The influence of HCO(3) -induced and CO(2) -induced pH changes on the action of non-depolarizing muscle relaxants in rats

The influences of HCO(3)-induced and CO(2)-induced pH changes on the action of nondepolarizing muscle relaxants (MRs) were investigated using a rat phrenic nervehemidiaphragm preparation. Changes in pH were induced by varying HCO(3) concentration of the modified Krebs' solution (mK') or by varying the CO(2) concentration in the gas aerating the mK'. Changes in [Ca(++)] and [Mg(++)], associated with changes in HCO(3) concentration, were measured and corrected. A change in HCO(3) from 50mM to 13.5mM or a change in CO(2) from 2.5% to 9%, augmented the partial neromuscular blockade produced by d-Tc or vecuronium (p<0.01), while inhibiting that produced by metocurine or pancuronium (p<0.01). A change in HCO(3) from 13.5mM to 50mM or a change in CO(2) from 9% to 2.5%, inhibited the partial neuromuscular blockade produced by d-Tc or vecuronium (p<0.01), while augmenting that produced by metocurine or pancuronium (p<0.01). These findings suggest that the HCO(3)-induced and CO(2)-induced pH changes have a similar influence on the action of MRs. A difference exists between mono- and bisquaternary MRs with regard to their response to pH changes.