Hypertrophy of rat extensor digitorum longus muscle injected with bupivacaine. A sequential histochemical, immunohistochemical, histological and morphometric study.

Histochemical, immunohistochemical, histological and morphometric properties of bupivacaine-injected rat skeletal muscle were studied at times spanning the complete course of degeneration and regeneration to establish when, if ever, 'normality' is reached. This was achieved in a sequence of measurements made on the same series of rat fast-twitch extensor digitorum longus muscle (EDL), of the fibre type composition, myosin heavy chain content, fibre size, connective tissue content and myofibril size at 1-2 h and 2, 4, 8, 11, 21, 40, 60, 80 and 180 d after treatment. By 2 d after injection 86% of the fibres had undergone necrosis. A rapid restoration of histochemical, immunohistochemical and morphometric properties then occurred, being apparently complete by 21 d after injection. A pattern of ongoing changes recognised when regeneration was essentially 'complete' are reminiscent of changes that occur in muscles following compensatory hypertrophy produced by synergist ablation. These changes included an increase in muscle weight, a decline in normalised peak twitch and tetanic tensions, and normalised force in response to different stimulation frequencies (Rosenblatt, 1992), an increase in the relative number of type I fibres and of fibres reacting with the slow myosin heavy chain antibody, an increase in whole muscle cross-sectional area, an increase in type I and type II fibre cross-sectional area and diameter, an increase in myofibril cross-sectional area, density, number, and area fraction, and an increase in the relative proportion of intramuscular connective tissue collagen. This suggests that the EDL muscle is being made to do more active work and is being influenced by passive forces (stretch) imposed on it. These changes appeared permanent: they stabilised at about 60 d after injection and were maintained for at least the next 120 d.