The effect of low calcium and of tetraethylammonium (TEA) on the rat diaphragm.

Abstract

Summary. 1 The effect of lowering the calcium content on the neuromuscular transmission of the isolated phrenic nerve diaphragm preparation of the rat has been investigated at 37° C and at 22° C. The threshold concentration of calcium necessary to maintain minimal activity was found to be about 4 times lower at 22° C than at 37° C. This was not due to an action on the nerve, and was independent of the presence of potassium ions. 2 A concentration of 1 mM tetraethylammonium (TEA) completely restored the neuromuscular transmission depressed by a deficiency of calcium, provided the calcium content had been adjusted to that necessary for minimal activity at 37° C. At 22° C TEA restored transmission for more than an hour in a completely calcium-free medium. During prolonged action, however, TEA could not replace calcium completely at this lower temperature either. This effect was also independent of the presence of potassium. Neither the beta-hydroxy derivative of TEA nor choline had a restoring effect. 3 When the function of the rat diaphragm preparation was depressed because of sodium depletion, TEA had no restoring effect. 4 The end plate potential obtained in a calcium-free Tyrode solution was increased in amplitude without change in time. 5 In a calcium-deficient solution, the muscle contractions produced by rapid intravascular injection of acetylcholine and potassium were greatly enhanced. When neuromuscular transmission was restored with TEA, the sensitivity to acetylcholine returned to normal, but the enhanced sensitivity to potassium remained. It is suggested that TEA effects the release of acetylcholine from the presynaptic nerve endings like calcium, but does not seem to stabilize the muscle membrane to the same extent as calcium.