The peritoneal mesothelium is a barrier to ion transport in peritoneal dialysis. In the present study, we used Ussing chamber experiments to investigate the effect of amiloride on the transmesothelial electrical resistance (R(TM)) of isolated visceral sheep peritoneum. Peritoneal samples from the omentum of adult sheep were isolated directly after the death of the animals and were transferred to the laboratory within 30 minutes in a cooled Krebs-Ringer bicarbonate solution (4 degrees C, pH 7.5) bubbled with 95% O2/5% CO2. A visceral peritoneal planar sheet was mounted in an Ussing-type chamber and amiloride (10(-3) mol/L) was added apically and basolaterally. The R(TM) was measured before and serially for 30 minutes after the addition of amiloride. Because active ion transport is temperature dependent, the Ussing chambers were held at 37 degrees C. The results presented are the means + standard error of 12 experiments. The control R(TM) (before the addition of amiloride) was 21.86 +/- 0.46 omega x cm2. Basolateral addition of amiloride induced, within 1 minute, an increase in R(TM) to 27.26 +/- 0.39 omega x cm2, a level that persisted throughout the experiment. When amiloride was added apically, the results were similar with a rapid rise of R(TM) to 24.18 +/- 0.9 omega x cm2 and subsequent value persistence (p < 0.05). A clear association between R(TM) and active ion transport was shown in previous studies. The results of the present study indicate rapid action of amiloride on the permeability of the visceral peritoneum. The observed increase in the R(TM) indicates the existence of amiloride-sensitive sodium channels in the visceral peritoneal membrane. The clinical implications of these results should be further investigated.