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Radiocontrast molecules (RCM) used in coronaroangiography and/or urography diminished their osmotic fragility when they were incubated for 30 min with human erythrocytes. The shift of the osmotic fragility curve towards lower NaCl concentration is related to the hypertonicity of RCM, but in addition, at a given osmolality (100 ± 5 mosm/kg), RCM at the concentration of 4–10% v/v increase the resistance to osmotic lysis or even suppress it. Similar protection is observed towards erythrocyte lysis induced by a detergent (saponin), polyenic antibiotic (filipin) or non-polyenic cholesterol-specific agent (digitonin). The effect is (1) proportional to the amount of RCM present, (2) independent of hypertonicity of the molecule, and (3) related to the nature of acidic molecules. A weak insertion of RCM into the erythrocyte membrane is suggested since it was suppressed by a single washing of the cells.

Ion channels and pumps in cell membranes consist of multiple transmembrane segments that are thought to be critical for transport of ions. Channel structures constituted by these transmembrane segments are characteristic of ion channels, whereas such structures have not been identified in ion pumps until now. By applying atomic force microscopy on Na÷,K+-ATPase molecules in canine kidney membranes under tapping mode, we identified a hollow in the protein with a characteristic internal diameter of 6-20 ,/k and an external diameter of 20-55 A, depending upon treatment conditions. This hollow may be interpreted as a channel-like conformation of Na+,K÷-ATPase. In the regions where the proteins were absent, lipid head structures with 2/~ width and 6/~ length were imaged in an orthorhombic lattice.