Presynaptic muscarinic and adenosine receptors are involved in 2 Hz-induced train-of-four fade caused by antinicotinic neuromuscular relaxants in the rat

Train-of-four fade (TOFfade) is a clinically useful parameter to monitor the degree of block of neuromuscular transmission in curarized patients. Experimentally, TOFfade has been attributed to the blockade of facilitatory nicotinic receptors on motor nerve terminals. There is less information regarding the involvement of coexistent presynaptic receptors (e. g. muscarinic M-1 and M-2, adenosine A(1) and A(2A)) in the TOFfade produced by antinicotinic agents. In the present study, we evaluated the TOFfade caused by antinicotinic neuromuscular relaxants (hexamethonium, D-tubocurarine, vecuronium and rocuronium) as the ratio of the muscle tension produced in the rat diaphragm by the fourth to the first stimulus (T-4/T-1) of a train-of-four stimuli delivered to the phrenic nerve trunk at a frequency of 2 Hz. All antinicotinic agents, except hexamethonium, decreased the amplitude of muscle tension during the first stimulus. Hexamethonium, (5.47 mmol/L), D-tubocurarine-(1.1 mu mol/L), vecuronium (4.7 mu mol/L)- and rocuronium (9.8 mu mol/L)-induced TOFfade was attenuated by 10 nmol/L pirenzepine (an M-1 receptor antagonist), 1 mu mol/L methoctramine (an M-2 receptor antagonist) and 2.5 nmol/L 1,3-dipropyl-8-cyclopentylxanthine (an A(1) receptor antagonist). Blockade of the A(2A) receptor with 10 nmol/L ZM241385 partially reversed the TOFfade induced by D-tubocurarine, vecuronium and rocuronium, but not that caused by the 'pure' neuronal nicotinic receptor antagonist hexamethonium, unless one increased the concentration of ZM241385 to 50 nmol/L. The data indicate that presynaptic M-1, M-2, A(1) and A(2A) receptors play a role in neuromuscular TOFfade caused by antinicotinic neuromuscular relaxants. Such interplay depends on adenosine tonus and on the affinity of neuromuscular blocking agents for neuronal versus muscular nicotinic receptors.