Electrical potential, resistance, and fluid secretion across isolated ciliary body

Rabbit ciliary epithelium was mounted with the sclera and lens still attached. The potential measured in physiological bathing medium was 1.29 +/- 0.18 mV initially, but dropped to 0.66 +/- 0.34 mV, then became stable for 2 hr or more. The resistance of the epithelium was measured by comparing total trans-tissue resistance before and after treatment with Triton X-100, ethanol, or distilled water. The calculated resistance with correction for epithelial surface area was found to be 1504 +/- 452 omega, in six experiments. Fluid secretion was measureable against a hydrostatic gradient when identical solutions bathed both sides of the preparation. Ouabain caused opposite responses when applied to either side of the preparation, indicating that it does not readily cross the epithelial barrier. The evidence presented suggests that the ciliary epithelium is a 'tight' rather than a 'leaky' epithelium, and that fluid secretion in the normal ciliary epithelium occurs by active transport rather than by ultrafiltration.