Effects of morphine on canine intestinal absorption and blood flow

1 Intestinal absorption and blood flow were determined in anaesthetized fed or fasted dogs following rapid intravenous injections of morphine (0.01, 0.1, 1 mg/kg). 2 3H2O and 22Na were used to determine the unidirectional fluxes of Na+ and H2O from saline perfused through the ileal lumen and the clearances of 3H2O were used to determine total and absorptive site blood flow. 3 Net Na+ and H2O absorption were increased at each dose of morphine in fed but not in fasted dogs, due primarily to increased absorptive fluxes. 4 Arterial pressure was decreased by morphine but mesenteric vein pressure was little affected. Absorptive site blood flow was increased by morphine due to decreased blood flow resistance but total blood flow resistance was little affected by morphine. 5 The absorptive fluxes of Na+ and H2O were correlated with absorpitve site blood flow in both fed and fasted animals. The secretory fluxes of Na+ and H2O were correlated with estimated capiliary pressure in fasted dogs but morphine decreased the the secretory fluxes at a given capillary pressure in dogs which had been fed. 6 Naloxone (0.12 mg, i.v.) reversed the effects of morphine. The effects of morphine on the gut were reversed more slowly than on systemic blood pressure. 7 It was concluded that morphine can increase net absorption in fed dogs by a selective increase in intestinal absorptive site blood flow and thus increase absorptive fluxes by a washout effect but that there is also an epithelial effect, sensitized by feeding, which reduces the secretory fluxes of Na+ and H2O.