Effect of ionized calcium on the neuromuscular blocking actions of atracurium and vecuronium in the cat.

We have determined neuromuscular blocking effects of atracurium and vecuronium at normal, high and low plasma concentrations of ionized calcium ( [Ca2+] ) in the cat. Twitch responses were measured bilaterally in the anterior tibialis muscles, using intact central innervation in one preparation. Plateaus of high and low [Ca2+] were created by infusions of calcium chloride and citrate, respectively. The interactions with changes in [Ca2+] were similar for atracurium and vecuronium, and were unaffected by central muscle innervation. The median increase in [Ca2+] from 1.21 to 1.59 mmol litre-1 shifted the dose-response curves of the drugs to the right, increasing ED50 by 7-13%, whereas the decrease to median 0.78 mmol litre-1 potentiated the drugs by a similar order. This indicates a lesser influence of [Ca2+] on the action of neuromuscular blockers than reported in a previous in vitro study. Even though the interactions were statistically significant, their moderate magnitude suggests minor clinical significance.