The 2 Hz train-of-four ratio (TOFratio) is used to monitor the degree of patient curarization. Using a rat phrenic nervehemidiaphragm preparation, we showed that antinicotinic agents, such as hexamethonium, d-tubocurarine and pancuronium, but not cisatracurium, decreased contractions produced by physiological nerve activity patterns (50 Hz) more efficiently than those caused by 2 Hz trains. Uncertainty about the usefulness of the TOFratio to control safe recovery from curarization prompted us to investigate the muscarinic and adenosine neuromodulation of tetanic (50 Hz) fade induced by antinicotinic agents at concentrations that cause a 25% reduction in the TOFratio (TOFfade). Tetanic fade caused by d-tubocurarine (1.1 mu mol/L), pancuronium (3 mu mol/L) and hexamethonium (5.47 mmol/L) was attenuated by blocking presynaptic inhibitory muscarinic M2 and adenosine A1 receptors with methoctramine (1 mu mol/L) and 1,3-dipropyl-8-cyclopentylxanthine (2.5 nmol/L), respectively. These compounds enhanced rather than decreased tetanic fade induced by cisatracurium (2.2 mu mol/L), but they consistently attenuated cisatracurium-induced TOFfade. The effect of the M1 receptor antagonist pirenzepine (10 nmol/L) on fade produced by antinicotinic agents at 50 Hz was opposite to that observed with TOF stimulation. Blockade of adenosine A(2A) receptors with ZM 241385 (10 nmol/L) attenuated TOFfade caused by all antinicotinic drugs tested, with the exception of the pure presynaptic nicotinic antagonist hexamethonium. ZM 241385 was the only compound tested in this series that facilitated recovery from tetanic fade produced by cisatracurium. The data suggest that distinct antinicotinic relaxants interfere with fine-tuning neuromuscular adaptations to motor nerve stimulation patterns via activation of presynaptic muscarinic and adenosine receptors. These results support the use of A(2A) receptor antagonists together with atropine to facilitate recovery from antinicotinic neuromuscular blockade.
