Contraction and cation contents of skeletal soleus and EDL muscles in age-matched control and diabetic rats

Abstract

Skeletal muscle atrophy and neuropathy are two major long term complications of diabetes mellitus (DM). This study investigated the effect of streptozotocin (STZ) - induced type I DM on the characteristics of the animals and on contraction, cation contents, morphology and muscle proteins of Extensor Digitorum Longus (EDL) and soleus skeletal muscles compared to age-matched control rats.
Adult Wistar rats were humanely killed and EDL and soleus muscles were located and excised rapidly and placed in organ baths. The muscles were electrically stimulated (EFS, 50 V, I msec, 1-100 Hz) and isometric contraction monitored using
a chart recorder. At the end of the experiments the muscles were blotted, weighed and dissolved in concentrated nitric acid for the measurements of cation contents. In another set of experiments, the cation contents were measured using unstimulated muscles. In addition, EDL and soleus isolated muscles were fixed, stained and examined for any morphological changes and levels of actin distribution of muscle fibres in age-matched control and diabetic rats. In the next series of experiments, gel electrophoresis was used to separate actin and myosin.
The results showed that STZ-induced diabetic rats have significantly (*PC0.05) reduced body weights, significantly (*P<0.05) elevated blood glucose and significantly (*Fc0.05) reduced blood insulin as well as reduced protein contents compared to healthy age-matched control. EFS evoked frequency dependent contraction in both soleus and EDL muscles of healthy control rats. The response was significantly (*P< 0.05, n= 8) increased in EDL compared to soleus.
In diabetic soleus and EDL muscles the force of contraction was significantly (*pc0.05) reduced in both muscle types compared to healthy age-matched controls. Perturbation of extracellular Ca 2+ [Ca2 ]o resulted in a Ca 2 dependent increase in contraction with maximum effect with 5 mM Ca 2 and minimal effect with 0 mM Ca2 . The responses were much large in age-matched control muscle compared to diabetic muscle. Perturbation of extracellular Mg 2 [Mg2 ]0 on EFS-evoked
contraction had no significant effect in either control or diabetic muscles at all frequencies tested compared to the responses obtained in normal (1.2 mM) (Mg 24)o.
The levels of total sodium (Ni'), potassium (IC), calcium (Ca 2 '), magnesium (Mg2 '), zinc (Zn2 '), copper (Cu 2 ') and iron (Fe2 ') were significantly (*PC0.05) reduced in diabetic soleus and EDL muscles compared to controls. The LDH assay showed an increase in the % lysis of STZ-induced diabetic muscles compared to the age-matched controls. Morphological studies have also demonstrated a reduction in the number and distribution of muscle fibres in both EDL and soleus skeletal muscles of STZ-induced diabetic rats compared to control. Similarly, diabetic EDL and soleus skeletal muscles contain less actin than muscles from healthy age-matched control rats. The results for gel electrophoresis did not show any major significant changes.
The results indicate that STZ - induced skeletal muscle atrophy is associated with marked reductions in the force of contraction cation contents, the number of muscle fibres and their distribution and levels of actin compared to the healthy agematched controls in EDL and soleus muscles of the rats.
It is postulated that these changes in the muscle properties may be responsible for the diabetes-induced muscle atrophy.