This paper explores the role of the calcium entry blocker nifedipine in the explanation of eccentric exercise-induced fibre damage by changes in skeletal muscle microcirculation. Eccentric exercise (EE) was induced by indirect stimulation of rat soleus muscle in its lengthening phase during cycling. Muscle damage was assessed by histology, electron microscopy and muscle tension 48 h later. Diameters of arterioles and venules, their response to dilator and constrictor stimuli and pattern of capillary flow were measured in epiiluminated muscles using intravital microscopy. Tetanic tension developed by EE muscles was lower (8.60 +/- 1.02, means +/- SEM, n = 8 N g(-1) wet weight compared to 12.25 +/- 0.56 in controls, P < 0.01). Electron microscopy showed changes similar to those in muscles exposed to EE by downhill running (Z line streaming, disruption of sarcolemma, swollen tubules). A total of 16% of muscle fibres were damaged, and fibre areas and interstitial space were enlarged. Capillary red blood cell flow showed tendency to a greater intermittency. Large venules were narrower, but arterioles and smaller venules had diameters similar to control muscles. Vessel dilatation to topically applied 10(-4) M adenosine was attenuated. Daily administration of calcium entry blocker nifedipine by gavage (2 mg/kg/day in two equal doses) removed the narrowing of venules, restored the dilator response of all vessels to adenosine and increased capillary:fibre ratio. The percentage of damaged fibres decreased to 4.7 and the size of the interstitial space and fibre areas was normalized. Thus muscle damage caused by eccentric exercise was attenuated by nifedipine due to its beneficial effect on muscle microcirculation, which was impaired by eccentric exercise.

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