Effects of different patterns of long-term stimulation on blood flow, fuel uptake and enzyme activities in rabbit fast skeletal muscles.

Long-term electrical stimulation (14-28 days) of rabbit fast muscles (tibialis anterior, TA and extensor digitorum longus, EDL) using intermittent high frequency (3 trains per min of 5 s duration at 40 Hz, for 8 h per day) produced changes in enzyme activities similar to those found with continuous stimulation at a frequency occurring in nerves to slow muscles (10 Hz). The activity of citrate synthetase, 3-hydroxyacyl-CoA dehydrogenase and succinate dehydrogenase increased two to 3-fold within 28 days. There was a 4-fold increase in hexokinase whereas phosphofructokinase, pyruvate kinase, lactate dehydrogenase and fructose-1,6-diphosphatase decreased to about 60% of the activity levels in the contralateral unstimulated muscles. Blood flow and oxygen consumption at rest were not changed even after 28 days of stimulation, but were increased during contractions in muscles stimulated at either frequency, the level being twice as high as in control muscles. Glucose uptake was similar to that in control muscles both at rest and during contractions and the output of lactate was similar to that found in control muscles in muscles stimulated at 40 Hz. Muscles stimulated at 10 Hz had smaller lactate output. Thus intermittent stimulation at high frequency (40 Hz) and continuous low frequency (10 Hz) produced similar changes in aerobic metabolism and fuel uptake provided that the total number of stimuli was comparable and that the stimulation was carried out for sufficiently long period.