Multiple conductance levels of calcium-permeable channels activated by epidermal growth factor in A431 carcinoma cells.

Single Ca(2+)-permeable channels were studied in membrane patches from A431 carcinoma cells. Amplitudes of channel openings fell into three major groups with mean unitary conductances of 1.3, 2.4 and 5.1 pS (105 mM Ca2+ in the pipette as charge carrier). All three groups of events were activated with epidermal growth factor (EGF) from the outside and by GTP non-hydrolyzable analogues from the inside of the patch membrane. As a rule, channel openings were uniform in amplitude in each individual patch but sometimes transitions between openings of different conductance levels were seen. It is concluded that the plasma membrane of A431 cells contains a single type of EGF- and GTP-dependent Ca(2+)-permeable channel (or channel complex) that can display, at least, three conductance levels.