Abstract
1. The ionic selectivity of A-current K+ channels has been measured in single Helix aspersa neurones by recording the reversal potential shift in test solutions containing various monovalent cations. 2. The A-current channel is permeable to Tl+, K+, Rb+, NH4+ and Cs+. The channels may also be sparingly permeable to Na+ and Li+. Organic cations have an apparent small permeability as judged from their reversal potentials, but this may be an artifact of K+ accumulation. 3. A large patch electrode (3 microns tip) isolated a region that appeared to contain only A-current channels. This may indicate that A-current channels are found in the membrane as rafts of at least 3 microns in diameter. 4. The single-channel conductance calculated from single-channel current steps was 14 pS.