Aqueous pores created in thin lipid membranes by the antibiotics nystatin, amphotericin B and gramicidin A: implications for pores in plasma membranes
Since the days of Bärlund and Collander (see Collander, 1937), physiologists have suggested that ions, water, and small hydrophilic non-electrolytes cross the lipoidal plasma membrane through polar pores or channels.’ Recent work with model membranes has established that indeed unmodified lipid bilayers are so poorly permeable to polar non-electrolytes and especially to small ions that these species require parallel pathways via either carriers or pores for significant transport (Mueller & Rudin, 1969). On the other hand, water permeability coefficients of unmodified lipid bilayers span almost the entire range of values reported for plasma membranes, making it unnecessary to invoke aqueous pores in explanation of large water permeabilities of some cells. In point of fact, however, the assumption that water crosses plasma membranes through aqueous pores depends less on the magnitude of the water permeability than on two other factors: first, the significant differences between the osmotic permeability coefficient (Pf) and the tagged water permeability coefficient (Pd) (for example, Prescott & Zeuthen, 1953; Nevis, 1958) and secondly, the occurrence of water—solute interaction during osmosis (for example, Koefoed-Johnsen & Ussing, 1953; Goldstein & Solomon, 1960). These two findings, however, have been challenged as artifactual, because of the presence of unstirred aqueous layers in the experiments (Dainty, 1963).
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