Ussing chamber apparatus to measure barrier ion flux/permeability across multiple tissues in vitro. This includes 4xUSS5SD and USS4SD USSING SYSTEM W/DRAIN, with the EVC4000-4 PRECISION V/I CLAMP 4 CHANNEL voltage/current clamp, 505063 Circulating Water Bath 13L 230V 50HZ, and LAB-TRAX-4 DATA ACQUISITION SYS 4 CHANNEL.