EXTRACELLULAR COMPARTMENTS OF FROG SKELETAL-MUSCLE

  • 1 January 1979
    • journal article
    • research article
    • Vol. 288  (MAR) , 45-70
Abstract
Detailed studies of solute efflux from frog sartorius muscle and single muscle fibers were carried out in order to characterize a special region in the extracellular space of muscle and determine whether this special region is the sarcoplasmic reticulum. The efflux of radioactive Na, Cl, glucose, 3-O-methylglucose, xylose, glycine, leucine, cycloleucine, Rb, K, inulin, (MW 5000) and dextran (MW 17,000) from previously loaded muscles was studied. In all cases except dextran the curve had 3 components, a rapid (A) component which could be equated with efflux from the extracellular space proper, a slow (C) component representing cellular solute and an intermediate (B) component. The distribution space for the B component was 8% of muscle volume in summer frogs and 12% in winter frogs and appeared to be equal for all compounds studied. The hypothesis that the B component originated from the sarcoplasmic reticulum was tested. The C component was missing from the dextran curves. Both dextran and inulin entered the compartment of origin of the B component (compartment B) to the same extent as small molecules. For all compounds studied, the efflux rate constant for the A component could be predicted from the diffusion coefficient. For the B component the efflux rate constant was 6-10 .times. slower than that for the A component but was still proportional to the diffusion coefficient for the solute in question. When Na and sucrose efflux from single fibers was followed, a B component was usually observed. The average distribution space for this component was small, averaging 1.5% of fiber volume. There was no difference between the average efflux rate constants for Na and sucrose. In an appendix, the constraints placed on the properties of a hypothetical channel between the sarcoplasmic reticulum and the T-system by the linear electrical parameters of frog skeletal muscle were derived. The conductance of such a channel must be less than 0.06 .times. 10-3 mhos/cm2 of fiber membrane. The conductance between compartment B and the extracellular space can be calculated from the efflux rate constants for Na, K and Cl. The value obtained was 5 .times. 10-3 mhos/cm2 of fiber membrane or 100 .times. the limiting value for the conductance of the T-SR junction. The finding that there is a B component in the efflux curves for large molecular weight substances like inulin and dextran and the small size of the B component in efflux curves from single muscle fibers indicate that the special region of the extracellular space of frog muscle is not the sarcoplasmic reticulum. This conclusion is confirmed by a calculation of the conductance between the B compartment and the extracellular space. The value obtained is incompatible with predicted electrical properties of the SR-T-tubule junction. These results, in conjunction with the cellular ion concentrations which can be derived from the efflux data on whole muscle, are consistent with data from other laboratories which indicate that the ionic composition of the fluid in the sarcoplasmic reticulum resembles intracellular more than extracellular fluid.