The peritoneal mesothelium is a barrier to ion transport in peritoneal dialysis. In this study, we used Ussing-chamber experiments to investigate the effect of HgCl2, an aquaporin-1 inhibitor, on the transmesothelial electrical resistance (RTM) of isolated sheep parietal peritoneum. Peritoneal samples from the diaphragm of adult sheep were isolated immediately after the death of the animal and were transferred within 30 minutes to the laboratory in a cooled Krebs-Ringer bicarbonate solution (4 degrees C, pH 7.5) bubbled with 95% O(2)/5% CO2. A planar sheet of the parietal peritoneum was mounted in an Ussing-type chamber and HgCl2 (10(-4) mol/L) was added apically or basolaterally. The RTM was measured before and serially after the addition of the HgCl2. The entire experimental apparatus was held at 37 degrees C, because active ion transport is temperature-dependent. The results presented are the mean +/- standard error of 12 experiments. The control RTM (that is, before the addition of HgCl) was 19.3 +/- 0.38 omega x cm2. Addition of HgCl2 apically induced a decrease in the RTM to 16.25 +/- 0.86 omega x cm2 within 1 minute. When added basolaterally, HgCl2 action was similar, with a rapid reduction in the RTM to 18.1 +/- 0.51 omega x cm2 (p < 0.05). A clear association between the RTM and the active transmesothelial ion transport was shown in previous studies. In the present study, rapid action of HgCl2 on the permeability ofthe parietal peritoneum was observed, resulting in a reduction in the RTM Taken together, these findings indicate that inhibition of aquaporin-1 alters the ionic permeability of the parietal peritoneal membrane.