Leslie R. Schwarz

Cholestatic steroid hormones inhibit taurocholate uptake into isolated rat hepatocytes

Abstract The effect of three cholestatic steroids (norethandrolone, 17-β-estradiol and progesterone) on hepatocellular uptake and secretion of taurocholate was studied in isolated rat liver cells. The steroids decreased the rate of taurocholate uptake. Norethandrolone inhibited uptake noncompetitively with a Ki of 18 μM, but had no effect on the activation energy of uptake. 17-β-estradiol and progesterone reduced taurocholate uptake by 50 per cent at concentrations between 40 μM and 50 μM. The secretion of taurocholate from taurocholate-loaded cells was slightly increased by all three steroids at concentrations below 100 μM. A 60 per cent inhibition of secretion was observed in the presence of 500μM norethandrolone. Interference of cholestatic steroids with hepatocellular bile acid uptake may be an important step in the pathogenesis of intrahepatic cholestasis.

Taurolithocholate inhibits taurocholate uptake by isolated hepatocytes at low concentrations.

SummaryThe cholestatic bile acid taurolithocholate inhibits taurocholate uptake by isolated liver cells non-competitively. Inhibition is instantaneous and inversely related to the cell number in the incubate. The Kiamounts to 7 μM in the presence of 2 mg cellular protein per ml. Secretion of taurocholate by isolated liver cells is not affected by taurolithocholate up to a concentration of 50 μM. This indicates a difference between the carrier for taurocholate uptake and the carrier for taurocholate secretion. Inhibition of bile acid uptake by liver cells may be involved in the pathogenesis of lithocholate-induced cholestasis.

Uptake of taurocholate, a vecuronium-like organic cation, org 9426, and ouabain into carcinogen-induced diploid and polyploid hepatocytes obtained by centrifugal elutriation

Bile acid uptake, an important function of differentiated hepatocytes, is decreased in hepatocellular carcinomas and gamma-glutamyltranspeptidase-positive, putatively preneoplastic hepatocytes. Whether hepatic uptake is also changed in carcinogen-induced diploid hepatocytes versus polyploid hepatocytes is unknown. The present study has determined whether the hepatic uptake of three model compounds, an anionic bile acid, an organic cation and a neutral organic compound, into diploid cells is different from that in polyploid hepatocytes. These two hepatocyte populations were separated from the parent freshly isolated hepatocyte suspension by centrifugal elutriation. Flow cytometric analysis indicated that the diploid fraction contained approximately 83% diploid cells and that the polyploid fraction had about 84% polyploid hepatocytes. Initial uptake velocity was determined for taurocholate (1-50 microM), ORG 9426 (20-400 microM), a vecuronium-like cation, and ouabain (20-500 microM). Apparent Km was not different between diploid and polyploid cells for the three tested substrates, whereas apparent Vmax was decreased in diploid hepatocytes for taurocholate and ouabain by 42 and 55%, respectively. There were no changes in the hepatic uptake of ORG 9426. These data indicate that uptake by the bile acid/multispecific carrier is compromised in carcinogen-induced diploid cells.

Preincubation accelerates taurocholate uptake into isolated liver cells

Initial rates of taurocholate uptake into isolated hepatocytes stored at 0 degrees C increased 3-fold during a 25 min preincubation. Concomitantly, V increased while Km remained unaffected. There are several possible explanations for the preincubation effects, such as new synthesis of carrier protein, altered fluidity of the membrane or stimulation of the sodium-dependent taurocholate uptake via a change in the cation distribution. The experiments presented strongly favor the latter explanation as the sodium gradient as well as the uptake of the bile acid reach their steady state within 20-30 min and replacement of sodium by potassium in the medium abolished the effect.

Formation of L-ascorbic acid in perfused rat liver.

SummaryIn perfused rat liver L-ascorbic acid is secreted into the perfusion medium until a concentration of about 0.07 mM is reached corresponding with the concentration in rat serum. This suggests that the concentration of L-ascorbic acid in the blood predominately regulates L-ascorbic acid formation.