The Effect of Tetraethylammoninum (TEA) and Temperature on the Neuromuscular Block Produced by Magnesium

Summary. 1) An investigation of the effect of temperature on the neuromuscular block caused by magnesium in the isolated phrenic nerve diaphragm preparation from the rat has been carried out. The effect of tetraethylammonium (TEA) on the neuromuscular depression produced by magnesium has been investigated in the rat diaphragm preparation as well as in nerve muscle preparations in situ in rabbits and cats. 2) A concentration of 6 mM magnesium in the bath just produced complete neuromuscular block of the responses to single nerve shocks at 37°C. At 22° C 10 mM magnesium was needed. In the presence of a neuromuscular block due to magnesium at 37° C, lowering the temperature to 22° C restored transmission. 3) TEA in a concentration of 1 mM in the bath partly restored neuromuscular transmission depressed by magnesium. The β-hydroxy-derivative of TEA and choline only increased the magnesium block. In the rabbit and cat TEA in a dose of 4 to 7 mg per kg body weight injected intravenously restored neuromuscular transmission strongly depressed by continuous infusion of magnesium. In spite of continuous magnesium infusion, the restoration of muscle contractions obtained with the single dose of TEA was far more sustained than the blood pressure fall caused by TEA. 4) The sensitivity of the motor end plate to externally applied acetylcholine was not increased after TEA had restored transmission in the magnesium-blocked rat diaphragm. 5) The end plate potential obtained under a magnesium block was increased in amplitude without significant change in time course, when TEA was added. It is concluded that TEA like calcium antagonizes the neuromuscular block caused by magnesium by effecting an increased output of acetylcholine from the presynaptic nerve terminals.