Poststerone increases muscle fibre size partly similar to its metabolically parent compound, 20-hydroxyecdysone

In mice, poststerone is a major in vivo metabolite of the worldwide popular anabolic food supplement 20-hydroxyecdysone (20E). Here we present the first study on this ecdysteroid in view of the in vivo anabolic effect of its parent compound, 20E in mammals. We have monitored muscle fibre type cross sectional areas (CSA) of developing rats after treatment with poststerone as we did in a previous study with 20E. The muscle mass and fibre CSAs of soleus and EDL were increased by poststerone in a muscle specific manner as by 20E but there were some differences. Notably, the CSAs of type I and type IIa fibres in the soleus were less elevated by poststerone than by 20E. However poststerone increased the CSA of each four fibre types (I, IIa, IIx, IIb) in the EDL more effectively than 20E did. Poststerone, like 20E, also increased the number of myonuclei in the EDL of both hind limbs. Overall, this shows for the first time that poststerone having steroid nucleus and no side chain of 20E has a partly overlapping effect with that of 20E.