Kinetic differences between embryonic- and adult-type acetylcholine receptors in rat myotubes.

Abstract

The burst structures of embryonic-type (low-.gamma.) and adult-type (high-.gamma.) nicotinic acetylcholine (ACh) receptors (AChRs) in rat myotubes were investigated with the patch clamp technique. The channels were activated with the agonists ACh and suberyldicholine (SubCh). With either agonist, the distribution of burst durations showed two exponential components for both channel types: a ''long'' component that corresponds to bursts of one or more openings and a ''brief'' component that includes short, isoilated openings. For low-.gamma. channels, the percentage of all openings associated with the brief component decreased from .apprx.40% at 10-100 nM-ACh to 10,000 s-1 for both channel types at root temperature. Estimates of the channel opening rate inferred from the rising phase of miniature end-plate currents recorded from rat soleus fibres are consistent with this interpretation. Both high-.gamma. and low-.gamma. channels apparently operate via similar gating mechanisms. Differences in their gating behaviour can be explained in terms of faster kinetic rate constants for high-.gamma. channels.