Vaginal permeability and enzymatic activity studies in normal and ovariectomized rabbits

Purpose. This study was initiated to develop an animal model, using ovariectomized rabbits, for the post-menopausal human, based on in vitro vaginal tissue permeability and aminopeptidase activity. Methods. An enkephalin derivative [D-ala(2),N-methyl-phe(4)-glycol(5)] [tyrosyl-3,5-H-3] enkephalin ([H-3] RX 783006), which has relative enzymatic stability to aminopeptidases and dipeptidyl peptidase, was used as a model peptide drug for permeability experiments. Aminopeptidase activity in vaginal homogenates, as well as in tissue pieces, was determined using 4-methoxy-2-naphthylamides of leucine, alanine, arginine, and glutamic acid as specific substrates. In addition, histological examination of normal and ovariectomized vaginal tissues was performed. Results. Vaginal permeability of the drug was significantly increased in the ovariectomized compared to the intact animal. The full vaginal tissue became thinner and mucosal epithelial thickness was reduced about two-fold after ovariectomization and vaginal cells from the castrated rabbit were typically immature. Aminopeptidase activity, leucine aminopeptidase, aminopeptidase B and A, was the same in vaginal tissue homogenates and whole-tissue specimens in both normal and ovariectomized rabbits whereas the activity of aminopeptidase N was significantly decreased in ovariectomized as compared to normal rabbits. Conclusions. Based on the present data, the ovariectomized rabbit may be useful as an animal model for postmenopausal vaginal studies.