Time-dependent increase of succinate dehydrogenase activity in low-frequency stimulated rabbit muscle: a comparison between microphotometric and biochemical methods.

We present an improved method for microphotometric measurement of enzyme activity in muscle fibres by determining maximum reaction rates using computer-assisted image analysis. The method was used to determine absolute and relative activities of succinate dehydrogenase (SDH) in 4801 whole-fibre cross-sections of rabbit tibialis anterior muscles stimulated at low frequency (10 Hz) for different time periods of up to 50 days. Measurements were performed on cross-sections of composite blocks from stimulated and contralateral control muscles. The validity of the method was checked by determining SDH activity in homogenates of the same muscles using a standard photometric assay. Both methods yielded similar results for the time-dependent increases of SDH activity in response to chronic low-frequency stimulation. Significant increases in catalytic activity were detected by the two methods only in muscles stimulated for longer than 8 days. According to homogenate measurements, overall SDH activity was 7.4-fold elevated in 50-day-stimulated total muscles. Depending on whether or not measurements were corrected for the so-called nothing-dehydrogenase activity, the average increase in microphotometrically determined SDH activity amounted to approximately 8-fold or 10-fold, respectively. Microphotometry revealed pronounced scattering of SDH activities within the fibre populations of both normal and stimulated muscles. The heterogeneity of the fibre population with regard to SDH activity increased in long-term stimulated muscles ranging between 5-fold and 15-fold elevations.