Paradox response of frog muscle membrane to changes in external potassium

Effects of chloride transport on bistable behaviour of the membrane potential in mouse skeletal muscle

The lumbrical skeletal muscle fibres of mice exhibited electrically bistable behaviour due to the nonlinear properties of the inwardly rectifying potassium conductance. When the membrane potential (V(m)) was measured continuously using intracellular microelectrodes, either a depolarization or a hyperpolarization was observed following reduction of the extracellular potassium concentration (K+o) from 5.7 mM to values in the range 0.76-3.8 mM, and V(m) showed hysteresis when K+o was slowly decreased and then increased within this range. Hypertonicity caused membrane depolarization by enhancing chloride import through the Na+-K+-2Cl- cotransporter and altered the bistable behaviour of the muscle fibres. Addition of bumetanide, a potent inhibitor of the Na+-K+-2Cl- cotransporter, and of anthracene-9-carboxylic acid, a blocker of chloride channels, caused membrane hyperpolarization particularly under hypertonic conditions, and also altered the bistable behaviour of the cells. Hysteresis loops shifted with hypertonicity to higher K+o values and with bumetanide to lower values. The addition of 80 microM BaCl2 or temperature reduction from 35 to 27 degrees C induced a depolarization of cells that were originally hyperpolarized. In the K+o range of 5.7-22.8 mM, cells in isotonic media (289 mmol x kg(-1)) responded nearly Nernstianly to K+o reduction, i.e. 50 mV per decade; in hypertonic media this dependence was reduced to 36 mV per decade (319 mmol x kg(-1)) or to 31 mV per decade (340 mmol x kg(-1)). Our data can explain apparent discrepancies in DeltaV(m) found in the literature. We conclude that chloride import through the Na+-K+-2Cl- cotransporter and export through Cl- channels influenced the V(m) and the bistable behaviour of mammalian skeletal muscle cells. The possible implication of this bistable behaviour in hypokalaemic periodic paralysis is discussed.

Effects of methylene blue and ascorbate on transmembrane potential in frog skeletal muscle

1. The effects of methylene blue (MB, 10(-4) M) and ascorbate (ASC, 10(-4) M) on the resting membrane potential of frog skeletal muscle fibers were studied in Cl(-)-free medium at various external K+ concentrations. 2. Muscle fibers formed two distinct populations according to their resting potentials (hyperpolarized to -104 mV or depolarized to -31 mV) after a 90 min period of K+ withdrawal. ASC increased the relative contribution of hyperpolarized fibers, while in the presence of MB, all fibers depolarized during the 90 min of K+ withdrawal. 3. In the presence of 2.5 mM K+, ASC had no significant effect on the resting potential, while MB moved half of the fibers into the depolarized pool. Elevation of [K+]0 to 10 mM caused repolarization of all previously depolarized fibers (to -63 mV) in spite of the continuous presence of MB. 4. Ionic currents were measured during a 60 mV depolarization step using the double sucrose-gap technique. MB significantly increased the peak inward current, while ASC had no effect. The steady-state outward current was not affected by MB or ASC. 5. The results suggest that ionic conductances may be under redox control in frog skeletal muscle.

REPETITIVE ELECTRICAL ACTIVITY OF THE MUSCLE MEMBRANE INDUCED IN CHLORIDE-FREE MEDIUM

1. Superficial fibres of frog skeletal muscle were electrically stimulated in Ringer solution where the chloride content had been replaced by various weakly permeant anions. Changes of the membrane potential were recorded at three different time scales. The complex response was initiated by a volley of fast repetitive action potentials (10-20 ms cycle length) superimposed on the ascending phase of a transient depolarization to -35 mV. The transient depolarization was followed by a membrane potential oscillation (0.3-0.6 s cycle length). The parameters of the volley and membrane potential oscillation were not greatly affected by the substituent anion. 2. The transient depolarization was fully abolished by tetrodotoxin (3 mumol/L), but left unaffected by nifedipine (5 mumol/L), or by the replacement of extracellular Ca for Ni or Co. Tetraethylammonium (20-40 mmol/L) increased the duration and amplitude of the transient depolarization. The shape of transient depolarization was uniform in a given fibre, in spite of its marked variability under different experimental conditions. 3. Single outward current pulses applied in chloride-free solution containing tetraethylammonium (20-40 mmol/L) evoked prolonged depolarizations to positive membrane potentials accompanied by increases in the specific membrane conductance. This slow response, which was also frequently observed in the absence of TEA, was mediated by Ca ions, as it was insensitive to tetrodotoxin (3 mumol/L) but abolished by nifedipine (10 mumol/L). 4. Two populations of the muscle fibres were observed during the slow response. Some fibres repolarized completely, while others failed to produce complete repolarization but formed a plateau between -20 and -30 mV, lasting for several minutes. When the external K concentration was abruptly increased to 5 or 10 mmol/L during the plateau of the slow response, repolarization and increase in membrane conductance were observed. 5. In muscle fibres, having osmotically disrupted T-system, the duration of transient depolarization was in the range of minutes, in contrast to the range of seconds observed in intact fibres. The volley was preserved in glycerol treated fibres, however, the baseline of discharges was close to the resting potential and the rate of depolarization of the baseline was significantly less in glycerol treated than in intact fibres. 6. These results are consistent with the existence of a second stable membrane potential level in skeletal muscle between -40 and -30 mV. The depolarization and repolarization during the membrane potential oscillation and transient depolarization can be regarded as a partial or full transition, respectively, between these two stable membrane potential levels, possibly due to the conductance changes of the inward rectifier K channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Different actions of aconitine and veratrum alkaloids on frog skeletal muscle

1. The electrophysiological effects of veratridine, cevadine and aconitine (10(-8)-2 x 10(-4), 2 x 10(-7)-2 x 10(-6) and 2 x 10(-6)-10(-4) mol/l, respectively) were compared on frog muscle membrane using conventional microelectrodes. 2. Veratridine and aconitine were equally effective in depolarizing the resting membrane with the threshold concentration of 5 x 10(-5) mol/l. 3. Volleys of repetitive discharges and slow transient depolarizations were observed when single electrical stimuli were applied in the presence of veratridine (5 x 10(-8)-2 x 10(-5) mol/l), but not aconitine. Volleys with aconitine could be evoked only by repetitive stimulation; however no tendency of repolarization was observed following these volleys. Two orders of magnitude more aconitine than veratridine was required to induce volleys with similar parameters. 4. The effects of cevadine were similar to those of the corresponding concentrations of veratridine. 5. The observed differences between the electrophysiological actions of aconitine and veratrum alkaloids may be explained in part with differences in Na+ channel inactivation produced by these toxins, in addition to differences in their use-dependent behavior.