Muscle postjunctional membrane: changes in chemosensitivity produced by calcium.

Increases in the extracellular concentration of calcium ions above 1.8 millimoles per liter caused a reversible decrease in the sensitivity of tmuscle postiunctional membrane to carbamylcholine. A quantitative study of the inhibitory effect of calcium ions on membrane depolarization produced by carbamylcholine indicates that calcium ions compete with carbamylcholine for some common binding sites on the postjunctional membrane. Calcium ions (20 millimoles per liter) caused a neuromuscular block wherein prolonged end plate potentials were produced after nerve stimulation. Calcium ions applied ionophoretically to the postjunctional membrane decreased the amplitude and prolonged the time course of the transient depolarization produced by ionophoretically applied carbamylcholine.