Distribution of potassium and chloride permeability over the surface and T-tubule membranes of mammalian skeletal muscle

The distribution of K and Cl permeability, PK and PCl, over the surface and T-tubule membranes of red rat sternomastoid fibers has been determined. Membrane potential, Vm, was recorded with 3-M KCl-filled glass microelectrodes. Changes in Vm with changes in [K]o or [Cl]o were used to estimate PCl/PK in normal and detubulated preparations. The results show that the T-tubule membrane has a high PCl and is therefore different from the T-tubule membrane of amphibian fibers. Analysis of the time course of depolarization when [K]o was raised (in SO4 solutions) showed that PK was distributed over the surface and T-tubule membranes. Two observations suggested that T-tubule PCl was higher than the surface PCl. Firstly, in normal fibers, the depolarization caused by an increase in [K]o was 3.5 times greater in SO4 solutions than in Cl solutions. In marked contrast, the depolarization in glycerol-treated fibers was independent of [Cl]o. Secondly, the rapid change in Vm when [Cl]o was changed was reduced by 80% after glycerol treatment. Both observations suggest that PCl was low in glycerol-treated fibers. PCl/PK was calculated from the Vm data using Goldman, Hodgkin and Katz equations for Na and K or for Na, K, and Cl. In normal fibers PCl/PK = 4.5 and in glycerol-treated fibers PCl/PK = 0.28. Since it is unlikely that glycerol treatment would increase PK, the reduction in the ratio must follow the loss of Cl permeability "channels" in the T-tubule membrane.

The dependence of membrane potential on extracellular chloride concentration in mammalian skeletal muscle fibres

1. The steady-state intracellular membrane potential of fibres in thin bundles dissected from mouse extensor digitorum longus or soleus muscles or rat sternomastoid muscles was measured with 3 M-KCl glass micro-electrodes. The steady-state membrane potential was found to depend on the extracellular concentrations of Na, K and Cl ions. 2. The resting membrane potential (3.5 mM-[K]o, 160 mM-[Cl]o) was -74 +/- 1 mV (mean +/- S.E.) and a reduction in [Cl]o to 3.5 mM caused a reversible steady-state hyperpolarization to -94 +/- 1 mV (mean +/- S.E.). 3. The steady-state membrane potentials recorded in fibres exposed to different [K]o and zero [Cl]o were consistent with potentials predicted by the Goldman, Hodgkin & Katz (GHK) equation for Na and K. The results of similar experiments done with Cl as the major external anion could not be fitted by the same equation. 4. The GHK equation for Na, K and Cl did fit data obtained from fibres in solutions containing different [K]o with Cl as the major external anion if the intracellular Cl concentration was allowed to be out of equilibrium with the steady-state membrane potential. 5. It is suggested that an active influx of Cl ions controls the intracellular Cl concentrations in these fibres and hence maintains the Cl equilibrium potential at a depolarized value with respect to the resting membrane potential. 6. The steady-state membrane potential of rat diaphragm fibres was independent of [Cl]o and it seems likely that the intracellular Cl concentration of these fibres is not controlled by active Cl transport.

The passive electrical properties of frog skeletal muscle fibres at different sarcomere lengths

1. The passive electrical properties of frog skeletal muscle fibres have been measured at a number of different sarcomere lengths (from 2-1 to 4-0 micron). The geometrical outline of each fibre was determined from optical cross-sections and sarcomere length was measured by laser beam diffraction. 2. When fibres were stretched to long sarcomere lengths the membrane capacity, Cm, of both normal and detubulated (glycerol-treated) fibres was significantly less than the Cm of fibres at rest length. A significant reduction in membrane conductance of fibres held at long sarcomere lengths was only seen with detubulated fibres. 3. Membrane capacity and membrane conductance have a significant dependence on the cross-sectional area of normal fibres but are independent of cross-sectional area after detubulation. 4. It has been shown that membrane geometry depends on the sarcomere length of the fibre and it is suggested that the passive membrane properties are related to sarcomere length because they depend on membrane geometry. 5. The specific membrane capacity, calculated from the data from detubulated fibres, is 0-8 micronF/cm2. 6. The internal resistivity, Ri, of normal fibres, also depends on sarcomere length between 2-1 and 3-0 micron. At a sarcomere length of 2-1 micron the average Ri is 122 +/- 3 omega. cm (mean +/- S.E. of mean) and at a sarcomere length of 3-0 micron the average Ri is 210 +/- 17 omega. cm (mean +/- S.E. of mean). No further increase in Ri was observed with further increases in sarcomere length.

Low resistance junctions in crayfish. Structural changes with functional uncoupling

Electrical uncoupling of crayfish septate lateral giant axons is paralleled by structural changes in the gap junctions. The changes are characterized by a tighter aggregation of the intramembrane particles and a decrease in the overall width of the junction and the thickness of the gap. Preliminary measurements indicate also a decrease in particle diameter. The uncoupling is produced by in vitro treatment of crayfish abdominal cords either with a Ca++, Mg++-free solution containing EDTA, followed by return to normal saline (Van Harreveld's solution), or with VAn Harreveld's solution containing dinitrophenol (DNP). The uncoupling is monitored by the intracellular recording of the electrical resistance at a septum between lateral giant axons. The junctions of the same septum are examined in thin sections; those of other ganglia of the same chain used for the electrical measurements are studied by freeze-fracture. In controls, most junctions contain a more or less regular array of particles repeating at a center to center distance of approximately 200 A. The overall width of the junctions is approximately 200 A and the gap thickness is 40-50 A. Vesicles (400-700 A in diameter) are closely apposed to the junctional membranes. In uncoupled axons, most junctions contain a hexagonal array of particles repeating at a center to center distance of 150-155 A. The overall width of the junctions is approximately 180 A and the gap thickness is 20-30 A. These junctions are usually curved and are rarely associated with vesicles. Isolated, PTA-stained junctions, also believed to be uncoupled, display similar structural features. There are reasons to believe that the changes in structure and permeability are triggered by an increase in the intracellular free Ca++ concentration. Most likely, the changes in permeability are caused by conformational changes in some components of the intramembrane particles at the gap junctions.

The relative contributions of the folds and caveolae to the surface membrane of frog skeletal muscle fibres at different sarcomere lengths

The plasmalemmal area of striated muscle fibres is greater than the apparent surface area (A = circumference x length) because of variable folds and the invaginations of the caveolae and T-tubules. Freeze-fracture replicas of the surface membrane of sartorius and semitendinosus muscles from Rana pipiens have been used to determine the numbers and distribution of folds and caveolae at different sarcomere lengths. (1) The plasmalemma folds are variable in size and shape, but are always oriented perpendicular to the long axis of the fibre. The folds vary with stretch, being more prominent at short sarcomere lengths. The caveolae are elliptical invaginations of the plasmalemma which open to the outside by a narrow "neck" of approximately 20 nm. The caveolar lumen has an average long dimension of 81.6 +/- 11.7 nm and an average short dimension of 66.9 +/- 7.9 nm. The caveolar "necks" only can be seen in freeze-fracture replicas and these are distributed in two circumferential bands on either side of the Z-line, and in longitudinal bands separated by distances of 1-5 mum. In the sartorius muscle, at a sarcomere length of 2.8 mum, there is an average number of thirty-seven caveolae per square micrometer of fibre surface. (2) During passive stretch the opening of folds provides membrane for the necessary increase in surface area up to a sarcomere length of about 3.0 mum. This length is defined as the critical sarcomere length (Sc). The number of caveolae remains constant at all sarcomere lengths less than Sc and thus their "necks" have been used as membrane markers to determine the amount of folding at different sarcomere lengths. The membrane area contained in folds and caveolae is expressed as a fraction of the apparent surface area (A). For example, in the sartorius muscle, at a sarcomere length of 2.4 mum, the membrane area, excluding the T-tubules, is: A + 0.1A (folding) + 0.7A (caveolae) = 1.8A. (3) For stretch beyond Sc membrane is provided by the opening of caveolae. At a sarcomere length of about 8 mum all the caveolae are open and the fibres rupture with further stretch. (4) The relative contributions of folds and caveolae vary with sarcomere length in a way that is consistent with assumptions of constant volume and plasmalemma area. The maintenance of constant plasmalemma area, even after excessive stretch, suggests that the plasmalemma is relatively inelastic in this situation.

Differential effects of glycerol treatment on membrane capacity and excitation-contraction coupling in toad sartorius fibres

Changes in membrane capacity and excitation-contraction coupling caused by glycerol movements have been investigated in toad sartorius fibres using a standard glycerol-Ringer solution containing 400 mM glycerol.1. The rates of glycerol movement, in and out of fibres, were determined by measuring diameter changes in single fibres. Glycerol equilibrated across the surface membrane within 20-25 min after changes in extracellular glycerol concentration.2. The reduction in membrane capacity, which occurs when glycerol-loaded fibres are returned to Ringer solution, was slower than, and not dependent on, changes in fibre volume.3. To obtain the maximum reduction in membrane capacity, it was necessary to expose fibres to glycerol-Ringer for 50-60 min and then to return them to Ringer for at least 60 min. If preparations were not kept in Ringer for the full 50-60 min, the reduction in membrane capacity could be partially or completely reversed by returning the fibres to glycerol-Ringer.4. When fibres were exposed to glycerol-Ringer there was an initial transient contracture: twitches and K contractures were rapidly inhibited, and then slowly recovered over the next 40-50 min. In some preparations, eventual potentiation of twitches was seen.5. When returned to Ringer solution after 60 min in glycerol-Ringer, preparations lost twitches and K contractures within 5-10 min. The time course of this effect was very similar to the time course of the recovery of the normal volume after the initial swelling.6. The briefer the exposure to the glycerol-Ringer, the slower the loss of twitches and K contractures on return to Ringer. In contrast to the loss of membrane capacity, the lesion of excitation-contraction coupling was essentially irreversible. Exposure times as brief as 5 min were eventually effective in producing paralysed fibres which, however, still responded to caffeine.7. The differences in the glycerol load-times required to produce decoupling of excitation and contraction, and disconnexion of the transverse tubules, show that the two effects are separable and that the first does not depend on the second.8. It is proposed that the two lesions obtained in glycerol-treated fibres may be related to distension of the transverse tubular system. It is shown in an Appendix that the outward movement of glycerol from sarcoplasm to transverse tubules would be expected to produce some early swelling of the tubules and this is related to the loss of contraction. Furthermore, much greater swelling would occur if a slow-loading compartment (presumed to be the sarcoplasmic reticulum) dumped glycerol into the transverse tubules: it is thought that this is related to the disconnexion of the transverse tubules.

Electrical properties of toad sartorius muscle fibres in summer and winter

1. The area and circumference of surface fibres of sartorius muscles were measured from photomicrographs of frozen sections of whole muscles, and compared with the values obtained assuming a circular cross-section. The latter assumption gave an over-estimate of the mean area of 28%, but only a 2% over-estimate of the circumference. In isolated, single fibres, the assumption gave over-estimates of 25 and 6%, of area and circumference respectively.2. The passive electrical properties of fibres were different in summer and winter. The mean internal resistivity, membrane resistance and membrane capacitance were 147 Omega.cm, 7.6 kOmega.cm(2) and 4 muF/cm(2) in summer, and 194 Omega.cm, 3.9 kOmega.cm(2) and 6.7 muF/cm(2) in winter, in fibres of comparable diameters in situ. In single fibres in summer, the mean values were 120 Omega.cm, 8.6 kOmega.cm(2) and 3.6 muF/cm(2).3. In glycerol-treated fibres the mean specific membrane capacitance was 1.0 muF/cm(2) in summer and 2.0 muF/cm(2) in winter. The internal resistivity and specific membrane resistance were 167 Omega.cm and 8.9 kOmega.cm(2) in summer, and 232 Omega.cm and 3.9 kOmega.cm(2) in winter.4. Early after-depolarizations were recorded in glycerol-treated fibres which had a low membrane capacitance, did not twitch and showed little ;creep'. Electron micrographs of glycerol-treated fibres showed disruption of the transverse tubular system and sarcoplasmic reticulum.5. After exposure of muscles to 400 mM urea or acetamide for 1 hr, muscle fibres did not twitch and had a reduced membrane capacitance in Ringer solution.