A time course study of the isometric contractile properties of rat extensor digitorum longus muscle injected with bupivacaine.

1. The effect of intramuscular injections of the myotoxin bupivacaine on the isometric contractile properties of rat extensor digitorum longus (EDL) muscle was studied. 2. Immediately after injection the EDL muscle did not contract when stimulated. The absolute (mN) and normalized (N/g) peak twitch (Pt) and tetanic (PO) tensions gradually returned to normal by 21 days after injection. 3. Thereafter, the injected muscles grew at an accelerated rate and by 80 days were 70% heavier than controls. 4. The hypertrophy was not paralleled by a proportional increase in absolute Pt and Po and consequently the normalized Pt and Po were 30% less than controls.