Distribution of acetylcholine receptors at frog neuromuscular junctions with a discussion of some physiological implications.

Publisher Website
Google Scholar
Abstract
The distribution of acetylcholine receptors (AChR) at frog cutaneous pectoris neuromuscular junctions was studied quantitatively using (125I].alpha.-bungarotoxin (.alpha.-BTX) labeling and EM autoradiography. As in mouse end-plates, the AChR is localized uniformly along the thickened post-junctional membrane. In the frog muscle this specialized membrane constitutes approximately the top 50% of the junctional folds. The receptor site density is .apprx. 26,000 .+-. 6000 sites/.mu.m2 on the thickened post-junctional membrane and falls sharply to .apprx. 50 sites/.mu.m2 within 15 .mu.m from the axon terminal. .alpha.-BTX site density on the presynaptic axonal membrane was directly determined to be at most 5% of the value on the thickened post-junctional membrane. The high post-junctional AChR site density shows that each quantum of ACh needs to spread only over a very small post-junctional area (to be called the critical area) before it encounters as many AChR (plus AChE) sites as there are ACh molecules in the quantum; the average concentration of ACh prevailing in the cleft over this critical area during a quantal response is .apprx. 10-3 M (independent of the size of the quantal packet) and since 10-3 M-ACh is large compared to any estimates of the dissociation constant Kd for ACh binding to the AChR, the ACh should essentially saturate the AChR within the critical area. The total receptive surface for a frog end-plate is calculated to be .apprx. 1500 .mu.m2, and therefore an end-plate potential resulting from 300 quanta should be due to the activation of < 10% of the total receptive area. Free diffusion would allow each small post-junctional critical area to be reached in < 15 .mu.s. Either the recorded rise time of the miniature end-plate current is not predominantly a function of ACh diffusion time, or, as suggested by Gage et McBurney (1975), the net rate of movement of ACh in the cleft is much slower than indicated by the free diffusion constant.

This publication has 53 references indexed in Scilit: