Bioelectric Activity as a Quantifiable Index of Acute Spermicide (Nonoxynol‐9) Actions on Rat Vaginal Epithelial Function during the Oestrous Cycle

The acute effect of instillation of the spermicide 2% nonylphenoxypolyethoxyethanol (nonoxynol-9) on the bioelectric activity of the vaginal epithelium was assessed using as an index the time taken in seconds to reduce the transvaginal potential difference (p.d.), measured in anaesthetised rats, to half its initial value (t1/2). The surfactant caused rapid falls in the p.d. at dioestrus (t1/2 = 35 +/- 25 sec., n = 16, mean +/- S.D.) and metoestrus (t1/2 = 32 +/- 27 sec., n = 11) with a slower fall at oestrus (t1/2 = 130 +/- 104 sec., n = 14). At pro-oestrus however, nonoxynol-9 had no effect (n = 18). The dye Nigrosin (0.5%), when instilled alone, did not stain untreated vaginas of anaesthetized rats at any stage of the oestrous cycle, but in the presence of 2% nonoxynol-9 heavily stained those in metoestrus and dioestrus. Vaginas in pro-oestrus were unstained while those in oestrus were stained lightly. While nonoxynol-9 has a dramatic effect on the bioelectric activity of the vaginal epithelium, both the electrical and dye data show that the pro-oestrus vaginal epithelium presents a significant diffusion barrier to the passage of nonoxynol-9 and nigrosin. Thus, in assessing the actions of vaginally instilled nonoxynol-9 in rats the variation in the permeability of the vaginal epithelium during the oestrous cycle must be taken into account.