1. The aim of this study was to examine the effect of chronic alcohol ingestion on working capacity and on the expression of myosin heavy chain isoforms in fibre types of human skeletal muscle.

2. Six alcoholic men having drunk more than 240 g of alcohol/day for more than 10 years underwent a test for working capacity and a muscle biopsy on the first day of alcohol abstinence (test 1) and again after 4 weeks of abstinence (test 2). The biopsies were analysed using histochemical, immunochemical and gel-electrophoretic techniques, and the results were compared with those from eight age-matched nonalcoholic control subjects.

3. The area of type IIB muscle fibres was decreased by 33% in the alcoholic patients compared with normal control subjects at both test 1 and test 2. The area of type IIA fibres was lower (13%) in alcoholic patients at test 1 than in the control group, and increased to the normal level at test 2.

4. The relative proportion of fibres expressing only myosin heavy chain type IIB isoforms was one-third of normal in the alcoholic patients at both tests 1 and 2. The relative proportion of fibres expressing only myosin heavy chain type IIA isoforms was the same in alcoholic patients at test 1 and in normal control subjects, but increased by 25% between test 1 and 2 in the alcoholic group.

5. The relative proportion of fibres showing co-expression of myosin heavy chain type IIA/IIB isoforms was about two-thirds of normal in alcoholic patients at both test 1 and test 2, whereas fibres with co-expression of myosin heavy chain type I/IIA were not seen in the control group, but were found in the alcoholic group, where they doubled from test 1 to test 2.

6. Thus, chronic heavy alcohol consumption modulates the expression of myosin heavy chain isoforms-in human skeletal muscle by decreasing the expression of myosin heavy chain type IIB and increasing the expression of myosin heavy chain type I. After 4 weeks of alcohol abstinence a shift in the expression of myosin heavy chain type I towards myosin heavy chain type IIA is evident, indicating that the effect of alcohol on myosin heavy chain expression is a reversible process. However, to obtain complete recovery with a normal level of myosin heavy chain type IIB expression, more than 4 weeks of abstinence is necessary.

7. Working capacity was low and unchanged by 4 weeks of alcohol abstinence. The low proportion of type IIB (fast twitch glycolytic) fibres at both test 1 and 2 coincided with a low lactate production during maximal exercise. The increased proportion of type I (slow twitch oxidative) fibres at test 1 coincided with an preferential lipid oxidation during the working period.

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