Changes in isometric force of mouse soleus muscle during the oestrous cycle.

Muscles excised from young female mice at known phases of the oestrous cycle were studied in vitro to determine if there are variations in force analogous to those that occur in vivo during the menstrual cycle in women. Oestrous phase was determined from vaginal smears. The maximum isometric and eccentric forces of pairs of isolated soleus muscles were measured. The first muscle was studied immediately after dissection, the second after incubation in Ringer solution for up to 2 h. Normalised isometric muscle force in the first muscle of each pair depended on the oestrous phase, the force being greatest during dioestrus. There was a negative correlation between normalised force and the eccentric/isometric force ratio. Neither of these phenomena was found with the second muscle of each pair. These results show that in mouse soleus muscle cross-bridge function does vary according to the phase of the oestrous cycle. However, the rise in force does not follow the pattern of the rise in blood oestrogen levels as it does in humans, and in the mouse the effect on cross-bridge function washes out after a few hours in vitro.