Potassium channel selectivity in mouse pancreatic B cells

High-resistance microelectrodes were used to measure membrane potential changes in response to increased extracellular K+ concentration ([K+]o; or a test cation X+ such as Li+, Rb+, Cs+, NH+4) in B cells from mouse islets of Langerhans. In the absence of glucose, a sudden increase in [K+]o (or [X+]o), keeping the sum [Na+]o + [K+]o constant (or [Na+]o + [K+]o + [X+]o), induced a rapid depolarization of the membrane. The membrane potential changes were essentially unchanged in the presence of 20 mM tetraethylammonium (TEA). The Goldman-Hodgkin-Katz equation was fitted to the experimental relationship between membrane potential and [K+]o (or [X+]o), and permeability (P) ratios were estimated. In the absence of TEA, P Na/PK was estimated to be approximately 0.046. In the presence of TEA the following ratios were estimated: P Rb/PK = 0.74, P Cs/PK = 0.62, and P NH4/PK = 0.36. From these ratios the following sequence of permeabilities was obtained, PK greater than P Rb greater than P Cs greater than P NH4 greater than P Na. It is proposed that this sequence reflects the selectivity of the intracellular [Ca2+]-activated K+ channel of the pancreatic B cell.