Acute in vitro neuromuscular effects of carbamazepine and carbamazepine-10,11-epoxide.

We examined the acute neuromuscular effects of the anticonvulsant, carbamazepine, and its major metabolite, carbamazepine-10,11-epoxide, using the in vitro rat phrenic nerve-hemidiaphragm muscle preparation. Carbamazepine produced 8.8% +/- 2.2% (n = 12) neuromuscular paralysis as its concentration was increased from 1 to 50 microg/mL (4.2-210 microM). In contrast, carbamazepine-10,11-epoxide produced maximum paralysis of 65% +/- 8% (n = 10) in the concentration range 1-100 microg/mL (4-400 microM) and the concentration required to produce half this paralysis was 36 +/- 7 microg/mL (144 +/- 28 microM). Carbamazepine 10 microg/mL (42 microM) shifted the response-concentration curve for both a depolarizing (succinylcholine) and a nondepolarizing (atracurium) neuromuscular blocker, reducing their concentrations required for 50% paralysis by approximately 30%. In contrast, the metabolite, which was a more potent neuromuscular blocker by itself, failed to alter either succinylcholine or atracurium effect. These results concur with previous clinical reports where anticonvulsants have acutely reduced neuromuscular blocker dose requirements.