Relative importance of the enzymic hydrolysis of sexamethonium in plasma and tissues: studies in Rhesus monkeys

1. Suxamethonium was given by intravenous infusion to anaesthetized Rhesus monkeys and the infusion rate required to produce after 10 min a 50% reduction in twitch tension of the indirectly stimulated gastrocnemius or tibialis anterior muscle (IR50) was calculated from results obtained with paired infusions of two concentrations, repeated at intervals. The IR50 values in three monkeys were (388, 958 and 1,010 nmol/kg)/minute.2. In one experiment the cholinesterase activity in plasma was increased to 125 and then to 182% of the initial value by infusions of purified human cholinesterase (usual enzyme). This increased the IR50 by 34 and then by 60%, respectively.3. Inhibition by iso-OMPA of cholinesterase activity in plasma and tissues (four monkeys) by more than 90% lowered the mean IR50 value to (69 +/- 11 (S.E.M.) nmol/kg)/minute. Restoration of cholinesterase activity in the plasma of these animals by infusion of purified human cholinesterase raised the IR50's to values within the normal range.4. It is concluded that in the Rhesus monkey most of the infused suxamethonium is hydrolyzed by the cholinesterase in plasma before reaching the motor endplates. This agrees with observations in man with usual cholinesterase and contrasts with experiments in the cat where the hydrolysis of suxamethonium by cholinesterase in tissues was found to be more important than the hydrolysis by cholinesterase in plasma.

An original approach to the diagnosis of scoline-induced apnoea

A new technique is described for measuring the rate of scoline hydrolysis by plasma cholinesterase, which may prove useful in the diagnosis of scoline sensitivity. The importance of extensive family screening is emphasized and three cases possessing a silent gene are described.

Relative importance of the enzymic hydrolysis of suxamethonium in plasma and tissues: studies in cats

1. The effects of intravenous infusions of suxamethonium on the twitch tension of the indirectly stimulated tibialis and gastrocnemius muscles of anaesthetized cats were recorded. From these data the infusion rate giving a 50% reduction in twitch tension after 15 min (IR50), was calculated.2. Marked inhibition of cholinesterase activity in plasma with less inhibition of cholinesterase activity in tissues, obtained by repeatedly withdrawing blood samples, incubating them with DFP and reinjecting them, had only a small effect on the IR50 of suxamethonium.3. Injection of iso-OMPA produced marked inhibition of cholinesterase activity in plasma and tissues, and lowered the IR50 to 10% of that in controls. The IR50 in cats treated with iso-OMPA could be restored to normal only by raising the suxamethonium hydrolysing capacity of plasma 10-50 times above normal by the intravenous injection of purified cholinesterase of human plasma.4. Exchange blood transfusion between normal cats and cats treated with iso-OMPA failed to affect the IR50 of suxamethonium in either of the two.5. It is concluded that in cats, unlike in man, the effectiveness of suxamethonium is not determined by the cholinesterase activity in plasma but by the cholinesterase activity in tissue. The role in this of cholinesterase at the neuromuscular junction and other sites is discussed.

Hydrolysis of suxamethonium by different types of plasma

1. A method, based on the use of the isolated frog rectus abdominis preparation, is described for studying quantitatively the hydrolysis of suxamethonium at low concentrations.2. Rates of hydrolysis of butyrylcholine, 10 mM, benzoylcholine, 0.05 mM, and suxamethonium, 0.025 mM, by plasma from different species and human plasma with genetic variants of cholinesterase were measured.3. The rates of hydrolysis of suxamethonium by different types of plasma vary widely. Human plasma with usual cholinesterase and monkey plasma hydrolyse suxamethonium more speedily than do the plasma of cats, dogs and rats, and human plasma with either atypical or fluoride resistant cholinesterase. This is only to a small extent attributable to differences in enzyme concentration and not explained by the presence of an inhibitor.4. The K(m) values for butyrylcholine are very similar for different types of plasma but the K(I) values for suxamethonium in the system plasma-butyrylcholine-suxamethonium vary greatly.5. These results and observations on the inhibition by decamethonium of the hydrolysis of butyrylcholine are consistent with the interpretation that the rates of hydrolysis of suxamethonium, 0.025 mM, obtained with different types of plasma vary because they are a function of two variables, the affinity of the ester for cholinesterase and the stability of the monosuccinyl derivative of the cholinesterase. It seems that human plasma with atypical cholinesterase hydrolyses suxamethonium much slower than does human plasma with usual cholinesterase mainly or solely because of differences in affinity of the ester for the two enzymes. On the other hand, cat plasma appears to hydrolyse suxamethonium much slower than does human plasma with usual cholinesterase mainly because the monosuccinyl derivative of cholinesterase in cat plasma is much more stable than that in human plasma. The reverse might apply for monkey plasma.6. Inhibition by dibucaine or sodium fluoride of the hydrolysis of benzoylcholine is not a general guide to rates of hydrolysis of suxamethonium by different types of plasma.