The Effects of Ethidium Bromide on Purinergic Modulation of Myoneural Transmission and Skeletal Muscle Contraction

The problem of changes in myoneural transmission in the presence of an intercalating agent, ethidium bromide, which has a known inhibitory effect on neuromuscular transmission, has been investigated, but the nature of such an effect remains unclear. To solve the question of the possible participation in this process of known modulators of synaptic transmission, that is, purines (ATP and adenosine), we evaluated their effects in the presence of this agent. After holding a frog neuromuscular preparation in a perfusing solution containing ethidium bromide, the amplitude of postsynaptic responses and muscle contraction forces decreased. Under these conditions, both purines additionally exerted their usual suppressive effect on both the amplitude of postsynaptic responses and the strength of skeletal muscle contraction. Thus, the inhibitory effect of ethidium bromide on neuromuscular transmission is not associated with an increase in the inhibitory effect of endogenous purines caused by the quantum release of the neurotransmitter.