mu-Opioid stimulation of isolated parietal sheep peritoneum decreases peritoneal permeability in vitro

The peritoneal mesothelium is one of the main barriers to ion transport in peritoneal dialysis. In a previous study, we showed the existence of a micro-opioid influence on the in vitro ionic permeability of serosal membranes (specifically, pleura and pericardium), which become less permeable to ionic currents after the action of morphine. In the present study, we used Ussing chamber experiments to investigate the effect of morphine on the transmesothelial electrical resistance (RTM) of isolated parietal sheep peritoneum. Peritoneal samples from the diaphragm 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 planar sheet of parietal peritoneum was mounted in an Ussing-type chamber and morphine (10(-9) mol/L) was added apically and basolaterally. The RTM was measured before and serially for 30 minutes after the addition of morphine. Because active ion transport is temperature dependent, the Ussing chamber was held at 37 degrees C. Results presented are the mean +/- standard error of 6 experiments. The control RTM (before the addition of morphine) was 20.26 +/- 0.57 Omega x cm2. Addition of morphine basolaterally induced, within 1 minute, an increase in RTM of 24% +/- 4.8%, which declined thereafter (p < 0.01). When morphine was added apically, the results were not similar, because no significant change occurred in the RTM. The RTM is an established surrogate of peritoneal permeability. The results of the present study indicate rapid action of basolaterally added morphine on the permeability of the parietal peritoneum. The observed increase in the RTM indicates the existence in the parietal peritoneum of micro-opioid receptors that seem to prevail basolaterally. The clinical implications of these results should be further investigated.