THE INITIATION OF THE MUSCLE ACTION-POTENTIAL

The specific functional properties of the nicotinic acetylcholine rece ptors (AChR) and the particular oligomeric membrane organization of AC hR are suggested to be the basis for the steep electrical depolarisati on, required for the initiation of the postsynaptic action potentials causing muscle contraction and discharge of electric organs. The dimer (M(r) approximate to 580,000) and the monomer (M(r) approximate to 29 0,000) of the detergent-solubilized, affinity-purified AChR of Torpedo californica electrocytes exhibit different channel conductances and l arger oligochannels. Patch clamp data of the dimer, reconstituted in l arge lipid vesicles, show that the dimer is a double-channel protein c ausing single-channel events of conductance G(D) = 84 +/- 6 pS at 0.11 M K+ and 0.1 mM Ca2+ at 293 K (20 degrees C). At the same ionic condi tions the vesicle-reconstituted monomer, if prevented from aggregation , exhibits a channel conductance, G(M) = 42 +/- 3 pS, which is only ha lf of that of the dimer. The dimer conductivity event thus reflects th e synchronous switching of its two constituent monomeric parts. The K-conductance of the reconstituted Torpedo dimer is the same, and shows the same inhibitory Ca2+-dependence, as that of the Torpedo AChR expr essed in Xenopus laevis oocytes (Imoto et al., Nature, 324, 670-674, 1 986). In terms of Ca2+-binding, reducing K+-transport, the equilibrium constant is K-Ca = 0.48 mM at 0.11 M K+, 20 degrees C; G(0) ([Ca] --> 0) = 98 +/- 6 pS and G(infinity) ([Ca] --> infinity = 27 +/- 6 pS. Th e ratio G(0)/G(infinity) and an estimate of the lateral surface area o f the channel vestibule yields about 16 negatively charged groups in a n average distance of 1.8 nm. These negative charges cause an accumula tion of K+ ions in the channel vestibule by a factor of about 4. Our r esults and the comparison with the oocyte data reveal that it is also the dimer which is the physiological opening-closing unit of the AChR in the oocyte membrane. The larger macrochannel events are multiples o f the dimer or of the monomer conductances. The occurrence of such oli gochannels from AChR protein oligomers could guarantee the steep elect rical depolarisation necessary to generate the action potential by the Na+-channel system.