Single channel properties of human alpha 3 AChRs: impact of beta 2, beta 4and alpha 5 subunits

1. We performed single channel analysis on human alpha 3 acetylcholine rece ptors (AChRs) in Xenopus oocytes and native AChRs from the human neuroblast oma cell line IMR-32. alpha 3 AChRs exhibit channel properties that reflect subunit composition. 2. alpha 3 beta 2 AChR open times were 0.71 +/- 0.14 and 3.5 +/- 0.4 ms wit h a predominant conductance of 26 pS. alpha 3 beta 4 AChRs had open times o f 1.4 +/- 0.2 and 6.5 +/- 0.8 ms and a predominant conductance of 31 pS. Bu rst times were 0.82 +/- 0.12 and 5.3 +/- 0.7 ms for alpha 3 beta 2 and 1.7 +/- 0.1 and 16 +/- 1 ms for alpha 3 beta 4. Desensitization was faster for AChRs with the beta 2 subunit than for those with the beta 4 subunit. 3. One open time for alpha 3 alpha 5 beta 2 AChRs (5.5 +/- 0 3 ms) was diff erent from those of alpha 3 beta 2 AChRs. For alpha 3 alpha 5 beta 4 AChRs, an additional conductance, open time and burst time (36 pS, 22 +/- 3 ms an d 43 +/- 4 ms, respectively) were different from those for alpha 3 beta 4 A ChRs. 4. alpha 3 AChRs were inhibited by hexamethonium or mecamylamine. The rate constants for block of alpha 3 beta 4 by hexamethonium and of alpha 3 beta 2 by mecamylamine were 1.2 x 10(7) and 4.6 x 10(7) M-1 s(-1), respectively. 5. AChRs from IMR-32 cells had a predominant conductance of 32 pS and open times of 1.5 +/- 0.3 and 9.6 +/- 1.2 ms. These properties mere most similar to those of alpha 3 beta 4 AChRs expressed in oocytes. Antibodies revealed that 5 +/- 2% of IMR-32 alpha 3 AChRs contained alpha 5 subunits and 6 +/- 2% contained beta 2 subunits. IMR-32 alpha 3 AChRs are primarily alpha 3 b eta 4 AChRs.