Developmental changes in the open time and conductance of acetylcholine receptors in aneural cultured Xenopus myocytes treated with cycloheximide or tunicamycin.

Aneural cultures of Xenopus myocytes were treated with cycloheximide or tunicamycin in order to determine the influence of continued protein synthesis and insertion of glycosylated acetylcholine (ACh) receptor proteins on developmental changes in ACh-activated single-channel events. The developmental stage of the cultures was estimated from the stage of intact embryos maintained at the same temperature as the cultures. Single-channel recordings (5 x 10(-7) M ACh) from untreated cultures revealed primarily low conductance (27.4 +/- 0.6 pS) events for the first 14 h in culture (temperature = 23-25 degrees C) and a second high conductance class of events (42.5 +/- 0.9 pS) at later developmental stages (after Stage 28). Treatment with cycloheximide (at Stage 28) or tunicamycin (at Stage 22) produced significantly (P less than 0.01) fewer high conductance events at later stages (Stages 31-47), but had no effect on the conductance or open time of the low conductance event. A significant decrease (Chi square, P less than 0.05) in ACh sensitivity was observed at Stage 27 in myocytes treated with tunicamycin at Stage 22. The results strongly suggest that the low and high conductance events represent two distinct receptor molecules synthesized at different developmental stages. The tunicamycin results also suggest that a developmental reduction in the mean open time of low conductance events in untreated cultures does not depend upon the continued insertion of newly synthesized receptors.