title: Extracellular space and diffusion barriers in muscle fibres from Megabalanus psittacus (Darwin).
abstract: 1. Muscle fibres from Megabalanus psittacus (Darwin) were used to measure the exchange of Na+ and Ca2+ between intracellular and extracellular compartments. 2. The size of the extracellular space was evaluated directly from electron micrographs at 6.1 +/- 0.5% (percentage of the fibre volume). The more conventional estimates using Na+ and 134Cs+ as space markers gave higher values, namely, 8.9 +/- 0.7 and 9.5 +/- 1.5%. 3. An average value of 9.2% was used to correct the total Na+ and K+ in the muscle fibres and to estimate the intracellular concentrations of Na+ and K+ as 39 +/- 4 and 202 +/- 11 mM respectively. 4. Na+ (and similar Ca2+) washout curves could be described using a three compartments diffusion model. The data analysed in terms of this model enabled us to estimate membrane permeabilities as PNa+ = 2.7 x 10(-7) and PCa2+ = 2.7 x 10(-7) cm/sec.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002463:Cell Membrane Permeability; D004058:Diffusion; D005110:Extracellular Space / Q000378:metabolism; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D007700:Kinetics; D008854:Microscopy, Electron; D009132:Muscles / Q000378:metabolism* / Q000648:ultrastructure; D012964:Sodium / Q000378:metabolism; D001468:Thoracica / Q000378:metabolism*
chemical_list: D012964:Sodium; D002118:Calcium