Localisation and quantification of dehydrogenase activities in single muscle fibres of mdx gastrocnemius

The kinetics of succinate (SDH) and lactate (LDH) dehydrogenases were deter mined in single muscle fibres in unfixed sections of the gastrocnemius of d ystrophic mdx mice (with an X-linked genetic disorder lacking a cytoskeleta l protein, dystrophin) and age-matched C57BL/10 control mice. Quantitative gel substrate-film techniques and a real-time image analysis system were us ed. Three main fibre types were observed in regenerated mdx gastrocnemius a nd in corresponding controls: small fibres (S) with high SDH and LDH initia l reaction velocities and activities, large fibres (L) with low activities of these dehydrogenases and intermediate-sized fibres (I) with intermediate enzyme activities. The small and intermediate fibres in both mdx and contr ol muscles exhibited respectively high and moderate subsarcolemmal SDH and LDH activities attributable to accumulated mitochondria. The ratios of the initial velocities of the intrinsic enzyme reactions in the sarcoplasm, exc luding the subsarcolemmal regions, of mdx muscle fibres compared to those i n control fibres were 0.958 (S), 1.09 (I) and 0.959 (L) for SDH, and 1.03 ( S), 1.06 (I) and 1.07 (L) for LDH. A parameter a, a measure of the diffusio n of LDH out of muscle sections during incubation on gel substrate films, w as found to be 0.981 and 1.00 in mdx and control muscles, respectively. Thu s there are no significant differences in the activities and microenvironme nts of the enzymes between regenerated mdx muscle fibres and normal control muscle fibres. These data suggest that dystrophin deficiency in mdx muscle s has no effects on the interactions of LDH with cytoskeletal proteins or o n SDH activities in mitochondria whose number and morphology differ in mdx muscle fibres compared to those in normal controls. SDH and LDH activities were also found in the mitochondria clustered on two longitudinally directe d poles of each central nucleus in regenerated mdx muscle fibres, They were proportional to the activities in the sarcoplasm excluding the subsarcolem mal regions.