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The effect of stevioside, an inhibitor of long-chain fatty acid transport, on ketogenesis and on [14C]CO2 production from [1-14C]palmitate (100-300 microM) was investigated in the isolated and hemoglobin-free perfused rat liver. Stevioside (2.5 mM), a sweet glycoside found in Stevia rebaudiana leaves, inhibited both parameters, but had a lower effect on [14C]CO2 production. At 300 microM palmitate and 150 microM albumin, for example, ketogenesis was inhibited by 66.3%, whereas no significant inhibition of [14C]CO2 was demonstrable. These results were interpreted to reflect 1) different degrees of saturation of the citric acid cycle and the ketogenic pathway and 2) changes in the redox state of the mitochondrial NAD(+)-NADH couple which may also occur upon stevioside infusion.

Binding of the anti-inflammatory drug niflumic acid to serum albumin was measured by equilibrium dialysis and the dissociation constants were determined. The maximal binding capacity was 36 mol niflumic acid per mol albumin. Most of the binding sites were of low affinity, only six having dissociation constants below 1 mM. At the plasma concentrations most frequently used in experimental work, the high affinity sites account for more than 99% of the albumin-bound niflumic acid.

Production of ketone bodies, activation of oxygen uptake and production of [14C] carbon dioxide due to fatty acids of various chain lengths were measured in the isolated perfused rat liver. As ketogenic substrates, the fatty acids investigated can be ordered in the following sequence: stearate less than palmitate less than laureate less than decanoate less than miristate less than octanoate. For activation of oxygen consumption, the same sequence is observed. A good linear correlation between the increase in oxygen uptake (delta O2) and production of ketone bodies was found, with a mean delta O2 of 0.66 mol per mol ketone bodies. This is a relatively low value when confronted with the known stoichiometry of beta-oxidation, and may be an indication of complexities in the metabolism of exogenously added fatty acids in the perfused liver.

The multiple indicator dilution technique was used to investigate octanoate transport in the isolated, perfused rat liver. The form of the octanoate outflow profiles depends on albumin concentration. The steady-state intracellular octanoate concentration and the rate of uptake increase when the albumin concentration is decreased. At physiological albumin concentrations, the intracellular concentration of octanoate is only 7% of the extracellular concentration. Exchange between the intra- and extracellular space, however, is very rapid, irrespective of the albumin concentration and consequently exchange is not rate limiting for metabolism.

Unidirectional fluxes (influx and efflux) and the net flux of palmitate across the hepatocyte membrane were measured in the intact rat liver employing the multiple indicator dilution technique. At albumin concentrations in the range between 0.1 and 0.5 mM the influx rate was 2.3 times greater than that of the net flux. The rate of efflux was somewhat higher than the net flux, indicating that palmitate undergoes an exchange process across the liver cell membranes with efflux to the extracellular albumin site being significant. At lower albumin concentrations, however, the influx/net flux ratio approached unity, implying that transport becomes a rate-limiting factor for metabolism.