Voltage clamp records have been obtained from bovine adrenal chromaffin cells in the outside-out and whole-cell configurations, in response to step changes of acetylcholine (ACh) concentration. The concentrations used ranged from 50 nM to 20 mM. 2. At high acetylcholine concentrations, the activation and desensitization kinetics of the nicotinic receptor, as observed in outside-out patches, may be described by a model incorporating a single, fast agonist binding step, and relatively slow isomerization to the open state. The affinity of the closed receptor for ACh is 310 microM, the channel opening rate constant is 460 s-1, and the closing rate constant is 29 s-1. 3. Single channel events, observed when nanomolar ACh concentrations are applied to whole cells, have two distinct channel lifetimes: 0.6 ms and 11-15 ms. The variation of the frequencies of the events with ACh concentration, suggests that the short lifetimes are openings of a singly liganded receptor and the longer lifetimes are openings of a doubly liganded receptor. 4. Only a single exponential associated with receptor desensitization is seen with outside-out patches, but two are seen with whole cells. It is postulated that there are two nicotinic receptor types present on adrenal chromaffin cells. 5. The rate of desensitization (9 s-1 and 26 s-1, whole cells; 24 s-1, patches), is fast enough to be significant in determining the open channel lifetime. 6. A sudden increase in current (rebound) is observed when a high concentration of ACh is abruptly removed from outside-out patches. This is evidence for a blocked state. The affinity of the blocking site for ACh is 1400 microM (outside-out patches). 7. The total number of activatable nicotinic channels per whole cell is estimated to be 2600.
