Interactions of vecuronium and atracurium in an in vitro nerve-muscle preparation

Atracurium and vecuronium were compared when given alone and in combination in the in vitro rat phrenic nerve-hemidiaphragm preparation stimulated via the phrenic nerve. The slopes of the log dose response curves of atracurium and vecuronium were parallel; their ED50s were 1.12 +/- 0.0035.10(-5)M and 5.89 +/- 0.16.10(-6)M, respectively. The combination's log dose response curves were significantly shifted to the left when compared with those of either relaxant alone; an increased potency is displayed by the combination. These observations indicate nondepolarizing muscle relaxant synergy for the combination of equal proportions of vecuronium and atracurium. The synergistic interaction of vecuronium and atracurium in this in vitro-model is not dependent on pharmacokinetic factors such as uptake, distribution, and biodegradation as are present in the in vivo animal models and in humans. Synergy of vecuronium and atracurium in vitro is a new finding and is consistent with hypotheses of multiple receptor sites and different modes of action of the "competitive" neuromuscular blocking agents. This degree of synergy, seen in the in vitro animal data, if extrapolatable to humans, is probably of little clinical significance.