Influence of Morphine Tolerance and Withdrawal on Intestinal Salt and Water Transport in the Rat In Vivo and In Vitro

Abstract

Intestinal water and electrolyte transport was investigated in vivo and in vitro in rats made tolerant to morphine and subsequently withdrawn with naloxone. Rats were rendered tolerant by injection of a slow-release emulsion containing morphine (75 mg/rat over 48 h), and when challenged with naloxone, they exhibited a characteristic withdrawal syndrome that included acute diarrhea. Morphine tolerance did not influence water absorption from ileal or colonic loops in vivo but naloxone-induced withdrawal provoked a rapid and sustained reduction in absorption. Naloxone did not affect absorption in control animals. In further experiments, sodium and chloride fluxes were measured in isolated stripped ileal mucosa derived from animals made tolerant to morphine and withdrawn with naloxone. Net sodium and chloride absorption was markedly reduced in mucosa derived from withdrawn animals compared with that derived from tolerant animals (Na+: 1.12 +/- 0.76 vs. 5.52 +/- 0.46, and Cl-: 0.60 +/- 0.52 vs. 4.70 +/- 0.80 microEq/cm2 X h, in withdrawn and tolerant animals, respectively; n = 8; p less than 0.001 and p less than 0.005). When an attempt was made to induce withdrawal by adding naloxone in vitro to isolated mucosa derived from tolerant animals, no effect on transport was detected whether the mucosa was stripped of muscle layers or not. Thus naloxone had to be given in vivo to produce withdrawal effects in mucosa studied subsequently in vitro. Naloxone also had an effect in control rats when a 50% reduction in net chloride absorption was observed in isolated ileal mucosa derived from these animals. Net sodium absorption was unaffected. These data support a role for endogenous opiates in the control of intestinal transport and provide a mechanism for the diarrhea associated with opiate withdrawal.