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

Inotropic effects of the K+ channel blocker 3,4-diaminopyridine: differential responses of rat soleus and extensor digitorum longus

The K+ channel blocker 3,4-diaminopyrindine (DAP) increases diaphragm force, use of which could potentially improve muscle performance during functional neuromuscular stimulation. To determine the extent of hindlimb muscle force augmentation, and delineate whether DAP effects vary in muscles comprised of mainly slow versus fast fibers, rat soleus, extensor digitorum longus (EDL) and diaphragm muscle samples were studied in vitro. DAP increased force of all three muscles, but at high concentrations the force increases were transient and were followed by declines in force below baseline. The maximum DAP-induced twitch force increase was smaller for soleus (38 +/-7%) than both EDL (94+/-12%) (P < 0.05) and diaphragm (93+/-13%) (P < 0.01). During fatigue-inducing 20 Hz stimulation (tested at an intermediate DAP concentration), force of soleus muscle remained significantly elevated by DAP for the entire testing period, force of DAP-treated EDL muscle rapidly declined to values in untreated muscle, and force of DAP-treated diaphragm had an intermediate force-time profile. Muscles varied in extent to which isometric contractile kinetics were altered by DAP. Thus, the K+ channel blocker DAP improves contractile performance of limb muscles, but the profile of improvement is distinct between the soleus and EDL muscles.

Modulation of diaphragm action potentials by K(+) channel blockers

K(+) channels regulate diaphragm contractility. The present study examined the electrophysiological mechanisms accounting for diversity among K(+) channel blockers in their inotropic actions on the diaphragm. Rat diaphragmatic muscle fibers were recorded intracellularly in vitro at 37 degrees C. Apamin and charybdotoxin (Ca2+)-activated K(+) channel blockers) did not alter resting membrane potential or action potentials. Glibenclamide (ATP-sensitive K(+) channel blocker) slowed action potential repolarization by 12% (P<0.05) and increased action potential area by 25% (P<0.005). Tetraethylammonium (which blocks several types of K(+) channels) increased action potential overshoot by 20% (P<0.01) and prolonged action potential rise time by 17% (P<0.02). 4-Aminopyridine and 3,4-diaminopyridine (which also block several types of K(+) channels) slowed action potential repolarization by 163% (P<0.0001) and 253% (P<0.0001), and increased action potential area by 183% (P<0.0001) and 298% (P<0.0001), respectively. Slowing of repolarization for the aminopyridines was especially marked at voltages approaching resting membrane potential, thereby changing action potential repolarization from a first to a second order decay. Previously reported variability in inotropic effects among K(+) channel blockers correlated significantly with the extent to which they slowed action potential repolarization and increased action potential area, but not with changes in other action potential properties.

Development of muscle-specific features in cultured frog embryonic skeletal myocytes

To study the development of muscle-specific features during myogenesis, we analysed the ultrastructure and voltage-dependent currents of frog embryonic skeletal myocytes maintained in culture for 10 days. The cells were maintained under culture conditions that prevented cell division, fusion and cell contacts with neuroblasts. The cell surface was estimated morphometrically and from cell capacity and the values obtained were used to calculate ion current densities. It was shown that the expression of all main types of voltage dependent ionic currents occurs during the first 3-5 days. Na+ maximum specific conductance at days 1-2 was low but by day 7 it showed a 20-fold increase. The magnitude of Na+ current densities increased 16-fold from day 1 (3.6 microA/cm) to the day 7 (58.1 microA/cm). The maximum specific K+ conductance increased almost 3-fold during the first 5 days. In contrast to the other types of currents, I(K) undergoes qualitative changes. Sodium action potentials, whose amplitude and time course depend on gNa/gK ratio, appeared from day 4 in culture, when myofibrils and the T-system also developed. The amplitude of DHP-sensitive slow I(Ca) increased in parallel with the development of the T-membrane. I(Ca,S) density per unit of T-membrane area reached an equilibrium of ca., 17 microA/cm2 on the day 4 and then remained stable until the end of the period of observation. These studies demonstrate that muscle-specific characteristics including morphology and excitatory properties begin to develop on the third day and resemble those of adult muscle cells by the sixth day in culture.

Peptide toxin blockers of voltage-sensitive K+ channels: inotropic effects on diaphragm

Agents that block many types of K+ channels (e.g., the aminopyridines) have substantial inotropic effects in skeletal muscle. Specific blockers of ATP-sensitive and Ca2+-activated K+ channels, on the other hand, do not, or minimally, alter the force of nonfatigued muscle, consistent with a predominant role for voltage-gated K+ channels in regulating muscle force. To test this more directly, we examined the effects of peptide toxins, which in other tissues specifically block voltage-gated K+ channels, on rat diaphragm in vitro. Twitch force was increased in response to alpha-, beta-, and gamma-dendrotoxin and tityustoxin Kalpha (17 +/- 6, 22 +/- 5, 42 +/- 14, and 13 +/- 5%; P < 0.05, < 0.01, < 0.05, < 0.05, respectively) but not in response to delta-dendrotoxin or BSA (in which toxins were dissolved). Force during 20-Hz stimulation was also increased significantly by alpha-, beta-, and gamma-dendrotoxin and tityustoxin Kalpha. Among agents, increases in twitch force correlated with the degree to which contraction time was prolonged (r = 0.88, P < 0.02). To determine whether inotropic effects could be maintained during repeated contractions, muscle strips underwent intermittent 20-Hz train stimulation for a duration of 2 min in presence or absence of gamma-dendrotoxin. Force was significantly greater with than without gamma-dendrotoxin during repetitive stimulation for the first 60 s of repetitive contractions. Despite the approximately 55% higher value for initial force in the presence vs. absence of gamma-dendrotoxin, the rate at which fatigue occurred was not accelerated by the toxin, as assessed by the amount of time over which force declined by 25 and 50%. These data suggest that blocking voltage-activated K+ channels may be a useful therapeutic strategy for augmenting diaphragm force, provided less toxic blockers of these channels can be found.

Transient outward K+ channels in vesicles derived from frog skeletal muscle plasma membranes

Whole-cell voltage-clamp experiments were performed in vesicles derived from frog skeletal muscle plasma membranes. Capacitance measurements showed that these vesicles lack invaginations. In solutions containing K+, transient outward currents with reversal potentials close to EK were recorded with a maximum potassium conductance of 0.3 mS/cm2. These currents inactivated in a voltage-dependent manner with a time constant of decay that reached a limiting value of 26 ms at large depolarizations. The steady-state inactivation reached half-maximum values at -66 mV. Transient currents were completely blocked with 5 mM 4-aminopyridine. Single-channel recordings made in inside-out excised patches from the vesicles had ensemble averages with characteristics similar to those of the macroscopic currents, although with significantly faster inactivation time constants. The single-channel chord conductance was 21 pS when the pipette and bath solutions contained 2.5 mM and 120 mM KCl, respectively. It is concluded that these vesicles contain potassium channels that are very similar to A channels found in neurons and other cells.

Characterization of the outward rectifying potassium channel in a novel mouse intestinal smooth muscle cell preparation

1. The outward rectifying K+ conductance and underlying single channel behaviour in mouse small intestine (MSI) smooth muscle cells was studied using microelectrode impalement and the patch clamp technique. 2. At 37 degrees C, smooth muscle cells in MSI explants had a resting membrane potential around -65 mV and showed spontaneous electrical and mechanical activity. 3. Under whole-cell voltage clamp, depolarization of smooth muscle cells in the explants evoked a methoxyverapamil (D600)-sensitive, partially inactivating inward current and a non-inactivating outward current. The outward current was also observed in enzymatically dispersed cells from neonatal mouse small intestine. 4. The reversal potential of the outward current as established in tail current experiments was -70.2 mV. Tail currents could be fitted with a single exponential, suggesting the participation of only one population of channels. 5. The outward current was sensitive to 4-aminopyridine (10(-4) M), Ba2+ (1 mM) and to the presence of Cs+ in the pipette, but not to D600 (10(-6) M), or the presence of ATP (1 mM) in the pipette. 6. In the cell-attached patch configuration, a unitary outward current was observed that showed increased activity upon depolarization of the patch. The current-voltage relationship was close to linear with a slope conductance of 186 pS. 7. With normal K+ (6 mM) in the pipette, the extrapolated reversal potential for the unitary current was around -75 mV, while with high K+ (120 mM) the reversal potential was close to 0 mV. 8. Averaging single channel traces recorded under a depolarizing pulse protocol resulted in a trace with similar time characteristics as the outward current observed in the whole-cell configuration. 9. The burst behaviour of the channel was described by a simple model consisting of two closed states, Cf (intraburst closed state) and Cs (interburst closed state) and an open state (O). The rate constants in the model showed differential sensitivity to potential changes, channel blockade by Ba2+ and equimolar K+ conditions. 10. It was concluded that the outward rectifying potassium current in MSI smooth muscle cells is mediated by a 186 pS bursting channel. Voltage dependency and Ba2+ blockade are mainly reflected by changes in the transition rate from the open channel state to the interburst closed state.

Two types of 4-aminopyridine-sensitive potassium current in rabbit Schwann cells

1. Delayed rectifier K+ currents were studied in Schwann cells cultured from neonatal rabbit sciatic nerves with the whole-cell patch-clamp technique. 2. Depolarizing voltage steps (40 ms duration) activated two types of K+ current: type I, whose apparent activation threshold was about -60 mV (half-maximal conductance at -40 +/- 1 mV, n = 10); and type II, whose apparent activation threshold was about -25 mV (half-maximal conductance at + 11 +/- 1 mV, n = 9). 3. Type I current was blocked by alpha-dendrotoxin (alpha-DTX) with an apparent equilibrium dissociation constant (KD) of 1.3 nM, whereas the type II current was unaffected by exposure to 500 nM toxin. The action of alpha-DTX on the type I current was reversible. 4. Most cells exhibited both types of current, but occasionally some cells displayed just type I or just type II. 5. Type I current activated rapidly and then showed a much slower fade, which became more noticeable with larger depolarizations. Activation of type II current was slower than that of type I and depended less steeply on voltage. The time constants of activation for type I and type II currents were derived with a Hodgkin-Huxley formalism (based on second-power activation and deactivation kinetics). The longest activation time constant for type II gating was more than twice the corresponding time constant for type I; however, the time constants determined from tail current decays at potentials more negative than -60 mV were shorter for the type II currents than for the type I currents. 6. Both type I and type II currents were sensitive to micromolar concentrations of 4-aminopyridine (4-AP). The KD for 4-AP blockade of type II current was 630 microM (pH 7.2, membrane potential (Em) = -10 mV), which is about 6 times higher than the corresponding value for 4-AP blockade of type I current at negative membrane potentials. The differential sensitivity of the type I and type II currents to 4-AP may account for the apparent voltage dependence of 4-AP block of delayed rectifier K+ currents. 7. In addition to types I and II, a third type of outward K+ current (type III) was generated in most cells at positive membrane potentials. This latter current was insensitive to millimolar concentrations of 4-AP. 8. Similarities between Schwann cell and neuronal potassium channels are discussed.

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)

A voltage-dependent potassium current in rabbit coronary artery smooth muscle cells

1. Voltage- and time-dependent outward currents were recorded from relaxed enzymatically isolated smooth muscle cells from the rabbit left descending coronary artery using a single pipette voltage clamp technique. The calcium-activated potassium current was blocked by inclusion of EGTA in the pipette solution and CdCl2 in the extracellular bath. 2. Outward currents were elicited with depolarizing voltage steps to potentials positive to -20 mV. Long (5 s) voltage steps revealed slow inactivation of the current with a time constant of nearly 3 s at +60 mV. Potassium was identified as the predominant charge carrier by reversal potential measurements in potassium substitution experiments. 3. The results of kinetic analyses compared favourably with the Hodgkin-Huxley model for a delayed rectifier with some deviations. The sigmoid current onset was best fitted by raising the activation variable (n) to the second power. Deactivation tail currents were consistently found to be comprised of two exponential components. The kinetics of activation and deactivation were strongly voltage-dependent from -80 to +60 mV. 4. Envelope of tails experiments showed that the scaled tail current amplitudes followed the kinetic behaviour of current activation. The contribution of each of the two exponential tail components was also measured in these experiments. They did not reveal kinetically separable currents, nor were they differentially altered by 4-aminopyridine (4-AP), tetraethylammonium (TEA), or elevated [K+]o. 5. The steady-state voltage-dependence curves for both activation and inactivation were well fitted by a Boltzmann distribution with V1/2 = -5.60 mV and k = -8.66 mV for n infinity act and V1/2 = -24.20 mV and k = 5.16 mV for n infinity act. Super-imposition of the two curves revealed a 'window' of voltage where channels are available for activation without completely inactivating. 6. Neither of the commonly used potassium channel blockers, TEA or 4-AP, were particularly effective blockers of IK, reducing current by only 50-70% at an extracellular concentration of 10 mM. TEA block was mildly voltage-dependent and was more effective in reducing current towards the end of a 500 ms depolarization. 4-AP, on the other hand, demonstrated considerable voltage-dependence and preferentially reduced early currents. 7. Outward currents recorded from guinea-pig and human coronary artery myocytes under the same conditions as in the rabbit cell experiments displayed similar characteristics.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitrate and chloride ions have different permeation pathways in skeletal muscle fibers of Rana pipiens

The effects of pH on the permeability and conductance of the membranes to nitrate and to chloride of semitendinosus and lumbricalis muscle fibers were examined. Membrane potential responses to quick solution changes were recorded in semitendinosus fibers initially equilibrated in isotonic, high K2SO4 solutions. External solutions were first changed to ones in which either Rb+ or Cs+ replaced K+ and then to solutions containing either NO3- or Cl- to replace SO4(2-). The hyperpolarizations produced by Cl- depend on external pH, being smaller in acid than in alkaline solutions. By contrast, hyperpolarizations produced by NO3- were independent of external pH over a pH range from 5.5 to 9.0. In addition, voltage-clamp measurements were made on short lumbricalis muscle fibers. Initially they were equilibrated in isotonic solutions containing mainly K2SO4 plus Na2SO4. KCl or KNO3 were added to the sulfate solutions and the fibers were equilibrated in these new solutions. When finally equilibrated the fibers had the same volume they had in the sulfate solutions before the additions. Constant hyperpolarizing voltage pulses of 0.6-sec duration were applied when all external K+ was replaced by TEA+. For these conditions, inward currents flowing during the voltage pulses were largely carried by Cl- or NO3- depending on the final equilibrating solution. Cl- currents during voltage pulses were both external pH and time dependent. By contrast, NO3- currents were independent of both external pH and time. The voltage dependence of NO3- currents could be fit by constant field equations with a PNO3 of 3.7.10(-6) cm/sec. The voltage dependence of the initial or "instantaneous" Cl- currents at pH 7.5 and 9.0 could also be fit by constant field equations with PCl of 5.8 x 10(-6) and 7.9 x 10(-6) cm/sec, respectively. At pH 5.0, no measurable "instantaneous" Cl- currents were found. From these results we conclude that NO3- does not pass through the pH, time-dependent Cl- channels but rather passes through a distinct set of channels. Furthermore, Cl- ions do not appear to pass through the channels which allow NO3- through. Consequently, the measured ratio of PCl/PNO3 based on membrane potential changes to ionic changes made on intact skeletal muscle fibres is not a measure of the selectivity of a single anion channel but rather is a measure of the relative amounts of different channel types.(ABSTRACT TRUNCATED AT 400 WORDS)

An emerging pharmacology of peptide toxins targeted against potassium channels

Voltage-dependent ion channels are a difficult class of proteins to approach biochemically. Many such channels are present at low density in relevant tissues and exist as multiple subtypes that can be distinguished electrophysiologically. In particular, K channels appear to be a diverse family of proteins characterized by many different conductance properties, gating behaviors and regulatory phenomena. Fortunately, specific peptide toxins for K channels are present in the venoms of insects, scorpions, snakes and possibly other species. The available sequences of these peptides define several different families of toxins. Electrophysiological and radioligand binding studies suggest that these toxins can be used to distinguish subclasses of K channels that share similar toxin binding sites. The growing databank of sequence homologies for both toxins and channels is, in essence, a codebook for identifying common elements of structure and function. The continuing development of toxins as biochemical probes should help to uncover the molecular basis and physiological significance of K-channel diversity.

Macroscopic Ca2+ -Na+ and K+ currents in single heart and aortic cells

The whole-cell voltage clamp technique was used to study the slow inward currents and K+ outward currents in single heart cells of embryonic chick and in rabbit aortic cells. In single heart cells of 3-day-old chick embryo three types of slow inward Na+ currents were found. The kinetics and the pharmacology of the slow INa were different from those of the slow ICa in older embryos. Two types of slow inward currents were found in aortic single cells of rabbit; angiotensin II increased the sustained type and d-cAMP and d-cGMP decreased the slow transient component. Two types of outward K+ currents were found in both aortic and heart cells. Single channel analysis demonstrated the presence of a high single K+ channel conductance in aortic cells. In cardiac and vascular smooth muscles, slow inward currents do share some pharmacological properties, although the regulation of these channels by cyclic nucleotides and several drugs seems to be different.

Appropriate conditions to record activation of fast Ca2+ channels in frog skeletal muscle (Rana pipiens)

Frog skeletal muscle fibres possess slow Ca++ channels which are currently studied. Only recently a fast Ca++ current in frog skeletal muscle was reported. This finding is related to the fact that this current is very labile. Here we report the appropriate conditions to record the fast calcium current (ICa-f) in skeletal muscle of the frog Rana pipiens, in isotonic solutions and resting fibre length.

Biochemical separation of delayed rectifier currents in frog short skeletal muscle fibres

Frog short (1-1.5 mm) skeletal muscle fibres in sucrose hypertonic Ringer at 5 degrees C were voltage clamped employing a two-electrode technique. Decreasing pH from 7.0 to 5.0 dramatically increased the rate of turn on of the slow delayed rectifier (GK,s) current, so that it became similar to the rapidly activated form. The accelerating effects of decreased pH upon slow current kinetics had a pKa of 5.8. Decreased pH also appeared to shift the voltage dependence of fast and GK,s gating to more positive potentials. In addition, a decrease in pH from 7.0 to 5.0 shifted the reversal potential of GK,s by over 11 mV in the positive direction. GK,s was selectively and irreversibly abolished by applying 1 mM-diethylpyrocarbonate (DEP), a histidine reagent. This effect occurred even after GK,s had been accelerated by low pH. DEP had no effect upon fast delayed rectifier current except for a small positive shift in voltage dependence. Application of 1 or 2 mM-N-ethylmaleimide, a sulphydryl reagent, depressed the fast delayed rectifier while sparing the GK,s currents. However, the muscle fibres also developed markedly increased leakage currents.