Differential effects of pregnancy on contractile behavior of rat fast and slow skeletal muscles

The effect of pregnancy on skeletal muscle still has not been clearly established. The aim of the present study was to investigate the effect of pregnancy on muscle weight and contractile properties of soleus and plantaris muscles.

The female Sprague-Dawley rats were divided into two groups: nonpregnant (NP, 250 ± 4 g, n = 8) and late-pregnant (LP, 305 ± 13 g, n = 8). The right plantaris and soleus muscles were liberated from the surrounding tissues. Each muscle was placed on setup to mechanical recording with electric stimulation. In the optimum length of each muscle were recorded single twitches, tetani and fatigue.

The weight and cross-sectional area of soleus and plantaris muscles from pregnant rats was increased respect to nonpregnant rats. The maximal twitch tension decreased in both muscles during pregnancy respect to nonpregnant group. The soleus muscle of LP group developed lesser tetanic tension than NP group. However, the plantaris muscle showed a different behavior: to lower frequencies (5-30 Hz) the NP group developed greater tetanic tension than LP group, and for higher frequencies (40-100 Hz), the LP group developed greater tetanic tension than NP group. Finally, the soleus muscle was more resistance to fatigue than plantaris muscle in pregnant rats.

The fast and slow skeletal muscles show a differential contractile response during pregnancy, in tetanic tension and fatigue (Fig. 4, Ref. 48).