Sodium transport by the acetylcholine receptor of cultured muscle cells.

Activation of the acetylcholine receptors of cultured muscle cells by carbamylcholine increases the rate of passive 22-Na+ uptake into the muscle cells up to 20-fold. The Na+ transport activity of the receptor desensitizes during exposure to carbamylcholine. The rate and extent of desensitization is reduced by lowering the assay temperature from 36 degrees to 2 degrees, allowing accurate measurements of initial rates of Na+ transport by the receptor. Activation of the receptor by carbamylcholine and acetylcholine is significantly cooperative (Hill coefficients of 1.4 to 2.0). Inhibition by D-tubocurarine is not cooperative. The carbamylcholine-induced Na+ transport activity of the receptor is inhibited 50% by 4 muM D-tubocurarine, 100 muM atropine, or 1.6 nM diiodo-alpha-bungarotoxin but is not affected by tetrodotoxin. The initial rate of Na+ transport by the receptor is temperature-independent between 2 degrees and 36 degrees. Receptor Na+ transport is saturable by Na+ at 2 degrees with an apparent Km of 150 plus and minus 20 mM. Saturation by Na+ not observed at 36 degrees at the concentrations tested. Saturation by Na+ is observed at 2 degrees both under conditions of net Na+ influx and under conditions of isotopic exchange at equilibrium. The receptor does not catalyze obligatory exchange diffusion at a detectable rate. Comparison of binding of [125-I]diiodo-alpha-bungarotoxin with rates of Na+ transport indicates a turnover number of 2 times 10-7 ions per min per receptor. These results are discussed in terms of the mechanism of Na+ transport by the receptor.