described by a squared two-state (l2) Hodgkin-Huxley model and have a Q10 of 2.8. These currents inactivated with a time constant of 5-7 s at 0 mV. Isolated GK,s with identical kinetics was also observed in certain sartorius fibres studied with the three-electrode voltage clamp. The fractional amount of GK,s in the combined delayed rectifier (GK,s + GK,f) currents could be estimated from analysis of the late activation phase with depolarization. Combined delayed currents were described by summing GK,f currents using a n4 model with GK,s currents defined by the l2 model.

A voltage-gated potassium channel in human T lymphocytes

Human peripheral T lymphocytes were studied at 20-24 degrees C using the gigaohm seal recording technique in whole-cell or outside-out patch conformations. The predominant ion channel present under the conditions employed was a voltage-gated K+ channel closely resembling delayed rectifier K+ channels of nerve and muscle. The maximum K+ conductance in ninety T lymphocytes ranged from 0.7 to 8.9 nS, with a mean of 4.2 nS. The estimated number of K+ channels per cell is 400, corresponding to a density of about three channels/micron2 apparent membrane area. The activation of K+ currents could be fitted by Hodgkin-Huxley type n4 kinetics. The K+ conductance in Ringer solution was half-maximal at -40 mV. The time constant of K+ current inactivation was practically independent of voltage except near the threshold for activating the K+ conductance. Recovery from inactivation was slow and followed complex kinetics. Steady-state inactivation was half-maximal at -70 mV, and was complete at positive potentials. Permeability ratios, relative to K+, determined from reversal potential measurements were: K+(1.0) greater than Rb+(0.77) greater than NH4+(0.10) greater than Cs+ (0.02) greater than Na+(less than 0.01). Currents through K+ channels display deviations from the independence principle. The limiting outward current increases when external K+ is increased, and Rb+ carries less inward current than expected from its relative permeability. Tail current kinetics were slowed about 2-fold by raising the external K+ concentration from 4.5 to 160 mM, and were 5 times slower in Rb+ Ringer solution than in K+ Ringer solution. Single K+ channel currents had two amplitudes corresponding to about 9 and 16 pS in Ringer solution. Replacing Ringer solution with isotonic K+ Ringer solution increased the unitary conductance and resulted in inward rectification of the unitary current-voltage relation. Comparable effects of external K+ were seen in the whole-cell conductance and instantaneous current-voltage relation. Several changes in the K+ conductance occurred during the first few minutes after achievement of the whole-cell conformation. Most are explainable by dissipation of a 10-20 mV junction potential between pipette solution and the cytoplasm, and by the use of a holding potential more negative than the resting potential. However, inactivation of K+ currents became faster and more complete, changes not accounted for by these mechanisms. K+ efflux through open K+ channels in intact lymphocytes, calculated from measured properties of K+ channels, can account for efflux values reported in resting lymphocytes, and for the increase in K+ efflux upon mitogenic stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Basolateral potassium channel in turtle colon. Evidence for single-file ion flow

Treatment of the apical surface of the isolated, ouabain-inhibited turtle colon with the polyene antibiotic amphotericin B permitted the properties of a barium-sensitive potassium conductance in the basolateral membrane to be discerned from the measurements of transepithelial fluxes and electrical currents. Simultaneous measurements of potassium currents and 42K fluxes showed that the movement of potassium was not in accord with simple diffusion. Two other cations, thallium and rubidium, were also permeable and, in addition, exhibited strong interactions with the potassium tracer fluxes. The results indicate that permeant cations exhibit positive coupling, which is consistent with a single-file mechanism of ion translocation through a membrane channel.

The cation selectivity and voltage dependence of the light-activated potassium conductance in scallop distal photoreceptor

Light-dependent voltage and current responses were measured from the distal hyperpolarizing photoreceptors of the scallop (Pecten irradians) retina. In normal external solution, the hyperpolarizing receptor potential was caused by a light-dependent K+ outward current. The magnitude of the hyperpolarizing receptor potential and the light-dependent outward current, measured at the resting potential, was graded with light intensity. In normal external solution, during prolonged illumination the light-dependent K+ outward current was characterized by an early peak and a subsequent plateau. Current responses to brief light flashes were reduced progressively during background illumination. In the absence of external Na+ ions, the reversal potential for the receptor potential changed 58 mV per 10-fold change in the extracellular K+ concentration. The estimated internal K+ concentration was 385 mM. The hyperpolarizing receptor potential produced by prolonged bright illumination consists of an early peak which decays to a plateau. This decay was determined by a decrease in the light-dependent K+ conductance during maintained illumination. The light-dependent conductance pathway passed outward currents better than inward K+ currents. The light-dependent K+ conductance was estimated to increase e-fold per 23-34 mV depolarization at the peak and during the plateau of the light response. The light-dependent conductance pathway was highly selective for K+ ions. The selectivity sequence for monovalent cations was T1+, K+ greater than Rb+ greater than NH4 greater than Cs+, Li+, Na+. External caesium and tetraethylammonium blocked inward but not outward K+ currents through the light-dependent K+ conductance pathway. The data suggest that K+ ions move through an aqueous pore which is controlled by light.

Selectivity of the Ca2+-activated and light-dependent K+ channels for monovalent cations

The ionic selectivity of the Ca(2+)-activated K(+) channel of Aplysia neurons and of the light-dependent K(+) channel of Pecten photoreceptors to metal and organic cations was studied. The selectivity sequence determined from reversal potential measurements is T1(+) K(+) > Rb(+) > NH(+) (4) > Cs(+) > Na(+), Li(+) and is identical to the sequence determined previously for voltage-dependent K(+) channels in a variety of tissues. Our results suggest that some physical aspect of the K(+) channel is conserved in phyllogenetically different tissues and cells.

Effects of nystatin-mediated intracellular ion substitution on membrane currents in calf purkinje fibres

1. Calf cardiac Purkinje fibres were exposed briefly to the ionophore nystatin to promote exchange of caesium for intracellular potassium. The effects of Cs loading were stable for at least 30 min, but they could be reversed by nystatin-mediated K loading.2. After Cs loading, the resting potential shifted to about -20 mV and the current-voltage relationship showed a strong inhibition of inwardly rectifying K channels.3. Anodal break stimulation evoked normal action potential upstrokes and twitch contractions. The early repolarization (phase 1) was markedly slowed.4. Cs loading simplified the pattern of current changes evoked by step depolarizations over the plateau range. Membrane current reached an inward peak and then declined monotonically.5. The current signal showed no hint of the transient outward current found in untreated or K-loaded preparations. Furthermore, Cs loading abolished the outward tails associated with deactivation of transient outward current, and occluded the blocking effect of the K-channel inhibitor 4-aminopyridine.6. Inhibition of transient outward current revealed a maximal inward current of about 5 muA/muF in 5.4 mM-Ca(o), which is considerably larger than the net inward current without Cs loading.7. The inward current was attributed to Ca channels on the basis of its sensitivity to membrane potential, extracellular Ca, D600, Mn and Cd.8. Cs loading also reduced slow current changes associated with delayed rectification and pace-maker depolarization.9. The results support the hypothesis that the transient outward current is carried by K(+) ions, while providing a method for unmasking inward Ca current.

Ion conductance and ion selectivity of potassium channels in snail neurones

Delayed potassium channels were studied in internally perfused neurone somata from land snails. Relaxation and fluctuation analysis of this class of ion channels revealed Hodgkin-Huxley type K channels with an average single channel conductance (gamma K) of 2.40 +/- 0.15 pS. The conductance of open channels is independent of voltage and virtually all K channels seem to be open at maximum K conductance (gk) of the membrane. Voltage dependent time constants of activation of gK, calculated from K current relaxation and from cut-off frequencies of power spectra, are very similar indicating dominant first-order kinetics. Ion selectivity of K channels was studied by ion substitution in the external medium and exhibited the following sequence: Tl+ greater than K+ greater than Rb+ greater Cs+ greater than NH4+ greater Li+ greater than Na+. The sequence of the alkali cations does not conform to any of the sequences predicted by Eisenman's theory. However, the data are well accommodated by a new theory assuming a single rate-limiting barrier that governs ion movement through the channel.