Effect of fasting on the enterohepatic circulation of bile acids in rats.

Fasting Sprague-Dawley rats for 72 h resulted in inhibition of bile salt synthesis, reduced bile flow and bile salt secretion rate, and reduced duodenal and portal venous bile acid concentrations. The initial rate of ileal brush border membrane (BBM) taurocholate (TC) uptake was markedly reduced in the fasted group (45% of control). TC uptake by BBM was saturable, with similar maximal transport velocity for the fasted rats and controls (1.69 +/- 0.06 and 1.62 +/- 0.017 nmol.mg protein-1.min-1, respectively) but higher Michaelis-Menten constant for fasted rats than for controls (96.9 +/- 20.0 and 54.9 +/- 10.2 microM, respectively). Hepatic basolateral membrane (BLM) TC uptake was enhanced by 65% in fasted animals. Transport kinetics in BLM had a similar Michaelis-Menten constant for fasted rats and controls (31.6 +/- 8.2 and 27.3 +/- 4.7 microM, respectively), and maximal transport velocity was higher for fasted rats than for controls (1.84 +/- 0.017 and 1.11 +/- 0.014 nmol.mg protein-1.min-1). The cholesterol-to-phospholipid ratio and fluorescence anisotropy in BLM of fasted rats decreased, and the cholesterol-to-phospholipid ratio and fluorescence anisotropy increased in ileal BBM. Alterations in the enterohepatic circulation of bile acids with fasting may alter expression of transport proteins for bile salts by direct effects on synthesis or indirectly through membrane lipid compositional changes.