An inexpensive meter to measure differences in electrical resistance in the rat vagina during the ovarian cycle

The inherent electrical resistance of the rat vaginal wall rises markedly near the beginning of estrus and then falls again to low levels for the remainder of the ovarian cycle. Accordingly, special instruments have been developed to measure such resistances (within seconds) on simply inserting a small probe fitted with a pair of recording electrodes into the vagina (i.e., the MK-10A impedance checker and the EC40 estrus cycle monitor). As described herein, these two instruments are far more convenient for monitoring individual cycles than more laborious methods in which vaginal smears are inspected for changes in numbers of cornified (C), nucleated (N), and leukocytic (L) cells. However, they are also expensive and their use has essentially remained uncited in the literature. Thus we sought to determine whether a simple, inexpensive electrical meter (with resistance-measuring capacity), as commonly used by professional electricians, would serve the same purpose. We chose a standard multifunctional meter (model 22-178, RadioShack) and attached leads to it fabricated from the internal wiring of a shielded audio cable (model 42-2387A, RadioShack), one male terminal of which was used as a vaginal probe. In rats from which vagina smears revealed cell numbers in the order of C > N > L (typical of early estrus) electrical resistances were high, 488 +/- 130 k Omega (18 rats). In rats from which vagina smears revealed all other possible cell distributions, electrical resistances (combined) were much lower (P < 0.05), 124 +/- 23 k Omega (32 rats). Thus readily accessible, inexpensive electrical meters may be useful in assessing the status of estrus in female rats, either to improve reproductive efficiencies and/or for other purposes involving experiments in which such information is desirable.