Intramembrane charge movements in frog skeletal muscle in strongly hypertonic solutions.

A hypothesis in which intramembrane charge reflects a voltage sensing process allosterically coupled to transitions in ryanodine receptor (RyR)‐Ca2+ release channels as opposed to one driven by release of intracellularly stored Ca2+ would predict that such charging phenomena should persist in skeletal muscle fibres unable to release stored Ca2+. Charge movement components were accordingly investigated in intact voltage‐clamped amphibian fibres treated with known sarcoplasmic reticular (SR) Ca2+‐ATPase inhibitors. Cyclopiazonic acid (CPA) pretreatment abolished Ca2+ transients in fluo‐3‐loaded fibres following even prolonged applications of caffeine (10 mM) or K+ (122 mM). Both CPA and thapsigargin (TG) transformed charge movements that included delayed (qγ) ‘hump’ components into simpler decays. However, steady‐state charge‐voltage characteristics were conserved to values (maximum charge, Qmax∼ 20–25 nC μF−1; transition voltage, V*∼−40 to −50 mV; steepness factor, k∼ 6–9 mV; holding voltage −90 mV) indicating persistent qγ charge. The features of charge inactivation similarly suggested persistent qβ and qγ charge contributions in CPA‐treated fibres. Perchlorate (8.0 mm) restored the delayed kinetics shown by ‘on’qγ charge movements, prolonged their ‘off’ decays, conserved both Qmax and k, yet failed to restore the capacity of such CPA‐treated fibres for Ca2+ release. Introduction of perchlorate (8.0 mm) or caffeine (0.2 mm) to tetracaine (2.0 mm)‐treated fibres, also known to restore qγ charge, similarly failed to restore Ca2+ transients. Steady‐state intramembrane qγ charge thus persists with modified kinetics that can be restored to its normally complex waveform by perchlorate, even in intact muscle fibres unable to release Ca2+. It is thus unlikely that qγ charge movement is a consequence of SR Ca2+ release rather than changes in tubular membrane potential.

Section: Methodssupporting

confidence: 78%

Differential effects of sarcoplasmic reticular Ca²⁺‐ATPase inhibition on charge movements and calcium transients in intact amphibian skeletal muscle fibres

Chawla

Skepper

Huang

2002

“…Electrical recordings were made at 2-4 'C in either of two bathing solutions. Both have been used in earlier work (Huang, 1991(Huang, , 1992. The Ca2+-containing solution consisted of 80 mm tetraethylammonium sulphate, 15 mm tetraethylammonium chloride, 2-5 mm Rb2SO4, 8 mM CaSO4, 500 mm sucrose and 3 mm N-2-hydroxyethylpiperazine-N'-2-ethanesulphonic acid (Hepes).…”

Section: Charge Inactivation In Musclementioning

confidence: 99%

“…Thirdly, repeated control runs were interposed within sequences of obtaining five test averages to monitor the consistency of fibre cable constants. Values of specific membrane constants computed directly from successive control records could thus be scrutinized to assess fibre condition and stability (see Huang, 1991Huang, , 1992. Their values at the beginning, and at the end of each run are provided in the figure legends.…”

Section: Capacitative Charge In Test and Control Voltage Stepsmentioning

confidence: 99%

Charge inactivation in the membrane of intact frog striated muscle fibers.

Huang

1993

Self Cite

SUMMARY1. Charge movements were compared in normally polarized and depolarized intact frog muscle fibres under voltage clamp.2. The membrane capacitance was linear through positive control steps made consistently from a holding voltage of -10 mV, in agreement with earlier reports from cut fibres.3. A shift in holding voltage from -90 to -1O mV reduced both the absolute amount and the voltage dependence of charge movement elicited by voltage steps imposed from a fixed conditioning voltage of -180 mV. The charge transferred by steps from -180 to -20 mV was 43 8+ 1-14 nC/1sF in fully polarized fibres and 217 +149 nC/,uF in the same depolarized fibres (means+s.E. of the mean; four fibres).4. Charge movement in response to steps from -90 to -20 mV increased from 104+160 nC/,uF to 284 + 242 nC/,uF (five fibres) within 30s of changing the holding voltage from -10 to -90 mV.5. The same fibres also showed significant charge movement between voltages of -180 and -90 mV. However, shifts in holding voltage did not significantly alter the maximum value of this charge, around 10-11 nC/,uF.6. Membrane capacitance as measured by small steps to a voltage of -90 mV remained constant despite holding potential changes, or lidocaine (10 mM) treatment.7. The same results were obtained whether the above procedures were applied to fibres exposed to normal extracellular [Ca2+], or in Ca2+-free media. In both cases tubular cable corrections did not affect the results. 8. These findings suggest independent charge I and charge II systems in which inactivation of charge I is not associated with its interconversion into charge II.

“…However, at higher osmolalities, a reduction in the amount of Ca2+ released by depolarization probably contributes to the inhibition of the force responses (Parker & Zhu, 1987), although it is not clear whether this reduction in release results from depolarization being less able to open the Ca2+ release channels or from some initial translocation of Ca2+ causing partial depletion of the SR. Recently, Huang (1992) has shown that strongly hypertonic solutions abolish the delayed component of asymmetric charge movement (q,), but it is not certain whether the suppression of q, at such high tonicity is the result, or the cause, of reduced Ca2+ release.…”

Section: Introductionmentioning

confidence: 99%

Effects of osmolality and ionic strength on the mechanism of Ca2+ release in skinned skeletal muscle fibres of the toad.

Lamb

Stephenson

Stienen

1993

SUMMARY1. The effects of increased osmolality and ionic strength on the mechanism of Ca2+ release were examined in mechanically skinned skeletal muscle fibres of the toad at 23°C. Ca2+ release was induced by depolarizing the transverse tubular (T-) system by ionic substitution.2. Increasing the osmolality of the 'myoplasmic' solution about four times (to 955 mosmol/kg), by addition of 700 mm sucrose to the standard potassium (K-)HDTA solution (HDTA: hexamethylenediamine-tetraacetate), only depressed the depolarization-induced response by about 46 %. Much of this decrease could be attributed to a reduction in the Ca2+-sensitivity of the contractile proteins at this high osmolality.3. Addition of > 400 mm sucrose itself often induced substantial Ca2+ release and a transient tension response. This 'spontaneous' release was (a) greatly enhanced when the sarcoplasmic reticulum (SR) had been heavily loaded with Ca2 , (b) little affected by inactivation of the voltage sensors by prolonged or permanent depolarization of the T-system and (c) blocked by Ruthenium Red (10 ,lM).4. When both the osmolality and ionic strength were increased, by increasing the K-HDTA concentration, the depolarization-induced force was greatly reduced (to 35% at 818 mosmol/kg and 5% at 1095 mosmol/kg). Most of this reduction could be directly attributed to the substantially reduced maximum force and Caa2+ sensitivity of the contractile apparatus.5. The small amount of releasable Ca2' remaining in the SR after a single depolarization in a high-HDTA solution with 1 mm EGTA (to chelate the released Ca2+), indicated that depolarization could still elicit massive Ca2`release at high ionic strength and osmolality (at 1 mm free Mg2+).6. In contrast, when the total Mg2+ and ATP concentrations were raised about threefold (free [Mg2+] increased 2-7-fold) along with the osmolality and ionic strength, the ability of depolarization to elicit Ca2+ release was greatly hindered.7. Osmotic compression of the skinned fibres to their in situ diameter by addition of 4 % polyvinylpyrrolidone (PVP-40), substantially potentiated the depolarization-* To whom correspondence should be addressed.