Angiotensin II-induced formation of ionic channels in bilayer lipid membranes

The interaction of angiotensin II (ANG II) with membrane was studied by measuring conductance and current-voltage characteristics (IVC) of bilayer lipid membranes (BLM) prepared of a mixture of egg lecithin with cholesterol, and of gramicidin D-modified membranes of the same composition. Addition of physiological concentrations of ANG II (approx. 15 mumol/l) into the electrolyte (1 mol/l KCl, pH = 7) in contact with one side of BLM resulted in the appearance of discrete membrane conductance (symbol; see text) = (39.5 +/- 1.07) pS with a duration of the conductivity state tau = (52.15 +/- 6.44) s. Raising ANG II concentration to 75 mumol/l resulted in an additional conductance level of approx. 130 pS with a lifetime of approx. 1s. The electrolyte pH markedly influenced ANG II modified BLM conductance. A decrease of the electrolyte pH to 2.8 resulted in a reduction of the discrete conductance level to approx. 14 pS, whereas ANG did not induce any conductivity at pH = 11.5. The results obtained suggest that ion channels are formed consisting at least of two ANG II molecules. IVC of ANG II-modified BLM are superlinear within the range of electrolyte concentrations studied (between 0.01 and 3 mol/l KCl), i.e, the limiting stage of ion transport is the internal area of the conducting pore. ANG II affects in a cooperative manner the gramicidin D (GRD)-mediated transport, most likely by forming ANG II aggregates in the area of local inhomogeneities in the BLM structure of GRD channels.