Effects of exposure to cigarette smoke at different dose levels on extensor digitorum longus muscle fibres in Wistar-Kyoto and spontaneously hypertensive rats.

1. The fibre type distributions, cross-sectional areas (CSA) and succinate dehydrogenase (SDH) activities in the deep (EDLd) and superficial (EDLs) regions of the extensor digitorum longus (EDL) muscle in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) were determined after exposure to cigarette smoke at three different dose levels using a smoking machine. 2. Rats were exposed to cigarette smoke at a rate of 15 puffs/min for 20 min/day with 23 cigarettes (low-dose level), 26 cigarettes (medium-dose level) or 30 cigarettes (high-dose level) for 8 weeks. 3. No changes in fibre type distribution, CSA or SDH activity were observed after exposure to cigarette smoke at the low- and medium-dose levels, irrespective of the muscle region or strain. 4. No change in fibre type distribution was observed after exposure to cigarette smoke at the high-dose level, irrespective of the muscle region or strain. A decreased CSA of type IIA and type IIB fibres in EDLd and increased SDH activity of all types of fibres in EDLd and EDLs were observed in WKY rats after exposure to cigarette smoke at the high-dose level. In addition, a decreased CSA of type IIB fibres in EDLd and type IIA and type IIB fibres in EDLs and increased SDH activity of type IIB fibres in EDLd were observed in SHR after exposure to cigarette smoke at the high-dose level. 5. The smaller CSA and higher SDH activity of fibres in EDL observed in WKY rats and SHR after exposure to cigarette smoke at the high-dose level, but not at the low- and medium-dose levels, are adaptive responses, indicating that heavy cigarette smoke affects the morphological and metabolic properties of fibres in skeletal muscle.