Ultrastructural changes accompanying development of fatigue in frog twitch skeletal muscle fibres

The aim of the present study was to characterise and compare alterations in the ultrastructure of the functionally identified isolated twitch skeletal muscle fibres of the frog after repeated tetanic stimulation and under experimental conditions which modified their fatigability.

Single isolated twitch muscle fibres of m. iliofibularis of adult frogs Rana temporaria were subjected to intermittent tetanic stimulation. Fibres at specified degree of fatigue were processed for electron microscopic observation and ultrastructural examination.

The fatigue-resistant (FR) fibres that developed 90% of the control tetanic tension after 10 min stimulation in ordinary Ringer's solution showed regions with dilated intermyofibrillar spaces containing small vesicles and swollen mitochondria. In addition to the changes observed in FR fibres, the easily fatigued (EF) fibres that produced 50% of the original tension after 3 min stimulation showed small vacuoles in the sarcoplasm. In EF fibres that preserved 10% of the control tension after 10 min stimulation and showed swelling of the longitudinal sarcoplasmic reticulum, the central element of triads and mitochondria, large vacuoles were present. FR fibres exposed to low Ca2+ medium containing 0.02 mmol/l verapamil, lost their resistance to fatigue. Their contractile responses fell down to 20 % within 0.5 min of stimulation. Those fibres displayed large vacuoles and changes in mitochondria as observed in EF fibres after 10 min stimulation.

These results suggest that morphological changes accompanying reduction of the contractile force (i) appear earlier than the reduction of the contractile ability, (ii) correlate with the degree of reduction of the contractile capacity but not with the duration of contractile activity, (iii) are not specific for the fatigue fibre type.