SUFMARYThe rate of movement of non-electrolytes and tritiated water (THO) across the muscle cell membrane of the giant barnacle Balanus nubilus has been studied and permeability coefficient calculated. The rate of permeation is more closely related to the oil-water partition coefficient than to size of the molecule or degree of hydrogen bonding. Calculations based on efflux from an ideal cylinder suggest that the membrane acts as a significant barrier to movement of these molecules. The cell was unable to concentrate dimethyl sulphoxide (DMSO); the steady state was reached at about 60 % of the extracellular concentration. The energies of activation for water, urea and DMSO are 7 5, 20-3 and 26-1 kcal/mol. At 40 C the apparent pore size measured with urea, glycerol and DMSO was 315 A. At 250 C the apparent pore size for urea and glycerol is unchanged but that for DMSO is 14 A.
INTRODUCTIONThe permeability properties of a cell membrane may be characterized in several ways. Studies on the movements of molecules give permeability coefficients. The effects of temperature on the permeability give an energy of activation for the over-all permeability process. Measurements of the ability of the membrane to discriminate between molecules of varying sizes can be used to estimate an effective pore size; this technique, first used by Goldstein & Solomon (1960), has been used to characterize several biological and artificial membranes (Katchalsky & Curran, 1967). In each calculation certain arbitrary assumptions are made; the combined results describe, therefore, the properties of a sort of idealized membrane.In this paper the results of studies using all of these approaches are presented to characterize the membrane of the barnacle muscle cell. The
684WILTON BUNCH AND CHARLES EDWARDS large size of the single muscle fibres of the barnacle permits direct injection of substances and thus allows direct studies leading to the measurement of the permeability coefficient and the energy of activation. Further, the muscles can be used for the measurement of concentrations of various solutes required to eliminate water movement in solutions hypo-osmotic in the usual salts; this data is the basis of the calculation of the effective pore size. The molecules examined include water, of obvious biological interest, urea, widely used in permeability studies, and dimethyl sulphoxide (DMSO), which has become of interest recently because of its reported ability to rapidly penetrate biological membranes. In addition, data was collected on movements of glycerol, also widely used in permeability studies, and acetylsalycilic acid (ASA), a larger organic molecule.
METHODS
Injection 8tudies8Single muscle fibres of the giant barnacle, Balanus nubilwe, were dissected free from the surrounding tissue and suspended on hooked metal rods. A 0-13 mm needle was inserted axially and while it was being withdrawn, 5 1l. solution was injected along a distance of about 1 cm. The basic injection solution contained 25 m-equiv NaCl, 75m-equiv K2S04 and 725 m-osmo...
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