Rapid integrity assessment of rat and human epidermal membranes for in vitro dermal regulatory testing: correlation of electrical resistance with tritiated water permeability.

An approach is presented that allows for rapid selection of robust rat and human epidermal membranes for use on in vitro dermal regulatory studies. Tritiated water (THO) permeability was correlated with electrical resistance (ER) and the results used to propose ER values to judge membrane integrity. Rat and human epidermal membranes were prepared and mounted onto in vitro glass static diffusion cells (0.64 cm(2)) maintained at 32 degrees C. THO permeability coefficients (Kp) were determined and compared with ER measurements. Electrical resistance was also determined for various in vitro cell exposure areas from 0.64 cm(2) to 2.54 cm(2). Our results show that rat epidermal membrane THO Kp values exhibited a lognormal distribution with a median value of 2.76 x 10(-3) cm/h. Human epidermal membrane THO Kp values were best described by a Weibull distribution with a median value of 1.13 x 10(-3) cm/h. The corresponding median electrical resistance measurements were 5.59 kOmega for rat and 23 kOmega for human epidermal membranes. Based on the widely used and accepted single point THO Kp thresholds of </=2.5 x 10(-3) cm/h (rat) and </=1.5 x 10(-3) cm/h (human), the corresponding ER values of >/=5.87 kOmega and >/=17.1 kOmega were calculated and proposed as acceptable benchmarks for pre-qualifying membranes. In our research exploring the relationship between ER and exposure area we report that an inverse relationship exists between ER and in vitro cell exposure area; as cell area increased, ER decreased. The use of electrical resistance provides a rapid and reliable method for evaluating the integrity of rat and human epidermal membranes for in vitro dermal kinetic testing.