Permeability of Ropivacaine to the Maxilla in the Rat

Methods: Subjects were Wister strain rats, 7-9 weeks old and weighing 220-350g. The rats were anesthetized with 50-60mg/kg of pentobarbital (NembutalⓇ) intraperitoneally. Spontaneous respiration was maintained. Twenty ìl of 0.5% 3H-labeled ropivacaine was injected into the palatal gum proximal to the right premolar. The rats were put to sleep at 0.5, 2, 5, 10, 20, 30 and 60 minutes after the injection. The maxilla (divided into 6 parts), maxillary nerve, brain and liver were taken from the deceased rats. Each sample (100-300mg wet weight) was transferred to a scintillation vial, with 1ml of solvent (SolvableⓇ, Perkin Elmer) added, incubated for 3 hours at 60 degrees Celsius, and neutralized with 50ìl of acetic acid. The samples were mixed with 10ml of a scintillation cocktail (AQUASOL-2Ⓡ, Perkin Elmer), and left in a dark place for 24 hours at room temperature. The radioactivity in the samples was measured by liquid scintillation counting. The pattern of changes of radioactivity in ropivacaine was compared with that in lidocaine.

Results: The radioactivity remained low in the maxilla, maxillary nerve, brain, and liver throughout the time course of measurements, and high in palatal mucosa till 10 minutes after the injection. The radioactivity in the right maxilla lower part including the injection site was lower than in the ipsilateral palatal mucosa at 30 minutes. The radioactivity in the serum showed little changes until 60 minutes after the injection.

Conclusions: The results of this study indicate that ropivacaine localizes in palatal mucosa and hardly permeates into the maxilla, and its absorption into the circulation is very slow.