DIFFERENTIAL INTERACTIONS OF GENTAMICIN WITH MOUSE JUNCTIONAL AND EXTRAJUNCTIONAL ACH RECEPTORS EXPRESSED IN XENOPUS-OOCYTES

The nicotinic acetylcholine receptors (AChRs) from Torpedo electric or gan and mouse muscles when expressed in Xenopus oocytes desensitize wi th different time courses. Initially, the role of cAMP-dependent phosp horylation on the gamma subunits in the different desensitization rate s was investigated by expressing normal and mutant AChRs in the oocyte s cultured in the presence of gentamicin. Mutant Torpedo AChRs lacking the potential cAMP-dependent phosphorylation sites in the gamma subun it appear to desensitize like normal Torpedo AChRs. Similarly, mutant mouse extrajunctional AChRs containing a newly created phosphorylation site in the gamma subunit appeared to desensitize like normal mouse A ChRs, which lack the potential cAMP-dependent phosphorylation site in the gamma subunit. These results suggest that different rates of desen sitization between the Torpedo and muscle extrajunctional AChRs are no t attributable to differential cAMP-dependent phosphorylation of these AChRs. Subsequently, to determine whether gentamicin used in culturin g oocytes differentially interacts with muscle junctional and extrajun ctional AChRs, we analyzed rates of current decay following different gentamicin treatments. Both chronic and acute treatment with gentamici n profoundly accelerated the decay of whole-cell currents mediated by both types of AChR. The effect of prolonged gentamicin treatment on ju nctional AChRs was long lasting when compared to treatment on extrajun ctional AChRs. Although the two types of AChR still desensitize differ ently in the absence of gentamicin, these results suggest that the cha racteristic desensitization of junctional and extrajunctional AChRs ob served previously is largely due to differential interactions of genta micin with the two types of AChR.