The release of acetylcholine from mammalian motor nerve endings.

The release of acetylcholine from rat and guinea-pig isolated diaphragm preparations stimulated through the phrenic nerve was optimal at 37 degrees in Krebs solution with 5x10(-6) neostigmine methylsulphate. The amount of acetylcholine released by a 20 min. tetanus was reduced by cooling. At frequencies of stimulation above 6/sec. the release was less than that predicted. This "failure" was unaffected by the addition of 1x10(-6) choline. The acetylcholine release declined with continued stimulation at 25/sec. In the absence of nerve stimulation, there was a small continuous resting release of acetylcholine which seemed to originate in the muscle fibres. These results are discussed in the light of current electrophysiological knowledge of the quantal release of acetylcholine.