Human acetylcholine receptors desensitize much faster than rat acetylcholine receptors.

The process of acetylcholine receptor (AChR) desensitization in the presence of transmitter was evaluated for human myoballs and medullo-blastoma cells. The cell under investigation was placed in one of two parallel streams of solution ejected from a double-barrelled pipette. Ultrafast (less than 50 ms) application of acetylcholine (ACh) in a specific concentration was accomplished by a shift of the cell into the other stream. ACh-induced current was measured in the whole-cell mode of patch clamping. Parameters for AChR desensitization were almost identical for the two types of human cells. Desensitization could be described by two time constants. With an ACh concentration of 3 microM, the fast time constant, tau f, was about 0.4 s and the slow time constant, tau s, was about 3.6 s. When the ACh concentration was raised to 10 microM, desensitization became faster. We also measured desensitization of the AChRs in rat myoballs and compared the values to our results from human cells. With 3 microM ACh, tau f in rat myoballs was 2.3 s and tau s was 12.7 s. Thus, rat AChRs desensitize more slowly than human AChRs. However, desensitization times for rat AChRs obtained in our experiments are much faster than published values. Therefore, ultrafast solution change would seem to be requisite for correct assessment of the desensitization process.