Effect of activity on performance and morphology in ischaemic rat slow muscles

Muscle performance and structure was studied in rat soleus muscle with limited blood supply in combination with chronic muscle stimulation. Blood supply to the lower leg was restricted by ligation of the common iliac artery, electrodes were implanted in the vicinity of the sciatic nerve and ankle flexors were denervated. Three days later, soleus and gastrocnemius muscles were stimulated at 4 Hz four times a day for a period of 20 min with 2 h intervals between stimulations; this procedure was continued for 4 days. Muscle performance, histochemistry and ultrastructure were studied on the eighth day after operation in these muscles and in ischaemic unstimulated muscles with denervated ankle flexors. Both were compared with control animals. Muscles with limited blood supply developed less isometric twitch tension than control muscles (peak twitch tension in ischaemic muscle was 60.3 +/- 4.8 g g-1 muscle, mean +/- S.E.M., compared to 79.7 +/- 6.9 g g-1 in control muscle; tensions after 5 min contraction were 54.5 +/- 5.5 g g-1 in ischaemic muscle compared to 70.6 +/- 6 g g-1 in controls). Stimulated muscles with limited blood supply had higher peak (85 +/- 16.6 g g-1) and final (87 +/- 12 g g-1) tensions, and also fatigued less than muscles with limited blood supply but no stimulation. Histochemical estimation of capillary density (by staining for alkaline phosphatase) and slow (SO) and fast (FOG) fibres (by myosin ATPase staining) revealed similar capillary to fibre ratios (2.5) and a similar proportion of FOG fibres (around 18%) in all muscles. The proportion of glycogen-depleted fibres (estimated from the periodic acid Schiff reaction, PAS) in muscles removed from animals 10 min after a 5 min period of isometric twitches was significantly lower in ischaemic muscles (45.1 +/- 1.9%) than in control (80.5 +/- 1.5%) or chronically stimulated ischaemic muscles (67.3 +/- 4.0%). Electron microscopy showed disorganised myofibrils with Z-line streaming in 7.48 +/- 3.04% of fibres in muscles with limited blood supply. Swollen and degenerated mitochondria, dilated sarcoplasmic reticulum and areas of disrupted sarcolemma were also observed. Stimulated ligated muscles showed a significantly lower proportion of fibres with disorganised filaments (0.65 +/- 0.32%) and other signs of damage were much less frequent. The reduced damage and improved performance of chronically stimulated slow muscle may be the result of improved microcirculation, preventing accumulation of lactate.