Meeting Abstract
Live bearing is a defining feature of all eutherian mammals. In order to accommodate the fetus, the female body undergoes radical hormonal, morphological and mechanical changes. Yet, we know very little about the effects of pregnancy on skeletal muscle-tendon units and the whole organism. We used the gastrocnemius muscle of rats as a model system to examine the organ and whole animal level effects of pregnancy, by comparing animals that had never been pregnant, primiparous animals near the end of pregnancy, and postpartum animals. We found that muscle mass did not change significantly with pregnancy, even though pregnant animals were approximately 30% heavier than non-pregnant ones. However, in postpartum animals muscle mass was reduced by 22%. Muscle fiber size did not vary among conditions but postpartum animals were the only ones with muscle fibers larger than 0.20 mm². We predicted that muscle vascularization would increase with pregnancy due to the action of the hormone relaxin. Preliminary histological data support our prediction. Since relaxin is also an antifibrotic agent, we predicted that it would affect the mechanical properties of collagenous tissues associated with muscle. The stiffness of the proximal aponeurosis of the lateral gastrocnemius, as measured by the tangent slope at strain=0.13, did not vary by condition. Interestingly, there was high variation in all conditions. All females tended to have lower Achilles tendon stiffness than males, although this was not statistically significant. However, tendon stiffness had surprisingly low variability in the postpartum animals that was highly significant (P< 0.001). Ongoing videographic analysis in our laboratory is characterizing the gait of animals as they perform a demanding task (20% incline) to examine the effects of pregnancy on the whole body.
