The Effects of Macrocyclic Compounds on Cation Transport in Sheep Red Cells and Thin and Thick Lipid Membranes

For some years, work in this laboratory has been directed toward elucidation of the mechanism of selectivity of high-potassium (HK) and low-potassium (LK) sheep red cell membranes for Na + and K + (I-4). One recent approach has been to compare the ionic permeability of the intact ceil membranes with thin artificial bilayer membranes prepared from lipids extracted from these cells (5). It was found that the thin lipid membranes have ionic permeability properties which are vastly different from those of intact HK and LK sheep red cell membranes. For example, the PC electrical resistance of the bilayers is about 2 X 10 s ohm cm 2, while that of the red cell membranes is estimated to be from 10 to 100 ohm cm 2. Furthermore, the bilayers are more permeable to both Na + and K + than they are to Cl-, while the ratio of Clto K + or Na + permeability of red cells is of the order of 106 (6). Finally, the thin lipid membranes do not distinguish between Na + and K +, and the ionic transport properties of such membranes prepared from HE and LK sheep red cell lipids are identical. The intact cell membranes do distinguish between these alkali metal ions, and, furthermore, the Na + and K + transport properties of the two genetic types of sheep red cells are significantly different. Apparently, the thin lipid membranes lack essential components which are responsible for the distinctive cation permeability properties of the intact cell membranes. In the course of continuing attempts to identify such components, we became aware of reports of the effect of the macrocyclic depsipeptide antibiotic, valinomycin, on the ionic permeabil i ty of lecithin (7) and mixed brain lipid (8) bilayer membranes. This compound had previously been reported to stimulate the respiration of mitochondria in the presence of K + but not in the presence of Na + (9). Valinomycin was found to produce specific permeabili ty of intact H K and L K sheep red cell membranes, as well as thin artificial membranes prepared from lipids extracted from these cells, to K + but not to Na + ( 10, 11). In the presence of K + ( 10 t M) and valinomycin (5 X 10 -~ M), the electrical resistance of the lipid bilayer membranes is about 103 ohm cm ~, which is in the range of biological membranes. The significance of these observations derives from the unusual structure of valino-