Abstract
The modulation of acetylcholine‐activated current (I~ACh~) by protein kinase C (PKC) was studied in Xenopus laevis oocytes microinjected with either mRNA extracted from C2C12 myotubes (C2C12 mRNA) or RNAs encoding murine αβγδ subunits of the nicotinic ACh receptor (nAChR). Voltage‐clamped oocytes were treated for 90 sec with 12‐0‐tetradecanoylphorbol‐13‐acetate (TPA, 300 nM), a potent PKC activator. Transient increase in the amplitude and acceleration in the decay of I~ACh~ were invariably observed within minutes of TPA application, and were independent of extracellular Ca^2+^ concentration. Both parameters recovered to control within 20–30 min; then a slight depression of I~ACh~ developed. By this time, an initial PKC down regulation was observed. At the peak of TPA‐induced potentiation, dose‐response relations suggested an increased binding affinity of nAChR for the neurotransmitter. 4a‐phorbol 12,13‐didecanoate (300 nM), a biologically inactive analogue of TPA, did not affect I~ACh~, while staurosporine (5–10 μM), a potent inhibitor of PKC activity, suppressed the action of TPA on I~ACh~. In oocytes co‐injected with C2C12 mRNA and with rat brain mRNA, I~ACh~ was potentiated by 5‐hydroxytryptamine (10 μM), whose receptors are coupled to phosphoinositide hydrolysis. The nAChR‐channel activity in cell‐attached patches increased when TPA was applied to the oocytes. In 50% of the oocytes examined, a sustained depression of the single channel activity followed. We conclude that in Xenopus oocytes an endogenous PKC system regulates the function of embryonic‐type muscle nAChRs. © 1995 Wiley‐Liss, Inc.