Fasting increases the sensitivity of hepatic harmol glucuronidation to hypoxia.

Livers from fasted (N = 16) and fed (N = 22) rats were perfused with harmol (50 microM) for an initial 30 min with normal oxygen delivery (6-10 mumol/min/g liver), then for 45 min with perfusate equilibrated with O2/N2 mixtures, which reduced hepatic oxygen delivery to 0.9-6 mumol/min/g liver, and finally for a further 30 min period of normal oxygenation. Seventy per cent of the harmol eliminated was accounted for as the glucuronide conjugate and approximately 5% as the sulphate conjugate. During the hypoxia phase with fed preparations, decreasing oxygenation did not reduce harmol clearance or harmol glucuronide formation clearance until oxygen delivery was less than 2.5 mumol/min/g liver, whereas with fasted preparations this hypoxic threshold was much higher (5 mumol/min/g liver). Below the hypoxic threshold, harmol clearance was linearly related to oxygen delivery in both groups. Hepatic tissue concentrations of unchanged harmol at the end of the hypoxia phase were double those after the same period of normal oxygenation, whereas tissue harmol glucuronide concentrations were similar. By establishing a hypoxic threshold for reduced oxygen availability this study shows that harmol glucuronidation is relatively insensitive to hypoxia, but sensitivity increases markedly in fasted animals.