Acetylcholine Content of Tyrode Solution Perfused Through Muscles as Affected by Calcium and Procaine Hydrochloride.

Summary 1. Dogs'denervated fibrillating muscles when perfused with Tyrode solution, Ca++ free Tyrode solution, or Ca++ free Tyrode solution containing 1 millimol of procaine hydrochloride per liter, continue to fibrillate and the perfusate from such muscles contains a heat labile material capable of inhibiting the heart of Venus mercenaria in vitro. 2. Intact muscle perfused with Tyrode solution does not usually exhibit electrical activity and the perfusate from such inactive muscles does not inhibit the heart of Venus mercenaria. 3. Innervated muscle perfused with eserin-ized Ca++ free Tyrode solution displays electrical activity similar to the fibrillation of denervation and the perfusate from such muscles contains a heat labile material capable of inhibiting the heart of Verms mercenaria in vitro. 4. Perfusates subjected to alkaline hydrolysis obtained from denervated nbrillating muscle, or from innervated muscle rendered active by perfusion with Ca++ free solutions, do not contain a cardio-inhibitor substance when tested for such an inhibitor on the heart of Venus mercenaria in vitro; these solutions often augment the contractions of the isolated clam heart. 5. Electrical activity of innervated muscle perfused with Ca++ free Tyrode solution ceases when procaine hydrochloride is added to the perfusing fluid and the perfusate from the muscle then contains no heat labile cardio-inhibiting substance. 6. A new hypothesis is discussed in which it is postulated that motor nerve endings at rest are in part responsible for the removal of acetylcholine at the neuromuscular junction.