Effect of streptozotocin-induced diabetes on growth and proteolytic activity of different muscles in rats

Fibre and non-fibre components and proteolytic activity of fore- and hindlimb muscles with different fibre composition and functions were examined in streptozotocin-induced diabetic rats. From hindlimb, extensor digitorum longus (EDL), medial gastrocnemius (MG) and soleus (SOL) were selected to represent muscles rich in fast oxidative-glycolytic, fast glycolytic and slow oxidative fibres, respectively. The biceps brachii (BB) and extensor carpi radialis longus (ECRL), represented forelimb muscles having similar fibre composition but performing flexor and extensor functions respectively. Among the five muscles studied, all but SOL showed decrease in muscle weight and protein in diabetic rats indicating the atrophy of these muscles. Atrophy was greater in EDL and MG as compared to ECRL and BB. This differential atrophy could be negatively correlated to the percentage of slow oxidative fibres. Despite the loss of fibre mass in diabetic group, elevated fibre protein concentration was observed in BB (flexor) but not in ECRL (extensor). There was a significant reduction in non-fibre protein concentration with markedly enhanced alkaline proteolytic activity in EDL of diabetic rats. Non-protein solid concentration was also increased in EDL and MG. These observations showed that fast muscles are more susceptible to diabetes induced atrophic changes and diabetes affected various muscle components differently in muscles with different fibre composition and functions. Therefore, fibre composition and function of a muscle may be the factors in determining muscle growth in diabetes.