Influence of lanthanum on transmitter release at the neuromuscular junction

Abstract

Abstract The presynaptic influence of lanthanum on neuromuscular transmission was studied in the isolated sciatic nerve‐sartorius muscle preparation of the frog, Rana pipiens . Twitch tension output from indirectly stimulated muscles progressively decreased with increasing concentrations of La 3+ from 0.01 to 1.0 mM and was abolished in the presence of 0.5 mM and 1 mM La 3+ after approximately 50 min and 15 min, respectively. Studies on directly stimulated muscle indicated that the inhibition by La 3+ of tension development during indirect stimulation was not primarily of myogenic origin. The decrease in indirectly stimulated twitch tension by 0.5 mM La 3+ was reduced by pretreatment in 10 mM Ca 2+ , but was not overcome by 1 mM TEA or 5 mM guanidine. In contrast, blockade produced by 8.2 mM Mg 2+ was alleviated by TEA or guanidine. In Mg 2+ poisoned preparations, microperfusion of 0.001 mM La 3+ decreased m by 70% (5 fibers). In curarized preparations, the presence of 0.001 mM La 3+ caused the EPP to decrease by 57% (3 fibers), and 0.5 mM Mg 2+ decreased the EPP by 61% (4 fibers). Although the EPP amplitude was reduced by La 3+ , the MEPP frequency was markedly potentiated in the presence of 0.001 to 0.5 mM La 3+ . The potentiation of MEPP frequency by La 3+ appeared to be enhanced in the absence of calcium and reduced by pretreatment in a 10 mM Ca 2+ Ringer solution. It is suggested that La 3+ blocks transmission presynaptically by decreasing the quantal content of the EPP, and that it is a more efficient blocker of evoked release than Mg 2+ .