Changes provoked by multiparity on the fiber type composition of pelvic and perineal striated muscles in the female rabbit

We have previously reported that pelvic and perineal striated muscles have considerable effects on the contractile alterations due to multiparity (Fajardo et al,2008). Such changes are associated to the increase of muscle damage markers as ?-glucuronidase and satellite cells provoked by several pregnancies and deliveriesin the rabbit doe (López-García et al, 2014). Now, we analyze the effect of multiparity (4 consecutive deliveries) on the fiber type composition of the pubococcygeus (Pc) and bulbospongiosus (Bs) muscles of female rabbits. We use five young female rabbits (Oryctolagus cuniculus) induced to copulation at the age of 6 months and allowed to have four consecutive pregnancies and deliveries. Pcand Bs muscles were dissected 20 days after the last delivery and histochemicallystained with the ATPase (9.4 pH) technique. The identification of fibers in musclesection was based on the following criteria: transparent or light fibers were classified as slow (S); gray as fast (F) and black as intermediate (I). The totalnumber and the cross sectional area of fiber types were measured by using theImageJ software and the percentage of each fiber type was determined. Findingswere compared with those obtained from nulliparous rabbits of the same agepreviously reported (López-García et al, 2013). The results showed that Pcmuscles from multiparous rabbits have a higher proportion of slow andintermediate but lower proportion of fast fibers (S = 41.4 3 %, I = 40.7 8 %and F = 17.1 4 %, respectively; P > 0.05) while Pc muscles from nulliparousrabbits showed a similar proportion of slow, intermediate and fast fibers (S = 38.4 1.2 %, I = 31.3 3.3 % and F = 30.2 2.4 %; P > 0.05). Meanwhile multiparousBs muscles had a higher proportion of intermediate fibers than slow and fast fibers(I = 73.6 9%, S = 17.4 5 %, and F = 9 3%; P<0.05) compared withnulliparous Bs muscles that had less proportion of slow and intermediate fibersthan that of fast fibers (S = 16.4 1.9 %, I = 34.0 1.5 % and F = 50.4 1.2 %;P>0.05). In both muscles, Bs and Pc, slow fibers showed larger cross sectional areathan intermediate and fast fibers, confirming the higher susceptibility of such fibersto damage. Our results suggest that multiparity affects the fiber composition of PCand Bs muscles by inducing an increment of intermediate fibers with aconcomitant decrement of fast fibers.